Laser

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Laser 

A laser beam used for welding

 

Red (660 & 635 nm), green (532 & 520 nm) and blue-violet (445 & 405 nm) lasers

A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The first laser was built in 1960 by Theodore H. Maiman at Hughes Research Laboratories, based on theoretical work by Charles Hard Townes and Arthur Leonard Schawlow.

A laser differs from other sources of light in that it emits light which is coherent. Spatial coherence allows a laser to be focused to a tight spot, enabling applications such as laser cutting and lithography. Spatial coherence also allows a laser beam to stay narrow over great distances (collimation), enabling applications such as laser pointers and lidar. Lasers can also have high temporal coherence, which allows them to emit light with a very narrow spectrum. Alternatively, temporal coherence can be used to produce ultrashort pulses of light with a broad spectrum but durations as short as a femtosecond.

Lasers are used in optical disc drives, laser printers, barcode scanners, DNA sequencing instruments, fiber-optic, semiconducting chip manufacturing (photolithography), and free-space optical communication, laser surgery and skin treatments, cutting and welding materials, military and law enforcement devices for marking targets and measuring range and speed, and in laser lighting displays for entertainment. They have been used for car headlamps, by using a blue laser and a phosphor to produce highly directional white light.

Fundamentals

Modern telescopes use laser technologies to compensate for the blurring effect of the Earth's atmosphere.

Lasers are distinguished from other light sources by their coherence. Spatial (or transverse) coherence is typically expressed through the output being a narrow beam, which is diffraction-limited. Laser beams can be focused to very tiny spots, achieving a very high irradiance, or they can have very low divergence in order to concentrate their power at a great distance. Temporal (or longitudinal) coherence implies a polarized wave at a single frequency, whose phase is correlated over a relatively great distance (the coherence length) along the beam. A beam produced by a thermal or other incoherent light source has an instantaneous amplitude and phase that vary randomly with respect to time and position, thus having a short coherence length.

Lasers are characterized according to their wavelength in a vacuum. Most "single wavelength" lasers actually produce radiation in several modes with slightly different wavelengths. Although temporal coherence implies some degree of monochromaticity, there are lasers that emit a broad spectrum of light or emit different wavelengths of light simultaneously. Some lasers are not single spatial mode and have light beams that diverge more than is required by the diffraction limit. All such devices are classified as "lasers" based on the method of producing light by stimulated emission. Lasers are employed where light of the required spatial or temporal coherence can not be produced using simpler technologies.

Terminology

Laser beams in fog, reflected on a car windshield

The first device using amplification by stimulated emission operated at microwave frequencies, and was named "maser", an acronym for "microwave amplification by stimulated emission of radiation". When similar optical devices were developed they were first known as "optical masers", until "microwave" was replaced by "light" in the acronym.

All such devices operating at frequencies higher than microwaves are called lasers (including infrared laser, ultraviolet laser, X-ray laser and gamma-ray laser). All devices operating at microwave or lower radio frequencies are called masers.

A laser that produces light by itself is technically an optical oscillator rather than an optical amplifier as suggested by the acronym. It has been humorously noted that the acronym LOSER, for "light oscillation by stimulated emission of radiation", would have been more correct. With the widespread use of the original acronym as a common noun, optical amplifiers have come to be referred to as "laser amplifiers".

The back-formed verb to lase is frequently used in the field, meaning "to give off coherent light," especially in reference to the gain medium of a laser; when a laser is operating it is said to be "lasing". The words laser and maser are also used in cases where there is a coherent state unconnected with any manufactured device, as in astrophysical maser and atom laser.

Design

Components of a typical laser:

1. Gain medium
2. Laser pumping energy
3. High reflector
4. Output coupler
5. Laser beam

A laser consists of a gain medium, a mechanism to energize it, and something to provide optical feedback. The gain medium is a material with properties that allow it to amplify light by way of stimulated emission. Light of a specific wavelength that passes through the gain medium is amplified (increases in power). Feedback enables stimulated emission to amplify predominantly the optical frequency at the peak of the gain-frequency curve. As stimulated emission grows, eventually one frequency dominates over all others, meaning that a coherent beam has been formed. The process of stimulated emission is analogous to that of an audio oscillator with positive feedback which can occur, for example, when the speaker in a public-address system is placed in proximity to the microphone. The screech one hears is audio oscillation at the peak of the gain-frequency curve for the amplifier.

For the gain medium to amplify light, it needs to be supplied with energy in a process called pumping. The energy is typically supplied as an electric current or as light at a different wavelength. Pump light may be provided by a flash lamp or by another laser.

The most common type of laser uses feedback from an optical cavity—a pair of mirrors on either end of the gain medium. Light bounces back and forth between the mirrors, passing through the gain medium and being amplified each time. Typically one of the two mirrors, the output coupler, is partially transparent. Some of the light escapes through this mirror. Depending on the design of the cavity (whether the mirrors are flat or curved), the light coming out of the laser may spread out or form a narrow beam. In analogy to electronic oscillators, this device is sometimes called a laser oscillator.

Most practical lasers contain additional elements that affect properties of the emitted light, such as the polarization, wavelength, and shape of the beam.

Laser physics

Electrons and how they interact with electromagnetic fields are important in our understanding of chemistry and physics.

Stimulated emission

Play media

Animation explaining stimulated emission and the laser principle

In the classical view, the energy of an electron orbiting an atomic nucleus is larger for orbits further from the nucleus of an atom. However, quantum mechanical effects force electrons to take on discrete positions in orbitals. Thus, electrons are found in specific energy levels of an atom, two of which are shown below:

An electron in an atom can absorb energy from light (photons) or heat (phonons) only if there is a transition between energy levels that matches the energy carried by the photon or phonon. For light, this means that any given transition will only absorb one particular wavelength of light. Photons with the correct wavelength can cause an electron to jump from the lower to the higher energy level. The photon is consumed in this process.

When an electron is excited from one state to that at a higher energy level with energy difference ΔE, it will not stay that way forever. Eventually, a photon will be spontaneously created from the vacuum having energy ΔE . Conserving energy, the electron transitions to a lower energy level which is not occupied, with transitions to different levels having different time constants. This process is called "spontaneous emission". Spontaneous emission is a quantum-mechanical effect and a direct physical manifestation of the Heisenberg uncertainty principle. The emitted photon has random direction, but its wavelength matches the absorption wavelength of the transition. This is the mechanism of fluorescence and thermal emission.

A photon with the correct wavelength to be absorbed by a transition can also cause an electron to drop from the higher to the lower level, emitting a new photon. The emitted photon exactly matches the original photon in wavelength, phase, and direction. This process is called stimulated emission.

Gain medium and cavity

A helium–neon laser demonstration. The glow running through the center of the tube is an electric discharge. This glowing plasma is the gain medium for the laser. The laser produces a tiny, intense spot on the screen to the right. The center of the spot appears white because the image is overexposed there.

 

Spectrum of a helium–neon laser. The actual bandwidth is much narrower than shown; the spectrum is limited by the measuring apparatus.

The gain medium is put into an excited state by an external source of energy. In most lasers this medium consists of a population of atoms which have been excited into such a state by means of an outside light source, or an electrical field which supplies energy for atoms to absorb and be transformed into their excited states.

The gain medium of a laser is normally a material of controlled purity, size, concentration, and shape, which amplifies the beam by the process of stimulated emission described above. This material can be of any state: gas, liquid, solid, or plasma. The gain medium absorbs pump energy, which raises some electrons into higher-energy ("excited") quantum states. Particles can interact with light by either absorbing or emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, population inversion is achieved. In this state, the rate of stimulated emission is larger than the rate of absorption of light in the medium, and therefore the light is amplified. A system with this property is called an optical amplifier. When an optical amplifier is placed inside a resonant optical cavity, one obtains a laser.

In a few situations it is possible to obtain lasing with only a single pass of EM radiation through the gain medium, and this produces a laser beam without any need for a resonant or reflective cavity; thus, reflection in a resonant cavity is usually required for a laser, but is not absolutely necessary.

The optical resonator is sometimes referred to as an "optical cavity", but this is a misnomer: lasers use open resonators as opposed to the literal cavity that would be employed at microwave frequencies in a maser. The resonator typically consists of two mirrors between which a coherent beam of light travels in both directions, reflecting back on itself so that an average photon will pass through the gain medium repeatedly before it is emitted from the output aperture or lost to diffraction or absorption. If the gain (amplification) in the medium is larger than the resonator losses, then the power of the recirculating light can rise exponentially. But each stimulated emission event returns an atom from its excited state to the ground state, reducing the gain of the medium. With increasing beam power the net gain (gain minus loss) reduces to unity and the gain medium is said to be saturated. In a continuous wave (CW) laser, the balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the applied pump power is too small, the gain will never be sufficient to overcome the cavity losses, and laser light will not be produced. The minimum pump power needed to begin laser action is called the lasing threshold. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons in a spatial mode supported by the resonator will pass more than once through the medium and receive substantial amplification.

The light emitted

In most lasers, lasing begins with spontaneous emission into the lasing mode. This initial light is then amplified by stimulated emission in the gain medium. Stimulated emission produces light that matches the input signal in direction, wavelength, and polarization, whereas the phase of emitted light is 90 degrees in lead of the stimulating light. This, combined with the filtering effect of the optical resonator gives laser light its characteristic coherence, and may give it uniform polarization and monochromaticity, depending on the resonator's design. The fundamental laser linewidth of light emitted from the lasing resonator can be orders of magnitude narrower than the linewidth of light emitted from the passive resonator. Some lasers use a separate injection seeder to start the process off with a beam that is already highly coherent. This can produce beams with a narrower spectrum than would otherwise be possible.

In 1963, Roy J. Glauber showed that coherent states are formed from combinations of photon number states, for which he was awarded the Nobel Prize in physics. A coherent beam of light is formed by single-frequency quantum photon states distributed according to a Poisson distribution. As a result, the arrival rate of photons in a laser beam is described by Poisson statistics.

Many lasers produce a beam that can be approximated as a Gaussian beam; such beams have the minimum divergence possible for a given beam diameter. Some lasers, particularly high-power ones, produce multimode beams, with the transverse modes often approximated using Hermite–Gaussian or Laguerre-Gaussian functions. Some high power lasers use a flat-topped profile known as a "tophat beam". Unstable laser resonators (not used in most lasers) produce fractal-shaped beams. Specialized optical systems can produce more complex beam geometries, such as Bessel beams and optical vortexes.

Near the "waist" (or focal region) of a laser beam, it is highly collimated: the wavefronts are planar, normal to the direction of propagation, with no beam divergence at that point. However, due to diffraction, that can only remain true well within the Rayleigh range. The beam of a single transverse mode (gaussian beam) laser eventually diverges at an angle which varies inversely with the beam diameter, as required by diffraction theory. Thus, the "pencil beam" directly generated by a common helium–neon laser would spread out to a size of perhaps 500 kilometers when shone on the Moon (from the distance of the earth). On the other hand, the light from a semiconductor laser typically exits the tiny crystal with a large divergence: up to 50°. However even such a divergent beam can be transformed into a similarly collimated beam by means of a lens system, as is always included, for instance, in a laser pointer whose light originates from a laser diode. That is possible due to the light being of a single spatial mode. This unique property of laser light, spatial coherence, cannot be replicated using standard light sources (except by discarding most of the light) as can be appreciated by comparing the beam from a flashlight (torch) or spotlight to that of almost any laser.

A laser beam profiler is used to measure the intensity profile, width, and divergence of laser beams.

Diffuse reflection of a laser beam from a matte surface produces a speckle pattern with interesting properties.

Quantum vs. classical emission processes

The mechanism of producing radiation in a laser relies on stimulated emission, where energy is extracted from a transition in an atom or molecule. This is a quantum phenomenon discovered by Albert Einstein who derived the relationship between the A coefficient describing spontaneous emission and the B coefficient which applies to absorption and stimulated emission. However, in the case of the free electron laser, atomic energy levels are not involved; it appears that the operation of this rather exotic device can be explained without reference to quantum mechanics.

Continuous and pulsed modes of operation

Lidar measurements of lunar topography made by Clementine mission.

 

Laserlink point to point optical wireless network

 

Mercury Laser Altimeter (MLA) of the MESSENGER spacecraft

A laser can be classified as operating in either continuous or pulsed mode, depending on whether the power output is essentially continuous over time or whether its output takes the form of pulses of light on one or another time scale. Of course even a laser whose output is normally continuous can be intentionally turned on and off at some rate in order to create pulses of light. When the modulation rate is on time scales much slower than the cavity lifetime and the time period over which energy can be stored in the lasing medium or pumping mechanism, then it is still classified as a "modulated" or "pulsed" continuous wave laser. Most laser diodes used in communication systems fall in that category.

Continuous wave operation

Some applications of lasers depend on a beam whose output power is constant over time. Such a laser is known as continuous wave (CW). Many types of lasers can be made to operate in continuous wave mode to satisfy such an application. Many of these lasers actually lase in several longitudinal modes at the same time, and beats between the slightly different optical frequencies of those oscillations will, in fact, produce amplitude variations on time scales shorter than the round-trip time (the reciprocal of the frequency spacing between modes), typically a few nanoseconds or less. In most cases, these lasers are still termed "continuous wave" as their output power is steady when averaged over any longer time periods, with the very high-frequency power variations having little or no impact in the intended application. (However, the term is not applied to mode-locked lasers, where the intention is to create very short pulses at the rate of the round-trip time.)

For continuous wave operation, it is required for the population inversion of the gain medium to be continually replenished by a steady pump source. In some lasing media, this is impossible. In some other lasers, it would require pumping the laser at a very high continuous power level which would be impractical or destroy the laser by producing excessive heat. Such lasers cannot be run in CW mode.

Pulsed operation

Pulsed operation of lasers refers to any laser not classified as continuous wave, so that the optical power appears in pulses of some duration at some repetition rate. This encompasses a wide range of technologies addressing a number of different motivations. Some lasers are pulsed simply because they cannot be run in continuous mode.

In other cases, the application requires the production of pulses having as large an energy as possible. Since the pulse energy is equal to the average power divided by the repetition rate, this goal can sometimes be satisfied by lowering the rate of pulses so that more energy can be built up in between pulses. In laser ablation, for example, a small volume of material at the surface of a work piece can be evaporated if it is heated in a very short time, while supplying the energy gradually would allow for the heat to be absorbed into the bulk of the piece, never attaining a sufficiently high temperature at a particular point.

Other applications rely on the peak pulse power (rather than the energy in the pulse), especially in order to obtain nonlinear optical effects. For a given pulse energy, this requires creating pulses of the shortest possible duration utilizing techniques such as Q-switching.

The optical bandwidth of a pulse cannot be narrower than the reciprocal of the pulse width. In the case of extremely short pulses, that implies lasing over a considerable bandwidth, quite contrary to the very narrow bandwidths typical of CW lasers. The lasing medium in some dye lasers and vibronic solid-state lasers produces optical gain over a wide bandwidth, making a laser possible which can thus generate pulses of light as short as a few femtoseconds (10⁻¹⁵ s).

Q-switching

In a Q-switched laser, the population inversion is allowed to build up by introducing loss inside the resonator which exceeds the gain of the medium; this can also be described as a reduction of the quality factor or 'Q' of the cavity. Then, after the pump energy stored in the laser medium has approached the maximum possible level, the introduced loss mechanism (often an electro- or acousto-optical element) is rapidly removed (or that occurs by itself in a passive device), allowing lasing to begin which rapidly obtains the stored energy in the gain medium. This results in a short pulse incorporating that energy, and thus a high peak power.

Mode-locking

A mode-locked laser is capable of emitting extremely short pulses on the order of tens of picoseconds down to less than 10 femtoseconds. These pulses will repeat at the round trip time, that is, the time that it takes light to complete one round trip between the mirrors comprising the resonator. Due to the Fourier limit (also known as energy-time uncertainty), a pulse of such short temporal length has a spectrum spread over a considerable bandwidth. Thus such a gain medium must have a gain bandwidth sufficiently broad to amplify those frequencies. An example of a suitable material is titanium-doped, artificially grown sapphire (Ti:sapphire) which has a very wide gain bandwidth and can thus produce pulses of only a few femtoseconds duration.

Such mode-locked lasers are a most versatile tool for researching processes occurring on extremely short time scales (known as femtosecond physics, femtosecond chemistry and ultrafast science), for maximizing the effect of nonlinearity in optical materials (e.g. in second-harmonic generation, parametric down-conversion, optical parametric oscillators and the like). Due to the large peak power and the ability to generate phase-stabilized trains of ultrafast laser pulses, mode-locking ultrafast lasers underpin precision metrology and spectroscopy applications.

Pulsed pumping

Another method of achieving pulsed laser operation is to pump the laser material with a source that is itself pulsed, either through electronic charging in the case of flash lamps, or another laser which is already pulsed. Pulsed pumping was historically used with dye lasers where the inverted population lifetime of a dye molecule was so short that a high energy, fast pump was needed. The way to overcome this problem was to charge up large capacitors which are then switched to discharge through flashlamps, producing an intense flash. Pulsed pumping is also required for three-level lasers in which the lower energy level rapidly becomes highly populated preventing further lasing until those atoms relax to the ground state. These lasers, such as the excimer laser and the copper vapor laser, can never be operated in CW mode.

History

Foundations

In 1917, Albert Einstein established the theoretical foundations for the laser and the maser in the paper Zur Quantentheorie der Strahlung (On the Quantum Theory of Radiation) via a re-derivation of Max Planck's law of radiation, conceptually based upon probability coefficients (Einstein coefficients) for the absorption, spontaneous emission, and stimulated emission of electromagnetic radiation. In 1928, Rudolf W. Ladenburg confirmed the existence of the phenomena of stimulated emission and negative absorption. In 1939, Valentin A. Fabrikant predicted the use of stimulated emission to amplify "short" waves. In 1947, Willis E. Lamb and R.C. Retherford found apparent stimulated emission in hydrogen spectra and effected the first demonstration of stimulated emission. In 1950, Alfred Kastler (Nobel Prize for Physics 1966) proposed the method of optical pumping, experimentally confirmed, two years later, by Brossel, Kastler, and Winter.

Maser

Aleksandr Prokhorov

In 1951, Joseph Weber submitted a paper on using stimulated emissions to make a microwave amplifier to the June 1952 Institute of Radio Engineers Vacuum Tube Research Conference at Ottawa, Ontario, Canada. After this presentation, RCA asked Weber to give a seminar on this idea, and Charles Hard Townes asked him for a copy of the paper.

Charles H. Townes

In 1953, Charles Hard Townes and graduate students James P. Gordon and Herbert J. Zeiger produced the first microwave amplifier, a device operating on similar principles to the laser, but amplifying microwave radiation rather than infrared or visible radiation. Townes's maser was incapable of continuous output. Meanwhile, in the Soviet Union, Nikolay Basov and Aleksandr Prokhorov were independently working on the quantum oscillator and solved the problem of continuous-output systems by using more than two energy levels. These gain media could release stimulated emissions between an excited state and a lower excited state, not the ground state, facilitating the maintenance of a population inversion. In 1955, Prokhorov and Basov suggested optical pumping of a multi-level system as a method for obtaining the population inversion, later a main method of laser pumping.

Townes reports that several eminent physicists—among them Niels Bohr, John von Neumann, and Llewellyn Thomas—argued the maser violated Heisenberg's uncertainty principle and hence could not work. Others such as Isidor Rabi and Polykarp Kusch expected that it would be impractical and not worth the effort. In 1964 Charles H. Townes, Nikolay Basov, and Aleksandr Prokhorov shared the Nobel Prize in Physics, "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser–laser principle".

Laser

In April 1957, Japanese engineer Jun-ichi Nishizawa proposed the concept of a "semiconductor optical maser" in a patent application.

That same year, Charles Hard Townes and Arthur Leonard Schawlow, then at Bell Labs, began a serious study of infrared "optical masers". As ideas developed, they abandoned infrared radiation to instead concentrate on visible light. In 1958, Bell Labs filed a patent application for their proposed optical maser; and Schawlow and Townes submitted a manuscript of their theoretical calculations to the Physical Review, which was published in 1958.

LASER notebook: First page of the notebook wherein Gordon Gould coined the acronym LASER, and described the elements required to construct one

Simultaneously, at Columbia University, graduate student Gordon Gould was working on a doctoral thesis about the energy levels of excited thallium. When Gould and Townes met, they spoke of radiation emission, as a general subject; afterwards, in November 1957, Gould noted his ideas for a "laser", including using an open resonator (later an essential laser-device component). Moreover, in 1958, Prokhorov independently proposed using an open resonator, the first published appearance of this idea. Meanwhile, Schawlow and Townes had decided on an open-resonator laser design – apparently unaware of Prokhorov's publications and Gould's unpublished laser work.

At a conference in 1959, Gordon Gould first published the acronym "LASER" in the paper The LASER, Light Amplification by Stimulated Emission of Radiation. Gould's intention was that different "-ASER" acronyms should be used for different parts of the spectrum: "XASER" for x-rays, "UVASER" for ultraviolet, etc. "LASER" ended up becoming the generic term for non-microwave devices, although "RASER" was briefly popular for denoting radio-frequency-emitting devices.

Gould's notes included possible applications for a laser, such as spectrometry, interferometry, radar, and nuclear fusion. He continued developing the idea, and filed a patent application in April 1959. The U.S. Patent Office denied his application, and awarded a patent to Bell Labs, in 1960. That provoked a twenty-eight-year lawsuit, featuring scientific prestige and money as the stakes. Gould won his first minor patent in 1977, yet it was not until 1987 that he won the first significant patent lawsuit victory, when a Federal judge ordered the U.S. Patent Office to issue patents to Gould for the optically pumped and the gas discharge laser devices. The question of just how to assign credit for inventing the laser remains unresolved by historians.

On May 16, 1960, Theodore H. Maiman operated the first functioning laser at Hughes Research Laboratories, Malibu, California, ahead of several research teams, including those of Townes, at Columbia University, Arthur Schawlow, at Bell Labs, and Gould, at the TRG (Technical Research Group) company. Maiman's functional laser used a flashlamp-pumped synthetic ruby crystal to produce red laser light at 694 nanometers wavelength. The device was only capable of pulsed operation, due to its three-level pumping design scheme. Later that year, the Iranian physicist Ali Javan, and William R. Bennett, and Donald Herriott, constructed the first gas laser, using helium and neon that was capable of continuous operation in the infrared (U.S. Patent 3,149,290); later, Javan received the Albert Einstein Award in 1993. Basov and Javan proposed the semiconductor laser diode concept. In 1962, Robert N. Hall demonstrated the first laser diode device, which was made of gallium arsenide and emitted in the near-infrared band of the spectrum at 850 nm. Later that year, Nick Holonyak, Jr. demonstrated the first semiconductor laser with a visible emission. This first semiconductor laser could only be used in pulsed-beam operation, and when cooled to liquid nitrogen temperatures (77 K). In 1970, Zhores Alferov, in the USSR, and Izuo Hayashi and Morton Panish of Bell Telephone Laboratories also independently developed room-temperature, continual-operation diode lasers, using the heterojunction structure.

Recent innovations

Graph showing the history of maximum laser pulse intensity throughout the past 40 years.

Since the early period of laser history, laser research has produced a variety of improved and specialized laser types, optimized for different performance goals, including:

• new wavelength bands
• maximum average output power
• maximum peak pulse energy
• maximum peak pulse power
• minimum output pulse duration
• minimum linewidth
• maximum power efficiency
• minimum cost

and this research continues to this day.

In 2015, researchers made a white laser, whose light is modulated by a synthetic nanosheet made out of zinc, cadmium, sulfur, and selenium that can emit red, green, and blue light in varying proportions, with each wavelength spanning 191 nm.

In 2017, researchers at TU Delft demonstrated an AC Josephson junction microwave laser. Since the laser operates in the superconducting regime, it is more stable than other semiconductor-based lasers. The device has potential for applications in quantum computing. In 2017, researchers at TU Munich demonstrated the smallest mode locking laser capable of emitting pairs of phase-locked picosecond laser pulses with a repetition frequency up to 200 GHz.

In 2017, researchers from the Physikalisch-Technische Bundesanstalt (PTB), together with US researchers from JILA, a joint institute of the National Institute of Standards and Technology (NIST) and the University of Colorado Boulder, established a new world record by developing an erbium-doped fiber laser with a linewidth of only 10 millihertz.

Types and operating principles

Wavelengths of commercially available lasers. Laser types with distinct laser lines are shown above the wavelength bar, while below are shown lasers that can emit in a wavelength range. The color codifies the type of laser material (see the figure description for more details).

Gas lasers

Following the invention of the HeNe gas laser, many other gas discharges have been found to amplify light coherently. Gas lasers using many different gases have been built and used for many purposes. The helium–neon laser (HeNe) is able to operate at a number of different wavelengths, however the vast majority are engineered to lase at 633 nm; these relatively low cost but highly coherent lasers are extremely common in optical research and educational laboratories. Commercial carbon dioxide (CO₂) lasers can emit many hundreds of watts in a single spatial mode which can be concentrated into a tiny spot. This emission is in the thermal infrared at 10.6 µm; such lasers are regularly used in industry for cutting and welding. The efficiency of a CO₂ laser is unusually high: over 30%. Argon-ion lasers can operate at a number of lasing transitions between 351 and 528.7 nm. Depending on the optical design one or more of these transitions can be lasing simultaneously; the most commonly used lines are 458 nm, 488 nm and 514.5 nm. A nitrogen transverse electrical discharge in gas at atmospheric pressure (TEA) laser is an inexpensive gas laser, often home-built by hobbyists, which produces rather incoherent UV light at 337.1 nm. Metal ion lasers are gas lasers that generate deep ultraviolet wavelengths. Helium-silver (HeAg) 224 nm and neon-copper (NeCu) 248 nm are two examples. Like all low-pressure gas lasers, the gain media of these lasers have quite narrow oscillation linewidths, less than 3 GHz (0.5 picometers), making them candidates for use in fluorescence suppressed Raman spectroscopy.

Lasing without maintaining the medium excited into a population inversion was demonstrated in 1992 in sodium gas and again in 1995 in rubidium gas by various international teams. This was accomplished by using an external maser to induce "optical transparency" in the medium by introducing and destructively interfering the ground electron transitions between two paths, so that the likelihood for the ground electrons to absorb any energy has been cancelled.

Chemical lasers

Chemical lasers are powered by a chemical reaction permitting a large amount of energy to be released quickly. Such very high power lasers are especially of interest to the military, however continuous wave chemical lasers at very high power levels, fed by streams of gasses, have been developed and have some industrial applications. As examples, in the hydrogen fluoride laser (2700–2900 nm) and the deuterium fluoride laser (3800 nm) the reaction is the combination of hydrogen or deuterium gas with combustion products of ethylene in nitrogen trifluoride.

Excimer lasers

Excimer lasers are a special sort of gas laser powered by an electric discharge in which the lasing medium is an excimer, or more precisely an exciplex in existing designs. These are molecules which can only exist with one atom in an excited electronic state. Once the molecule transfers its excitation energy to a photon, its atoms are no longer bound to each other and the molecule disintegrates. This drastically reduces the population of the lower energy state thus greatly facilitating a population inversion. Excimers currently used are all noble gas compounds; noble gasses are chemically inert and can only form compounds while in an excited state. Excimer lasers typically operate at ultraviolet wavelengths with major applications including semiconductor photolithography and LASIK eye surgery. Commonly used excimer molecules include ArF (emission at 193 nm), KrCl (222 nm), KrF (248 nm), XeCl (308 nm), and XeF (351 nm). The molecular fluorine laser, emitting at 157 nm in the vacuum ultraviolet is sometimes referred to as an excimer laser, however this appears to be a misnomer inasmuch as F₂ is a stable compound.

Solid-state lasers

A 50 W FASOR, based on a Nd:YAG laser, used at the Starfire Optical Range

Solid-state lasers use a crystalline or glass rod which is "doped" with ions that provide the required energy states. For example, the first working laser was a ruby laser, made from ruby (chromium-doped corundum). The population inversion is actually maintained in the dopant. These materials are pumped optically using a shorter wavelength than the lasing wavelength, often from a flashtube or from another laser. The usage of the term "solid-state" in laser physics is narrower than in typical use. Semiconductor lasers (laser diodes) are typically not referred to as solid-state lasers.

Neodymium is a common dopant in various solid-state laser crystals, including yttrium orthovanadate (Nd:YVO₄), yttrium lithium fluoride (Nd:YLF) and yttrium aluminium garnet (Nd:YAG). All these lasers can produce high powers in the infrared spectrum at 1064 nm. They are used for cutting, welding and marking of metals and other materials, and also in spectroscopy and for pumping dye lasers. These lasers are also commonly frequency doubled, tripled or quadrupled to produce 532 nm (green, visible), 355 nm and 266 nm (UV) beams, respectively. Frequency-doubled diode-pumped solid-state (DPSS) lasers are used to make bright green laser pointers.

Ytterbium, holmium, thulium, and erbium are other common "dopants" in solid-state lasers. Ytterbium is used in crystals such as Yb:YAG, Yb:KGW, Yb:KYW, Yb:SYS, Yb:BOYS, Yb:CaF₂, typically operating around 1020–1050 nm. They are potentially very efficient and high powered due to a small quantum defect. Extremely high powers in ultrashort pulses can be achieved with Yb:YAG. Holmium-doped YAG crystals emit at 2097 nm and form an efficient laser operating at infrared wavelengths strongly absorbed by water-bearing tissues. The Ho-YAG is usually operated in a pulsed mode, and passed through optical fiber surgical devices to resurface joints, remove rot from teeth, vaporize cancers, and pulverize kidney and gall stones.

Titanium-doped sapphire (Ti:sapphire) produces a highly tunable infrared laser, commonly used for spectroscopy. It is also notable for use as a mode-locked laser producing ultrashort pulses of extremely high peak power.

Thermal limitations in solid-state lasers arise from unconverted pump power that heats the medium. This heat, when coupled with a high thermo-optic coefficient (dn/dT) can cause thermal lensing and reduce the quantum efficiency. Diode-pumped thin disk lasers overcome these issues by having a gain medium that is much thinner than the diameter of the pump beam. This allows for a more uniform temperature in the material. Thin disk lasers have been shown to produce beams of up to one kilowatt.

Fiber lasers

Solid-state lasers or laser amplifiers where the light is guided due to the total internal reflection in a single mode optical fiber are instead called fiber lasers. Guiding of light allows extremely long gain regions providing good cooling conditions; fibers have high surface area to volume ratio which allows efficient cooling. In addition, the fiber's waveguiding properties tend to reduce thermal distortion of the beam. Erbium and ytterbium ions are common active species in such lasers.

Quite often, the fiber laser is designed as a double-clad fiber. This type of fiber consists of a fiber core, an inner cladding and an outer cladding. The index of the three concentric layers is chosen so that the fiber core acts as a single-mode fiber for the laser emission while the outer cladding acts as a highly multimode core for the pump laser. This lets the pump propagate a large amount of power into and through the active inner core region, while still having a high numerical aperture (NA) to have easy launching conditions.

Pump light can be used more efficiently by creating a fiber disk laser, or a stack of such lasers.

Fiber lasers have a fundamental limit in that the intensity of the light in the fiber cannot be so high that optical nonlinearities induced by the local electric field strength can become dominant and prevent laser operation and/or lead to the material destruction of the fiber. This effect is called photodarkening. In bulk laser materials, the cooling is not so efficient, and it is difficult to separate the effects of photodarkening from the thermal effects, but the experiments in fibers show that the photodarkening can be attributed to the formation of long-living color centers.

Photonic crystal lasers

Photonic crystal lasers are lasers based on nano-structures that provide the mode confinement and the density of optical states (DOS) structure required for the feedback to take place. They are typical micrometer-sized and tunable on the bands of the photonic crystals.

Semiconductor lasers

A 5.6 mm 'closed can' commercial laser diode, such as those used in a CD or DVD player

Semiconductor lasers are diodes which are electrically pumped. Recombination of electrons and holes created by the applied current introduces optical gain. Reflection from the ends of the crystal form an optical resonator, although the resonator can be external to the semiconductor in some designs.

Commercial laser diodes emit at wavelengths from 375 nm to 3500 nm. Low to medium power laser diodes are used in laser pointers, laser printers and CD/DVD players. Laser diodes are also frequently used to optically pump other lasers with high efficiency. The highest power industrial laser diodes, with power up to 20 kW, are used in industry for cutting and welding. External-cavity semiconductor lasers have a semiconductor active medium in a larger cavity. These devices can generate high power outputs with good beam quality, wavelength-tunable narrow-linewidth radiation, or ultrashort laser pulses.

In 2012, Nichia and OSRAM developed and manufactured commercial high-power green laser diodes (515/520 nm), which compete with traditional diode-pumped solid-state lasers.

Vertical cavity surface-emitting lasers (VCSELs) are semiconductor lasers whose emission direction is perpendicular to the surface of the wafer. VCSEL devices typically have a more circular output beam than conventional laser diodes. As of 2005, only 850 nm VCSELs are widely available, with 1300 nm VCSELs beginning to be commercialized, and 1550 nm devices an area of research. VECSELs are external-cavity VCSELs. Quantum cascade lasers are semiconductor lasers that have an active transition between energy sub-bands of an electron in a structure containing several quantum wells.

The development of a silicon laser is important in the field of optical computing. Silicon is the material of choice for integrated circuits, and so electronic and silicon photonic components (such as optical interconnects) could be fabricated on the same chip. Unfortunately, silicon is a difficult lasing material to deal with, since it has certain properties which block lasing. However, recently teams have produced silicon lasers through methods such as fabricating the lasing material from silicon and other semiconductor materials, such as indium(III) phosphide or gallium(III) arsenide, materials which allow coherent light to be produced from silicon. These are called hybrid silicon laser. Recent developments have also shown the use of monolithically integrated nanowire lasers directly on silicon for optical interconnects, paving the way for chip level applications. These heterostructure nanowire lasers capable of optical interconnects in silicon are also capable of emitting pairs of phase-locked picosecond pulses with a repetition frequency up to 200 GHz, allowing for on-chip optical signal processing. Another type is a Raman laser, which takes advantage of Raman scattering to produce a laser from materials such as silicon.

Dye lasers

Close-up of a table-top dye laser based on Rhodamine 6G

Dye lasers use an organic dye as the gain medium. The wide gain spectrum of available dyes, or mixtures of dyes, allows these lasers to be highly tunable, or to produce very short-duration pulses (on the order of a few femtoseconds). Although these tunable lasers are mainly known in their liquid form, researchers have also demonstrated narrow-linewidth tunable emission in dispersive oscillator configurations incorporating solid-state dye gain media. In their most prevalent form these solid state dye lasers use dye-doped polymers as laser media.

Free-electron lasers

The free-electron laser FELIX at the FOM Institute for Plasma Physics Rijnhuizen, Nieuwegein

Free-electron lasers, or FELs, generate coherent, high power radiation that is widely tunable, currently ranging in wavelength from microwaves through terahertz radiation and infrared to the visible spectrum, to soft X-rays. They have the widest frequency range of any laser type. While FEL beams share the same optical traits as other lasers, such as coherent radiation, FEL operation is quite different. Unlike gas, liquid, or solid-state lasers, which rely on bound atomic or molecular states, FELs use a relativistic electron beam as the lasing medium, hence the term free-electron.

Exotic media

The pursuit of a high-quantum-energy laser using transitions between isomeric states of an atomic nucleus has been the subject of wide-ranging academic research since the early 1970s. Much of this is summarized in three review articles. This research has been international in scope, but mainly based in the former Soviet Union and the United States. While many scientists remain optimistic that a breakthrough is near, an operational gamma-ray laser is yet to be realized.

Some of the early studies were directed toward short pulses of neutrons exciting the upper isomer state in a solid so the gamma-ray transition could benefit from the line-narrowing of Mössbauer effect. In conjunction, several advantages were expected from two-stage pumping of a three-level system. It was conjectured that the nucleus of an atom, embedded in the near field of a laser-driven coherently-oscillating electron cloud would experience a larger dipole field than that of the driving laser. Furthermore, nonlinearity of the oscillating cloud would produce both spatial and temporal harmonics, so nuclear transitions of higher multipolarity could also be driven at multiples of the laser frequency.

In September 2007, the BBC News reported that there was speculation about the possibility of using positronium annihilation to drive a very powerful gamma ray laser. Dr. David Cassidy of the University of California, Riverside proposed that a single such laser could be used to ignite a nuclear fusion reaction, replacing the banks of hundreds of lasers currently employed in inertial confinement fusion experiments.

Space-based X-ray lasers pumped by a nuclear explosion have also been proposed as antimissile weapons. Such devices would be one-shot weapons.

Living cells have been used to produce laser light. The cells were genetically engineered to produce green fluorescent protein (GFP). The GFP is used as the laser's "gain medium", where light amplification takes place. The cells were then placed between two tiny mirrors, just 20 millionths of a meter across, which acted as the "laser cavity" in which light could bounce many times through the cell. Upon bathing the cell with blue light, it could be seen to emit directed and intense green laser light.

Natural lasers

Like astrophysical masers, irradiated planetary or stellar gases may amplify light producing a natural laser. Mars, Venus and MWC 349 exhibit this phenomenon.

Uses

Lasers range in size from microscopic diode lasers (top) with numerous applications, to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion, nuclear weapons research and other high energy density physics experiments.

When lasers were invented in 1960, they were called "a solution looking for a problem". Since then, they have become ubiquitous, finding utility in thousands of highly varied applications in every section of modern society, including consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military. Fiber-optic communication using lasers is a key technology in modern communications, allowing services such as the Internet.

The first widely noticeable use of lasers was the supermarket barcode scanner, introduced in 1974. The laserdisc player, introduced in 1978, was the first successful consumer product to include a laser but the compact disc player was the first laser-equipped device to become common, beginning in 1982 followed shortly by laser printers.

Some other uses are:

• Communications: besides fiber-optic communication, lasers are used for free-space optical communication, including laser communication in space.
• Medicine: see below.
• Industry: cutting including converting thin materials, welding, material heat treatment, marking parts (engraving and bonding), additive manufacturing or 3D printing processes such as selective laser sintering and selective laser melting, non-contact measurement of parts and 3D scanning, and laser cleaning.
• Military: marking targets, guiding munitions, missile defense, electro-optical countermeasures (EOCM), lidar, blinding troops, firearms sight. See below
• Law enforcement: LIDAR traffic enforcement. Lasers are used for latent fingerprint detection in the forensic identification field
• Research: spectroscopy, laser ablation, laser annealing, laser scattering, laser interferometry, lidar, laser capture microdissection, fluorescence microscopy, metrology, laser cooling.
• Commercial products: laser printers, barcode scanners, thermometers, laser pointers, holograms, bubblegrams.
• Entertainment: optical discs, laser lighting displays, laser turntables

In 2004, excluding diode lasers, approximately 131,000 lasers were sold with a value of US$2.19 billion. In the same year, approximately 733 million diode lasers, valued at $3.20 billion, were sold.

In medicine

Lasers have many uses in medicine, including laser surgery (particularly eye surgery), laser healing, kidney stone treatment, ophthalmoscopy, and cosmetic skin treatments such as acne treatment, cellulite and striae reduction, and hair removal.

Lasers are used to treat cancer by shrinking or destroying tumors or precancerous growths. They are most commonly used to treat superficial cancers that are on the surface of the body or the lining of internal organs. They are used to treat basal cell skin cancer and the very early stages of others like cervical, penile, vaginal, vulvar, and non-small cell lung cancer. Laser therapy is often combined with other treatments, such as surgery, chemotherapy, or radiation therapy. Laser-induced interstitial thermotherapy (LITT), or interstitial laser photocoagulation, uses lasers to treat some cancers using hyperthermia, which uses heat to shrink tumors by damaging or killing cancer cells. Lasers are more precise than traditional surgery methods and cause less damage, pain, bleeding, swelling, and scarring. A disadvantage is that surgeons must have specialized training. It may be more expensive than other treatments.

As weapons

A laser weapon is a laser that is used as a directed-energy weapon.

The US–Israeli Tactical High Energy weapon has been used to shoot down rockets and artillery shells.

Hobbies

In recent years, some hobbyists have taken interests in lasers. Lasers used by hobbyists are generally of class IIIa or IIIb, although some have made their own class IV types. However, compared to other hobbyists, laser hobbyists are far less common, due to the cost and potential dangers involved. Due to the cost of lasers, some hobbyists use inexpensive means to obtain lasers, such as salvaging laser diodes from broken DVD players (red), Blu-ray players (violet), or even higher power laser diodes from CD or DVD burners.

Hobbyists also have been taking surplus pulsed lasers from retired military applications and modifying them for pulsed holography. Pulsed Ruby and pulsed YAG lasers have been used.

Examples by power

Laser application in astronomical adaptive optics imaging

Different applications need lasers with different output powers. Lasers that produce a continuous beam or a series of short pulses can be compared on the basis of their average power. Lasers that produce pulses can also be characterized based on the peak power of each pulse. The peak power of a pulsed laser is many orders of magnitude greater than its average power. The average output power is always less than the power consumed.

Safety

Top: European laser warning symbol required for Class 2 lasers and higher. Bottom: US laser warning label, in this case for a Class 3B laser

Even the first laser was recognized as being potentially dangerous. Theodore Maiman characterized the first laser as having a power of one "Gillette" as it could burn through one Gillette razor blade. Today, it is accepted that even low-power lasers with only a few milliwatts of output power can be hazardous to human eyesight when the beam hits the eye directly or after reflection from a shiny surface. At wavelengths which the cornea and the lens can focus well, the coherence and low divergence of laser light means that it can be focused by the eye into an extremely small spot on the retina, resulting in localized burning and permanent damage in seconds or even less time.

Lasers are usually labeled with a safety class number, which identifies how dangerous the laser is:

• Class 1 is inherently safe, usually because the light is contained in an enclosure, for example in CD players.
• Class 2 is safe during normal use; the blink reflex of the eye will prevent damage. Usually up to 1 mW power, for example laser pointers.
• Class 3R (formerly IIIa) lasers are usually up to 5 mW and involve a small risk of eye damage within the time of the blink reflex. Staring into such a beam for several seconds is likely to cause damage to a spot on the retina.
• Class 3B can cause immediate eye damage upon exposure.
• Class 4 lasers can burn skin, and in some cases, even scattered light can cause eye and/or skin damage. Many industrial and scientific lasers are in this class.

The indicated powers are for visible-light, continuous-wave lasers. For pulsed lasers and invisible wavelengths, other power limits apply. People working with class 3B and class 4 lasers can protect their eyes with safety goggles which are designed to absorb light of a particular wavelength.

Infrared lasers with wavelengths longer than about 1.4 micrometers are often referred to as "eye-safe", because the cornea tends to absorb light at these wavelengths, protecting the retina from damage. The label "eye-safe" can be misleading, however, as it applies only to relatively low power continuous wave beams; a high power or Q-switched laser at these wavelengths can burn the cornea, causing severe eye damage, and even moderate power lasers can injure the eye.

Lasers can be a hazard to both civil and military aviation, due to the potential to temporarily distract or blind pilots. See Lasers and aviation safety for more on this topic.

Cameras based on charge-coupled devices may actually be more sensitive to laser damage than biological eyes.

Ultimate fate of the universe

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Ultimate_fate_of_the_universe 

Physical cosmology

The ultimate fate of the universe is a topic in physical cosmology, whose theoretical restrictions allow possible scenarios for the evolution and ultimate fate of the universe to be described and evaluated. Based on available observational evidence, deciding the fate and evolution of the universe has become a valid cosmological question, being beyond the mostly untestable constraints of mythological or theological beliefs. Several possible futures have been predicted by different scientific hypotheses, including that the universe might have existed for a finite and infinite duration, or towards explaining the manner and circumstances of its beginning.

Observations made by Edwin Hubble during the 1920s–1950s found that galaxies appeared to be moving away from each other, leading to the currently accepted Big Bang theory. This suggests that the universe began – very small and very dense – about 13.82 billion years ago, and it has expanded and (on average) become less dense ever since. Confirmation of the Big Bang mostly depends on knowing the rate of expansion, average density of matter, and the physical properties of the mass–energy in the universe.

There is a strong consensus among cosmologists that the universe is considered "flat" and will continue to expand forever.

Factors that need to be considered in determining the universe's origin and ultimate fate include the average motions of galaxies, the shape and structure of the universe, and the amount of dark matter and dark energy that the universe contains.

Emerging scientific basis

Theory

The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1915 theory of general relativity. General relativity can be employed to describe the universe on the largest possible scale. There are several possible solutions to the equations of general relativity, and each solution implies a possible ultimate fate of the universe.

Alexander Friedmann proposed several solutions in 1922, as did Georges Lemaître in 1927. In some of these solutions, the universe has been expanding from an initial singularity which was, essentially, the Big Bang.

Observation

In 1929, Edwin Hubble published his conclusion, based on his observations of Cepheid variable stars in distant galaxies, that the universe was expanding. From then on, the beginning of the universe and its possible end have been the subjects of serious scientific investigation.

Big Bang and Steady State theories

In 1927, Georges Lemaître set out a theory that has since come to be called the Big Bang theory of the origin of the universe. In 1948, Fred Hoyle set out his opposing Steady State theory in which the universe continually expanded but remained statistically unchanged as new matter is constantly created. These two theories were active contenders until the 1965 discovery, by Arno Penzias and Robert Wilson, of the cosmic microwave background radiation, a fact that is a straightforward prediction of the Big Bang theory, and one that the original Steady State theory could not account for. As a result, the Big Bang theory quickly became the most widely held view of the origin of the universe.

Cosmological constant

Einstein and his contemporaries believed in a static universe. When Einstein found that his general relativity equations could easily be solved in such a way as to allow the universe to be expanding at the present and contracting in the far future, he added to those equations what he called a cosmological constant ⁠— ⁠essentially a constant energy density, unaffected by any expansion or contraction ⁠— ⁠whose role was to offset the effect of gravity on the universe as a whole in such a way that the universe would remain static. However, after Hubble announced his conclusion that the universe was expanding, Einstein would write that his cosmological constant was "the greatest blunder of my life."

Density parameter

An important parameter in fate of the universe theory is the density parameter, omega (${\displaystyle \Omega }$), defined as the average matter density of the universe divided by a critical value of that density. This selects one of three possible geometries depending on whether ${\displaystyle \Omega }$ is equal to, less than, or greater than ${\displaystyle 1}$. These are called, respectively, the flat, open and closed universes. These three adjectives refer to the overall geometry of the universe, and not to the local curving of spacetime caused by smaller clumps of mass (for example, galaxies and stars). If the primary content of the universe is inert matter, as in the dust models popular for much of the 20th century, there is a particular fate corresponding to each geometry. Hence cosmologists aimed to determine the fate of the universe by measuring ${\displaystyle \Omega }$, or equivalently the rate at which the expansion was decelerating.

Repulsive force

Starting in 1998, observations of supernovas in distant galaxies have been interpreted as consistent with a universe whose expansion is accelerating. Subsequent cosmological theorizing has been designed so as to allow for this possible acceleration, nearly always by invoking dark energy, which in its simplest form is just a positive cosmological constant. In general, dark energy is a catch-all term for any hypothesized field with negative pressure, usually with a density that changes as the universe expands.

Role of the shape of the universe

The ultimate fate of an expanding universe depends on the matter density ${\displaystyle \Omega _{M}}$ and the dark energy density ${\displaystyle \Omega _{\Lambda }}$

The current scientific consensus of most cosmologists is that the ultimate fate of the universe depends on its overall shape, how much dark energy it contains and on the equation of state which determines how the dark energy density responds to the expansion of the universe. Recent observations conclude, from 7.5 billion years after the Big Bang, that the expansion rate of the universe has likely been increasing, commensurate with the Open Universe theory. However, other recent measurements by Wilkinson Microwave Anisotropy Probe suggest that the universe is either flat or very close to flat.

Closed universe

If ${\displaystyle \Omega >1}$, the geometry of space is closed like the surface of a sphere. The sum of the angles of a triangle exceeds 180 degrees and there are no parallel lines; all lines eventually meet. The geometry of the universe is, at least on a very large scale, elliptic.

In a closed universe, gravity eventually stops the expansion of the universe, after which it starts to contract until all matter in the universe collapses to a point, a final singularity termed the "Big Crunch", the opposite of the Big Bang. Some new modern theories assume the universe may have a significant amount of dark energy, whose repulsive force may be sufficient to cause the expansion of the universe to continue forever—even if ${\displaystyle \Omega >1}$.

Open universe

If ${\displaystyle \Omega <1}$, the geometry of space is open, i.e., negatively curved like the surface of a saddle. The angles of a triangle sum to less than 180 degrees, and lines that do not meet are never equidistant; they have a point of least distance and otherwise grow apart. The geometry of such a universe is hyperbolic.

Even without dark energy, a negatively curved universe expands forever, with gravity negligibly slowing the rate of expansion. With dark energy, the expansion not only continues but accelerates. The ultimate fate of an open universe is either universal heat death, a "Big Freeze" (not to be confused with heat death, despite seemingly similar name interpretation ⁠— ⁠see §Theories about the end of the universe below), or a "Big Rip", where the acceleration caused by dark energy eventually becomes so strong that it completely overwhelms the effects of the gravitational, electromagnetic and strong binding forces.

Conversely, a negative cosmological constant, which would correspond to a negative energy density and positive pressure, would cause even an open universe to re-collapse to a big crunch.

Flat universe

If the average density of the universe exactly equals the critical density so that ${\displaystyle \Omega =1}$, then the geometry of the universe is flat: as in Euclidean geometry, the sum of the angles of a triangle is 180 degrees and parallel lines continuously maintain the same distance. Measurements from Wilkinson Microwave Anisotropy Probe have confirmed the universe is flat within a 0.4% margin of error.

In the absence of dark energy, a flat universe expands forever but at a continually decelerating rate, with expansion asymptotically approaching zero; with dark energy, the expansion rate of the universe initially slows down, due to the effects of gravity, but eventually increases, and the ultimate fate of the universe becomes the same as that of an open universe.

Theories about the end of the universe

The fate of the universe is determined by its density. The preponderance of evidence to date, based on measurements of the rate of expansion and the mass density, favors a universe that will continue to expand indefinitely, resulting in the "Big Freeze" scenario below. However, observations are not conclusive, and alternative models are still possible.

Big Freeze or Heat Death

The Big Freeze (or Big Chill) is a scenario under which continued expansion results in a universe that asymptotically approaches absolute zero temperature. This scenario, in combination with the Big Rip scenario, is gaining ground as the most important hypothesis. It could, in the absence of dark energy, occur only under a flat or hyperbolic geometry. With a positive cosmological constant, it could also occur in a closed universe. In this scenario, stars are expected to form normally for 10¹² to 10¹⁴ (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. As existing stars run out of fuel and cease to shine, the universe will slowly and inexorably grow darker. Eventually black holes will dominate the universe, which themselves will disappear over time as they emit Hawking radiation. Over infinite time, there would be a spontaneous entropy decrease by the Poincaré recurrence theorem, thermal fluctuations, and the fluctuation theorem.

A related scenario is heat death, which states that the universe goes to a state of maximum entropy in which everything is evenly distributed and there are no gradients—which are needed to sustain information processing, one form of which is life. The heat death scenario is compatible with any of the three spatial models, but requires that the universe reach an eventual temperature minimum.

Big Rip

The current Hubble constant defines a rate of acceleration of the universe not large enough to destroy local structures like galaxies, which are held together by gravity, but large enough to increase the space between them. A steady increase in the Hubble constant to infinity would result in all material objects in the universe, starting with galaxies and eventually (in a finite time) all forms, no matter how small, disintegrating into unbound elementary particles, radiation and beyond. As the energy density, scale factor and expansion rate become infinite the universe ends as what is effectively a singularity.

In the special case of phantom dark energy, which has supposed negative kinetic energy that would result in a higher rate of acceleration than other cosmological constants predict, a more sudden big rip could occur.

Big Crunch

The Big Crunch. The vertical axis can be considered as expansion or contraction with time.

The Big Crunch hypothesis is a symmetric view of the ultimate fate of the universe. Just as the Big Bang started as a cosmological expansion, this theory assumes that the average density of the universe will be enough to stop its expansion and the universe will begin contracting. The end result is unknown; a simple estimation would have all the matter and space-time in the universe collapse into a dimensionless singularity back into how the universe started with the Big Bang, but at these scales unknown quantum effects need to be considered. Recent evidence suggests that this scenario is unlikely but has not been ruled out, as measurements have been available only over a short period of time, relatively speaking, and could reverse in the future.

This scenario allows the Big Bang to occur immediately after the Big Crunch of a preceding universe. If this happens repeatedly, it creates a cyclic model, which is also known as an oscillatory universe. The universe could then consist of an infinite sequence of finite universes, with each finite universe ending with a Big Crunch that is also the Big Bang of the next universe. A problem with the cyclic universe is that it does not reconcile with the second law of thermodynamics, as entropy would build up from oscillation to oscillation and cause the eventual heat death of the universe. Current evidence also indicates the universe is not closed. This has caused cosmologists to abandon the oscillating universe model. A somewhat similar idea is embraced by the cyclic model, but this idea evades heat death because of an expansion of the branes that dilutes entropy accumulated in the previous cycle.

Big Bounce

The Big Bounce is a theorized scientific model related to the beginning of the known universe. It derives from the oscillatory universe or cyclic repetition interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe.

According to one version of the Big Bang theory of cosmology, in the beginning the universe was infinitely dense. Such a description seems to be at odds with other more widely accepted theories, especially quantum mechanics and its uncertainty principle. It is not surprising, therefore, that quantum mechanics has given rise to an alternative version of the Big Bang theory. Also, if the universe is closed, this theory would predict that once this universe collapses it will spawn another universe in an event similar to the Big Bang after a universal singularity is reached or a repulsive quantum force causes re-expansion.

In simple terms, this theory states that the universe will continuously repeat the cycle of a Big Bang, followed up with a Big Crunch.

Big Slurp

This theory posits that the universe currently exists in a false vacuum and that it could become a true vacuum at any moment.

In order to best understand the false vacuum collapse theory, one must first understand the Higgs field which permeates the universe. Much like an electromagnetic field, it varies in strength based upon its potential. A true vacuum exists so long as the universe exists in its lowest energy state, in which case the false vacuum theory is irrelevant. However, if the vacuum is not in its lowest energy state (a false vacuum), it could tunnel into a lower energy state. This is called vacuum decay. This has the potential to fundamentally alter our universe; in more audacious scenarios even the various physical constants could have different values, severely affecting the foundations of matter, energy, and spacetime. It is also possible that all structures will be destroyed instantaneously, without any forewarning.

Cosmic uncertainty

Each possibility described so far is based on a very simple form for the dark energy equation of state. However, as the name is meant to imply, very little is currently known about the physics of dark energy. If the theory of inflation is true, the universe went through an episode dominated by a different form of dark energy in the first moments of the Big Bang, but inflation ended, indicating an equation of state far more complex than those assumed so far for present-day dark energy. It is possible that the dark energy equation of state could change again, resulting in an event that would have consequences which are extremely difficult to predict or parametrize. As the nature of dark energy and dark matter remain enigmatic, even hypothetical, the possibilities surrounding their coming role in the universe are currently unknown. None of these theoretic endings for the universe are certain.

Observational constraints on theories

Choosing among these rival scenarios is done by 'weighing' the universe, for example, measuring the relative contributions of matter, radiation, dark matter, and dark energy to the critical density. More concretely, competing scenarios are evaluated against data on galaxy clustering and distant supernovas, and on the anisotropies in the cosmic microwave background. 

Speculative evolution

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Speculative_evolution 

 

Surviving dinosaurs and Mesozoic creatures are a common theme in alternative evolution. One example is the 2001-2005 Speculative Dinosaur Project and its invention of many speculative animals.

Speculative evolution is a genre of speculative fiction and an artistic movement focused on hypothetical scenarios in the evolution of life, and a significant form of fictional biology. It is also known as speculative biology and it is referred to as speculative zoology in regards to hypothetical animals. Works incorporating speculative evolution may have entirely conceptual species that evolve on a planet other than Earth, or they may be an alternate history focused on an alternate evolution of terrestrial life. Speculative evolution is often considered hard science fiction because of its strong connection to and basis in science, particularly biology.

Speculative evolution is a long-standing trope within science fiction, often recognized as beginning as such with H. G. Wells's 1895 novel The Time Machine, which featured several imaginary future creatures. Although small-scale speculative faunas were a hallmark of science fiction throughout the 20th century, ideas were only rarely well-developed, with some exceptions such as Edgar Rice Burroughs's Barsoom, a fictional rendition of Mars and its ecosystem published through novels from 1912 to 1941, and Gerolf Steiner's Rhinogradentia, a fictional order of mammals created in 1957.

The modern speculative evolution movement is generally agreed to have begun with the publication of Dougal Dixon's 1981 book After Man, which explored a fully realized future Earth with a complete ecosystem of over a hundred hypothetical animals. The success of After Man spawned several "sequels" by Dixon, focusing on different alternate and future scenarios. Dixon's work, like most similar works that came after them, were created with real biological principles in mind and were aimed at exploring real life processes, such as evolution and climate change, through the use of fictional examples.

Speculative evolution's possible use as an educational and scientific tool has been noted and discussed through the decades following the publication of After Man. Speculative evolution can be useful in exploring and showcasing patterns present in the present and in the past. By extrapolating past trends into the future, scientists can research and predict the most likely scenarios of how certain organisms and lineages could respond to ecological changes. In some cases, creatures first imagined within speculative evolution have since been discovered, such as an imaginary filter-feeding anomalocarid illustrated by artist John Meszaros in the 2012 book All Yesterdays by John Conway, C. M. Kosemen and Darren Naish being proven as having existed through fossils discovered in 2014 of the real anomalocarid Tamisiocaris.

History

Early works

The Time Machine (1895) by H. G. Wells is seen by some as an early instance of speculative evolution and has been cited as an inspiration by later creators within the field.

Explorations of hypothetical worlds featuring future, alternate or alien lifeforms is a long-standing trope in science fiction. One of the earliest works usually recognized as representing one of speculative evolution is H. G. Wells's science fiction novel The Time Machine, published in 1895. The Time Machine, set over eight hundred thousand years in the future, features post-human descendants in the form of the beautiful but weak Eloi and the brutish Morlocks. Further into the future, the protagonist of the book finds large crab-monsters and huge butterflies. Science fiction authors who wrote after Wells often used fictional creatures in the same vein, but most such imaginary faunas were small and not very developed.

A four-armed "Green Martian" riding a "throat" from Edgar Rice Burroughs's Barsoom, a fictional version of the planet Mars. Illustration by James Allen St. John (1920).

Edgar Rice Burroughs, who wrote in the early 20th century, can like Wells be considered an early speculative evolution author. Although his fictional ecosystems were still relatively small in scope, they were the settings of many of his novels and as such quite well-developed. In particular, Burroughs's Barsoom, a fictional version of the planet Mars which appeared in ten novels published from 1912 to 1941, featured a Martian ecosystem with a variety of alien creatures and several distinct Martian cultures and ethnic groups.

A mock taxidermy of a rhinograde, using its nasorium to catch fish. Rhinogrades, created by Gerolf Steiner in 1957, are one of the earliest concrete examples of speculative zoology.

In 1930, Olaf Stapledon published a "future history", Last and First Men: A Story of the Near and Far Future, describing the history of humanity from the present onwards, across two billion years and eighteen human species, of which Homo sapiens is the first. The book anticipates the science of genetic engineering, and is an early instance of the fictional group mind idea. Published in 1957, German zoologist Gerolf Steiner's book Bau und Leben der Rhinogradentia (translated into English as The Snouters: The Form and Life of the Rhinogrades) described the fictional evolution, biology and behavior of an imaginary order of mammals, the Rhinogradentia or "rhinogrades". The Rhinogrades are characterized by a nose-like feature called a "nasorium", the form and function of which vary significantly between species, akin to Darwin's finches and their beak specialization. This diverse group of fictional animals inhabits a series of islands in which they have gradually evolved, radiating into most ecological niches. Satirical papers have been published continuing Steiner's imagined world. Although the work does feature an entire speculative ecosystem, its impact is dwarfed by the later works due to its limited scope, only exploring the life of an island archipelago.

In 1976, the Italian author and illustrator Leo Lionni published Parallel Botany, a "field guide to imaginary plants", presented with academic-style mentions of genuine people and places. Parallel Botany has been compared to the 1972 book Invisible Cities by Italo Calvino, in which Marco Polo in a dialogue with Kublai Khan describes 55 cities, which, like Lionni's "parallel" plants, are "only as real as the mind's ability to conceptualize them".

Movement

Author Dougal Dixon with a model of a "Strida", one of the creatures featured in his 2010 book Greenworld.

One of the significant "founding" works of speculative evolution is After Man by Dougal Dixon, published in 1981. To this day, After Man is recognized as the first truly large-scale speculative evolution project involving a whole world and a vast array of species. Furthering its significance is the fact that the book was made very accessible by being published by mainstream publishers and being fully illustrated with color images. As such, After Man is often seen as having firmly established the idea of creating entire speculative worlds. Through the decades following After Man's publication, Dixon remained one of the sole authors of speculative evolution, publishing two more books in the same vein as After Man; The New Dinosaurs in 1988 and Man After Man in 1990. Dixon cited The Time Machine as his primary inspiration, being unaware of Steiner's work, and devised After Man as a popular-level book on the processes of evolution that instead of using the past to tell the story projected the processes into the future. A central idea of After Man, besides a wave of extinction following humans, is convergent evolution as new species bear a close resemblance to their unrelated predecessors.

When designing the various animals of the book, Dixon looked at the different types of biomes on the planet and what adaptations animals living there have, designing new animals descended from modern day ones with the same set of adaptations. The success of After Man inspired Dixon to continue writing books that explained factual scientific processes through fictional examples. The New Dinosaurs was in essence a book about zoogeography, something the general public would be unfamiliar with, using a world in which the non-avian dinosaurs had not gone extinct. Man After Man, explored climate change over the course of the next few million years by showcasing its effects through the eyes of future human descendants.

Today, many artists and writers work on speculative evolution projects online, often in the same vein as Dixon's works. Speculative evolution continues to endure a somewhat mainstream presence through TV shows featuring hypothetical and imaginary creatures, such as The Future is Wild (2002), Primeval (2007–2011) and Terra Nova (2011) and films such as Avatar (2009) and After Earth (2013). The modern explosion of speculative evolution has been termed by British paleontologist Darren Naish as the "Speculative Zoology Movement".

As an educational and scientific tool

Reconstruction of Tamisiocaris (top), an anomalocarid from the Cambrian which was discovered to have been a filter-feeder in 2014. A hypothetical filter-feeding anomalocarid was featured in the book All Yesterdays (2012).

Although primarily characterized as entertainment, speculative evolution can be used as educational tool to explain and illustrate real natural processes through using fictional and imaginary examples. The worlds created are often built on ecological and biological principles inferred from the real evolutionary history of life on Earth and readers can learn from them as such. For example, all of Dixon's speculative works are aimed at exploring real processes, with After Man exploring evolution, The New Dinosaurs zoogeography and both Man After Man and Greenworld (2010) exploring climate change, offering an environmental message.

In some cases, speculative evolution artists have successfully predicted the existence of organisms that were later discovered to resemble something real. Many of the animals featured in Dixon's After Man are still considered plausible ideas, with some of them (such as specialized rodents and semi-aquatic primates) being reinforced with recent biology studies. A creature dubbed "Ceticaris", conceived by artist John Meszaros as a filter-feeding anomalocarid, was published in the 2012 book All Yesterdays and two years later, in 2014, the actual Cambrian anomalocarid Tamisiocaris was discovered to have been a filter-feeder. In honor of Meszaros's prediction, Tamisiocaris was included in a new clade named the Cetiocaridae.

Dougal Dixon's The New Dinosaurs was heavily influenced by paleontological ideas developing during its time, such as the ongoing dinosaur renaissance, and as such many of the dinosaurs in the book are energetic and active creatures rather than sluggish and lumbering. Dixon extrapolated on the ideas of paleontologists such as Robert Bakker and Gregory S. Paul when creating his creatures and also used patterns seen in the actual evolutionary history of the dinosaurs and pushing them to an extreme. Perhaps because of this, many of the animals in the book are similar to actual Mesozoic animals that were later discovered. Many of the dinosaurs in it are feathered, something not widely accepted at the time of its publication but seen as likely today. Similarly, After Man in 1981 represents a sort of time capsule of geological thought before global warming was fully discerned, but Dixon also portrays a sixth mass extinction or Anthropocene before it was commonplace to do so.

Hypothetical restoration of Dromaeosauroides bornholmensis, which is known from two teeth. Its appearance is inferred from related genera.

 

Speculative reconstruction of Sinopliosaurus fusuiensis with generalized spinosaurid morphology, and unique coloration pattern.

Speculative evolution can be useful in exploring and showcasing patterns present in the present and in the past, and there is a useful aspect to hypothesizing on the form of future and alien life. By extrapolating past trends into the future, scientists could research and predict the most likely scenarios of how certain organisms and lineages could respond to ecological changes. As such, speculative evolution facilitates authors and artists to develop realistic hypotheses of the future. In some scientific fields, speculation is essential in understanding what is being studied. Paleontologists apply their own understanding of natural processes and biology to understand the appearances and lifestyles of extinct organisms that are discovered, varying in how far their speculation goes. For instance, All Yesterdays and its sequel All Your Yesterdays (2017) explores highly speculative renditions of real (and in some cases hypothetical) prehistoric animals that do not explicitly contradict any of the recovered fossil material. The speculation undertaken for All Yesterdays and its sequel has been compared to that of Dixon's speculative evolution works, though its objective was to challenge modern conservative perceptions and ideas of how dinosaurs and other prehistoric creatures lived, rather than designing whole new ecosystems. The books have inspired a modern artistic movement of artists going beyond conventional paleoart tropes, expanding into increasingly speculative renditions of prehistoric life.

Additionally, the evolutionary history of fictional organisms has been used as a tool in biology education. Caminalcules, named after Joseph H. Camin, are a group of animal-like lifeforms, consisting of 77 purported extant and fossil species that were invented as a tool for understanding phylogenetics. The classification of Caminalcules, as well as other fictional creatures like dragons and aliens, have been used as analogies to teach concepts in evolution and systematics.

Speculative evolution is sometimes presented in museum exhibitions. For instance, both After Man and The Future is Wild has been presented in exhibition form, educating museum visitors on the principles of biology and evolution through using their own fictional future creatures.

Subsets

Alien life

The "Hellfire wasp", a wasp-like alien creature designed for James Cameron's film Avatar (2009).

A popular subset of speculative evolution is the exploration of possible realistic extraterrestrial life and ecosystems. Speculative evolution writings focusing on extraterrestrial life, like the blog Furahan Biology, use realistic scientific principles to describe the biomechanics of hypothetical alien life. Although commonly identified with terms such as "astrobiology", "xenobiology" or "exobiology", these terms designate actual scientific fields largely unrelated to speculative evolution. Though 20th century work in exobiology sometimes formulated "audacious" ideas about extraterrestrial forms of life. Astrophysicists Carl Sagan and Edwin Salpeter speculated that a "hunters, floaters and sinkers" ecosystem could populate the atmospheres of gas giant planets like Jupiter, and scientifically described it in a 1976 paper.

In extraterrestrial-focused speculative biology, lifeforms are often designed with the intention to populate planets wildly different from Earth, and in such cases concerns like chemistry, astronomy and the laws of physics become just as important to consider as the usual biological principles. Very exotic environments of physical extremes may be explored in such scenarios. For example, Robert Forward's 1980 Dragon's Egg develops a tale of life on a neutron star, and the resulting high-gravity, high-energy environment with an atmosphere of iron vapor and mountains 5-100 millimeters high. Once the star cools down and stable chemistry develops, life evolves extremely quickly, and Forward imagines a civilization of "cheela" that lives a million times faster than humans.

In some cases, artists and writers exploring possible alien life conjure similar ideas independent of each other, often attributed to studying the same biological processes and ideas. Such occasions can be called "convergent speculation", similar to the scientific idea of convergent evolution.

Perhaps the most famous speculative work on a hypothetical alien ecosystem is Wayne Barlowe's 1990 book Expedition, which explores the fictional planet Darwin IV. Expedition was written as a report of an 24th-century expedition that had been led to the planet by a team composed of both humans and intelligent aliens and used paintings and descriptive texts to create and describe a fully realized extraterrestrial ecosystem. Barlowe later served as an executive producer of a TV adaptation of the book, Alien Planet (2005) where exploration of Darwin IV is instead carried out by robotic probes and the segments detailing the ecosystems of the planet are intercut with interviews with scientists, such as Michio Kaku, Jack Horner and James B. Garvin.

Other examples of speculative evolution focused on extraterrestrial life include Dougal Dixon's 2010 book Greenworld, TV programmes such as 1997 the BBC2/Discovery Channel special Natural History of an Alien and the 2005 Channel 4/National Geographic programme Extraterrestrial as well as a variety of personal web-based artistic projects, such as C. M. Kosemen's "Snaiad" and Gert van Dijk's "Furaha", envisioning the biosphere of entire alien worlds.

Through science fiction, the speculative biology of extraterrestrial organisms has a strong presence in popular culture. The eponymous monster of Alien (1979), particularly its life cycle from egg to parasitoid larva to 'Xenomorph', is thought to be based on the real habits of parasitoid wasps in biology. Further, H. R. Giger's design of the Alien incorporated the features of insects, echinoderms and fossil crinoids, while concept artist John Cobb suggested acid blood as a biological defense mechanism. James Cameron's 2009 film Avatar constructed a fictional biosphere full of original, speculative alien species; a team of experts ensured that the lifeforms were scientifically plausible. The creatures of the movie took inspiration from Earth species as diverse as pterosaurs, microraptors, great white sharks, and panthers, and combined their traits to create an alien world.

Alternative evolution

Speculative zoology can examine sometimes overlooked prehistoric animals in an evolutionary context. The Speculative Dinosaur Project focused as much on mammals, squamates, and crocodylomorphs as on dinosaurs. Pictured are metatherian marsupials that have converged on our world's mustelids.

Similar to alternate history, alternative evolution is the exploration of possible alternate scenarios that could have played out in the Earth's past to give rise to alternate lifeforms and ecosystems, popularly the survival of non-avian dinosaurs to the present day. As humanity is often not a part of the worlds envisioned through alternative evolution, it has sometimes been characterized as non-anthropocentric.

Although dinosaurs surviving to the age of humans has been adapted as a plot point in numerous science fiction stories since at least 1912, beginning with Arthur Conan Doyle's The Lost World, the idea of exploring the fully fledged alternate ecosystems that would develop in such a scenario truly began with the publication of Dixon's The New Dinosaurs in 1988, in which dinosaurs were not some lone stragglers of known species that had survived more or less unchanged for the last 66 million years, but diverse animals that had continued to evolve beyond the Cretaceous. In the vein of Dixon's The New Dinosaurs imagination, a now largely defunct, but creatively significant collaborative online project the Speculative Dinosaur Project followed in the same zoological worldbuilding tradition.

Since 1988, alternative evolution has sometimes been applied in popular culture. The creatures in the 2005 film King Kong were fictitious descendants of real animals, with Skull Island being inhabited by dinosaurs and other prehistoric fauna. Inspired by Dougal Dixon's works, the designers imagined what 65 million years or more of isolated evolution might have done to dinosaurs. Concept art for the film was published in the book The World of Kong: A Natural History of Skull Island (2005), which explored the world of the film from a biological perspective, envisioning Skull Island as a surviving fragment of ancient Gondwana. Prehistoric creatures on a declining, eroding island had evolved into "a menagerie of nightmares".

A hypothetical natural history of dragons is a popular subject of speculative zoology, being explored in works such as Peter Dickinson's The Flight of Dragons (1979), the 2004 mockumentary The Last Dragon and the Dragonology series of books.

Future evolution

The evolution of organisms in the Earth's future is a popular subset of speculative evolution. A relatively common theme in future evolution is civilizational collapse and/or humans becoming extinct due to an anthropogenic extinction event caused by environmental degradation. After such a mass extinction event, the remaining non-human fauna and flora evolve into a variety of new forms. Although the foundations of this subset were laid by Wells's The Time Machine already in 1895, it is generally agreed to have been definitely founded through Dixon's After Man in 1981, which explored a fully realized future ecosystem set 50 million years from the present. Dixon's third work on speculative evolution, Man After Man (1990) is also an example of future evolution, this time exploring an imagined future evolutionary path of humanity.

Peter Ward's Future Evolution (2001) makes a scientifically accurate approach to the prediction of patterns of evolution in the future. Ward compares his predictions with those of Dixon and Wells. He tries to understand the mechanism of mass extinctions and the principles of recovery of ecosystems. A key point is that "champion supertaxa" who diversify and speciate at a greater rate, will inherit the world after mass extinctions. Ward quotes the paleontologist Simon Conway Morris, who points out that the fantastical or even whimsical creatures devised by Dougal Dixon's, echo nature's tendency to converge on the same body plans. While Ward calls Dixon's visions "semi-whimsical" and compares them to Wells' initial visions in The Time Machine, he nonetheless continues the use of analogous evolution, which is a larger trend in speculative zoology.

Future evolution has also been explored on TV, with the mockumentary series The Future is Wild in 2002, for which Dixon was a consultant (and author of the companion book), and the series Primeval (2007–2011), a drama series in which imagined future animals occasionally appeared. Ideas of future evolution are also frequently explored in science fiction novels, such as in Kurt Vonnegut's 1985 science fiction novel Galápagos, which imagines the evolution of a small surviving group of humans into a sea lion-like species. Stephen Baxter's 2002 science fiction novel Evolution follows 565 million years of human evolution, from shrewlike mammals 65 million years in the past to the ultimate fate of humanity (and its descendants, both biological and non-biological) 500 million years in the future. C. M. Kosemen's 2008 All Tomorrows similarly explores the future evolution of humanity. Speculative biology and the future evolution of the human species are significant in bio art.