Gödel metric

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/G%C3%B6del_metric

The Gödel metric is an exact solution of the Einstein field equations in which the stress–energy tensor contains two terms, the first representing the matter density of a homogeneous distribution of swirling dust particles (dust solution), and the second associated with a nonzero cosmological constant (see lambdavacuum solution). It is also known as the Gödel solution or Gödel universe.

This solution has many unusual properties—in particular, the existence of closed timelike curves that would allow time travel in a universe described by the solution. Its definition is somewhat artificial in that the value of the cosmological constant must be carefully chosen to match the density of the dust grains, but this spacetime is an important pedagogical example.

The solution was found in 1949 by Kurt Gödel.

Definition

Like any other Lorentzian spacetime, the Gödel solution presents the metric tensor in terms of some local coordinate chart. It may be easiest to understand the Gödel universe using the cylindrical coordinate system (presented below), but this article uses the chart that Gödel originally used. In this chart, the metric (or equivalently the line element) is

${\displaystyle g={\frac {1}{2\omega ^{2}}}\left[-(dt+e^{x}\,dy)^{2}+dx^{2}+{\tfrac {1}{2}}e^{2x}\,dy^{2}+dz^{2}\right],\qquad -\infty <t,x,y,z<\infty ,}$

where ${\displaystyle \omega }$ is a nonzero real constant, which turns out to be the angular velocity of the surrounding dust grains around the y axis, as measured by a "non-spinning" observer riding one of the dust grains. "Non-spinning" means that the observer does not feel centrifugal forces, but in this coordinate frame it would actually be turning on an axis parallel to the y axis. As can be seen, the dust grains stay at constant values of x, y, and z. Their density in this coordinate chart increases with x, but their density in their own frames of reference is the same everywhere.

Properties

To study the properties of the Gödel solution, the frame field can be adopted (dual to the coframe read off the metric as given above),

${\displaystyle {\vec {e}}_{0}={\sqrt {2}}\omega \,\partial _{t}}$

${\displaystyle {\vec {e}}_{1}={\sqrt {2}}\omega \,\partial _{x}}$

${\displaystyle {\vec {e}}_{2}={\sqrt {2}}\omega \,\partial _{y}}$

${\displaystyle {\vec {e}}_{3}=2\omega \,\left(\exp(-x)\,\partial _{z}-\partial _{t}\right).}$

This frame defines a family of inertial observers who are 'comoving with the dust grains'. However, computing the Fermi–Walker derivatives with respect to ${\displaystyle {\vec {e}}_{0}}$ shows that the spatial frames are spinning about ${\displaystyle {\vec {e}}_{2}}$ with angular velocity ${\displaystyle -\omega }$. It follows that the 'nonspinning inertial frame' comoving with the dust particles is

${\displaystyle {\vec {f}}_{0}={\vec {e}}_{0}}$

${\displaystyle {\vec {f}}_{1}=\cos(\omega t)\,{\vec {e}}_{1}-\sin(\omega t)\,{\vec {e}}_{3}}$

${\displaystyle {\vec {f}}_{2}={\vec {e}}_{2}}$

${\displaystyle {\vec {f}}_{3}=\sin(\omega t)\,{\vec {e}}_{1}+\cos(\omega t)\,{\vec {e}}_{3}.}$

Einstein tensor

The components of the Einstein tensor (with respect to either frame above) are

${\displaystyle G^{{\hat {a}}{\hat {b}}}=\omega ^{2}\operatorname {diag} (-1,1,1,1)+2\omega ^{2}\operatorname {diag} (1,0,0,0).}$

Here, the first term is characteristic of a lambdavacuum solution and the second term is characteristic of a pressureless perfect fluid or dust solution. The cosmological constant is carefully chosen to partially cancel the matter density of the dust.

Topology

The Gödel spacetime is a rare example of a 'regular' (singularity-free) solution of the Einstein field equation. Gödel's original chart (given here) is geodesically complete and singularity free; therefore, it is a global chart, and the spacetime is homeomorphic to R⁴, and therefore, simply connected.

Curvature invariants

In any Lorentzian spacetime, the fourth-rank Riemann tensor is a multilinear operator on the four-dimensional space of tangent vectors (at some event), but a linear operator on the six-dimensional space of bivectors at that event. Accordingly, it has a characteristic polynomial, whose roots are the eigenvalues. In the Gödel spacetime, these eigenvalues are very simple:

• triple eigenvalue zero,
• double eigenvalue ${\displaystyle -\omega ^{2}}$,
• single eigenvalue ${\displaystyle \omega ^{2}}$.

Killing vectors

This spacetime admits a five-dimensional Lie algebra of Killing vectors, which can be generated by 'time translation' ${\displaystyle \partial _{t}}$, two 'spatial translations' ${\displaystyle \partial _{y},\;\partial _{z}}$, plus two further Killing vector fields:

${\displaystyle \partial _{x}-z\,\partial _{z}}$

and

${\displaystyle -2\exp(-x)\,\partial _{t}+z\,\partial _{x}+\left(\exp(-2x)-z^{2}/2\right)\,\partial _{z}.}$

The isometry group acts 'transitively' (since we can translate in ${\displaystyle t,y,z}$, and using the fourth vector we can move along ${\displaystyle x}$ as well), so the spacetime is 'homogeneous'. However, it is not 'isotropic', as can be seen.

It is obvious from the generators just given that the slices ${\displaystyle x=x_{0}}$ admit a transitive abelian three-dimensional transformation group, so a quotient of the solution can be reinterpreted as a stationary cylindrically symmetric solution. The slices ${\displaystyle y=y_{0}}$ admit an SL(2,R) action, and the slices ${\displaystyle t=t_{0}}$ admit a Bianchi III (c.f. the fourth Killing vector field). This can be restated by saying that the symmetry group includes three-dimensional subgroups examples of Bianchi types I, III and VIII. Four of the five Killing vectors, as well as the curvature tensor, do not depend upon the coordinate y. The Gödel solution is the Cartesian product of a factor R with a three-dimensional Lorentzian manifold (signature −++).

It can be shown that the Gödel solution is, up to local isometry, the only perfect fluid solution of the Einstein field equation admitting a five-dimensional Lie algebra of Killing vectors.

Petrov type and Bel decomposition

The Weyl tensor of the Gödel solution has Petrov type D. This means that for an appropriately chosen observer, the tidal forces have Coulomb form.

To study the tidal forces in more detail, the Bel decomposition of the Riemann tensor can be computed into three pieces, the tidal or electrogravitic tensor (which represents tidal forces), the magnetogravitic tensor (which represents spin-spin forces on spinning test particles and other gravitational effects analogous to magnetism), and the topogravitic tensor (which represents the spatial sectional curvatures).

Observers comoving with the dust particles find that the tidal tensor (with respect to ${\displaystyle {\vec {u}}={\vec {e}}_{0}}$, which components evaluated in our frame) has the form

${\displaystyle {E\left[{\vec {u}}\right]}_{{\hat {m}}{\hat {n}}}=\omega ^{2}\operatorname {diag} (1,0,1).}$

That is, they measure isotropic tidal tension orthogonal to the distinguished direction ${\displaystyle \partial _{y}}$.

The gravitomagnetic tensor vanishes identically

${\displaystyle {B\left[{\vec {u}}\right]}_{{\hat {m}}{\hat {n}}}=0.}$

This is an artifact of the unusual symmetries of this spacetime, and implies that the putative "rotation" of the dust does not have the gravitomagnetic effects usually associated with the gravitational field produced by rotating matter.

The principal Lorentz invariants of the Riemann tensor are

${\displaystyle R_{abcd}\,R^{abcd}=12\omega ^{4},\;R_{abcd}{{}^{\star }R}^{abcd}=0.}$

The vanishing of the second invariant means that some observers measure no gravitomagnetism, which is consistent with what was just said. The fact that the first invariant (the Kretschmann invariant) is constant reflects the homogeneity of the Gödel spacetime.

Rigid rotation

The frame fields given above are both inertial, ${\displaystyle \nabla _{{\vec {e}}_{0}}{\vec {e}}_{0}=0}$, but the vorticity vector of the timelike geodesic congruence defined by the timelike unit vectors is

${\displaystyle -\omega {\vec {e}}_{2}}$

This means that the world lines of nearby dust particles are twisting about one another. Furthermore, the shear tensor of the congruence ${\displaystyle {\vec {e}}_{0}}$ vanishes, so the dust particles exhibit rigid rotation.

Optical effects

If the past light cone of a given observer is studied, it can be found that null geodesics moving orthogonally to ${\displaystyle \partial _{y}}$ spiral inwards toward the observer, so that if he looks radially, he sees the other dust grains in progressively time-lagged positions. However, the solution is stationary, so it might seem that an observer riding on a dust grain will not see the other grains rotating about himself. However, recall that while the first frame given above (the ${\displaystyle {\vec {e}}_{j}}$) appears static in the chart, the Fermi–Walker derivatives show that, in fact, it is spinning with respect to gyroscopes. The second frame (the ${\displaystyle {\vec {f}}_{j}}$) appears to be spinning in the chart, but it is gyrostabilized, and a nonspinning inertial observer riding on a dust grain will indeed see the other dust grains rotating clockwise with angular velocity ${\displaystyle \omega }$ about his axis of symmetry. It turns out that in addition, optical images are expanded and sheared in the direction of rotation.

If a nonspinning inertial observer looks along his axis of symmetry, he sees his coaxial nonspinning inertial peers apparently nonspinning with respect to himself, as would be expected.

Shape of absolute future

According to Hawking and Ellis, another remarkable feature of this spacetime is the fact that, if the inessential y coordinate is suppressed, light emitted from an event on the world line of a given dust particle spirals outwards, forms a circular cusp, then spirals inward and reconverges at a subsequent event on the world line of the original dust particle. This means that observers looking orthogonally to the ${\displaystyle {\vec {e}}_{2}}$ direction can see only finitely far out, and also see themselves at an earlier time.

The cusp is a nongeodesic closed null curve. (See the more detailed discussion below using an alternative coordinate chart.)

Closed timelike curves

Because of the homogeneity of the spacetime and the mutual twisting of our family of timelike geodesics, it is more or less inevitable that the Gödel spacetime should have closed timelike curves (CTCs). Indeed, there are CTCs through every event in the Gödel spacetime. This causal anomaly seems to have been regarded as the whole point of the model by Gödel himself, who was apparently striving to prove, and arguably succeeded in proving, that Einstein's equations of spacetime are not consistent with what we intuitively understand time to be (i. e. that it passes and the past no longer exists, the position philosophers call presentism, whereas Gödel seems to have been arguing for something more like the philosophy of eternalism), much as he, conversely, succeeded with his incompleteness theorems in showing that intuitive mathematical concepts could not be completely described by formal mathematical systems of proof. See the book A World Without Time.

Einstein was aware of Gödel's solution and commented in Albert Einstein: Philosopher-Scientist that if there are a series of causally-connected events in which "the series is closed in itself" (in other words, a closed timelike curve), then this suggests that there is no good physical way to define whether a given event in the series happened "earlier" or "later" than another event in the series:

In that case the distinction "earlier-later" is abandoned for world-points which lie far apart in a cosmological sense, and those paradoxes, regarding the direction of the causal connection, arise, of which Mr. Gödel has spoken.

Such cosmological solutions of the gravitation-equations (with not vanishing A-constant) have been found by Mr. Gödel. It will be interesting to weigh whether these are not to be excluded on physical grounds.

Globally nonhyperbolic

If the Gödel spacetime admitted any boundaryless temporal hyperslices (e.g. a Cauchy surface), any such CTC would have to intersect it an odd number of times, contradicting the fact that the spacetime is simply connected. Therefore, this spacetime is not globally hyperbolic.

A cylindrical chart

In this section, we introduce another coordinate chart for the Gödel solution, in which some of the features mentioned above are easier to see.

Derivation

Gödel did not explain how he found his solution, but there are in fact many possible derivations. We will sketch one here, and at the same time verify some of the claims made above.

Start with a simple frame in a cylindrical type chart, featuring two undetermined functions of the radial coordinate:

${\displaystyle {\vec {e}}_{0}=\partial _{t},\;{\vec {e}}_{1}=\partial _{z},\;{\vec {e}}_{2}=\partial _{r},\,{\vec {e}}_{3}={\frac {1}{b(r)}}\,\left(-a(r)\,\partial _{t}+\partial _{\varphi }\right)}$

Here, we think of the timelike unit vector field ${\displaystyle {\vec {e}}_{0}}$ as tangent to the world lines of the dust particles, and their world lines will in general exhibit nonzero vorticity but vanishing expansion and shear. Let us demand that the Einstein tensor match a dust term plus a vacuum energy term. This is equivalent to requiring that it match a perfect fluid; i.e., we require that the components of the Einstein tensor, computed with respect to our frame, take the form

${\displaystyle G^{{\hat {i}}{\hat {j}}}=\mu \operatorname {diag} (1,0,0,0)+p\operatorname {diag} (0,1,1,1)}$

This gives the conditions

${\displaystyle b^{\prime \prime \prime }={\frac {b^{\prime \prime }\,b^{\prime }}{b}},\;\left(a^{\prime }\right)^{2}=2\,b^{\prime \prime }\,b}$

Plugging these into the Einstein tensor, we see that in fact we now have ${\displaystyle \mu =p}$. The simplest nontrivial spacetime we can construct in this way evidently would have this coefficient be some nonzero but constant function of the radial coordinate. Specifically, with a bit of foresight, let us choose ${\displaystyle \mu =\omega ^{2}}$. This gives

${\displaystyle b(r)={\frac {\sinh({\sqrt {2}}\omega \,r)}{{\sqrt {2}}\omega }},\;a(r)={\frac {\cosh({\sqrt {2}}\omega r)}{\omega }}+c}$

Finally, let us demand that this frame satisfy

${\displaystyle {\vec {e}}_{3}={\frac {1}{r}}\,\partial _{\varphi }+O\left({\frac {1}{r^{2}}}\right)}$

This gives ${\displaystyle c=-1/\omega }$, and our frame becomes

${\displaystyle {\vec {e}}_{0}=\partial _{t},\;{\vec {e}}_{1}=\partial _{z},\;{\vec {e}}_{2}=\partial _{r},\;{\vec {e}}_{3}={\frac {{\sqrt {2}}\omega }{\sinh({\sqrt {2}}\omega r)}}\,\partial _{\varphi }-{\frac {{\sqrt {2}}\sinh({\sqrt {2}}\omega r)}{1+\cosh({\sqrt {2}}\omega r)}}\,\partial _{t}}$

Appearance of the light cones

From the metric tensor we find that the vector field ${\displaystyle \partial _{\varphi }}$, which is spacelike for small radii, becomes null at ${\displaystyle r=r_{c}}$ where

${\displaystyle r_{c}={\frac {\operatorname {arcosh} (3)}{{\sqrt {2}}\omega }}}$

This is because at that radius we find that ${\displaystyle {\vec {e}}_{3}={\tfrac {\omega }{2}}\,\partial _{\varphi }-\partial _{t},}$ so ${\displaystyle {\tfrac {\omega }{2}}\,\partial _{\varphi }={\vec {e}}_{3}+{\vec {e}}_{0}}$ and is therefore null. The circle ${\displaystyle r=r_{c}}$ at a given t is a closed null curve, but not a null geodesic.

Examining the frame above, we can see that the coordinate ${\displaystyle z}$ is inessential; our spacetime is the direct product of a factor R with a signature −++ three-manifold. Suppressing ${\displaystyle z}$ in order to focus our attention on this three-manifold, let us examine how the appearance of the light cones changes as we travel out from the axis of symmetry ${\displaystyle r=0}$:

Two light cones (with their accompanying frame vectors) in the cylindrical chart for the Gödel lambda dust solution. As we move outwards from the nominal symmetry axis, the cones tip forward and widen. Vertical coordinate lines (representing the world lines of the dust particles) are timelike.

When we get to the critical radius, the cones become tangent to the closed null curve.

A congruence of closed timelike curves

At the critical radius ${\displaystyle r=r_{c}}$, the vector field ${\displaystyle \partial _{\varphi }}$ becomes null. For larger radii, it is timelike. Thus, corresponding to our symmetry axis we have a timelike congruence made up of circles and corresponding to certain observers. This congruence is however only defined outside the cylinder ${\displaystyle r=r_{c}}$.

This is not a geodesic congruence; rather, each observer in this family must maintain a constant acceleration in order to hold his course. Observers with smaller radii must accelerate harder; as ${\displaystyle r\rightarrow r_{c}}$ the magnitude of acceleration diverges, which is just what is expected, given that ${\displaystyle r=r_{c}}$ is a null curve.

Null geodesics

If we examine the past light cone of an event on the axis of symmetry, we find the following picture:

The null geodesics spiral counterclockwise toward an observer on the axis of symmetry. This shows them from "above".

Recall that vertical coordinate lines in our chart represent the world lines of the dust particles, but despite their straight appearance in our chart, the congruence formed by these curves has nonzero vorticity, so the world lines are actually twisting about each other. The fact that the null geodesics spiral inwards in the manner shown above means that when our observer looks radially outwards, he sees nearby dust particles, not at their current locations, but at their earlier locations. This is just what we would expect if the dust particles are in fact rotating about one another.

The null geodesics are geometrically straight; in the figure, they appear to be spirals only because the coordinates are "rotating" in order to permit the dust particles to appear stationary.

The absolute future

According to Hawking and Ellis (see monograph cited below), all light rays emitted from an event on the symmetry axis reconverge at a later event on the axis, with the null geodesics forming a circular cusp (which is a null curve, but not a null geodesic):

Hawking and Ellis picture of expansion and reconvergence of light emitted by an observer on the axis of symmetry.

This implies that in the Gödel lambdadust solution, the absolute future of each event has a character very different from what we might naively expect.

Cosmological interpretation

Following Gödel, we can interpret the dust particles as galaxies, so that the Gödel solution becomes a cosmological model of a rotating universe. Besides rotating, this model exhibits no Hubble expansion, so it is not a realistic model of the universe in which we live, but can be taken as illustrating an alternative universe, which would in principle be allowed by general relativity (if one admits the legitimacy of a nonzero cosmological constant). Less well known solutions of Gödel's exhibit both rotation and Hubble expansion and have other qualities of his first model, but travelling into the past is not possible. According to S. W. Hawking, these models could well be a reasonable description of the universe that we observe, however observational data are compatible only with a very low rate of rotation. The quality of these observations improved continually up until Gödel's death, and he would always ask "is the universe rotating yet?" and be told "no, it isn't".

We have seen that observers lying on the y axis (in the original chart) see the rest of the universe rotating clockwise about that axis. However, the homogeneity of the spacetime shows that the direction but not the position of this "axis" is distinguished.

Some have interpreted the Gödel universe as a counterexample to Einstein's hopes that general relativity should exhibit some kind of Mach's principle, citing the fact that the matter is rotating (world lines twisting about each other) in a manner sufficient to pick out a preferred direction, although with no distinguished axis of rotation.

Others take Mach principle to mean some physical law tying the definition of nonspinning inertial frames at each event to the global distribution and motion of matter everywhere in the universe, and say that because the nonspinning inertial frames are precisely tied to the rotation of the dust in just the way such a Mach principle would suggest, this model does accord with Mach's ideas.

Many other exact solutions that can be interpreted as cosmological models of rotating universes are known. See the book Homogeneous Relativistic Cosmologies (1975) by Ryan and Shepley for some of these generalizations.

Metaverse

From Wikipedia, the free encyclopedia

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

Avatars socialising in the 2003 virtual world Second Life

The metaverse (a portmanteau of "meta-" and "universe") is a hypothesized iteration of the Internet, supporting persistent online 3-D virtual environments through conventional personal computing, as well as virtual and augmented reality headsets. Metaverses, in some limited form, have already been implemented in video games such as Second Life. Current metaverse ambitions are centered on addressing technological limitations with modern virtual and augmented reality devices, as well as expanding the use of metaverse spaces to business, education, and retail applications.Numerous entertainment and social media companies have invested in metaverse-related research and development.

The metaverse has come to be criticised as a method of public relations building using a purely speculative, "over-hyped" concept based on existing technology. Information privacy and user addiction are concerns within the metaverse, stemming from current challenges facing the social media and video game industries as a whole.

Elements

The metaverse is described as a means of manufacturing immersive digital spaces for a range of human activity. To achieve this, some iterations of the metaverse involve integration between virtual and physical spaces and virtual economies. Additional qualities include digital persistence and synchronicity in order to better establish a sense of presence in a realistic environment, along with implementing existing social media elements such as avatar identity, content creation, and social acceptability.

Current implementations

Video games

Several components of metaverse technologies have already been developed within modern internet-enabled video games. The 2003 video game Second Life is often described as the first metaverse, as it incorporated many aspects of social media into a persistent virtual world. Social functions are often an integral feature in many massively multiplayer online games. Technology journalist Clive Thompson has argued that the emergent, social-based gameplay of Minecraft represents an advanced implementation of the metaverse. Similar statements were made for the game Roblox, which has since employed significant usage of the term in marketing. Other claims of developing a metaverse include the games Active Worlds, Decentraland, and Fortnite in addition to a few early MUD games.

Virtual reality

Metaverse development has often focused on bettering virtual reality technologies due to benefits in establishing immersion in virtual environments. In 2019, the social network company Facebook launched a social VR world called Facebook Horizon. Facebook would later be renamed "Meta Platforms" in 2021. Its chairman Mark Zuckerberg declared a company commitment to developing a metaverse ecosystem. Much of the underlying virtual reality technology that Meta Platforms advertised remains to be developed. Microsoft acquired the VR company AltspaceVR in 2017, and is planning on integrating metaverse features into Microsoft Teams.

Potential implementations

Concept art of virtual office space called Nth Floor.

Some developers have proposed using metaverse technology for improvements in work productivity.

Within the education sector, metaverse technologies have been proposed as a way to allow for interactive environments for learning. The metaverse could also host virtual reality home tours in the real estate sector.

Technology

The metaverse is a proposed expansion to existing internet technologies. Access points for the metaverse include general-purpose computers and smartphones, in addition to augmented reality (AR), mixed reality, virtual reality (VR), and virtual world technologies.

Business and commercial interest in metaverse-related research and technology include Facebook, which bought VR company Oculus in 2014, and has announced plans to build a 3-D social space to connect varying services.

The metaverse's dependency on VR technology places limitations on its development and wide-scale adoption. Limitations stemming from the balance between cost and design include the lack of high quality graphics and a lack of mobility. Lightweight wireless headsets lack image quality, which is optimized for bulky, wired VR goggle systems. Another issue for wide-scale adoption of the technology is the cost, with the HTC Vive Pro 2 headset costing US$799 plus controllers in 2021.

In 2021, the South Korean government announced the creation of a national metaverse alliance with the goal to build a unified national VR and AR platform.

Technical standards

Common standards, interfaces, and communication protocols among virtual environments are in development. Collaborations and working groups are attempting to create standards and protocols to support interoperability between virtual environments, including:

• OpenXR, application programming interfaces (APIs) for interfacing with VR and AR devices, Khronos Group (2019–present)
• Virtual Worlds—Standard for Systems Virtual Components Working Group (P1828), IEEE (2010–Present)
• Information technology—Media context and control—Part 4: Virtual world object characteristics (ISO/IEC 23005-4:2011), ISO (2008–Present)
• Immersive Education Technology Group (IETG), Media Grid (2008–Present)
• Virtual World Region Agent Protocol (VWRAP), IETF (2009–2011)
• The Metaverse Roadmap, Acceleration Studies Foundation (2006–2007)
• The Open Source Metaverse Project (2004–2008)
• X3D, the successor to the Virtual Reality Modeling Language (VRML) as the open standard for interactive real-time 3D (web3D). X3D is an accepted standard for integrating virtual and augmented realities with the web.

Criticisms and concerns

The term arose in the early 1990s, and has come to be criticised as a method of public relations building using a purely speculative, "over-hyped" concept based on existing technology.

Information privacy in the metaverse is an area of concern because the companies involved will likely collect users' personal information through wearable devices and user interactions. Facebook is planning on persisting targeted advertising within the metaverse, raising further worries related to the spread of misinformation and loss of personal privacy. AR pioneer Louis Rosenberg warned that the metaverse could be more polarizing than social media, driving people from their information bubbles to their "own custom realities” and that users could selectively “reality block” the parts of society they don’t like.

User addiction and problematic social media use is another concern for the development of the metaverse. Internet addiction disorder, social media, and video game addiction can have mental and physical repercussions over a prolonged period of time, such as depression, anxiety, and obesity. Experts are also concerned that the metaverse could be used as an 'escape' from reality in a similar fashion to existing internet technologies.

The metaverse may magnify the social impacts of online echo chambers and digitally alienating spaces. Since metaverse developments may be made to algorithmically tailor virtual worlds based on each person's beliefs, the metaverse may further distort users' perceptions of reality with biased content to maintain or increase engagement.

Fiction

Snow Crash

The term metaverse was coined in Neal Stephenson's 1992 science fiction novel Snow Crash, where humans, as avatars, interact with each other and software agents, in a three-dimensional virtual space that uses the metaphor of the real world. Stephenson used the term to describe a virtual reality-based successor to the internet.

Neal Stephenson's metaverse appears to its users as an urban environment developed along a 100-meter-wide road, called the Street, which spans the entire 65536 km (2¹⁶ km) circumference of a featureless, black, perfectly spherical planet. The virtual real estate is owned by the Global Multimedia Protocol Group, a fictional part of the real Association for Computing Machinery, and is available to be bought and buildings developed thereupon.

Users of the metaverse access it through personal terminals that project a high-quality virtual reality display onto goggles worn by the user, or from grainy black and white public terminals in booths. The users experience it from a first-person perspective. Stephenson describes a sub-culture of people choosing to remain continuously connected to the metaverse; they are given the sobriquet "gargoyles" due to their grotesque appearance.

Within the metaverse, individual users appear as avatars of any form, with the sole restriction of height, "to prevent people from walking around a mile high". Transport within the metaverse is limited to analogs of reality by foot or vehicle, such as the monorail that runs the entire length of the Street, stopping at 256 Express Ports, located evenly at 256 km intervals, and Local Ports, one kilometer apart.

Ready Player One

Ready Player One is a 2011 dystopian science fiction novel by Ernest Cline, depicting the world in the year 2045 as being gripped by an energy crisis and global warming, causing widespread social problems and economic stagnation. The primary escape for people is a metaverse called the OASIS, which is accessed with a VR headset and wired gloves. It functions both as an MMORPG and as a virtual society. A film adaptation was released in 2018.

 

Technology in science fiction

From Wikipedia, the free encyclopedia

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

 

A conceptional science fiction prosthetic/robotic arm.

Technology in science fiction delves into the possibilities and implications of new technological concepts. Authors have taken, or created, new innovations and technologies, and elaborated on what they might be and how they might be used. This exchange goes in both directions – sometimes the technology appears first in science fiction, then becomes reality (such as space travel) and other times the real technology comes first, and science fiction authors speculate about how it might be used, and how it might affect the human condition. Likewise, the accuracy of the technology portrayed spans a wide range – sometimes it is existing technology, sometimes it is a physically realistic portrayal of a far-out technology, and sometimes it is simply a plot device that looks scientific, but has no basis in science. Examples drawn from space travel in science fiction include:

• Realistic case: Space suits. These are almost always based on existing suits, or near-term extrapolation of their capabilities.
• Extrapolation: Travel within the Solar System. As of 2019, humans have only traveled in Earth orbit or from Earth to Moon and back. However, travelling within the Solar System violates no physical principles.
• Plot device: faster-than-light drive. It is unsupported by physics as we know it, but needed for galaxy-wide or intergalactic plots with human lifespans.

Fictional technologies that have since been realized

Almost every new technology that becomes practical was previously used in science fiction. The following are a few examples, from a very large set:

Transparent Aluminum as featured in the Star Trek universe has since become a reality as Aluminium oxynitride (ALON™), patented in 1985, and as different from metallic aluminum as rust is from iron. Rather than being used as transparent blast shielding as in the fictional Enterprise class starships, this transparent ceramic is used, as the chemically similar (and similarly expensive) corundum (crystalline aluminum oxide) has long been used, in tough windows.

Tractor/Repulsor Beams have been realized as Laser-based Optical tweezers, and more recently as a pair of Bessel beams. These instruments use the radiation from the laser beam to manipulate microscopic particles in what is called an "optical trap" along the length of the beam as desired.

Fictional Tractor beams have been prominently used in the Star Wars universe and in the Star Trek universe. In an early scene of Star Wars: A New Hope a large spaceship uses such a beam to seize a small one, in order to capture the protagonists.

Artificial Vision/Prosthetic Eyes Visual prosthesis has been a topic of experiments since the late 20th century. Notable characters using artificial vision include all characters from the Ghost in the Shell series who use prosthetic bodies e.g. Batou's ranger eyes, Saito's left eye, and Motoko Kusanagi's artificial eyes, Geordi La Forge from the Star Trek: The Next Generation series who made use of a VISOR and later; ocular implants, RoboCop from the RoboCop series, Spike Spiegel from the Cowboy Bebop anime series, and the Illusive Man from the Mass Effect series of videogames.

Cell Phones the science fiction franchise Star Trek introduced the wireless communication device that has since become the cell phone. The actual idea of small, portable phones goes back decades before this. Science fiction author Robert A. Heinlein in his 1948 book Space Cadet, had a young man named Matt reach into his pouch, pull out his phone and answer it to talk to his father, and a young man answering a phone call on his horse in Heinlein's 1961 book Between Planets.

Tricorder The Lab-On-a-Chip Application Development Portable Test System (LOCAD-PTS) used by astronauts on the International Space Station is designed specifically to biochemical molecules with the purpose of "identifying microbes on space station services" through use of the Gram Staining Method.

Though less advanced than the fictional tricorder of the Star Trek series, the LOCAD-PTS is useful for quickly identifying bacteria and fungi on the International Space Station without having to send samples back to Earth, thus risking contamination or degradation. Fungi have proven to be a hazard if left unchecked on the space station as they managed to decompose some Russian electronics.

The Tricorder featured in the Star Trek universe was capable of measuring almost anything, from the chemical composition of explosives, to the life signs of a dying humanoid. The LOCAD-PTS does not differentiate between live and dead test material yet.

Communication technology has often been advanced in science fiction, usually with the idea of being able to see the person or place you are trying to reach, or make a call or talk to someone in a more convenient way.

A staple science fiction technology are phones or screens that allowed the user to see live feed of the person they were talking to while making a call. This was much used in 20th-century science fiction before the concept became a commercial reality. It was used as a futuristic type of content device for people to meet with each other from long distances in Stanley Kubrick’s film 2001: A Space Odyssey (film). In George Orwell’s 1984 (novel) it was a tool by which the socialist Ingsoc monitored and controlled the citizens of a dystopian future.

Holograms are the idea of being able to talk to someone by communicating with a 3-d light projection of their image. While it appears in many different works of science fiction, it is used most prominently in the Star Wars franchise to talk to someone face to face, and for conferences across long distances in the Marvel Cinematic Universe.

Another type of contact involves using androids that are controlled by other people, giving them the ability to actually interact with objects and people from long distances. This was most popularly seen in the thriller film Surrogates, which is set in a future where this type of interaction is so commonplace, people have forgotten what it is like to have social exchanges outside an android body.

In real-world science, it takes time for signals to travel through space. However, In Orson Scott Card’s Ender’s Game, a video screen allows people to talk to each other instantly across space no matter how far apart they are, effectively sending messages faster than the speed of light.

Sometimes science fiction communication crosses not space, but rather time. In the mystery film Don’t Let Go (film) a father discovers that his phone has the ability to call his daughter in the past. He used it to prevent her death. In the drama movie Frequency (2000 film), a solar storm temporarily gives a police officer’s old radio the ability to talk to people 30 years in the past, which he uses to save his father, bringing a fatal ripple in history that they must work together to fix.

Mechanical life/Androids/Robots

While now (as of 2017) there are companies that are fully devoted to creating robots and artificial intelligence, these ideas were long present in science fiction before they started to become real technology. Mechanical and artificial characters were derived both from extrapolations of real engineering efforts, and from the whims and imaginations of the authors. This technology has given writers, as well as other forms of art, the inspiration to create non-human characters.

Early fiction about mechanical life

C-3PO of Star Wars

• Olimpia – "The Sandman", a short story by E. T. A. Hoffmann (1816)
• Marius – R.U.R., a play by Karel Čapek. (1920)
• Maria – Metropolis film (1927)
• Revolt of the Pedestrian, novel by David H. Keller (1932)
• Asimov's Robots short stories (1954–1992)
• Robbie – The Forbidden Planet film (1956)
• Daleks – Doctor Who (1963)
• Cybermen – Doctor Who (1966)
• The Iron Man, novel by Ted Hughes (1968)
• The Stepford Wives by Ira Levin Novel (1972)
• The Questor Tapes (1974)
• The Bicentennial Man by Isaac Asimov (1976)
• C-3PO – Star Wars Episode IV: A New Hope (1977)
• Darth Vader – Star Wars Episode IV: A New Hope (1977)
• K9 – Doctor Who (1977)
• Marvin the Paranoid Android – The Hitch-Hiker's Guide to the Galaxy (1978)

Definitions

Artificial Intelligence (also known as machine intelligence and often abbreviated as AI) is intelligence exhibited by any manufactured (not grown) system. The term is often applied to general-purpose computers and also in the field of scientific investigation into the theory and practical application of AI.

A robot is an electro-mechanical or bio-mechanical device or group of devices that can perform autonomous or preprogrammed tasks.

An android is a robot made to resemble a human, usually both in appearance and behavior. The word derives from the Greek andr-, " meaning "man, male", and the suffix -eides, used to mean "of the species; alike" (from eidos "species").

A cyborg is a cybernetic organism which adds to or enhances its abilities by using technology.

A mecha is a large robotic mobile suit, sometimes piloted, sometimes remote-operated, and often used for combat or labor. Mechas usually come equipped with enhanced strength, rocket-propelled flight and an assortment of built-in weapons.

Early timeline of real-world technology

• 1957: Applied Physics Laboratory AIS begins with a focus on learning machines and self-organizing systems.
• 1961: MINOS 1 First perceptron machine, responds to a pattern of binary inputs using weights.
• 1966: Artificial Intelligence Center is formed
• 1966-1972: Shakey the Robot First autonomous mobile robots, controlled from radio and TV links.
• 1968: A*Algorithm Graph-searching algorithm used to route planning solver for navigation.
• 1969: STRIPS Planning engine for Shakey.
• 1969: QA3 and QA4 Automated problem-solving.

ESP/Psychic powers/Psi phenomena

With new developments in science and technology helping to study and promote parapsychology or Psi Phenomena, many SF writers felt the need to incorporate and elaborate on these subjects in their stories. While technology helped the investigation into Psi Phenomena it also created questions that many SF writers chose to answer, through their stories, in their own unique way. If we look at some of the examples of Psi Phenomena prominent in stories, they may have stemmed from how science would take this experimentation with Psi Phenomena and use it. In Stephen King's "The Dead Zone", we see how precognition was used to affect political candidates. The idea that someone could harness this power and use it for good or evil was one that many SF writer's elaborated on. In "The Foreign Hand Tie" by Randall Garret espionage takes on a new form via telepathy through twins. When science and technology can be used to anchor something in reality, via experimentation or exploration, and yield results, it creates controversy that society may fear or even fantasize about. Throughout SF history, Psi Phenomena can be seen to be used for good and evil, and through new science and technological discoveries, this genre then becomes more real and more elaborate.

Terms commonly used

• Telepathy: the ability to read minds
• Precognition: the ability to see the future
• Telekinesis: the ability to move objects with mental force (Psychokinesis (PK for short) or "mind over matter")
• Teleportation: the ability to move oneself from one place to the other, or back and forward in time
• Telempathy: Emotion-reading
• Remote viewing/Clairvoyance/Scrying: the ability for seeing things not actually before your eyes
• Psychometry: the ability to sense what has touched a certain physical object or the imprint it has left behind
• Bilocation: the ability to be in two places at the same time.
• Pyrokinesis: the capability to start fires by mental action alone

Writers to mention these topics

• G. H. Ryan: "Fifteen Months in the Moon" (1880)
• Fitz James O’Brien: "The Bohemian" (1885)
• Arthur C. Clarke: Childhood's End (1953) Parapsychology determines, in the end, the fate of the human race.
• Robert A. Heinlein: Time for the Stars, (1956): Telepathic twins
• Joanna Russ: "And Chaos Died" (1970): Telepathy
• Algis Budrys: Rogue Moon, (1960)
• Chester Aaron: "Out of Sight, Out of Mind" (1986)
• Stephen King: "The Dead Zone" (1979): Precognition affects political candidate
• James H. Schmitz: "These Are The Arts" (1962): Telepathic masters, we're slaves
• Isaac Asimov: "Belief" (1953): Physics versus levitation
• Mark Clifton & Alex Apostolide: "What Thin Partitions (1953): Industrial psychokinesis
• Randall Garrett: "The Foreign Hand Tie" (1961): Espionage via telepathy between identical twins
• Robert A. Heinlein: "Project Nightmare" (1953): Clairvoyance and A-bombs
• Zenna Henderson: "Ararat" (1952: The first of "The People" stories, about psi-gifted aliens who live on Earth)
• Murray Leinster: "The Leader" (1960): Long-distance mass-hypnotism

Brief history of psi phenomena in science

While ESP and belief in other powers were, in the beginning, mainly fueled by superstitions, religion and tradition, the dawn of science brought about a way to analyze and study these supposed "powers" giving them an anchor in reality. The Scientific Revolution featured ideas that life should be "led by reason" and that, "the universe as a mechanistic, deterministic system could eventually be known accurately and fully through observation and reason". While new science and technology gave rise to skepticism towards the existence of psi phenomena, it also gave way for new technologies to be applied in either proving or disproving such phenomena. One of the first experimental approaches to Psi Phenomena started in the 1930s and was conducted under the direction of J.B. Rhine (1895–1980). Rhine popularized the now famous methodology of card-guessing and dice-rolling experiments in a laboratory in attempt to find statistical validation for ESP. In 1957 the Parapsychological Association was formed at the preeminent society for parapsychology. Openness to new parapsychology studies and occult phenomena continued to rise in the 1970s.

Technological developments

• Ganzfeld Experiment: homogenous, unpatterned, sensory stimulation to produce an effect similar to sensory deprivation
• Development of statistical tools by R. A. Fisher in the 1920s

Timeline of probable influences

E. Dawson Rogers hopes to gain new respectability for spiritualism and founds Society for Psychical Research in 1882

Government investigations into parapsychology: Project Star Gate, formed in 1970 with cooperation from the Central Intelligence Agency and Defense Intelligence Agency, investigates remote viewing, sees nothing useful

Visitors from other planets

Extraterrestrial life is a familiar topic in fiction. In the centuries since astronomers discovered that planets are worlds, people have speculated on the possibility of life existing there, though xenobiology has remained a science without a subject. However, people from afar, or alien creatures with various powers and purposes, provided fresh new material for fiction. Some stories were about friendly visitors who got along with humans, such as the aliens in the Keroro Gunsou series, when they give up on attempting to take over planet Earth. Others made alien invasion their theme, as in the 1898 novel, War of the Worlds. Meteorites have long shown that foreign bodies sometimes enter Earth's atmosphere, and the term "flying saucer" was coined in 1947. Several science fiction novels used them.

Early writers

• The War of the Worlds by H. G. Wells, first serialised in 1897
• Martians, Go Home by Fredric Brown, 1956
• The Moon that Vanished by Leigh Brackett, 1950
• 3 From Out There by Leo Margulies, 1959
• To Outrun Doomsday by Kenneth Bulmer, 1957
• Venus Stories by Edgar Rice Burroughs, 1955

Other terms

• NTI: Non-Terrestrial Intelligence (A term for alien life that dwells in the oceans or otherwise not on land)
• UFO: Unidentified Flying Object
• Flying Saucer: A certain kind of spaceship

Timeline of non science fiction influences

• Antiquity onward: Philosophers have debated the existence of extraterrestrial life.
• 1609: Galileo, using a telescope to observe the heavens, discovers that planets are other worlds.
• 1877: Italian astronomer Giovanni Schiaparelli reported the appearance of certain long, thin lines he called canali, meaning channels in Italian.
• early 1900s: Astronomer Percival Lowell, a science popularizer, wrote the books "Mars" (1895), "Mars and Its Canals" (1906), and "Mars As the Abode of Life" (1908). This was considered science at the time, not fiction, but has been shown to be incorrect by modern missions to Mars.
• 2009: NASA's Kepler mission shows that an assumption of science fiction, that planets are common throughout the galaxy, is in fact true.

Parallel worlds

The notion of parallel worlds has always intrigued different types of genres, especially the science fiction aspect. Many authors have used the idea of travelling back into prehistoric times or traveling forwards to an unknown universe. The idea of entering a world that has not been touched or that has evolved into a new incomprehensible parallel, makes people ponder about what it could looks like or what it could be. Authors have used this notion of an alternate reality and have created their own worlds that have given readers a different view of alternate worlds.

Early writers

• Sideways in Time by Murray Leinster Novel (1934)
• Lest Darkness Fall By L. Sprague De Camp Novel (1939)
• Horsesense Hank in the Parallel Worlds by Nelson S. Bond Magazine (1942)
• The Alteration by Kingsley Amis Novel (1976)
• The Anubis Gates By Tim Powers Novel (1983)

Definition

Parallel Universe Parallel universe or alternate reality in science fiction and fantasy is a self-contained separate reality coexisting with our own

Other terms

Multiverse Set of many universes. There are many specific uses of the concept, as well as systems in which a multiverse is proposed to exist in.

Parallel universe alternate universes, worlds, realities and dimensions in fiction.

Alternate reality alternate universes, worlds, realities and dimensions in fiction.

Alternate future is a possible future which never comes to pass, typically because someone travels back into the past and alters it so that the events of the alternate future cannot occur.

Early timeline

Gravitational distortions caused by a black hole in front of the Large Magellanic Cloud (artistic interpretation Provided by: Black hole )

• 1905: Albert Einstein Proposes Special theory of Relativity
• 1905: Albert Einstein's special theory of relativity shows that space and time are relative, not absolute, and that time is actually a fourth dimension within what he calls "space-time."
• 1916: Einstein discovers that space-time is curved.
• 1920s: Heisenberg, Schrödinger, and Dirac reformulate mechanics into Quantum Mechanics, based on the Uncertainty Principle.
• 1922: Kaluza–Klein theory combined Einstein's General Relativity and Maxwell's electromagnetic field theory in 5 dimensions.
• 1937: Mathematician Kurt Gödel proposes that the universe itself may be a time machine.
• 1949: Gödel demonstrates mathematically that pathways through time are consistent with general relativity (see Gödel metric).
• 1967: U.S. physicist John Wheeler invents the name "black hole" to describe singularities in space and time.

Invisibility

The idea of being unseen and hence undetectable has fascinated humankind for generations. This concept has generated scientific pursuit towards defying our physical parameters. Many authors have toyed with the idea of gaining invisibility via both science-based and fictional means. Invisibility in the actual scientific world will be a very difficult achievement, one that will involve much more complication than we have begun to delve into. Further technological developments bring us closer to our goal, while also broadening the horizon for science fiction authors performing thought experiments on the topic of invisibility.

Mythology and folklore precursors

Many myths and legends include gods, spirits, angels, and demons that are often invisible or can choose to become invisible at will.

• Plato's The Republic was one of the first stories to explore the idea of invisibility. A peasant finds a ring in the tomb of a dead king that allows him to become invisible. He enters the palace, seduces the queen, and plots to kill the present king, showing that power such as invisibility corrupts.
• Perseus, the Greek mythic hero who helped establish the Twelve Olympians, was equipped with a cap of invisibility to kill Medusa.

Early writers

• H. G. Wells wrote The Invisible Man (1897) which was the first science fiction novel to explore the idea of invisibility. The invisible man is a scientist named Griffin who theorizes that if a person's refractive index is changed to exactly that of air and his body does not absorb or reflect light, then he will not be visible. He successfully carries out this procedure on himself, but cannot become visible again, leading to mental instability.
• J. R. R. Tolkien wrote The Lord of the Rings series which revolves around the function of a ring that renders the user invisible. Unfortunately, it had an evil influence with negative effects on the wearer's actions.
• Douglas Adams wrote The Hitchhiker's Guide to the Galaxy (1978) novels which encompass a humorous concept of a field which makes people believe the object in question is "somebody else's problem" and therefore do not see it. This concept as explained in the book, bases off of a statement to the effect that actual invisibility is impossible and that the field is merely a way to make something close to being invisible by actually making it hard to notice deliberately.
• Philip K. Dick wrote in his 1974 novel A Scanner Darkly of a "scramble suit." This is a flexible sheath covering the body of the wearer with a reflective/refractive coating on the inside surface that transfers the camouflaging pattern- projected by a holographic lens mounted on the wearer's head- onto the outside surface of the sheath causing camouflage-like invisibility.

Invisibility in science fiction

There is an undeniable link between science facts and the ideas that emerge in science fiction. Science fiction authors are inspired by actual scientific and technological discoveries, but allow themselves the freedom to project the possible future course of these discoveries and their potential impact on society, perhaps only weakly bound to the facts.

Invisibility in fiction

Authors are faced with obstacles presented by the realities of actual technology, however fiction allows a window for the opportunity of inventing completely imaginary technologies to move their storyline forward and maybe even still explore the outcomes of such power.

• Magic objects such as rings and cloaks can be worn to grant the wearer permanent invisibility.
• Spells and potions can be used or cast upon people or objects granting temporary invisibility.

Timeline of possible influences

• 17th century the refractive index was developed. Major advances near the end of the 19th century raised author's awareness.
• 1670s Emitting or reflecting light outside the wavelength range of visible light would result in a human-shaped black hole which would be completely opaque.
• 1930s Chroma key began to develop which is the removal of color from one image to reveal another image "behind it". The removed color becomes transparent, which is also called "color keying".
• 1938 Stealth technology began to develop. It is used with aircraft, ships, and missiles, in order to make them less visible to certain detection methods.
• 1966 An enemy in a Star Trek episode uses a Cloaking device. Other fiction has used a cloak of invisibility.