In a general sense, biospheres are any closed, self-regulating
systems containing ecosystems. This includes artificial biospheres such
as Biosphere 2 and BIOS-3, and potentially ones on other planets or moons.
Origin and use of the term
A beach scene on Earth, simultaneously showing the lithosphere (ground), hydrosphere (ocean) and atmosphere (air)
The term "biosphere" was coined in 1875 by geologist Eduard Suess, who defined it as the place on Earth's surface where life dwells.
Geochemists define the biosphere as being the total sum of living organisms (the "biomass" or "biota"
as referred to by biologists and ecologists). In this sense, the
biosphere is but one of four separate components of the geochemical
model, the other three being geosphere, hydrosphere, and atmosphere. When these four component spheres are combined into one system, it is known as the ecosphere. This term was coined during the 1960s and encompasses both biological and physical components of the planet.
The Second International Conference on Closed Life Systems defined biospherics as the science and technology of analogs and models of Earth's biosphere; i.e., artificial Earth-like biospheres. Others may include the creation of artificial non-Earth biospheres—for example, human-centered biospheres or a native Martian biosphere—as part of the topic of biospherics.
Earth's biosphere
Overview
Currently, the total number of living cells on the Earth is estimated to be 1030; the total number since the beginning of Earth, as 1040, and the total number for the entire time of a habitable planet Earth as 1041.This is much larger than the total number of estimated stars (and Earth-like planets) in the observable universe as 1024, a number which is more than all the grains of beach sand on planet Earth; but less than the total number of atoms estimated in the observable universe as 1082; and the estimated total number of stars in an inflationary universe (observed and unobserved), as 10100.
Age
Stromatolite fossil estimated at 3.2–3.6 billion years old
Every part of the planet, from the polar ice caps to the equator, features life of some kind. Recent advances in microbiology have demonstrated that microbes live deep beneath the Earth's terrestrial surface and that the total mass of microbial life in so-called "uninhabitable zones" may, in biomass,
exceed all animal and plant life on the surface. The actual thickness
of the biosphere on Earth is difficult to measure. Birds typically fly
at altitudes as high as 1,800 m (5,900 ft; 1.1 mi) and fish live as much
as 8,372 m (27,467 ft; 5.202 mi) underwater in the Puerto Rico Trench.
There are more extreme examples for life on the planet: Rüppell's vulture has been found at altitudes of 11,300 metres (37,100 feet; 7.0 miles); bar-headed geese migrate at altitudes of at least 8,300 m (27,200 ft; 5.2 mi); yaks live at elevations as high as 5,400 m (17,700 ft; 3.4 mi) above sea level; mountain goats live up to 3,050 m (10,010 ft; 1.90 mi). Herbivorous animals at these elevations depend on lichens, grasses, and herbs.
Life forms live in every part of the Earth's biosphere, including soil, hot springs, inside rocks at least 19 km (12 mi) deep underground, and at least 64 km (40 mi) high in the atmosphere. Marine life under many forms has been found in the deepest reaches of the world ocean while much of the deep sea remains to be explored.
Under certain test conditions, microorganisms have been observed to survive the vacuum of outer space. The total amount of soil and subsurface bacterial carbon is estimated as 5 × 1017 g. The mass of prokaryote microorganisms—which includes bacteria and archaea, but not the nucleated eukaryote microorganisms—may be as much as 0.8 trillion tons of carbon (of the total biosphere mass, estimated at between 1 and 4 trillion tons). Barophilic marine microbes have been found at more than a depth of 10,000 m (33,000 ft; 6.2 mi) in the Mariana Trench, the deepest spot in the Earth's oceans. In fact, single-celled life forms have been found in the deepest part of the Mariana Trench, by the Challenger Deep, at depths of 11,034 m (36,201 ft; 6.856 mi). Other researchers reported related studies that microorganisms thrive
inside rocks up to 580 m (1,900 ft; 0.36 mi) below the sea floor under
2,590 m (8,500 ft; 1.61 mi) of ocean off the coast of the northwestern United States, as well as 2,400 m (7,900 ft; 1.5 mi) beneath the seabed off Japan. Culturable thermophilic microbes have been extracted from cores drilled more than 5,000 m (16,000 ft; 3.1 mi) into the Earth's crust in Sweden, from rocks between 65–75 °C (149–167 °F). Temperature increases with increasing depth
into the Earth's crust. The rate at which the temperature increases
depends on many factors, including the type of crust (continental vs.
oceanic), rock type, geographic location, etc. The greatest known
temperature at which microbial life can exist is 122 °C (252 °F) (Methanopyrus kandleri Strain 116). It is likely that the limit of life in the "deep biosphere" is defined by temperature rather than absolute depth. On 20 August 2014, scientists confirmed the existence of microorganisms living 800 m (2,600 ft; 0.50 mi) below the ice of Antarctica.
Earth's biosphere is divided into several biomes, inhabited by fairly similar flora and fauna. On land, biomes are separated primarily by latitude. Terrestrial biomes lying within the Arctic and Antarctic Circles are relatively barren of plant and animal life. In contrast, most of the more populous biomes lie near the equator.
Annual variation
Artificial biospheres
Biosphere 2 in Arizona
Experimental biospheres, also called closed ecological systems,
have been created to study ecosystems and the potential for supporting
life outside the Earth. These include spacecraft and the following
terrestrial laboratories:
No biospheres have been detected beyond the Earth; therefore, the
existence of extraterrestrial biospheres remains hypothetical. The rare Earth hypothesis suggests they should be very rare, save ones composed of microbial life only. On the other hand, Earth analogs may be quite numerous, at least in the Milky Way galaxy, given the large number of planets. Three of the planets discovered orbiting TRAPPIST-1 could possibly contain biospheres. Given limited understanding of abiogenesis, it is currently unknown what percentage of these planets actually develop biospheres.
Based on observations by the Kepler Space Telescope
team, it has been calculated that provided the probability of
abiogenesis is higher than 1 to 1000, the closest alien biosphere should
be within 100 light-years from the Earth.
It is also possible that artificial biospheres will be created in the future, for example with the terraforming of Mars.
The formation of complex symmetrical and fractalpatterns in snowflakes exemplifies emergence in a physical system.A termite "cathedral" mound produced by a termite colony offers a classic example of emergence in nature.
In philosophy, systems theory, science, and art, emergence
occurs when a complex entity has properties or behaviors that its parts
do not have on their own, and emerge only when they interact in a wider
whole.
Philosophers often understand emergence as a claim about the etiology of a system's
properties. An emergent property of a system, in this context, is one
that is not a property of any component of that system, but is still a
feature of the system as a whole. Nicolai Hartmann (1882–1950), one of the first modern philosophers to write on emergence, termed this a categorial novum (new category).
Definitions
This concept of emergence dates from at least the time of Aristotle. In Heideggerian thought, the notion of emergence is derived from the Greek word poiein, meaning "to make", and refers to a bringing-forth that encompasses not just a process of crafting (techne) but also the broader sense of something coming into being or revealing itself. Heidegger used emerging blossoms and butterflies as examples to illustrate poiêsis as a threshold event where something moves from one state to another. Many scientists and philosophers have written on the concept, including John Stuart Mill (Composition of Causes, 1843) and Julian Huxley (1887–1975).
The philosopher G. H. Lewes coined the term "emergent" in 1875, distinguishing it from the merely "resultant":
Every resultant is either a sum or a difference of the
co-operant forces; their sum, when their directions are the same – their
difference, when their directions are contrary. Further, every
resultant is clearly traceable in its components, because these are homogeneous and commensurable.
It is otherwise with emergents, when, instead of adding measurable
motion to measurable motion, or things of one kind to other individuals
of their kind, there is a co-operation of things of unlike kinds. The
emergent is unlike its components insofar as these are incommensurable,
and it cannot be reduced to their sum or their difference.
Usage of the notion "emergence" may generally be subdivided into two
perspectives, that of "weak emergence" and "strong emergence". One paper
discussing this division is Weak Emergence, by philosopher Mark Bedau.
In terms of physical systems, weak emergence is a type of emergence in
which the emergent property is amenable to computer simulation or
similar forms of after-the-fact analysis (for example, the formation of a
traffic jam, the structure of a flock of starlings in flight or a
school of fish, or the formation of galaxies). Crucial in these
simulations is that the interacting members retain their independence.
If not, a new entity is formed with new, emergent properties: this is
called strong emergence, which it is argued cannot be simulated,
analysed or reduced.
David Chalmers
writes that emergence often causes confusion in philosophy and science
due to a failure to demarcate strong and weak emergence, which are
"quite different concepts".
Some common points between the two notions are that emergence
concerns new properties produced as the system grows, which is to say
ones which are not shared with its components or prior states. Also, it
is assumed that the properties are supervenient rather than metaphysically primitive.
Weak emergence describes new properties arising in systems as a
result of the interactions at a fundamental level. However, Bedau
stipulates that the properties can be determined only by observing or
simulating the system, and not by any process of a reductionist analysis. As a consequence the emerging properties are scale dependent:
they are only observable if the system is large enough to exhibit the
phenomenon. Chaotic, unpredictable behaviour can be seen as an emergent
phenomenon, while at a microscopic scale the behaviour of the
constituent parts can be fully deterministic.
Bedau notes that weak emergence is not a universal metaphysical solvent, as the hypothesis that consciousness is weakly emergent would not resolve the traditional philosophical questions
about the physicality of consciousness. However, Bedau concludes that
adopting this view would provide a precise notion that emergence is
involved in consciousness, and second, the notion of weak emergence is
metaphysically benign.
Strong emergence describes the direct causal action of a high-level system on its components; qualities produced this way are irreducible to the system's constituent parts. The whole is other than the sum of its parts. It is argued then that no
simulation of the system can exist, for such a simulation would itself
constitute a reduction of the system to its constituent parts. Physics lacks well-established examples of strong emergence, unless it is interpreted as the impossibility in practice
to explain the whole in terms of the parts. Practical impossibility may
be a more useful distinction than one in principle, since it is easier
to determine and quantify, and does not imply the use of mysterious
forces, but simply reflects the limits of our capability.
Viability of strong emergence
One of the reasons for the importance of distinguishing these two
concepts with respect to their difference concerns the relationship of
purported emergent properties to science. Some thinkers question the
plausibility of strong emergence as contravening our usual understanding
of physics. Mark A. Bedau observes:
Although strong emergence is
logically possible, it is uncomfortably like magic. How does an
irreducible but supervenient downward causal power arise, since by
definition it cannot be due to the aggregation of the micro-level
potentialities? Such causal powers would be quite unlike anything within
our scientific ken. This not only indicates how they will discomfort
reasonable forms of materialism. Their mysteriousness will only heighten
the traditional worry that emergence entails illegitimately getting
something from nothing.
The concern that strong emergence does so entail is that such a
consequence must be incompatible with metaphysical principles such as
the principle of sufficient reason or the Latin dictum ex nihilo nihil fit, often translated as "nothing comes from nothing".
Strong emergence can be criticized for leading to causal overdetermination.
The canonical example concerns emergent mental states (M and M∗) that
supervene on physical states (P and P∗) respectively. Let M and M∗ be
emergent properties. Let M∗ supervene on base property P∗. What happens
when M causes M∗? Jaegwon Kim says:
In our schematic example above, we
concluded that M causes M∗ by causing P∗. So M causes P∗. Now, M, as an
emergent, must itself have an emergence base property, say P. Now we
face a critical question: if an emergent, M, emerges from basal
condition P, why cannot P displace M as a cause of any putative effect
of M? Why cannot P do all the work in explaining why any alleged effect
of M occurred? If causation is understood as nomological
(law-based) sufficiency, P, as M's emergence base, is nomologically
sufficient for it, and M, as P∗'s cause, is nomologically sufficient for
P∗. It follows that P is nomologically sufficient for P∗ and hence
qualifies as its cause...If M is somehow retained as a cause, we are
faced with the highly implausible consequence that every case of
downward causation involves overdetermination (since P remains a cause
of P∗ as well). Moreover, this goes against the spirit of emergentism in
any case: emergents are supposed to make distinctive and novel causal
contributions.
If M is the cause of M∗, then M∗ is overdetermined because M∗ can
also be thought of as being determined by P. One escape-route that a
strong emergentist could take would be to deny downward causation.
However, this would remove the proposed reason that emergent mental
states must supervene on physical states, which in turn would call physicalism into question, and thus be unpalatable for some philosophers and physicists.
Carroll and Parola propose a taxonomy that classifies emergent
phenomena by how the macro-description relates to the underlying
micro-dynamics.
Type‑0 (Featureless) Emergence
A coarse-graining map Φ from a micro state space A to a macro state space B that commutes with time evolution, without requiring any further decomposition into subsystems.
Type‑1 (Local) Emergence
Emergence where the macro theory is defined in terms of localized
collections of micro-subsystems. This category is subdivided into:
Type‑1a (Direct) Emergence: When the emergence map Φ is
algorithmically simple (i.e. compressible), so that the macro behavior
is easily deduced from the micro-states.
Type‑1b (Incompressible) Emergence: When Φ is algorithmically
complex (i.e. incompressible), making the macro behavior appear more
novel despite being determined by the micro-dynamics.
Type‑2 (Nonlocal) Emergence
Cases in which both the micro and macro theories admit subsystem
decompositions, yet the macro entities are defined nonlocally with
respect to the micro-structure, meaning that macro behavior depends on
widely distributed micro information.
Type‑3 (Augmented) Emergence
A form of strong emergence in which the macro theory introduces
additional ontological variables that do not supervene on the
micro-states, thereby positing genuinely novel macro-level entities.
Objective or subjective quality
Crutchfield regards the properties of complexity and organization of any system as subjectivequalities determined by the observer.
Defining structure and detecting the emergence of
complexity in nature are inherently subjective, though essential,
scientific activities. Despite the difficulties, these problems can be
analysed in terms of how model-building observers infer from
measurements the computational capabilities embedded in non-linear
processes. An observer's notion of what is ordered, what is random, and
what is complex in its environment depends directly on its computational
resources: the amount of raw measurement data, of memory, and of time
available for estimation and inference. The discovery of structure in an
environment depends more critically and subtly, though, on how those
resources are organized. The descriptive power of the observer's chosen
(or implicit) computational model class, for example, can be an
overwhelming determinant in finding regularity in data.
The low entropy
of an ordered system can be viewed as an example of subjective
emergence: the observer sees an ordered system by ignoring the
underlying microstructure (i.e. movement of molecules or elementary
particles) and concludes that the system has a low entropy. On the other hand, chaotic, unpredictable behaviour can also be seen as
subjective emergent, while at a microscopic scale the movement of the
constituent parts can be fully deterministic.
In science
In physics, weak
emergence is used to describe a property, law, or phenomenon which
occurs at macroscopic scales (in space or time) but not at microscopic
scales, despite the fact that a macroscopic system can be viewed as a
very large ensemble of microscopic systems.
An emergent behavior of a physical system is a qualitative property
that can only occur in the limit that the number of microscopic
constituents tends to infinity.
According to Robert Laughlin, for many-particle systems, nothing can be calculated exactly from the
microscopic equations, and macroscopic systems are characterised by
broken symmetry: the symmetry present in the microscopic equations is
not present in the macroscopic system, due to phase transitions. As a
result, these macroscopic systems are described in their own
terminology, and have properties that do not depend on many microscopic
details.
Novelist Arthur Koestler used the metaphor of Janus
(a symbol of the unity underlying complements like open/shut,
peace/war) to illustrate how the two perspectives (strong vs. weak or holistic vs. reductionistic) should be treated as non-exclusive, and should work together to address the issues of emergence. Theoretical physicist Philip W. Anderson states it this way:
The ability to reduce everything to
simple fundamental laws does not imply the ability to start from those
laws and reconstruct the universe. The constructionist hypothesis breaks
down when confronted with the twin difficulties of scale and
complexity. At each level of complexity entirely new properties appear.
Psychology is not applied biology, nor is biology applied chemistry. We
can now see that the whole becomes not merely more, but very different
from the sum of its parts.
Meanwhile, others have worked towards developing analytical evidence of strong emergence. Renormalization
methods in theoretical physics enable physicists to study critical
phenomena that are not tractable as the combination of their parts. In 2009, Gu et al. presented a class of infinite physical systems that exhibits non-computable macroscopic properties.. More precisely, if one could compute certain macroscopic properties of
these systems from the microscopic description of these systems, then
one would be able to solve computational problems known to be
undecidable in computer science. These results concern infinite systems,
finite systems being considered computable. However, macroscopic
concepts which only apply in the limit of infinite systems, such as phase transitions and the renormalization group, are important for understanding and modeling real, finite physical systems. Gu et al. concluded that
Although macroscopic concepts are
essential for understanding our world, much of fundamental physics has
been devoted to the search for a 'theory of everything', a set of
equations that perfectly describe the behavior of all fundamental
particles. The view that this is the goal of science rests in part on
the rationale that such a theory would allow us to derive the behavior
of all macroscopic concepts, at least in principle. The evidence we have
presented suggests that this view may be overly optimistic. A 'theory
of everything' is one of many components necessary for complete
understanding of the universe, but is not necessarily the only one. The
development of macroscopic laws from first principles may involve more
than just systematic logic, and could require conjectures suggested by
experiments, simulations or insight.
Human beings are the basic elements of social systems, which
perpetually interact and create, maintain, or untangle mutual social
bonds. Social bonds in social systems are perpetually changing in the
sense of the ongoing reconfiguration of their structure. An early argument (1904–05) for the emergence of social formations can be found in Max Weber's most famous work, The Protestant Ethic and the Spirit of Capitalism. Recently, the emergence of a new social system is linked with the
emergence of order from nonlinear relationships among multiple
interacting units, where multiple interacting units are individual
thoughts, consciousness, and actions. In the case of the global economic system, under capitalism,
growth, accumulation and innovation can be considered emergent
processes where not only does technological processes sustain growth,
but growth becomes the source of further innovations in a recursive,
self-expanding spiral. In this sense, the exponential trend of the
growth curve reveals the presence of a long-term positive feedback
among growth, accumulation, and innovation; and the emergence of new
structures and institutions connected to the multi-scale process of
growth. This is reflected in the work of Karl Polanyi,
who traces the process by which labor and nature are converted into
commodities in the passage from an economic system based on agriculture
to one based on industry. This shift, along with the idea of the self-regulating market, set the
stage not only for another economy but also for another society. The
principle of emergence is also brought forth when thinking about
alternatives to the current economic system based on growth facing
social and ecological limits. Both degrowth and social ecological economics
have argued in favor of a co-evolutionary perspective for theorizing
about transformations that overcome the dependence of human wellbeing on
economic growth.
Economic trends and patterns which emerge are studied intensively by economists. Within the field of group facilitation and organization development,
there have been a number of new group processes that are designed to
maximize emergence and self-organization, by offering a minimal set of
effective initial conditions. Examples of these processes include SEED-SCALE, appreciative inquiry, Future Search, the world cafe or knowledge cafe, Open Space Technology, and others (Holman, 2010). In international development, concepts of emergence have been used within a theory of social change termed SEED-SCALE
to show how standard principles interact to bring forward
socio-economic development fitted to cultural values, community
economics, and natural environment (local solutions emerging from the
larger socio-econo-biosphere). These principles can be implemented
utilizing a sequence of standardized tasks that self-assemble in individually specific ways utilizing recursive evaluative criteria.
Looking at emergence in the context of social and systems change, invites us to reframe our thinking on parts and wholes and their interrelation. Unlike machines, living systems
at all levels of recursion - be it a sentient body, a tree, a family,
an organisation, the education system, the economy, the health system,
the political system etc - are continuously creating themselves. They
are continually growing and changing along with their surrounding
elements, and therefore are more than the sum of their parts. As Peter
Senge and co-authors put forward in the book Presence: Exploring profound change in People, Organizations and Society,
"as long as our thinking is governed by habit - notably industrial,
"machine age" concepts such as control, predictability, standardization,
and "faster is better" - we will continue to recreate institutions as
they have been, despite their disharmony with the larger world, and the
need for all living systems to evolve." While change is predictably constant, it is unpredictable in direction
and often occurs at second and nth orders of systemic relationality. Understanding emergence and what creates the conditions for different
forms of emergence to occur, either insidious or nourishing vitality, is
essential in the search for deep transformations.
The works of Nora Bateson and her colleagues at the International
Bateson Institute delve into this. Since 2012, they have been
researching questions such as what makes a living system ready to change? Can unforeseen ready-ness for change be nourished? Here being ready is not thought of as being prepared, but rather as nourishing the flexibility
we do not yet know will be needed. These inquiries challenge the common
view that a theory of change is produced from an identified preferred
goal or outcome. As explained in their paper An essay on ready-ing: Tending the prelude to change: "While linear managing or controlling of the direction of change may
appear desirable, tending to how the system becomes ready allows for
pathways of possibility previously unimagined." This brings a new lens
to the field of emergence in social and systems change as it looks to
tending the pre-emergent process. Warm Data Labs are the fruit of their praxis, they are spaces for transcontextual mutual learning in which aphanipoetic phenomena unfold. Having hosted hundreds of Warm Data processes with thousands of
participants, they have found that these spaces of shared poly-learning
across contexts lead to a realm of potential change, a necessarily
obscured zone of wild interaction of unseen, unsaid, unknown
flexibility. It is such flexibility that nourishes the ready-ing living systems
require to respond to complex situations in new ways and change. In
other words, this readying process preludes what will emerge. When
exploring questions of social change, it is important to ask ourselves,
what is submerging in the current social imaginary and perhaps, rather
than focus all our resources and energy on driving direct order
responses, to nourish flexibility with ourselves, and the systems we are
a part of.
Another approach that engages with the concept of emergence for
social change is Theory U, where "deep emergence" is the result of
self-transcending knowledge after a successful journey along the U
through layers of awareness. This practice nourishes transformation at the inner-being level, which
enables new ways of being, seeing and relating to emerge. The concept of
emergence has also been employed in the field of facilitation. In Emergent Strategy, adrienne maree brown
defines emergent strategies as "ways for humans to practice complexity
and grow the future through relatively simple interactions".
In linguistics, the concept of emergence has been applied in the domain of stylometry to explain the interrelation between the syntactical structures of the text and the author style (Slautina, Marusenko, 2014). It has also been argued that the structure and regularity of languagegrammar, or at least language change, is an emergent phenomenon. While each speaker merely tries to reach their own communicative goals,
they use language in a particular way. If enough speakers behave in
that way, language is changed. In a wider sense, the norms of a language, i.e. the linguistic
conventions of its speech society, can be seen as a system emerging from
long-time participation in communicative problem-solving in various
social circumstances.
In technology
The bulk conductive response of binary (RC) electrical networks with random arrangements, known as the universal dielectric response
(UDR), can be seen as emergent properties of such physical systems.
Such arrangements can be used as simple physical prototypes for deriving
mathematical formulae for the emergent responses of complex systems. Internet traffic can also exhibit some seemingly emergent properties. In the congestion control mechanism, TCP
flows can become globally synchronized at bottlenecks, simultaneously
increasing and then decreasing throughput in coordination. Congestion,
widely regarded as a nuisance, is possibly an emergent property of the
spreading of bottlenecks across a network in high traffic flows which
can be considered as a phase transition. Some artificially intelligent (AI) computer applications simulate emergent behavior. One example is Boids, which mimics the swarming behavior of birds.
In religion and art
In religion, emergence grounds expressions of religious naturalism and syntheism in which a sense of the sacred is perceived in the workings of entirely naturalistic processes by which more complex forms arise or evolve from simpler forms. Examples are detailed in The Sacred Emergence of Nature by Ursula Goodenough & Terrence Deacon and Beyond Reductionism: Reinventing the Sacred by Stuart Kauffman, both from 2006, as well as Syntheism – Creating God in The Internet Age by Alexander Bard & Jan Söderqvist from 2014 and Emergentism: A Religion of Complexity for the Metamodern World by Brendan Graham Dempsey (2022).
Michael J. Pearce has used emergence to describe the experience of works of art in relation to contemporary neuroscience. Practicing artist Leonel Moura, in turn, attributes to his "artbots" a real, if nonetheless rudimentary, creativity based on emergent principles.
In daily life and nature
Objects consist of components with properties differing from the
object itself. We call these properties emergent because they did not
exist at the component level. The same applies to artifacts (structures, devices, tools, and even works of art). They are created for a specific purpose and are therefore subjectively emergent: someone who doesn't understand the purpose can't use it.
The artifact is the result of an invention: through a clever combination of components, something new is created with emergent properties and functionalities. This invention is often difficult to predict and therefore usually based on a chance discovery. An invention based on discovery is often improved through a feedback loop, making it more applicable. This is an example of downward causation.
Example 1: A hammer is a combination of a head and a
handle, each with different properties. By cleverly connecting them, the
hammer becomes an artifact with new, emergent functionalities. Through
downward causation, you can improve the head and handle components in
such a way that the hammer's functionality increases. Example 2: A mixture of tin and copper produces the alloy
bronze, with new, emergent properties (hardness, lower melting
temperature). Finding the correct ratio of tin to copper is an example
of downward causation. Example 3: Finding the right combination of chemicals to create a superconductor
at high temperatures (i.e room temperature) is a great challenge for
many scientists, where chance plays a significant role. Conversely,
however, the properties of all these invented artifacts can be readily
explained reductionistically.
Something similar occurs in nature: random mutations in genes
rarely create a creature with new, emergent properties, increasing its
chances of survival in a changing ecosystem. This is how evolution
works. Here too, through downward causation, new creatures can sometimes
manipulate their ecosystem in such a way that their chances of survival
are further increased.
In both artifacts and living beings, certain components can be crucial to the emergent end result: the end result supervenes
on these components. Examples include: a construction error, a bug in a
software program, an error in the genetic code, or the absence of a
particular gene.
Both aspects: supervenience and the unpredictability of the emergent result are characteristic of strong emergence (see above). (This definition, however, differs significantly from the definition in philosophical literature).
Planetary Health is a multi- and transdisciplinary research paradigm, a science for exceptional action, and a global movement. Planetary health refers to "the health of human civilization and the
state of the natural systems on which it depends." In 2015, the
Rockefeller Foundation–Lancet Commission on Planetary Health launched
the concept which is currently being developed towards a new health science with over 25 areas of expertise.
Background and milestones
There are a number of ideas, concepts that can be understood as
precursors to the concept of planetary health. According to Susan
Prescott, the term "planetary health" emerged from the environmental and
holistic health movements of the 1970-80s. In 1980, Friends of the Earth
expanded the World Health Organization's definition of health, stating,
"health is a state of complete physical, mental, social and ecological
well-being and not merely the absence of disease - personal health
involves planetary health." James Lovelock created the term "Planetary Medicine" in 1986. In 1993 the Norwegian physician Per Fugelli
wrote: "The patient Earth is sick. Global environmental disruptions can
have serious consequences for human health. It's time for doctors to
give a world diagnosis and advise on treatment." In the 1990s, a model curriculum Terra Medicine (Planetary Medicine) was developed at the Catholic University of Eichstätt-Ingolstadt as part of the Altmühltal Agenda 21 project. In 2000, James Lovelock published his book Gaia: The Practical Science of Planetary Medicine.
Milestones
Fourteen years later, a commentary in the March 2014 issue of the medical journal The Lancet called to create a movement for planetary health to transform the field of public health,
which has traditionally focused on the health of human populations
without necessarily considering the surrounding natural ecosystems. The proposal recognized the emerging threats to natural and human-made systems that support humanity.
In 2015, the Rockefeller Foundation and The Lancet launched the concept with the Rockefeller Foundation–Lancet Commission on Planetary Health. The Planetary Health Alliance was founded in December 2015, by Harvard University, together with the Wildlife Conservation Society and other partner organizations. The Rockefeller Foundation Economic Council on Planetary Health
at the Oxford Martin School was established on 1 June 2017 to further
define the new discipline of planetary health. The open-access journal
"Lancet Planetary Health" published its inaugural issue in April 2017.
The Planetary Health Education Framework, developed in 2021
by the Planetary Health Alliance, aims to guide the education of global
citizens, practitioners, and professionals able and willing to address
complex Planetary Health challenges. The framework also seeks to inspire
all peoples across the globe to create, restore, steward, and conserve
healthy ecosystems for a thriving human civilization. The framework
considers five foundational domains that form the essence of
Planetary Health knowledge, values, and practice: (1) interconnection
with nature, (2) the Anthropocene and health, (3) equity and social
justice, (4) movement building and systems change, and (5) systems
thinking and complexity.
The São Paulo Declaration on Planetary Health is a
multi-stakeholder call to action co-created by the global Planetary
Health community at the 2021 Planetary Health Annual Meeting in São
Paulo, Brazil. The declaration calls on governments, the private sector,
civil society, and the general public to commit to the Great Transition
to safeguard a healthy and equitable future for humanity and protect
all life on Earth.
In 2022, on the occasion of the 50th anniversary of the first UN environmental conference "United Nations Conference on the Human Environment"
in Stockholm 1972, the UN published the report: 'UN Conference
Stockholm+50: A Healthy Planet for the Prosperity of All - Our
Responsibility, Our Opportunity'.
In 2023 the Association of Faculties of Medicine of Canada
published the "Academic Health Institutions' Declaration on Planetary
Health," which calls on all academic health institutions throughout the
world to take immediate action to halt both the negative impact of their
activities on the planet's natural systems, and to institute adaptive
and regenerative measures, including through advocacy. More than 40
academic health institutions have signed the declaration. These include medical schools, faculties of medicine, schools of
nursing, schools of public health, and other health-related academic
institutions from various countries including Canada, India, Finland,
Dominican Republic, South Africa, Germany, Portugal, Indonesia, and
others.
In April 2024, the Global Planetary Health Roadmap and Action
Plan, a map to guide a path forward for Planetary Health was created by
over 100 members of a worldwide community, building on the principles
and call to action of the 2021 São Paulo Declaration on Planetary
Health. The roadmap
encompasses key domains, such as governance, education, business, and
communications, providing a strategic framework to nurture this growing
movement and safeguard the health and well-being of all life on Earth.
Research paradigms and agenda
Drawing from the definition of health – "a state of complete physical, mental and social wellbeing and not merely the absence of disease or infirmity" – as well as principles articulated in the preamble of the constitution of the World Health Organization, The Lancet
Commission report elaborated that planetary health refers to the
"achievement of the highest attainable standard of health, wellbeing,
and equity worldwide through judicious attention to the human systems –
political, economic, and social – that shape the future of humanity and
the Earth's natural systems that define the safe environmental limits
within which humanity can flourish."
The report laid down the overarching principles guiding the idea
of planetary health. One is that human health depends on "flourishing
natural systems and the wise stewardship of those natural systems".
Human activities, such as energy generation and food production, have
led to substantial global effects on the Earth's systems, prompting
scientists to refer to the modern times as the Anthropocene.
The Rockefeller Foundation–Lancet Commission on Planetary Health
report concluded that urgent and transformative actions are needed to
protect present and future generations. One important area which
required immediate attention was the system of governance and
organization of human knowledge, which was deemed inadequate to address the threats to planetary health.
The report made several overarching recommendations. One was to
improve governance to aid the integration of social, economic, and
environmental policies and for the creation, synthesis, and application
of interdisciplinary knowledge. The authors called for solutions based on the redefinition of prosperity to focus on the enhancement of quality of life and delivery of improved health for all, together with respect for the integrity of natural systems.
International research agenda for planetary health
In June 2023, the Royal Netherlands Academy of Sciences presented their planetary health report Planetary Health, An emerging field to be developed
based on a two-year consultative process. Many knowledge gaps were
identified in the field of planetary health. A review of the literature
and subsequent consultation with experts resulted in a longlist of more
than one hundred specific knowledge gaps. Knowledge for the health impacts of global environmental change on
human health are incomplete, pathways are poorly understood, the
effectiveness of mitigation and adaptation measures are still unclear,
how timely policy and behaviour change can be realised. The Academy
concluded that: "Filling all Planetary Health knowledge gaps requires an
international collaborative effort in research funding". The Academy
will cooperate with international partner and 'umbrella academies' (such
as EASAC, FEAM and ALLEA) how to take this agenda forward."
In 2025 the United Nations Environment Programme (UNEP) report GEO-7
found that investing in planetary health can deliver trillions in
additional global GDP, avoid millions of deaths and reduce poverty and
hunger.
Issues
Nutrition and diet are important contributors to and indicators of
planetary health. Diets, agriculture, and technology must adjust to
sustain population projections upwards of 9 billion while reducing
harmful consequences on the environment through food waste and carbon-intensive
diets. A focus of planetary health research is nutritional solutions
that are sustainable for the human species and the environment, and the
generation of scientific research and political will to create and
implement desired solutions. In January 2019, an international commission created the planetary health diet.
Planetary health aims to seek out further solutions to global
human and environmental sustainability through collaboration and
research across all sectors, including the economy, energy, agriculture,
water, and health. Biodiversity loss,
exposure to pollutants, climate change, and fuel consumption are all
issues that threaten human and health, and are, as such, foci of the
field. A number of researchers think that it is actually humanity's
destruction of biodiversity and the invasion of wild landscapes that
creates the conditions for malaria, and new diseases such as COVID-19. Some propose incorporating concern for the impact of digital technology
in planetary health and health promotion, including the impact of generative AI on climate, biodiversity, and pollution.
Planetary Health Alliance
The Planetary Health Alliance is an informal global consortium of
over 470 universities, non-governmental organizations, government
entities, and research institutes with over 20,000 newsletter
subscribers.
Several PHA regional hubs function as locally rooted communities
that bring PHA members together in geographic clusters to
collaboratively advance planetary health research, education, policy,
and outreach relevant to specific local contexts.
The alliance's mission is "to promote, mobilize, and lead an
inclusive, transdisciplinary field of Planetary Health and its diverse
science, stories, solutions, and communities to achieve a comprehensive
shift in how human beings interact with each other and Nature, in order
to secure a livable future for humanity and the rest of life on Earth."
Since November 2023, the secretariat of PHA is based at Johns Hopkins University alongside the Johns Hopkins Institute for Planetary Health.
Regional Hubs
There are eight established Planetary Health Regional Hubs that
function as locally rooted communities which bring PHA members together
in geographic clusters to collaboratively advance planetary health
research, education, policy, and outreach relevant to specific local
contexts
While additional hubs are under development, the eight established Planetary Health Regional Hubs are:
Caribbean
East Africa
Europe
Japan
Latin America
Oceania
South & Southeast Asia
West Africa
In 2022, the inaugural Planetary Health Europe Regional Hub meeting was held in Amsterdam, with 72 institutions represented. The inaugural meeting was organized by the Planetary Health Alliance,
the European Environment and Sustainable Development Advisory Councils
Network (EEAC Network), and Natura Artis Magistra (ARTIS). The PHA
Europe Secretariat has been located in the Netherlands. It is jointly
coordinated by Maastricht University and the University Medical Center Utrecht (UMC Utrecht).
Next Generation Network
The Planetary Health Next Generation Network is composed of students
and next-generation leaders worldwide who are dedicated to advancing the
emerging field of Planetary Health through local community efforts,
educational events, and research projects. This open-access network
brings together the Planetary Health Campus Ambassadors (PHCAs),
Planetary Health student club leaders and members, former and current
Travel Scholars to Planetary Health Annual Meetings, and any youths who
would like to engage with the Planetary Health community. The Planetary
Health Alliance staff team and Impact Fellows work to support these
diverse efforts by providing introductory resources, workshop materials,
mentorship opportunities, and community-building platforms.
Campus Ambassador program
The Planetary Health Campus Ambassador program formally recognizes
next-generation leaders in planetary health on academic campuses and
within the international planetary health community at-large. During the
program, ambassadors build their planetary health network and gain
leadership and organizational skills with the support of their program
cohort, staff, fellows, and alliance members. Ambassadors are empowered
to take leadership on their campus and beyond, to educate their
community, and to facilitate collaborations between existing disciplines
and initiatives within the scope of human health and environmental
change. They also become part of the program's broader Next Generation
Network, composed of individuals from a variety of academic and cultural
backgrounds, career stages, and interests. They also have access to
leadership opportunities within other initiatives, such as the global
Planetary Health Annual Meeting, Planetary Health Regional Hubs,
Clinicians for Planetary Health, and various education projects.
Annual meeting
The Planetary Health Annual Meeting, convened by the Planetary Health
Alliance, is an international conference series established in 2017,
serving as a global forum for advancing the field of Planetary Health.
First launched at Harvard University, these meetings have evolved into
comprehensive gatherings connecting diverse stakeholders including
scientists, policymakers, healthcare professionals, educators, students,
and community leaders from over 130 countries. The meetings rotate
globally, having been hosted in the United States (Harvard University 2017, 2022; Stanford University 2019), Scotland (University of Edinburgh 2018), Brazil (University of São Paulo 2021, virtual), and Malaysia (Sunway University
2024), reflecting a commitment to geographic and cultural diversity in
addressing planetary health challenges. A meeting is planned for October
2025 in the Netherlands (Erasmus University).
The meetings consistently focus on planetary health themes, including climate change, biodiversity loss, food systems transformation, health equity, and education.
Each meeting has produced significant outcomes that have shaped the
field: from establishing foundational frameworks in the early meetings
to the São Paulo Declaration on Planetary Health (2021) and the Kuala
Lumpur Call to Action (2024), accompanied by the launch of the global
Planetary Health Roadmap and Action Plan. Through plenary sessions,
research presentations, workshops, and community engagement activities,
these meetings have been instrumental in building capacity, fostering
collaboration, and driving actionable solutions for planetary health
challenges.
While there may be competing definitions of global health, it is loosely defined as the health of populations in a global context,
a response to the cross-border movement of health drivers as well as
risks, and an improvement over the older concept of international health
with its new emphasis on achieving equity in health among all people. Some scholars hold that advocacy of planetary health amounts to an over-expansion and totalization of health.
The editor in chief of The Lancet, Richard Horton, wrote in a 2014 special issue of The Economist
on planetary health, that global health was no longer able to truly
meet the demands which societies face, as it was still too narrow to
explain and illuminate some pressing challenges."Global health does not
fully take into account the natural foundation on which humans live –
the planet itself. Nor does it factor in the force and fragility of
human civilizations."
In 2015, Judith Rodin, president of the Rockefeller Foundation, declared planetary health as a new discipline in global health.
In September 2024, the Consortium of Universities for Global Health
(CUGH) put forth a set of planetary health learning objectives, noting
"the knowledge of planetary health science, interventions, and
communication that is essential for future global health professionals." CUGH included planetary health in the updated edition of their Global Health Competencies Toolkit.
In 2026, Daniel Oerther proposed that the profession of engineering
modify the paramountcy clause to, "hold paramount the health, safety,
and welfare of the public and the planet,” in recognition of the
interconnectedness of all life.
The multiverse is the hypothetical set of all universes. Together, these universes are presumed to comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants
that describe them. The different universes within the multiverse are
called "parallel universes", "flat universes", "other universes",
"alternate universes", "multiple universes", "plane universes", "parent
and child universes", "many universes", or "many worlds". One common
assumption is that the multiverse is a "patchwork quilt of separate
universes all bound by the same laws of physics."
The concept of multiple universes, or a multiverse, has been
discussed throughout history. It has evolved and has been debated in
various fields, including cosmology, physics, and philosophy. Some
physicists have argued that the multiverse is a philosophical notion
rather than a scientific hypothesis, as it cannot be empirically falsified.
In recent years, there have been proponents and skeptics of multiverse
theories within the physics community. Although some scientists have
analyzed data in search of evidence for other universes, no statistically significant
evidence has been found. Critics argue that the multiverse concept
lacks testability and falsifiability, which are essential for scientific
inquiry, and that it raises unresolved metaphysical issues.
Max Tegmark and Brian Greene
have proposed different classification schemes for multiverses and
universes. Tegmark's four-level classification consists of Level I: an
extension of our universe, Level II: universes with different physical
constants, Level III: many-worlds interpretation of quantum mechanics,
and Level IV: ultimate ensemble.
Brian Greene's nine types of multiverses include quilted, inflationary,
brane, cyclic, landscape, quantum, holographic, simulated, and
ultimate. The ideas explore various dimensions of space, physical laws,
and mathematical structures to explain the existence and interactions of
multiple universes. Some other multiverse concepts include twin-world
models, cyclic theories, M-theory, and black-hole cosmology.
The anthropic principle
suggests that the existence of a multitude of universes, each with
different physical laws, could explain the asserted appearance of fine-tuning of our own universe
for conscious life. The weak anthropic principle posits that we exist
in one of the few universes that support life. Debates around Occam's razor
and the simplicity of the multiverse versus a single universe arise,
with proponents like Max Tegmark arguing that the multiverse is simpler
and more elegant. The many-worlds interpretation of quantum mechanics and modal realism,
the belief that all possible worlds exist and are as real as our world,
are also subjects of debate in the context of the anthropic principle.
History of the concept
According to some, the idea of infinite worlds was first suggested by the pre-Socratic Greek philosopher Anaximander in the sixth century BCE. However, there is debate as to whether he believed in multiple worlds,
and if he did, whether those worlds were co-existent or successive.
The first figures to whom historians can definitively attribute the concept of innumerable worlds are the Ancient Greek Atomists, beginning with Leucippus and Democritus in the 5th century BCE, followed by Epicurus (341–270 BCE) and the Roman Epicurean Lucretius (1st century BCE).In the third century BCE, the philosopher Chrysippus
suggested that the world eternally expired and regenerated, effectively
suggesting the existence of multiple universes across time. The concept of multiple universes became more defined in the Middle Ages. In the Renaissance, Giordano Bruno (1548–1600) expressed the concept of infinite worlds.
The American philosopher and psychologist William James used the term "multiverse" in 1895, but in a different context.
The concept first appeared in the modern scientific context in the course of the debate between Boltzmann and Zermelo in 1895.
In Dublin in 1952, Erwin Schrödinger
gave a lecture in which he jocularly warned his audience that what he
was about to say might "seem lunatic". He said that when his equations
seemed to describe several different histories, these were "not
alternatives, but all really happen simultaneously". This sort of duality is called "superposition".
Search for evidence
In the 1990s, after recent works of fiction about the concept gained
popularity, scientific discussions about the multiverse and journal articles about it gained prominence.
Around 2010, scientists such as Stephen M. Feeney analyzed Wilkinson Microwave Anisotropy Probe
(WMAP) data and claimed to find evidence suggesting that this universe
collided with other (parallel) universes in the distant past. However, a more thorough analysis of data from the WMAP and from the Planck satellite, which has a resolution three times higher than WMAP, did not reveal any statistically significant evidence of such a bubble universe collision. In addition, there was no evidence of any gravitational pull of other universes on ours.
In 2015, an astrophysicist may have found evidence of alternate or
parallel universes by looking back in time to a time immediately after
the Big Bang, although it is still a matter of debate among physicists. Dr. Ranga-Ram Chary, after analyzing the cosmic radiation spectrum, found a signal 4,500 times brighter than it should have been, based on the number of protons and electrons
scientists believe existed in the very early universe. This signal—an
emission line that arose from the formation of atoms during the era of
recombination—is more consistent with a universe whose ratio of matter
particles to photons is about 65 times greater than our own. There is a
30% chance that this signal is noise, and not really a signal at all;
however, it is also possible that it exists because a parallel universe
dumped some of its matter particles into our universe. If additional
protons and electrons had been added to our universe during
recombination, more atoms would have formed, more photons would have
been emitted during their formation, and the signature line that arose
from all of these emissions would be greatly enhanced. Chary said:
Many
other regions beyond our observable universe would exist with each such
region governed by a different set of physical parameters than the ones
we have measured for our universe.
— Ranga-Ram Chary, USA Today
Chary also noted:
Unusual claims like evidence for alternate universes require a very high burden of proof.
— Ranga-Ram Chary, "Universe Today"
The signature that Chary has isolated may be a consequence of incoming light from distant galaxies, or even from clouds of dust surrounding our own galaxy.
In his 2003 New York Times opinion piece, "A Brief History of the Multiverse", author and cosmologist Paul Davies offered a variety of arguments that multiverse hypotheses are non-scientific:
For
a start, how is the existence of the other universes to be tested? To
be sure, all cosmologists accept that there are some regions of the
universe that lie beyond the reach of our telescopes, but somewhere on
the slippery slope between that and the idea that there is an infinite
number of universes, credibility reaches a limit. As one slips down that
slope, more and more must be accepted on faith, and less and less is
open to scientific verification. Extreme multiverse explanations are
therefore reminiscent of theological discussions. Indeed, invoking an
infinity of unseen universes to explain the unusual features of the one
we do see is just as ad hoc as invoking an unseen Creator. The
multiverse theory may be dressed up in scientific language, but in
essence, it requires the same leap of faith.
George Ellis,
writing in August 2011, provided a criticism of the multiverse, and
pointed out that it is not a traditional scientific theory. He accepts
that the multiverse is thought to exist far beyond the cosmological horizon.
He emphasized that it is theorized to be so far away that it is
unlikely any evidence will ever be found. Ellis also explained that some
theorists do not believe the lack of empiricaltestability and falsifiability is a major concern, but he is opposed to that line of thinking:
Many physicists who talk about the multiverse, especially advocates of the string landscape, do not care much about parallel universes per se.
For them, objections to the multiverse as a concept are unimportant.
Their theories live or die based on internal consistency and, one hopes,
eventual laboratory testing.
Ellis says that scientists have proposed the idea of the multiverse as a way of explaining the nature of existence. He points out that it ultimately leaves those questions unresolved because it is a metaphysical
issue that cannot be resolved by empirical science. He argues that
observational testing is at the core of science and should not be
abandoned:
As
skeptical as I am, I think the contemplation of the multiverse is an
excellent opportunity to reflect on the nature of science and on the
ultimate nature of existence: why we are here. ...
In looking at this concept, we need an open mind, though not too open.
It is a delicate path to tread. Parallel universes may or may not exist;
the case is unproved. We are going to have to live with that
uncertainty. Nothing is wrong with scientifically based philosophical
speculation, which is what multiverse proposals are. But we should name
it for what it is.
Philosopher Philip Goff argues that the inference of a multiverse to explain the apparent fine-tuning of the universe is an example of Inverse Gambler's Fallacy.
Stoeger, Ellis, and Kircher
note that in a true multiverse theory, "the universes are then
completely disjoint and nothing that happens in any one of them is
causally linked to what happens in any other one. This lack of any
causal connection in such multiverses really places them beyond any
scientific support".
In May 2020, astrophysicist Ethan Siegel expressed criticism in a Forbes
blog post that parallel universes would have to remain a science
fiction dream for the time being, based on the scientific evidence
available to us.
Scientific American contributor John Horgan also argues against the idea of a multiverse, claiming that they are "bad for science."
Types
Max Tegmark and Brian Greene have devised classification schemes for the various theoretical types of multiverses and universes that they might comprise.
Max Tegmark's four levels
CosmologistMax Tegmark has provided a taxonomy of universes beyond the familiar observable universe.
The four levels of Tegmark's classification are arranged such that
subsequent levels can be understood to encompass and expand upon
previous levels. They are briefly described below.
Level I: An extension of our universe
A prediction of cosmic inflation is the existence of an infinite ergodic universe, which, being infinite, must contain Hubble volumes realizing all initial conditions.
Accordingly, an infinite universe will contain an infinite number of Hubble volumes, all having the same physical laws and physical constants. In regard to configurations such as the distribution of matter, almost all will differ from our Hubble volume. However, because there are infinitely many, far beyond the cosmological horizon,
there will eventually be Hubble volumes with similar, and even
identical, configurations. Tegmark estimates that an identical volume to
ours should be about 1010115 meters away from us.
Given infinite space, there would be an infinite number of Hubble volumes identical to ours in the universe. This follows directly from the cosmological principle, wherein it is assumed that our Hubble volume is not special or unique.
Level II: Universes with different physical constants
In the eternal inflation theory, which is a variant of the cosmic inflation theory, the multiverse or space as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles
(like gas pockets in a loaf of rising bread). Such bubbles are embryonic
level I multiverses.
In brief, one aspect of quantum mechanics is that certain
observations cannot be predicted absolutely. Instead, there is a range
of possible observations, each with a different probability. According to the MWI, each of these possible observations corresponds to a different "world" within the Universal wavefunction, with each world as real as ours. Suppose a six-sided die is thrown and that the result of the throw corresponds to observable
quantum mechanics. All six possible ways the die can fall correspond to
six different worlds. In the case of the Schrödinger's cat thought
experiment, both outcomes would be "real" in at least one "world".
Tegmark argues that a Level III multiverse does not contain more
possibilities in the Hubble volume than a Level I or Level II
multiverse. In effect, all the different worlds created by "splits" in a
Level III multiverse with the same physical constants can be found in
some Hubble volume in a Level I multiverse. Tegmark writes that, "The
only difference between Level I and Level III is where your doppelgängers
reside. In Level I they live elsewhere in good old three-dimensional
space. In Level III they live on another quantum branch in
infinite-dimensional Hilbert space."
Similarly, all Level II bubble universes with different physical
constants can, in effect, be found as "worlds" created by "splits" at
the moment of spontaneous symmetry breaking in a Level III multiverse. According to Yasunori Nomura, Raphael Bousso, and Leonard Susskind, this is because global spacetime appearing in the (eternally) inflating
multiverse is a redundant concept. This implies that the multiverses of
Levels I, II, and III are, in fact, the same thing. This hypothesis is
referred to as "Multiverse = Quantum Many Worlds". According to Yasunori Nomura, this quantum multiverse is static, and time is a simple illusion.
This level considers all universes to be equally real which can be described by different mathematical structures.
Tegmark writes:
Abstract mathematics is so general that any Theory Of Everything (TOE)
which is definable in purely formal terms (independent of vague human
terminology) is also a mathematical structure. For instance, a TOE
involving a set of different types of entities (denoted by words, say)
and relations between them (denoted by additional words) is nothing but
what mathematicians call a set-theoretical model, and one can generally find a formal system that it is a model of.
He argues that this "implies that any conceivable parallel universe
theory can be described at Level IV" and "subsumes all other ensembles,
therefore brings closure to the hierarchy of multiverses, and there
cannot be, say, a Level V."
Schmidhuber explicitly includes universe representations
describable by non-halting programs whose output bits converge after a
finite time, although the convergence time itself may not be predictable
by a halting program, due to the undecidability of the halting problem. He also explicitly discusses the more restricted ensemble of quickly computable universes.
The quilted multiverse works only in an infinite
universe. With an infinite amount of space, every possible event will
occur an infinite number of times. However, the speed of light prevents
us from being aware of these other identical areas.
Inflationary
The inflationary multiverse is composed of various pockets in which inflation fields collapse and form new universes.
Brane
The brane multiverse version postulates that our entire universe exists on a membrane (brane)
which floats in a higher dimension or "bulk". In this bulk, there are
other membranes with their own universes. These universes can interact
with one another, and when they collide, the violence and energy
produced is more than enough to give rise to a Big Bang.
The branes float or drift near each other in the bulk, and every few
trillion years, attracted by gravity or some other force we do not
understand, collide and bang into each other. This repeated contact
gives rise to multiple or "cyclic" Big Bangs. This particular hypothesis falls under the string theory umbrella as it requires extra spatial dimensions.
Cosmos animation of a cyclic universe
Cyclic
The cyclic multiverse has multiple branes that have collided, causing Big Bangs.
The universes bounce back and pass through time until they are pulled
back together and again collide, destroying the old contents and
creating them anew.
Landscape
The landscape multiverse relies on string theory's Calabi–Yau
spaces. Quantum fluctuations drop the shapes to a lower energy level,
creating a pocket with a set of laws different from that of the
surrounding space.
The holographic multiverse is derived from the theory that the surface area of a space can encode the contents of the volume of the region.
Simulated
The simulated multiverse
exists on complex computer systems that simulate entire universes. A
related hypothesis, as put forward as a possibility by astronomer Avi Loeb, is that universes may be creatable in laboratories of advanced technological civilizations who have a theory of everything. Other related hypotheses include brain in a vat-type
scenarios where the perceived universe is either simulated in a
low-resource way or not perceived directly by the virtual/simulated
inhabitant species.
Ultimate
The ultimate multiverse contains every mathematically possible universe under different laws of physics.
Twin-world models
Concept of a twin universe, with the beginning of time in the middle
There are models of two related universes that e.g. attempt to explain the baryon asymmetry – why there was more matter than antimatter at the beginning – with a mirror anti-universe. One two-universe cosmological model could explain the Hubble constant (H0) tension via interactions between the two worlds. The "mirror world" would contain copies of all existing fundamental particles. Another twin/pair-world or "bi-world" cosmology is shown to theoretically be able to solve the cosmological constant (Λ) problem, closely related to dark energy: two interacting worlds with a large Λ each could result in a small shared effective Λ.
In several theories, there is a series of, in some cases infinite, self-sustaining cycles – typically a series of Big Crunches (or Big Bounces).
However, the respective universes do not exist at once but are forming
or following in a logical order or sequence, with key natural
constituents potentially varying between universes (see § Anthropic principle).
A multiverse of a somewhat different kind has been envisaged within string theory and its higher-dimensional extension, M-theory.
These theories require the presence of 10 or 11 spacetime
dimensions, respectively. The extra six or seven dimensions may either
be compactified on a very small scale, or our universe may simply be
localized on a dynamical (3+1)-dimensional object, a D3-brane. This opens up the possibility that there are other branes which could support other universes.
Black-hole cosmology is a cosmological model in which the observable universe is the interior of a black hole existing as one of possibly many universes inside a larger universe. This includes the theory of white holes, which are on the opposite side of space-time.
The concept of other universes has been proposed to explain how our own universe appears to be fine-tuned for conscious life as we experience it.
If there were a large (possibly infinite) number of universes, each with possibly different physical laws (or different fundamental physical constants),
then some of these universes (even if very few) would have the
combination of laws and fundamental parameters that are suitable for the
development of matter, astronomical structures, elemental diversity, stars, and planets that can exist long enough for life to emerge and evolve.
The weak anthropic principle could then be applied to conclude that we (as conscious beings) would only exist in one of those few universes that happened to be
finely tuned, permitting the existence of life with developed
consciousness. Thus, while the probability might be extremely small that
any particular universe would have the requisite conditions for life (as we understand life), those conditions do not require intelligent design as an explanation for the conditions in the Universe that promote our existence in it.
An early form of this reasoning is evident in Arthur Schopenhauer's
1844 work "Von der Nichtigkeit und dem Leiden des Lebens", where he
argues that our world must be the worst of all possible worlds, because
if it were significantly worse in any respect it could not continue to
exist.
Occam's razor
Proponents and critics disagree about how to apply Occam's razor.
Critics argue that to postulate an almost infinite number of
unobservable universes, just to explain our own universe, is contrary to
Occam's razor. However, proponents argue that in terms of Kolmogorov complexity the proposed multiverse is simpler than a single idiosyncratic universe.
For example, multiverse proponent Max Tegmark argues:
[A]n entire ensemble is often much simpler than one of its members. This principle can be stated more formally using the notion of algorithmic information
content. The algorithmic information content in a number is, roughly
speaking, the length of the shortest computer program that will produce
that number as output. For example, consider the set of all integers.
Which is simpler, the whole set or just one number? Naively, you might
think that a single number is simpler, but the entire set can be
generated by quite a trivial computer program, whereas a single number
can be hugely long. Therefore, the whole set is actually simpler...
(Similarly), the higher-level multiverses are simpler. Going from our
universe to the Level I multiverse eliminates the need to specify initial conditions, upgrading to Level II eliminates the need to specify physical constants,
and the Level IV multiverse eliminates the need to specify anything at
all... A common feature of all four multiverse levels is that the
simplest and arguably most elegant theory involves parallel universes by
default. To deny the existence of those universes, one needs to
complicate the theory by adding experimentally unsupported processes and
ad hoc postulates: finite space, wave function collapse
and ontological asymmetry. Our judgment therefore comes down to which
we find more wasteful and inelegant: many worlds or many words. Perhaps
we will gradually get used to the weird ways of our cosmos and find its
strangeness to be part of its charm.
— Max Tegmark
Possible worlds and real worlds
In any given set of possible universes – e.g. in terms of histories
or variables of nature – not all may be ever realized, and some may be
realized many times. For example, over infinite time there could, in some potential
theories, be infinite universes, but only a small or relatively small
real number of universes where humanity could exist and only one where
it ever does exist (with a unique history). It has been suggested that a universe that "contains life, in the form it has on Earth, is in a certain sense radically non-ergodic, in that the vast majority of possible organisms will never be realized". On the other hand, some scientists, theories and popular works conceive
of a multiverse in which the universes are so similar that humanity
exists in many equally real separate universes but with varying
histories.
Possible worlds are a way of explaining probability and hypothetical statements. Some philosophers, such as David Lewis, posit that all possible worlds exist and that they are just as real as the world we live in. This position is known as modal realism.
