The Accidental Universe
JohnEB
JohnEB
This long and appealing trend may be coming to an end. Dramatic developments in cosmological findings and thought have led some of the world’s premier physicists to propose that our universe is only one of an enormous number of universes with wildly varying properties, and that some of the most basic features of our particular universe are indeed mere accidents—a random throw of the cosmic dice. In which case, there is no hope of ever explaining our universe’s features in terms of fundamental causes and principles.
JohnEB
world’s leading experts on Einstein’s general theory of relativity and co-author, with Stephen Hawking, of the seminal book The Large
Scale Structure of Space-Time (Cambridge University Press, 1975).
The notion of parallel universes leapt out of the pages of fiction into scientific journals in the 1990s.
Many scientists claim that megamillions of other universes, each with its own laws of physics, lie out
there, beyond our visual horizon. They are collectively known as the multiverse. The trouble is that
no possible astronomical observations can ever see those other universes. The arguments are indirect at best.
And even if the multiverse exists, it leaves the deep mysteries of nature unexplained.
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What Lies Beyond?
When astronomers peer into the universe, they see out to a distance of about 42 billion lightyears‚
our cosmic horizon, which represents how far light has been able to travel since the big bang (as well as how
much the universe has expanded in size since then). Assuming that space does not just stop there and may well
be infinitely big, cosmologists make educated guesses as to what the
rest of it looks like.
Level 1 Multiverse: Plausible
The most straightforward assumption is that our volume of space is a representative
sample of the whole. Distant alien beings see different volumes‚ but all of these look
basically alike, apart from random variations in the distribution of matter. Together these
regions, seen and unseen, form a basic type of multiverse.
Level 2 Multiverse: Questionable
Many cosmologists go further and speculate that, sufficiently far away, things look quite
different from what we see. Our environs may be one of many bubbles floating in an otherwise
empty background. The laws of physics would differ from bubble to bubble, leading to an almost
inconceivable variety of outcomes. Those other bubbles may be impossible to observe even in
principle. The author and other skeptics feel dubious about this type of multiverse.
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In the past decade an extraordinary claim has captivated cosmologists: that the expanding
universe we see around us is not the only one; that billions of other universes are out there,
too. There is not one universe—there is a multiverse. In Scientific American articles and
books such as Brian Greene’s latest, The Hidden Reality, leading scientists have spoken of a
super-Copernican revolution. In this view, not only is our planet one among many, but even
our entire universe is insignificant on the cosmic scale of things. It is just one of countless
universes, each doing its own thing.
of about 42 billion light-years, our cosmic visual horizon. We have no reason to suspect the universe
stops there. Beyond it could be many—even infinitely many—domains much like the one
we see. Each has a different initial distribution of matter, but the same laws of physics operate
in all. Nearly all cosmologists today (including me) accept this type of multiverse, which Max
Tegmark calls “level 1.” Yet some go further. They suggest completely different kinds of universes,
with different physics, different histories, maybe different numbers of spatial dimensions.
Most will be sterile, although some will be teeming with life. A chief proponent of this “level 2”
multiverse is Alexander Vilenkin, who paints a dramatic picture of an infinite set of universes with
an infinite number of galaxies, an infinite number of planets and an infinite number of people
with your name who are reading this article. Similar claims have been made since antiquity by many
cultures. What is new is the assertion that the multiverse is a scientific theory, with all that
implies about being mathematically rigorous and experimentally testable. I am skeptical about this
claim. I do not believe the existence of those other universes has been proved—or ever could be.
Proponents of the multiverse, as well as greatly enlarging our conception of physical reality, are
implicitly redefining what is meant by “science.”
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The proponents are telling us we can state in broad terms what happens 1,000 times as far as our cosmic
horizon, 10100 times, 101,000,000 times, an infinity—all from data we obtain within the horizon. It
is an extrapolation of an extraordinary kind. Maybe the universe closes up on a very large scale, and
there is no infinity out there. Maybe all the matter in the universe ends somewhere, and there is empty
space forever after. Maybe space and time come to an end at a singularity that bounds the universe. We
just do not know what actually happens, for we have no information about these regions and never will.
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SEVEN QUESTIONABLE ARGUMENTS
Most multiverse proponents are careful scientists who are quite aware of this problem but think we can
still make educated guesses about what is going on out there. Their arguments fall into seven broad
types, each of which runs into trouble.
Space has no end.
Few dispute that space extends beyond our cosmic horizon and that many other domains lie beyond what
we see. If this limited type of multiverse exists, we can extrapolate what we see to domains beyond
the horizon, with more and more uncertainty as regards the farther-out regions. It is then easy to imagine
more elaborate types of variation, including alternative physics occurring out where we cannot see.
But the trouble with this type of extrapolation, from the known to the ununknown, is that no one can prove
you wrong. How can scientists decide whether their picture of an unobservable region of spacetime is a
reasonable or an unreasonable extrapolation of what we see? Might other universes have different initial
distributions of matter, or might they also have different values of fundamental physical constants, such as
those that set the strength of nuclear forces? You could get either, depending on what you assume.
Known physics predicts other domains.
Proposed unified theories predict entities such as scalar fields, a
hypothesized relative of other space-filling fields such as the magnetic field. Such fields should drive cosmic
inflation and create universes ad infinitum. These theories are well grounded theoretically, but the
nature of the hypothesized fields is unknown, and experimentalists have yet to demonstrate their existence,
let alone measure their supposed properties. Crucially, physicists have not substantiated that the dynamics
of these fields would cause different laws of physics to operate in different bubble universes.
The theory that predicts an infinity of universes passes a key observational test.
The cosmic microwave background radiation reveals what the universe looked like at the end of its hot
early expansion era. Patterns in it suggest that our universe really did undergo a period of inflation.
But not all types of inflation go on forever and create an infinite number of bubble universes. Observations
do not single out the required type of inflation from other types. Some cosmologists such as Steinhardt even
argue that eternal inflation would have led to different patterns in the background radiation than we see
[see “The Inflation Debate,” by Paul J. Steinhardt; Scientific American, April]. Linde and others
disagree. Who is right? It all depends on what you assume about the physics of the inflationary field.
Fundamental constants are finely tuned for life.
A remarkable fact about our universe is that physical constants have just the right values needed to allow
for complex structures, including living things. Steven Weinberg, Martin Rees, Leonard Susskind and others
contend that an exotic multiverse provides a tidy explanation for this apparent coincidence: if all possible
values occur in a large enough collection of universes, then viable ones for life will surely be found somewhere.
This reasoning has been applied, in particular, to explaining the density of the dark energy that is speeding up
the expansion of the universe today. I agree that the multiverse is a possible valid explanation for the
value of this density; arguably, it is the only scientifically based option we have right now. But we have no
hope of testing it observationally. Additionally, most analyses of the issue assume the basic equations of physics
are the same everywhere, with only the constants differing—but if one takes the multiverse seriously, this
need not be so [see “Looking for Life in the Multiverse,” by Alejandro Jenkins and Gilad Perez; Scientific American,
January 2010].
Fundamental constants match multiverse predictions.
This argument refines the previous one by suggesting that the universe is no more finely tuned for life than it
strictly needs to be. Proponents have assessed the probabilities of various values of the dark energy density.
The higher the value is, the more probable it is, but the more hostile the universe would be to life.
The value we observe should be just on the borderline of uninhabitability, and it does appear to be so.
Where the argument stumbles is that we cannot apply a probability argument if there is no multiverse to apply the
concept of probability to. This argument thus assumes the desired outcome before it starts; it simply is not
applicable if there is only one physically existing universe. Probability is a probe of the consistency
of the multiverse proposal, not a proof of its existence.
String theory predicts a diversity of universes.
String theory has moved from being a theory that explains everything to a theory where almost anything is possible.
In its current form, it predicts that many essential properties of our universe are pure happenstance. If the
universe is one of a kind, those properties seem inexplicable. How can we understand, for example, the fact
that physics has precisely those highly constrained properties that allow life to exist? If the universe is one
of many, those properties make perfect sense. Nothing singled them out; they are simply the ones that arose in
our region of space. Had we lived elsewhere, we would have observed different properties, if we could indeed
exist there (life would be impossible in most places). But string theory is not a tried-and-tested theory; it is not
even a complete theory. If we had proof that string theory is correct, its theoretical predictions could be a
legitimate, experimentally based argument for a multiverse. We do not have such proof.
In seeking to explain why nature obeys certain laws and not others, some physicists and philosophers have speculated
that nature never made any such choice: all conceivable laws apply somewhere. The idea is inspired in part by quantum
mechanics, which, as Murray Gell-Mann memorably put it, holds that everything not forbidden is compulsory. A particle
takes all the paths it can, and what we see is the weighted average of all those possibilities. Perhaps the
same is true of the entire universe, implying a multiverse. But astronomers have not the slightest chance of
observing this multiplicity of possibilities. Indeed, we cannot even know what the possibilities are. We can only
make sense of this proposal in the face of some unverifiable organizing principle or framework that decides what is
allowed and what is not—for example, that all possible mathematical structures must be realized in some physical
domain (as proposed by Tegmark). But we have no idea what kinds of existence this principle entails, apart from the
fact that it must, of necessity, include the world we see around us. And we have no way whatsoever to verify the
existence or nature of any such organizing principle. It is in some ways an attractive proposition, but its proposed
application to reality is pure speculation.
ABSENCE OF EVIDENCE
Although the theoretical arguments fall short, cosmologists have also suggested various empirical tests for parallel
universes. The cosmic microwave background radiation might bear some traces of other bubble universes if, for example,
our universe has ever collided with another bubble of the kind implied by the chaotic inflation scenario. The
background radiation might also contain remnants of universes that existed before the big bang in an endless cycle of
universes. These are indeed ways one might get real evidence of other universes. Some cosmologists have even
claimed to see such remnants. The observational claims are strongly disputed, however, and many of the hypothetically
possible multiverses would not lead to such evidence. So observers can test only some specific classes of multiverse
models in this way. A second observational test is to look for variations in one or more fundamental constants, which
would corroborate the premise that the laws of physics are not so immutable after all. Some astronomers claim to have
found such variations [see “Inconstant Constants,” by John D. Barrow and John K. Webb; Scientific American, June 2005].
Most, though, consider the evidence dubious. A third test is to measure the shape of the observable universe:
Is it spherical (positively curved), hyperbolic (negatively curved) or “flat” (uncurved)? Multiverse scenarios
generally predict that the universe is not spherical, because a sphere closes up on itself, allowing for only a finite
volume. Unfortunately, this test is not a clean one. The universe beyond our horizon could have a different shape from
that in the observed part; what is more, not all multiverse theories rule out a spherical geometry. A better test is
the topology of the universe: Does it wrap around like a doughnut or pretzel? If so, it would be finite in size,
which would definitely disprove most versions of inflation and, in particular, multiverse scenarios based on chaotic
inflation. Such a shape would produce recurring patterns in the sky, such as giant circles in the cosmic microwave
background radiation [see “Is Space Finite?” by Jean-Pierre Luminet, Glenn D. Starkman and Jeffrey R. Weeks; Scientific
American, April 1999]. Observers have looked for and failed to find any such patterns. But this null result cannot be
taken as a point in favor of the multiverse. Finally, physicists might hope to prove or disprove some of the theories
that predict a multiverse. They might find observational evidence against chaotic versions of inflation or discover a
mathematical or empirical inconsistency that forces them to abandon the landscape of string theory. That scenario would
undermine much of the motivation for supporting the multiverse idea, although it would not rule the concept out altogether.
TOO MUCH WIGGLE ROOM
All in all, the case for the multiverse is inconclusive. The basic reason is the extreme flexibility of the proposal:
it is more a concept than a well-defined theory. Most proposals involve a patchwork of different ideas rather than a
coherent whole. The basic mechanism for eternal inflation does not itself cause physics to be different in each domain
in a multiverse; for that, it needs to be coupled to another speculative theory. Although they can be fitted together,
there is nothing inevitable about it. The key step in justifying a multiverse is extrapolation from the known to the
unknown, from the testable to the untestable. You get different answers depending on what you choose to extrapolate.
Because theories involving a multiverse can explain almost anything whatsoever, any observation can be accommodated
by some multiverse variant. The various “proofs,” in effect, propose that we should accept a theoretical explanation
instead of insisting on observational testing. But such testing has, up until now, been the central requirement of the
scientific endeavor, and we abandon it at our peril. If we weaken the requirement of solid data, we weaken the core
reason for the success of science over the past centuries. Now, it is true that a satisfactory unifying explanation of
some range of phenomena carries greater weight than a hodgepodge of separate arguments for the same phenomena. If the
unifying explanation assumes the existence of unobservable entities such as parallel universes, we might well feel
compelled to accept those entities. But a key issue here is how many unverifiable entities are needed. Specifically,
are we hypothesizing more or fewer entities than the number of phenomena to be explained? In the case of the multiverse,
we are supposing the existence of a huge number—perhaps even an infinity—of unobservable entities to explain just one
existing universe. It hardly fits 14th-century English philosopher William of Ockham’s stricture that “entities must not
be multiplied beyond necessity.” Proponents of the multiverse make one final argument: that there are no good
alternatives. As distasteful as scientists might find the proliferation of parallel worlds, if it is the best
explanation, we would be driven to accept it; conversely, if we are to give up the multiverse, we need a viable
alternative. This exploration of alternatives depends on what kind of explanation we are prepared to accept. Physicists’
hope has always been that the laws of nature are inevitable—that things are the way they are because there is
no other way they might have been—but we have been unable to show this is true. Other options exist, too. The universe
might be pure happenstance—it just turned out that way. Or things might in some sense be meant to be the way they
are—purpose or intent somehow underlies existence. Science cannot determine which is the case, because these are
metaphysical issues. Scientists proposed the multiverse as a way of resolving deep issues about the nature of
existence, but the proposal leaves the ultimate issues unresolved. All the same issues that arise in relation
to the universe arise again in relation to the multiverse. If the multiverse exists, did it come into existence through
necessity, chance or purpose? That is a metaphysical question that no physical theory can answer for either the universe
or the multiverse. To make progress, we need to keep to the idea that empirical testing is the core of science. We need
some kind of causal contact with whatever entities we propose; otherwise, there are no limits. The link can be a bit
indirect. If an entity is unobservable but absolutely essential for properties of other entities that are indeed verified,
it can be taken as verified. But then the onus of proving it is absolutely essential to the web of explanation. The
challenge I pose to multiverse proponents is: Can you prove that unseeable parallel universes are vital to explain the
world we do see? And is the link essential and inescapable? 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. It leads to new and interesting insights and so is a productive research program. 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.
JohnEB
http://www.scientificamerican.com/article.cfm?id=bad-boy-of-physics
"We may never be able to grasp that reality. The universe and its ingredients may be impossible to describe unambiguously."
The fact of the matter is that our ONE UNIQUE 4-SPACE UNIVERSE is totally unambiguous and coherent when viewed with Dr. Mills' CLASSICAL PHYSICS.
Leonard Susskind, more than anyone else, is responsible for the current QM based dominant physics paradigm. Susskind has finally realized that his paradigm has failed, but he seems unable to grasp why it has failed. To me, the reason for this failure is crystal clear. Just as in the Standard Model, Susskind chose to follow Bohr's thinking while rejecting Einstein. Thus, Susskind threw away Einstein's ONE UNIQUE 4-SPACE UNIVERSE. And then Susskind wonders why he has nothing left but ambiguity. I do not think Susskind is quite as smart as he thinks he is - he is certainly no Einstein.