new book titled Time and Science tells us the latest
theories about what time really is. Volume1 has articles by philosophers.
Volume 2 is dedicated to biology. Volume 3 contains thirteen articles by famous
physicists including Carlo Rovelli, Julian Barbour, Lee Smolin, and Gilles
Cohen-Tannoudji.
After decades of trying to figure out what space and time are, they tell us their conclusion: in their considered opinion, space and time don't really exist. They're not real, they say, just an illusion, or at best an emergent phenomenon, which means they're produced by something more fundamental.
Maybe you scoff. After all, aren't these are the same guys who did away with Pluto, declared gravity not to be a force, and did away with subatomic particles, saying they were just excitations in a field that extends through space? And don't they practically worship the General Theory of Relativity, which says gravity is a wave in spacetime? How, then, can they conclude that none of it is real?
Unlike time, this book exists and occupies space
Maybe they're decluttering. Why not get rid of space and time, since nobody's using most of it anyway?
The philosophical arguments
The key question philosophers ask about time is not whether it exists, but why we experience it as past, present, and future. Philosophers are roughly divided into permanentists, who believe in a static universe, where everything that exists has always existed and always will, there is no change and no free will; and temporaryists, who believe the opposite.
There are also sub-factions, like those who favor the growing block theory of time. This theory has generally fallen from favor, but in a book titled Nothing to Come, Correia and Rosenkranz say the arguments against it are not entirely valid. To discuss them, they invent a new formal language called propositional tense logic, which contains ‘non-metric temporal operators’ for expressions like “Sometimes in the past.” They emphasize that they don't believe the theory themselves.
But they run into a big problem. Einstein's Special Theory of Relativity pulled the rug out from under them. It proved that the order in which events occur depends on the observer's frame of reference, specifically his velocity. How can one say the future doesn't exist when tomorrow for one observer is yesterday for another?
In the last two chapters they try, in effect, to make their philosophy Lorentz-invariant. But physicists may be pulling the rug out from them again: what if time is not even real?
The scientific arguments
In science, the “problem of time,” as it's called, is motivated by the distinction between the objective and subjective viewpoints. Physical laws consider only the objective viewpoint, in which time does not appear. Time is only a factor in time-dependent phenomena such as entropy and diffusion, in which thermodynamic irreversibility can occur. This is taken by some scientists as evidence that time is an emergent phenomenon.
Watch gears, no longer needed
The chapter by Dean Rickles and Jules Rankin, who present an excellent discussion of the earlier work that motivates the field, gives a variety of other reasons physicists say time doesn't exist. They don't sound too convinced, and they shouldn't be: the universe is either eternal, in which case something other than time must be invoked to explain what it is; or it had a beginning, in which case something other than time must be invoked to explain where it came from. No matter what, we need an explanation.
Though all the chapters are worth reading, I will discuss only two: the one by Lee Smolin and the one by Daniele Oriti, because they present the quantum argument most clearly.
Lee Smolin
Smolin's chapter is available online. It's an outgrowth of his earlier theory where he said that space is emergent, while time (which he equates to causality) is fundamental. Since then his views have evolved: he now says time doesn't exist either.
These earlier papers are a bit easier to understand. Smolin asks (but doesn't fully answer) some of the questions that arise from this concept, such as what is the meaning of locality, how does dynamics (which is all about moving in space) evolve, and how can there be fields and spatial derivatives without spacetime.
The papers talked about cosmological time being a quantum observable, but the overall idea now is that time arises from a causality network. This marks a dramatic shift in the field toward causal set theory (CST), which builds on the idea from Reichenbach (who credits Leibniz) that time and causation are essentially the same. An excellent non-technical description of causal set theory can be found in Discrete Causal Theory: Emergent Spacetime and the Causal Metric Hypothesis by Benjamin F. Dribus. A shorter description can be found here. Smolin describes the new theory, called causal views theory (CVT), this way:
The world consists of nothing but a vast number of partial views of its past. Each view is associated to an event, and is a representation of the immediate causal past of that event. These consist mainly of processes that transfer energy, momentum and other charges to it from its past events.
Causal views theory, like CST, says that events don't occur in spacetime. They are spacetime. This is conceptually and philosophically difficult because it's hard to understand how a network of events could exist with no spacetime. Smolin's view is that a Minkowski spacetime emerges in the semiclassical limit, which means that as the events get bigger they somehow change into a three-dimensional manifold. He writes:
The Lagrange multipliers za have become coordinates on an emergent Minkowski spacetime. We see also that gab is a conformal metric on spacetime and the intervals
zaJ − zaK are null. So we see that a conformal metric emerges on spacetime, which is just the inverse of the metric on momentum space.
These zaI are elements of “momentum space”, one per event, and become the coordinates of an emergent space. This means that a single quantum particle is composed of a structured set of N events linked in a specific way by causal relations, where N is some huge number. Thus, what we thought was a simple, structureless elementary particle of matter or empty space actually has an enormously complicated structure. In CVT, says Smolin, events don't exist, they happen, and the universe is made only of events.
Daniele Oriti
This raises an interesting question: by ‘space’, Smolin means a real space-time continuum we can exist in. Is finding a structure that identifies as a metric sufficient to create that? Or could it be that all we have done is re-invent the ruler?
Before answering that, we must ask why we need a quantized spacetime in the first place instead of just a continuum. While there is debate over whether spacetime is fundamental, most scientists are convinced that it is probably quantized. But why? Oriti has a good answer. In an earlier paper he wrote:
A black hole horizon is nothing but a particular region of spacetime, so it is spacetime itself that has entropy. And something that has entropy has a microstructure, whose number of degrees of freedom that entropy counts. If spacetime was a continuum, that entropy would be infinite. So the finite value for the horizon entropy is a clear indication that there exist[s] a discrete microstructure for spacetime.
Infinite entropy is a bad thing because entropy is information, and a black hole cannot have an infinite amount of information, or anything else for that matter. Therefore time has to be quantized.
What does emergent spacetime really mean? The question is not as trivial as it sounds. In his chapter and previous papers, Oriti talks about spacetime being a quantum condensate or phase transition, as if spacetime is like water vapor evaporating off some ancient ocean. But throughout his work, he emphasizes that to produce a convincing emergent spacetime we need more than a one-to-one equivalence between a vector component and a quantum of spacetime. In his 2013 paper, he put it this way:
In our opinion, the spatio-temporal meaning of these data can be assessed only after a procedure for extracting a continuum spacetime manifold and geometry out of them has been identified. This is because it is the last notions only that define what space and time are, in our current understanding. . . .Consequently, one should be aware of the risks of the interpretation of lattices as “discrete spacetimes” over which things happen or move. [emphasis in the original].
What he's saying is that emergent spacetime is more than finding a testable model. There is also a separate conceptual leap that needs to be crossed—a separate mechanism he calls geometrogenesis. The biggest obstacle is not finding a theory, but getting our thinking oriented. In his 2013 paper he called on philosophers to help.
No one would ever claim that loop quantum gravity (LQG) and group field theory (GFT) are mathematically trivial, but if Oriti is right there's a whole 'nother bridge to be crossed once you're figured all that mathematical felgercarb out and made it work.
Other chapters
Julian Barbour is probably the one most closely associated with the idea that time is not real. In his article he discusses how early physicists “constructed” time, and then discusses his new and somewhat idiosyncratic idea of the Janus point.
Barbour says time itself does not exist at all, ever, but is “an abstraction derived from change.” The universe, according to Barbour, started not with a singularity, but at a Janus point of minimum complexity. A colossal explosion split it in half, creating an increasing level of complexity in the universe that we mistakenly think of as time.
Carlo Rovelli was another one who claimed that time doesn't exist. In 2009 Rovelli told us to just forget about time altogether and get cracking on a quantum theory of gravity where time does not appear at all.
Discussion
Major conceptual challenges still remain. How can we even say that something exists, even if it's only a causal event or a 4D tetrahedron in LQG, if it doesn't occupy space or move in time? And what happens to string theory, which posits ten spatial dimensions that must be compactified to match common observations? Is string theory now stuffed—again?
Just as important as physical time is the question, which only the philosophers are asking, of why subjectivity provides a privileged frame of reference in space and time. This is just as much an essential feature of time as anything else. Saying it's all in our head gives the feeling that a bait-and-switch has occurred: we asked one question and got the answer to another. To understand time, we need much more than a new quantum gravity. We will also need to know what consciousness and subjectivity really are. Just because only conscious beings can feel it doesn't mean it's an illusion.
Omitting the essential nature of time in order to get a viable model is reminiscent of Daniel Dennett's eliminativist approach to consciousness: deny the importance of subjective phenomena as illusory, then say that the problem is trivial. Dennett claimed that consciousness does not exist, but is only an illusion. That saved him a lot of effort in devising a theory, but only by ignoring its essential nature, so it was scientifically sterile.
On the other hand, the non-existence of space and time will have many benefits. If time and space are merely fields, we could in principle block them just as we can block electromagnetic fields. If that were possible (which it probably is not), it would be an enormous technological breakthrough—once we clear away all the pterodactyls.
Or when the ol' ball and chain says she wants to go somewhere right now, we can simply tell her that Barbour has proved there is no such thing as ‘now’ and there is nowhere to go because Lee Smolin says spacetime is just an illusion. For conspiracy theorists it will be a dream come true. If there is no space or time, then the Moon does not exist and there is nothing to fly through to get there, so the Apollo astronauts could not possibly have landed on it.
“Time is in trouble,” say Rickles and Rankin, the only skeptics in this book. They sound like they feel outgunned, and maybe they are, but they defiantly paraphrase Galileo: “Yet it flows.”
jan 09 2024, 4:35 am
