What is the current state of research on the nature of Time?

[samudra]

TL;DR Summary

(my question is fed by various bodies of science-fiction involving FTL, instantaneous communication, 'thoughts', 'dreams', 'connections') and asks: Have we made progress in defining the nature of Time?

As a non-Physics major, like many others, I read science-fiction, hard science-fiction, have undergraduate/graduate courses on Astronomy and college physics. I also participated in R&D that produced electric rockets able to power small space vehicles (very slowly, but surely). I deal with satellite communications to GEO and LEO space vehicles and came up with a multi-hop method for hypothetically connecting Mars/Earth stations with each other with broadband speeds all of the time, regardless of the orbital geometry of Sun, Earth, Mars. Those were and are fun projects. What I have realized after looking at the work done few years ago, and trying to plot it on any sort of a big scaled 3-D plot, such networks are -- well -- big in any number of units, and if by the end of this century, the human race will begin to colonize outer space, there needs to be some consideration of the concept of Time.

Like a few sci-fi authors, I somehow intuitively (and perhaps through personal events) think that if humans are intentionally isolated in confined environments (Space vehicle in microgravity, under medication in ICU, incarcerated in a prison, in a space simulator) without contact, their sense of times become fuddled, and somehow they feel they have more time in hand to do things (like think, monastic astronomers in medieval times) yet time is actually plodding along just fine outside of that environment. Who wouldn't want to spend their entire lifetime dreaming up ideas, only to "wake up" and notice they have been asleep for a few minutes. Yet those memories / thoughts are apparently real to an individual. How can biologics have different senses of time when encased in a physical container, in a physical world in which regular time marches continously - yet they can regress to earlier childhood memories and go forward with visions of the future. Is time in a cosmological sense fixed in nature or not?

Separately if the universe is expanding continuously, and light speed is constant (is it?) - is there a possibility of finding a single event/location/phenomenon in Minkowski spacetime (Mikowski space) that observers in different part of this universe can agree on as a common spacetime reference? And if it was expanding from the moment after Big Bang (whenever that was) was the speed of light constant even then? If both are correct (the speed of light is constant, with lightspeed = distance/time) when was the "fundamental unit" of time [whatever that might be] different in the small compressed (possibly finite) volume after big bang than presently expanded (possibly finite) universe? Or, must the "fundamental unit" of distance [whatever that might be] have been different then compared to present? Or is it that the equation above is really too simplistic and ignores relativistic mehanics - as "spacetime" is a four dimensional manifold combining three-dimensional Euclidean space and "time", in which case for both cases can be true, lightspeed can be constant always as long as they are on different points on the manifold. Then - the question I am struggling philosophically, does that not mean in a given universe's spacetime, isn't it likely that the fundamental units of time are different at each point (= time scale is different) - or is this too much of a reach?

 

[PeterDonis]

is there a possibility of finding a single event/location/phenomenon in Minkowski spacetime (Mikowski space) that observers in different part of this universe can agree on as a common spacetime reference?

No, because the spacetime of the universe is not Minkowski spacetime.

 

[PeterDonis]

light speed is constant (is it?)

Asking whether the speed of light is constant is not really well-defined, because the speed of light depends on your choice of units. The actual "constants" of physics are dimensionless; the one that most closely corresponds to the speed of light is the fine structure constant.

So a better question would be whether the fine structure constant was the same in the very early universe as it is now. As far as we can tell, the answer is yes.

 

[samudra]

Thanks for the hint:

the one that most closely corresponds to the speed of light is the fine structure constant.

Is this wikipedia entry relevant? Fine Structure Constant -

 

[PeterDonis]

Is this wikipedia entry relevant?

Yes.

 

[kimbyd]

I generally like what Sean Carroll has to say on the subject:
https://www.scientificamerican.com/article/sean-carroll-eternity-to-here/

 

[ohwilleke]

Honestly, this really calls for a book length treatment. I have at least two dead tree books on my shelf that do just that, and several more that address subparts of the issue. Usually the big topic headings include some or all of the following:

1. The rate at which time passes. This is observer dependent ad a function of the velocity of the observer relative to the universal constant speed of light "c" (which is also the speed of gravity and any other massless particles). It is also influenced by gravitational fields in a not entirely unrelated phenomena. This also includes observations designed to test if various fundamental physical constants, including those with a time element, have always had the same value. For want of a better category to mention it, cosmologists are also interested in a concept called "cosmological inflation", which which the amount of space in the universe becomes vastly greater in an almost infinitesimal period of time, which superficially defies our "common sense" understanding of the nature of space and time that applies in other contexts. A full and accurate discussion of cosmological inflation is beyond the scope of this brief issue spotting answer.

2. Time and space can be viewed as mostly equivalent for many purposes. This comes up in the geometry of space-time in general relativity and also in the fact that Feynman diagrams (which describe the motion, creation and decays of particles in a simplified schematic form) are true no matter how they are rotated in space-time. But, in many, if not most, applications, it is usually helpful to single out time for special treatment.

3. The direction of time and causality. There are two main "arrows of time". One is the second law of thermodynamics at a macro-level. The other is CP (charge-parity) violation, which is equivalent to time symmetry violation, which occurs only in W boson interactions involving fundamental fermions in the Standard Model (and in neutrino oscillation) and still preserves CPT symmetry. Also, many theories of quantum gravity adopt causality is an axiom. In many quantum mechanical systems it is possible to preserve only two of the following: causality, locality and reality, although which of the three to sacrifice, or even if it is meaningful to talk about the three as three fully independent and distinct quantities or if this is a category error, is unknown. The distinction between matter and antimatter comes into play here because mathematically, antimatter is equivalent to ordinary matter of the same type moving backwards in time, a concept which also ties into the E=mc^2 equivalence on general relativity and special relativity. The notion of causality is tricky in quantum mechanics because if quantum mechanics is truly stochastic (i.e. basically random) and follows laws that are the same except for the asymmetry introduced by CP violation, then taken to its logical conclusion, both the past and the future are statistically uncertain and that is very contrary to our intuition. Also related is the unanswerable question of how a particle "knows", for example, to emit something at a particular rate with a particular mean lifetime, and how it "decides" to do so at a particular time, when all particles are hypothetically identical. This also embraces the issue of whether anything can be superluminal.

4. Why do path integrals in quantum mechanical propagators consider photon paths at speeds other than "c"? The foundation is quantum mechanics is that you determine the probability that a fundamental particle goes from point A to point B by summing up the probabilities (called "amplitudes" by physicists) of every possible path using what is called a path integral. But, to get the right result when you are looking at the photon propagator, you need to consider paths that are more than and less than "c" in intermediate steps (the final result always ends up with a path speed of "c") discounted by a function of the magnitude of the difference from "c" of the path. Why this is so is a mystery. Could the absolute speed of light be only a classical approximation of a deeper quantum level truth?

5. Is time discrete or continuous? The path integral issue could hint at various kinds of beyond "core theory" physics. One is the possibility that the speed of light is really a constant and exact number of discrete hops per second in space-time and the distance of a "hop" is actually only almost exactly the relevant distance, rather than exactly that distance). Time, like space, could be discrete in some manner. The Heisenberg Uncertainty Principal which limits, not just in practice, but in theory, the resolution at which quantities that include a time factor can be observed, may also be related to this observation. This also related to whether the concept of "Planck time" and so called "natural units" is fundamentally all that significant, or just convenient.

6. Is time one dimensional? We usually think of time as a one dimensional quantity. But, it could be that we simply exist on a one dimensional timeline and that there are myriad time lines in parallel to ours, something associated with the Many World Hypothesis as an explanation of quantum mechanics. Likewise, we usually think of time as being represented by real number values, but in CP violation, quantities related to the arrow of time have complex number values. This also embraces the issue of the topology of time, i.e. is time topologically equivalent to an infinite line, or for example, are continuous time-like loops, or wormholes that seem like they are non-local in time, possible?

7. Is time infinite? Cosmologists wonder if time literally begins at the Big Bang, or if there is something that comes before it (for example, a universe where time runs in the opposite direction from the Big Bang, and also "Big Bounce" scenarios). Some cosmologists wonder if time can theoretically have an end (e.g. the "heat death of the universe.").

8. Does time exist? Some theorists question whether defining time as a "dimension" or something that has independent reality, is really an accurate way at the most fundamental level to think about it.

 

[PeterDonis]

The rate at which time passes. This is observer dependent...It is also influenced by gravitational fields...

Only if "time" here means coordinate time. The rate at which proper time passes is not observer-dependent and is not influenced by gravitational fields. Differential aging of observers who follow different trajectories in spacetime is due to the different geometric lengths of their trajectories, not due to different rates of passage of proper time.

Time and space can be viewed as mostly equivalent for many purposes.

But timelike and spacelike vectors, curves, intervals, etc., are not equivalent; they are physically different. (And there is a third category, null, which is neither timelike nor spacelike, and which the pre-relativistic categorization into "time" and "space" cannot account for.)

 

[ohwilleke]

Only if "time" here means coordinate time. The rate at which proper time passes is not observer-dependent and is not influenced by gravitational fields. Differential aging of observers who follow different trajectories in spacetime is due to the different geometric lengths of their trajectories, not due to different rates of passage of proper time.

But timelike and spacelike vectors, curves, intervals, etc., are not equivalent; they are physically different. (And there is a third category, null, which is neither timelike nor spacelike, and which the pre-relativistic categorization into "time" and "space" cannot account for.)

I don't disagree with any of these points. This is also why I used the weasel words "mostly equivalent" and "many purposes", instead of saying that they "are equivalent."

I am just trying to briefly sketch and outline he headline issues in a big topic in a very few words, rather than a book length and rigorous treatment, and you are right to note the slippery use of the terminology. These points lead to a couple more general observations:

9. The word "time" does not have a single consistent meaning that plain English implies in all contexts. Instead, time is a more complex and subtle thing that that which embraces multiple concepts like "coordinate time" as distinct from "proper time" in general relativity at a technical level.

10. Is everything "within" time and/or time-like? There is an interaction in the Standard Model of particle physics that is theoretically possible, but has never been observed because our experiments can't get to sufficiently high energies, called a "sphaleron" which is a "space-like" interaction with no "time-like" component that is important to understanding the matter-antimatter balance in the universe, because quantum numbers called "baryon number" and "lepton number" are not conserved in this interaction (although a combination of these two quantum numbers called "B-L" is conserved). Also, as @PeterDonis notes in addition to things that are "time-like" and space-like": "there there is a third category, null, which is neither timelike nor spacelike, and which the pre-relativistic categorization into "time" and "space" cannot account for."

 

[PeterDonis]

a "sphaleron" which is a "space-like" interaction with no "time-like" component

This is a misleading way of putting it. A better description would be that a sphaleron is a stationary solution, i.e., the state it describes does not change with time, and it does not have a perturbative description, which means that calling it an "interaction" is not really correct, since you can't describe it in terms of Feynman diagrams.

 

[ohwilleke]

This is a misleading way of putting it. A better description would be that a sphaleron is a stationary solution, i.e., the state it describes does not change with time, and it does not have a perturbative description, which means that calling it an "interaction" is not really correct, since you can't describe it in terms of Feynman diagrams.

Again, I don't really disagree and I am not trying to be misleading.

The point is that there is something related to time that is unique about a sphaleron solution that someone probing deeply into the nature of time in physics might find notable or informative in developing their understanding of time, since it differs from every other solution in the Standard Model in a respect related to time.

I wouldn't have thought that the term "interaction" is limited to things that can be described in a perturbative description with a Feynman diagram, but I'm not trying to quibble. If there is a better word, I'd welcome it. I don't much like the term "stationary solution" as that sounds too much like something that happens on a piece of paper doing math and too little like something that could happen in the real world involving things that exist. Would "transformation" be a better word that conveys a real world physical connotation?

This is assuming, of course, that a "sphaleron" is actually physical, and not merely a hypothetical non-physical solution to an equation that pops out of the equation because some necessary axiom or boundary condition is missing in the Standard Model. I have no affirmative reason to question that it is physical, but non-physical solutions often pop up in effective theories when they are applied outside their domain of applicability, and we don't yet know with any great confidence whether the Standard Model's domain of applicability extends to arbitrarily high energies.

 

[PeterDonis]

there is something related to time that is unique about a sphaleron solution

it differs from every other solution in the Standard Model in a respect related to time

I don't see how "time" is involved in the difference. Can you explain in more detail why you think that? What thing or respect related to time is involved here?

 

[ohwilleke]

I don't see how "time" is involved in the difference. Can you explain in more detail why you think that? What thing or respect related to time is involved here?

You use words related to time to describe it when you say (emphasis added): "A better description would be that a sphaleron is a stationary solution, i.e., the state it describes does not change with time[.]"

It is defined by its independence of time unlike all other solutions to the same equations:

A sphaleron is "a static (time-independent) solution to the electroweak field equations of the Standard Model of particle physics[.]"

The notion that something can be "time-independent" at all is not obvious. All other standard model solutions to those equations are dynamic. The reason that it can't be described in a Feynman diagram is because Feynman diagrams are two dimensional with one dimension corresponding to space and one to time, but the time dimension isn't applicable in the sphaleron solution.

 

[PeterDonis]

It is defined by its independence of time unlike all other solutions to the same equations

Unlike all perturbative solutions. There are other non-perturbative solutions besides sphalerons, such as instantons, which have a similar property. (I believe there are also non-perturbative solutions that are not "time independent" as instantons and sphalerons are.)

All other standard model solutions to those equations are dynamic.

Not quite. See above.

The notion that something can be "time-independent" at all is not obvious.

No real thing is exactly time-independent, yes; something that was truly time-independent would never change at all, ever, and no real thing is like that. All "time independent" solutions to the mathematical equations are approximations--basically they are approximating a case where the thing of interest changes so much more slowly than everything else in the problem that it can be treated as unchanging.

The reason that it can't be described in a Feynman diagram is because Feynman diagrams are two dimensional with one dimension corresponding to space and one to time, but the time dimension isn't applicable in the sphaleron solution.

No, the reason it can't be described in a Feynman diagram is that it is a non-perturbative solution, and Feynman diagrams can only describe perturbative solutions. Feynman diagrams can (and usually are) drawn in momentum space, where your description in terms of "dimensions" doesn't even apply.