Why is there a "now"?

[Perpetual Student]

It's not uniform (for example, those parts in gravitational potential wells age more slowly), although of course none of the part we can see is getting younger!



That actually is the case taking into account small variations - but they are indeed fairly small (except near strongly gravitating objects like black hole horizons).



That's a possibility, yes. Another is that either in the far past or the far future of our own piece the arrow was pointing the other way.

I think I understand that the first two comments are the result of GR.
What is the scientific basis for this last statement? Is it a consequence of GR, QM, both or some other theory? How would a layman gain some insight as to how this is possible?

 

[sol invictus]

What is the scientific basis for this last statement? Is it a consequence of GR, QM, both or some other theory? How would a layman gain some insight as to how this is possible?

That's a question that deserves a careful answer.

The laws of physics as they are currently understood are invariant under the symmetry t->-t. In words, they are identical going back in time as going forward in time.^* To understand the implications of that, let's first consider a more familiar example.

The laws of physics as they are currently understood are also invariant under rotations. And yet, when you look around you, you see that the room you're in is plainly not invariant under rotations (unless you happen to be sitting inside a perfect sphere). Neither is the solar system, neither is the galaxy, neither is the universe on the largest scales we can see (although it is close).

In fact, if the world were invariant under rotations, it would be extremely boring - because it would be perfectly spherical, with at most a dependence on radius from a single central point. But the laws of physics are also invariant under spatial translations - which would mean a totally symmetric world could even depend on that radius, but must be perfectly uniform and homogeneous. And since the laws of physics are also invariant under time translations, it could not depend on time either.

It turns out (perhaps it's obvious) that the only configuration that's invariant under all the symmetries of the laws of physics is a state that's completely empty of everything, one that's absolutely featureless and static. That state is known as the "vacuum". The reason we don't live in such a perfectly symmetrical state is, well, because it's completely empty.

So what good are these symmetries if they don't describe the world we actually do live in? The answer is that they tell us something very powerful. They tell us that starting from any solution, symmetric or not - for example, the world in the region around us - one can act on that solution with a symmetry (e.g. rotate it) and the result is also a solution. That is to say: since we have proof that our part of the universe is a possible world, we can therefore conclude that a rotated version of it is also a possible world, and might exist somewhere. Somewhere far away might be another copy of the earth and the solar system, but rotated by 90 degrees. Note that I'm not saying it's likely or unlikely, just that it's guaranteed by the laws of physics to be possible and consistent. Symmetries transform solutions into other solutions, that's their power.

So: as I said at the beginning, one such symmetry is time-reversal, flipping past and future. Our part of the universe is not symmetric under that, since its past differs from its future. But that means that a region of the universe similar or identical to ours, but running backwards in time, must also be a possible world, must also be a solution - and therefore might exist somewhere far away (and again I'm not saying it does, just that it might).


^*Strictly speaking the symmetry is not t->-t, it also involves charge conjugation C and parity P (the full, true symmetry is called CPT) - but C and P are not relevant for this post, so I will ignore them.

 

[Michael C]

So: as I said at the beginning, one such symmetry is time-reversal, flipping past and future. Our part of the universe is not symmetric under that, since its past differs from its future. But that means that a region of the universe similar or identical to ours, but running backwards in time, must also be a possible world, must also be a solution - and therefore might exist somewhere far away (and again I'm not saying it does, just that it might).

I'm trying to picture what a region of the world where time runs backwards would look like. Would that be a region where entropy is always decreasing?

 

[Perpetual Student]

s. i.:
Thanks for that excellent response. If I understand your post, t symmetry allows for the possibility of time running backwards somewhere/sometime but that it does not necessarily follow that there is/was ever such a place/event.

 

[sol invictus]

I'm trying to picture what a region of the world where time runs backwards would look like. Would that be a region where entropy is always decreasing?

Yes - decreasing towards the time direction we call the future, and increasing into our past. For example, it could look exactly like the earth etc., but running in reverse.

Weird, isn't it?

 

[Perpetual Student]

s. i.: Since we are not aware of time running backwards anywhere or anytime, is it not likely (or art least possible) that there is a yet to be discovered law of physics allowing time's arrow to have only one direction?

 

[Ron_Tomkins]

That's a question that deserves a careful answer.

The laws of physics as they are currently understood are invariant under the symmetry t->-t. In words, they are identical going back in time as going forward in time.^* To understand the implications of that, let's first consider a more familiar example.

The laws of physics as they are currently understood are also invariant under rotations. And yet, when you look around you, you see that the room you're in is plainly not invariant under rotations (unless you happen to be sitting inside a perfect sphere). Neither is the solar system, neither is the galaxy, neither is the universe on the largest scales we can see (although it is close).

In fact, if the world were invariant under rotations, it would be extremely boring - because it would be perfectly spherical, with at most a dependence on radius from a single central point. But the laws of physics are also invariant under spatial translations - which would mean a totally symmetric world could even depend on that radius, but must be perfectly uniform and homogeneous. And since the laws of physics are also invariant under time translations, it could not depend on time either.

It turns out (perhaps it's obvious) that the only configuration that's invariant under all the symmetries of the laws of physics is a state that's completely empty of everything, one that's absolutely featureless and static. That state is known as the "vacuum". The reason we don't live in such a perfectly symmetrical state is, well, because it's completely empty.

So what good are these symmetries if they don't describe the world we actually do live in? The answer is that they tell us something very powerful. They tell us that starting from any solution, symmetric or not - for example, the world in the region around us - one can act on that solution with a symmetry (e.g. rotate it) and the result is also a solution. That is to say: since we have proof that our part of the universe is a possible world, we can therefore conclude that a rotated version of it is also a possible world, and might exist somewhere. Somewhere far away might be another copy of the earth and the solar system, but rotated by 90 degrees. Note that I'm not saying it's likely or unlikely, just that it's guaranteed by the laws of physics to be possible and consistent. Symmetries transform solutions into other solutions, that's their power.

So: as I said at the beginning, one such symmetry is time-reversal, flipping past and future. Our part of the universe is not symmetric under that, since its past differs from its future. But that means that a region of the universe similar or identical to ours, but running backwards in time, must also be a possible world, must also be a solution - and therefore might exist somewhere far away (and again I'm not saying it does, just that it might).


^*Strictly speaking the symmetry is not t->-t, it also involves charge conjugation C and parity P (the full, true symmetry is called CPT) - but C and P are not relevant for this post, so I will ignore them.

This reminds me of Richard Feynman's lecture on irreversibility of nature's phenomena which used to be available on youtube and now it's not!!

 

[Michael C]

Yes - decreasing towards the time direction we call the future, and increasing into our past. For example, it could look exactly like the earth etc., but running in reverse.

Weird, isn't it?

If there were conscious beings in such a region, how would they experience time? Since all processes are reversed, peoples brains would be going backwards: they'd "remember" things that will happen in the future. Would the experience of time for a conscious being in a time-reversed region in fact be any different from our experience? I can't see how.

 

[The Man]

If there were conscious beings in such a region, how would they experience time? Since all processes are reversed, peoples brains would be going backwards: they'd "remember" things that will happen in the future. Would the experience of time for a conscious being in a time-reversed region in fact be any different from our experience? I can't see how.

I'm not sure if there would be any particular differences in how they experience time, again since as you note “all processes are reversed, peoples brains would be going backwards:” and they would remember what we would consider to be the future just as we remember what they would consider to be the future. There would be at least one physical difference that I can think of though, however irrelevant it is actually. For example Sol mentioned charge reversal in CPT symmetry. So although what we call a positive charge would become what we would call a negative charge, like charges would still be alike and different charges would still be different. Just as we remember unlike charges separated in the past coming together in the future and like charges together in the past becoming separated in the future their experience would be the opposite. Unlike charges together in their past would separate in their future and like charges separate in their past would come together in their future. The keys points being that we would both see unlike charges still as unlike, like charges would still be the same as each other and there would still be a difference in the behavior of like charges from that of unlike charges. The only difference being that we would claim unlike charges attract (come together in our future) and like charges repel (move apart in our future), while they would claim like charges attract (come together in their future) while unlike charges repel (move apart in their future).

 

[Zeph]

I can see that my referring to the quantum, relativistic, mechanistic and physiological "fuzziness" of the concept of now is being heard by some as sweeping a huge problem under the rug, like playing games. I'll try a little bit more to explain what I mean by fuzziness, and why it's called fuzziness rather than than being the essence.

Going back to an earlier analogy, if we look at the time dimension as a coast to coast highway, it appears that all our consciousnesses (analogized to cars) are driving along the highway in near lockstep.

It's not total lockstep, tho. Two major exceptions have come up. One is relativistic time dilation - some of the cars will have slightly different odometer readings (if they have very accurate and precise odometers), perhaps from taking more outer curves (ie: in relativistic terms they have experience some time dilation) Or maybe they took a fast side trip to a town off the highway and back (but they seem to have rejoined the pack now, even if their odometer differs). The other effect is the time of light delay between distinct locations, meaning that even at relative rest with each other, we cannot have exactly the same concept of now. In terms of cars, perhaps we could whimsically say that we can't tell whether the cars are "exactly" lined up parallel going down the highway, only that they are fairly close to all being at the same mileposts and are definitely not scattered randomly along the highway. (Yes, I'm aware that the odometers of wandering cars would read higher, while the chronometers of relativistic wanderers would read less time - it's just a metaphor to help bridge a conceptual gap, not a calibrated model).

These effects on the odometers and inexact perception of "are we exactly at the same location along the highway" are what I have called "fuzziness". However the phenomenon I was focusing on is that nobody's car gets to Chicago while the rest are still in Denver, nobody can stop their car, and nobody is driving in the opposite direction. Our consciousnesses tend to agree on the same "now", give or take a small fuzziness, at least when we are located at close to the same frames of reference. You can't move your "now" by an arbitrary distance ahead of mine or behind mine on the time axis (once we are relatively close again), the way you can move your "here" around.

If you move your "here" fast enough or far enough away, the relativistic fuzziness can get large - but it still doesn't allow you to arbitrarily stop or reverse or maintain a delta between your "now" and mine when we are together again.

[Aside - what is the maximum cumulative time dilation experienced by a human being relative to somebody who stays in one place on the planet? I assume it would be some cosmonaut who has orbited many times, but has that accumulated a millisecond of difference or much more or much less?]

This lack of independence is the enormous effect which I was pondering, and the admitted "fuzziness" of not having EXACTLY the same concept of "now" does not explain why we cannot drive our cars (or concepts of "now") in arbitrarily different places along the timeline.

It was once said that "the wonder sir, is not how well the bear dances, but that the bear dances at all". In this case, the wonder is that our conciousnesses seem to have a concept of "now" which under many circumstances stays very close to the same location on the time axis, versus being completely independent. That they sometimes temporarily diverge somewhat is not surprising; the wonder is that they synchronize at all, ever.

If one of us goes to another star, our concepts of "now" will have substantial time delays at least when we communicate. The surprise if that if we get together again, we will again both be watching the future be converted into the past at (very close to) the same location on the time axis. Once we are standing beside each other, you won't be watching this unfolding from a minute or a year in my future or my past.

This is so obvious that it seems daft for me to even question it; we live and think in this atmosphere and assume it's "just the way things have to be". I was just asking if physics has any explanation for it. I believe the answer is mostly no, tho there are some leads to continue following up. For the most part, what I'm asking has been considered scientifically meaningless, and I actually agree with that.

========

The discussion about parts of the universe having a reversed time arrow is interesting. One of the more whimsical background sources of my musing is the literary device of a character living "backwards in time", For example, Merlin in White's "The Once and Future King". When Arthur first meets Merlin, the latter knows him very well and knows all of Arthur's future but none of his past, and is indeed saying farewell to an old friend. While this is amusingly absurd, it raises the question of "just why is this so impossible?".

Of course, the major source of my question was actually an article in an issue of Scientific American some while back (special issue on Time). It was actually a working scientist who raised the point that "now" is not really covered by science, not me. It just struck me as a fascinating omission, and I was wondering if the more esoteric explorations of science in the intervening years had made any progress on it.

===

One of the interesting phenomena here is the reaction to my question. Given that these forums are largely about debate and disputation, it's not surprising that there has seemed to be somewhat more of a tendency to take the narrowest interpretation of what I have been saying, in order to discount or dispute, rather than actively seeking out any kernel of meaning that my words may have. For example, if any of my explanations are in any way inept, rather than grasping for the essence I am trying to convey and helping to bring it to light with one's presumed superior knowledge, there seems to be a tendency to leap upon that as a way to dismiss and discount. That's OK, it's the culture here and has its value, just as the adversarial justice system of jousting attorneys has its merits.

It does remind me of the earliest part of my marriage a bit. I had the unconscious and mistaken impression that it was possible to "win" an argument with my spouse, if I could create the more powerful framing of the situation and make use of any weakness in their exposition. I did get my way sometimes, but ultimately it didn't work, at least if "work" means creating a solid, loving, functional relationship. "Winning" an argument turns out to be a Pyrrhic victory. I discovered that I needed to engage my intellect even more towards working to actively understand my partner's viewpoint, as in expressing my own. And that fostered as similar response from the other side. While other dialogues are not within intimate relationship, I've nevertheless found a value in collaborative exploration of different viewpoints, where the goal is to actively reach out to try to understand all points even when disagreeing. And I do see a bit of that here.

Thanks,
Zeph

 

[sol invictus]

Going back to an earlier analogy, if we look at the time dimension as a coast to coast highway, it appears that all our consciousnesses (analogized to cars) are driving along the highway in near lockstep.

You keep asserting this over and over, but you've been entirely unable to provide any evidence for it (presumably because there is none). It's not that your point is "scientifically meaningless", it's simply incorrect.

The "present" is to time as "here" is to space.

Can you provide a coherent argument for why that analogy fails?

 

[schrodingasdawg]

Going back to an earlier analogy, if we look at the time dimension as a coast to coast highway, it appears that all our consciousnesses (analogized to cars) are driving along the highway in near lockstep.

There isn't a universal 'time' direction, thus the analogy fails.

Two major exceptions have come up. One is relativistic time dilation ... The other effect is the time of light delay between distinct locations

I don't think you're understanding the point about the relativity of simultaneity. It isn't a measurement error caused by time dilation and signal delays. There isn't a global present that everyone is experiencing in some objective way. What events are simultaneous depends on the observer. Which events constitute the present depends on the observer. This completely undermines the claim that there is a global present.

Of course, the major source of my question was actually an article in an issue of Scientific American some while back (special issue on Time). It was actually a working scientist who raised the point that "now" is not really covered by science, not me. It just struck me as a fascinating omission, and I was wondering if the more esoteric explorations of science in the intervening years had made any progress on it.

It's not the purpose of science to address that.

 

[Fontwell]

Joining in at this late stage it looks to me like the OP's reference to the hard problem of consciousness is still the main issue. The sense of now seems to be inextricably linked to consciousness - all consciousness is only ever now. The various theories on the physical nature of time are all good stuff but work equally well with or without consciousness (or am I wrong?), so in terms of the OP I think that now is more related to the hard problem of consciousness than to a model of the nature of time within physics.

 

[dlorde]

Maybe the problem is that Zeph is interpreting his own 'now' as everyone's 'now' because he is the perceiver. Naturally, whoever we interact with will seem to be in our 'now' because we are interacting with them. If we ask them, they'll say it's their 'now' too - as, of course it must be, if we're interacting with them. This is just a simple egocentric viewpoint. From any individual's POV, their 'now' seems to be everyone else's, because they are only aware of events occurring in their own 'now'.

Our personal 'now' drifts into the past with increasing distance, so the 'communal/shared now' sensation begins to break down when we try to communicate with someone or something a long way away, e.g. astronauts on the moon, or Mars rovers, where differences of seconds or minutes in communication offset our perceived 'now'. Looking out into the universe, our 'now' may be offset by millions of years. Even within our own bodies, our perceptual now may be offset between our own senses - if you stub your toe, the visual 'now' will precede the tactile 'now', which, in turn, precedes the painful 'now'. If you burn your hand on a hot surface, your reflexive 'now' (jerking it away) will precede your conscious tactile 'now' for the event. Your conscious awareness of 'now' lags behind all, including your subconscious 'now' (although the brain does clever tricks to make it appear cotemporal).

 

[sol invictus]

Why do you think that an individual has their own "now"? All the arguments above apply just as well within a human body or brain as they do between individuals.

The life of an individual human is simply a sequence of states describing a reasonably well localized collection of loosely bound particles. The subjective experience of consciousness that said sack of particles will describe when exposed to the stimulus of being asked about it is simply a feature of those states, just like the fact that the sack of particles will stop moving much if at some point in the sequence it is deprived of energy input for long enough.

 

[Fontwell]

Why do you think that an individual has their own "now"? All the arguments above apply just as well within a human body or brain as they do between individuals.

The life of an individual human is simply a sequence of states describing a reasonably well localized collection of loosely bound particles. The subjective experience of consciousness that said sack of particles will describe when exposed to the stimulus of being asked about it is simply a feature of those states, just like the fact that the sack of particles will stop moving much if at some point in the sequence it is deprived of energy input for long enough.

But subjective experience isn't even in the same arena as biology or physics. The behaviour of a sack of particles might well be simply a feature of those states but through personal experience we each know that every moment has its own qualia and that implies consciousness and nowness. Or may be that is a dangerous topic, I seem to remember that my last gap of not posting here for about 3 years was prompted by a bad tempered argument about the existence of qualia.

 

[sol invictus]

But subjective experience isn't even in the same arena as biology or physics. The behaviour of a sack of particles might well be simply a feature of those states but through personal experience we each know that every moment has its own qualia and that implies consciousness and nowness. Or may be that is a dangerous topic, I seem to remember that my last gap of not posting here for about 3 years was prompted by a bad tempered argument about the existence of qualia.

I'm not particularly interested in having that debate, because I generally find it very uninteresting. It always starts from a totally unsupported opinion (like "we each know that every moment has its own qualia and that implies consciousness and nowness") and descends from there.

As far as I can tell, every bit of evidence we have is consistent with my view. Since my view is the minimal one, the one that follows from the known laws of physics with nothing extra needed, I think it should be the null hypothesis. Therefore any discussion of consciousness ought to begin by trying to find evidence that falsifies that hypothesis - but as far as I know, none exists.

 

[Fontwell]

OK, I only mentioned consciousness because that was the trust of the OP.

 

[schrodingasdawg]

I'm not particularly interested in having that debate, because I generally find it very uninteresting. It always starts from a totally unsupported opinion (like "we each know that every moment has its own qualia and that implies consciousness and nowness") and descends from there.

As far as I can tell, every bit of evidence we have is consistent with my view. Since my view is the minimal one, the one that follows from the known laws of physics with nothing extra needed, I think it should be the null hypothesis. Therefore any discussion of consciousness ought to begin by trying to find evidence that falsifies that hypothesis - but as far as I know, none exists.

The problem here is that subjective experience isn't in the realm of science. It has nothing to do with the objective, material world. The most science can give us is the well-confirmed fact that the objective state of the brain causes particular subjective experiences, which makes the claim that there is such a substance as soul that is separate from the physical brain a superfluous claim.

That's not to mention that the hard problem of consciousness does apply to any metaphysics which says only physical facts are true.

Honestly, I'm not too interested in having the debate either. I think physicalists tend to use circular arguments---the existence of qualia does not follow from physical laws, therefore they don't exist, therefore physicalism is true---but as long as we're stating reasons we're not debating, I might as well too.

Anyway, the description by dlorde seems perfectly sensible to me. Even if you don't believe subjective experience "really" exists, his description still works for explaining the illusion of a "shared now." (Well, it's illusory either way, but in the eternalist framework even just the "now" is illusory.)

 

[Perpetual Student]

My layman's understanding of simultaneity (e.g. who and where else is in my "now") is based on:

[latex] t* = (t-\dfrac{vx}{c^2})}/{\sqrt{1-\frac{v^2}{c^2}}[/latex]

where t and t* are the time coordinates in question, v is the relative velocity between two frames and x is the distance between them. Since v can be either positive or negative, it seems that my "now" can be in someone else's future or past.
If this is true, "now" seems to be a very subjective concept and would seem to depend on one's awareness.