How would a reverse time universe work?

[Valk]

It is predicted that time will at some point reverse direction. It is funny to consider everyone walking backwards, but I wouldn't think that would happen.


Without a physics degree, I could think that a fundamental aspect of our universe reverses. In our universe mass is attracted to eachother, and energy tries spreads out.

In our forward-time universe, mass moves towards eachother. Two asteroids exert a force on eachother, and a = F / m, and v = a * t, and they move towards eachother... would they move away from eachother in a reverse time universe? ==> v = a * -t

In thermodynamics, putting a hot object and a cold object together would cause heat to flow from the hot object to the cold object at a rate proportional to the temperature difference. Would, in a reverse time universe, the hot object extract heat from the cold object at a rate proportional to the temperature difference?


If both are true, then in a reverse time universe mass will spread out, and energy will be attracted to eachother.
Would lifeforms in that universe look for 'cold' food - that causes manageable flows of energy, and excrete 'hot' waste that is causing excessive flows of energy for their systems?

Are there any (semi-)physicists available that can tell more about what would happen in a reverse time universe? Can we imagine what lifeforms would be like in that universe?

[Eternal_Freedom]

Predicted by whom? The best estimates I know of from the last three years all say we're in an open/accelerating universe, not a closed one. We'll end up with a Heat Death or a Big Rip not a Big Crunch.

To the bes tof my knowledge, the "reverse time and everything just runs backwards" idea is perfectly acceptable. After all, from our perspective if everything is running backwards heat does flow from cold to hot and like charges do attract.

[Kuroneko]

It is predicted that time will at some point reverse direction.

Context would be helpful. It is predicted that the Sun will rise tomorrow. It is predicted that the world will end in 2012.

In our forward-time universe, mass moves towards eachother. Two asteroids exert a force on eachother, and a = F / m, and v = a * t, and they move towards eachother... would they move away from eachother in a reverse time universe? ==> v = a * -t

No. Gravity is time-symmetric. For Newtonian gravity, all that's needed to verify this is to note that d(-t)² = dt², though perhaps more intuitively, if you're looking at a film of an orbit, you cannot tell if the film is running forwards or backwards: both are perfectly good orbits.

In thermodynamics, putting a hot object and a cold object together would cause heat to flow from the hot object to the cold object at a rate proportional to the temperature difference. Would, in a reverse time universe, the hot object extract heat from the cold object at a rate proportional to the temperature difference?

Say you have a magically powerful telescope and you're looking at a planet in a distant galaxy where time is, for some reason, reversed. Then you will see hot objects extract heat from cold objects, etc., as you say. As to why I avoided talking about a reverse-time universe, well...

Are there any (semi-)physicists available that can tell more about what would happen in a reverse time universe? Can we imagine what lifeforms would be like in that universe?

Probably exactly the same. There are very little physics that are not time-symmetric, and none that appear to have any relevance whatsoever to biological processes. What you are taking for granted is how memory will work in a time-reversed universe. Entropy is intimately connected to information, and taken literally, quantum mechanics teaches us that:
(1) measuring anything whatsoever is to have to have your state become correlated (entangled with) what you're measuring, i.e., to have mutual information between your states
(2) information is conserved, always.
Decreasing entropy would involve destroying any correlation between an observer and whatever systems they interact with. The experience of life in his universe will be that of forward time direction just like we do.

[Korto]

I would think that, if you're in a universe that is running backwards, your perceptions of it would also be running backwards, and therefore it would all appear the same.
I suspect that's what Kuroneko just said, but I'm not entirely sure.

[Valk]

Predicted by whom? The best estimates I know of from the last three years all say we're in an open/accelerating universe, not a closed one. We'll end up with a Heat Death or a Big Rip not a Big Crunch.

It is predicted that time will at some point reverse direction.

Context would be helpful. It is predicted that the Sun will rise tomorrow. It is predicted that the world will end in 2012.

I do not have a good source saying that this will happen; I am just curious what would happen if it did. Stephen Hawking don't thinks it will happen, I just found out googling.
(timeframe would be in billions to trillions of years)

In our forward-time universe, mass moves towards eachother. Two asteroids exert a force on eachother, and a = F / m, and v = a * t, and they move towards eachother... would they move away from eachother in a reverse time universe? ==> v = a * -t

No. Gravity is time-symmetric. For Newtonian gravity, all that's needed to verify this is to note that d(-t)² = dt², though perhaps more intuitively, if you're looking at a film of an orbit, you cannot tell if the film is running forwards or backwards: both are perfectly good orbits.

Yes offcourse, if I had continued to determine the location I would have seen that x = v * -t, so x = a * (-t)2

Thanks for proving that thought wrong

In thermodynamics, putting a hot object and a cold object together would cause heat to flow from the hot object to the cold object at a rate proportional to the temperature difference. Would, in a reverse time universe, the hot object extract heat from the cold object at a rate proportional to the temperature difference?

Say you have a magically powerful telescope and you're looking at a planet in a distant galaxy where time is, for some reason, reversed. Then you will see hot objects extract heat from cold objects, etc., as you say. As to why I avoided talking about a reverse-time universe, well...

Are there any (semi-)physicists available that can tell more about what would happen in a reverse time universe? Can we imagine what lifeforms would be like in that universe?

Probably exactly the same. There are very little physics that are not time-symmetric, and none that appear to have any relevance whatsoever to biological processes. What you are taking for granted is how memory will work in a time-reversed universe. Entropy is intimately connected to information, and taken literally, quantum mechanics teaches us that:
(1) measuring anything whatsoever is to have to have your state become correlated (entangled with) what you're measuring, i.e., to have mutual information between your states
(2) information is conserved, always.
Decreasing entropy would involve destroying any correlation between an observer and whatever systems they interact with. The experience of life in his universe will be that of forward time direction just like we do.

I have not considered memory at all, because I assumed all life would be destroyed when time reverses, until new primitive life evolved in the new time-reverse universe. That first new life would not have memory. I would prefer if, in any case, we only considered simple life without memory for now.

1) I'm not sure what quantum mechanics has to do with this. If I measure the temperature of hot water then my temperature probe does affect the measurement, but it does so in a way that is negligible for the purpose of this measurement.

2) Why would information need to reverse?
I roll a ball over a patch of grass, where it slowly decelerates and stops. It's speed has turned into heat, spread out over itself and many specific grass straws.
Now, if time reverses, what equations (with -t) would suggest that the 'hot' grass and 'hot' ball would start moving. Conservation of information does not imply that the ball would start moving, it could just as well say that the specific hot grass straws could do something else (in reverse time).

[Kuroneko]

That first new life would not have memory.

They bear information; that is enough. The argument is not even inherently about life, but any macroscopic systems whatsoever. It's just put that way because of your comment in the OP about people walking backwards, etc.

1) I'm not sure what quantum mechanics has to do with this.

Exactly as was stated: to measure something is to have information about its state, i.e., have your state be correlated with whatever is measured. But information is also conserved. That means as a system is perturbed by its environment, information about its state 'leaks' into the environment, and as a result, its state becomes more uncertain. In other words, entropy increases.

If I measure the temperature of hot water then my temperature probe does affect the measurement, but it does so in a way that is negligible for the purpose of this measurement.

Not true. For our purposes, it's the most important thing in the world. Specifically that in measuring with your macroscopic apparatus, you're increasing entropy, and conversely decreasing entropy would involve destroying these correlations and hence any record of your measurement.

[Purple]

Not a physics expert by any means but I have to ask something. Why should reversing time reverse the laws of physics like that? From what I understand when ever you use t in an equation what it signifies is a certain number of units of time that pass. But it is treated as a scalar and not a vector. So in theory the direction of said passage should be irrelevant. If time passed 5 seconds it passed time seconds, be they in reverse or forward its still 5 seconds no?

[Valk]

That first new life would not have memory.

They bear information; that is enough. The argument is not even inherently about life, but any macroscopic systems whatsoever. It's just put that way because of your comment in the OP about people walking backwards, etc.

Information is indeed the only thing that could explain a foot being forced off the ground, only to be caught by muscular action bringing it back to the ground (one backwards step).

That's why I took the example with the ball and the grass. I find it too fantastic to just accept that the ball would start rolling backwards... doing everything that happened before in perfect reverse. Temperature, a vibration, eventually focusing on one spot on the ball to make it move.

To me, it would seem more credible that when you reverse time, after rolling the ball, the 'hot' grass and the ball would eventually go 'supernova' from the reversed thermodynamics. Feelings have nothing to do with the scientific truth offcourse, but as I said I want to know why exactly that ball would start rolling.
I myself do not have the physics background to prove either that the ball will not roll, or to prove that the ball will roll, but I hope someone has something in that direction.

1) I'm not sure what quantum mechanics has to do with this.

Exactly as was stated: to measure something is to have information about its state, i.e., have your state be correlated with whatever is measured. But information is also conserved. That means as a system is perturbed by its environment, information about its state 'leaks' into the environment, and as a result, its state becomes more uncertain. In other words, entropy increases.

Yes, but it has no effect in a way that we care.

Let's say that I have a tank filled with hydrogen and oxygen, and I drop a 1 kg 1273 Kelvin iron rod into it. Me measuring the temperature of that rod, with a 1 gram 293 kelvin probe, will not reduce its temperature sufficiently to change the outcome of the action I am about to perform. In this case, the explosion might occur a pico-second later, but we don't care because we are looking at it in terms of seconds.

Quantum uncertainty (right word?) is an interesting line of thought offcourse, but I don't think it is relevant to this discussion.

If I measure the temperature of hot water then my temperature probe does affect the measurement, but it does so in a way that is negligible for the purpose of this measurement.

Not true. For our purposes, it's the most important thing in the world. Specifically that in measuring with your macroscopic apparatus, you're increasing entropy, and conversely decreasing entropy would involve destroying these correlations and hence any record of your measurement.

Are you making a point that 'reverse time life' would need to observe its environment in a wholly different way than we do?

Not a physics expert by any means but I have to ask something. Why should reversing time reverse the laws of physics like that? From what I understand when ever you use t in an equation what it signifies is a certain number of units of time that pass. But it is treated as a scalar and not a vector. So in theory the direction of said passage should be irrelevant. If time passed 5 seconds it passed time seconds, be they in reverse or forward its still 5 seconds no?

Reversing something not always restores its former state.

If you take a raw egg, introduce a positive energy flow into it for 5 minutes (putting it in boiling water), and introduce a negative energy flow into it (or energy flow out of it) for 5 minutes... then your egg is different even though you reversed the energy flow.

I am looking for laws of physics that either dictate a perfect reversal, or contradict it; when time reverses.

[Purple]

Reversing something not always restores its former state.

If you take a raw egg, introduce a positive energy flow into it for 5 minutes (putting it in boiling water), and introduce a negative energy flow into it (or energy flow out of it) for 5 minutes... then your egg is different even though you reversed the energy flow.

I am looking for laws of physics that either dictate a perfect reversal, or contradict it; when time reverses.

That's sort of my point. Such laws could not really exist without violating both everything we know of physics and common sense. For your egg analogy, the universe won't care if you cooked the egg for 10 minutes strait or for +5 and than -5, the egg still gets cooked for 10 minutes. To claim anything else would hit into the barrier of common sense. Let's look for example at a simple thermodynamic problem. You have a metal rod heated to 50C and a bowl of water at 10C. Put one in the other and apply enough time and they should balance out at something in the middle. What you are suggesting is creating some sort of law that will explain how and why when you put a metal rod inside a bowl of water on your table the rod heats up whilst the water cools down until they reach 50-10. That might sound possible at first but we hit a common sense problem. How does the universe know the ratio? There are after all an arbitrarily large amount of combinations to be made from the same state. Why 50-10 and not 60-0 or 40-20? If you want to make up a fantasy law, well I guess you have to account for that.

[Valk]

@Purple
It would not reach a balance, something will happen, such as below. Assuming a perfect reversal:

• We have a sandbox, the temperature is not entirely uniform.
• Slightly hotter patches of sand extract heat from surrounding sand
• Even hotter patches of sand extract even more heat from already hot sand
• The hottest patches of sand begin to vibrate visibly
• The vibration focuses under a metal ball that lies in the sand
• The metal ball absorbes all vibration and is launched into the sky
• The sands temperature is perfectly balanced

That would be the perfect time reversal of a metal ball falling into dry sand.


I must admit that it does sound somewhat reasonable

[Kuroneko]

Information is indeed the only thing that could explain a foot being forced off the ground, only to be caught by muscular action bringing it back to the ground (one backwards step).

I get the feeling you've interpreted me as arguing something either disconnected from or contrary to what I actually said...

To me, it would seem more credible that when you reverse time, after rolling the ball, the 'hot' grass and the ball would eventually go 'supernova' from the reversed thermodynamics.

Time-reversed thermodynamics is the same thermodynamics. Asymmetry can be found in different states, not the laws (with a special exception that's not apparently relevant to our discussion).

Yes, but it has no effect in a way that we care.

The effect is very fundamental and has every relevance to thermodynamics and exactly the situations you're trying to describe. Imagine a container with filled with a gas with the molecules arranged in such a way that the gas concentrates in one corner of the box. This situation breaks no laws of physics; it merely requires a very special initial state. But it is obviously a very unstable situation, for it requires the molecules to be arranged just so, and even very tiny perturbations from the surrounding environment will destroy it.

Which is completely unlike the opposite situation of a gas starting from a concentrated state being allowed to fill the rest of the container. That situation is very stable against perturbations from the environment. In general, information and uncertainty are the workhorses behind the second law of thermodynamics, so to dismiss their relevance is more than a little silly. What quantum mechanics does is to force the fact that that information is always physical: the state of an observer that acquired information is always physically different than without (which technically classical physics does not address).

Let's say that I have a tank filled with hydrogen and oxygen, and I drop a 1 kg 1273 Kelvin iron rod into it. Me measuring the temperature of that rod, with a 1 gram 293 kelvin probe, will not reduce its temperature sufficiently to change the outcome of the action I am about to perform.

Temperature reduction is not relevant. What's important is that you will never observe macroscopic entropy-reducing processes because interactions with you will destroy it. But there is nothing physically special about you either, and every macroscopic object is effectively always measured by its environment, or even effectively is its own observer.

Since entropy is intimately connected to the flow of information, anyone living in the hypothetical time-reversed universe will call the direction of lower entropy the past and will be completely unimpressed with your (as hypothetical completely external yet noninteracting observer of this universe) insistence that's it's actually the future but they're living in reverse time.

Quantum uncertainty (right word?) is an interesting line of thought offcourse, but I don't think it is relevant to this discussion.

You've misunderstood the issue.

[Cykeisme]

Doesn't time only seem to flow to our perception because our memories and information coming to an observer (and the observer itself) is governed by causality?
So if everything ran backwards, including causality, wouldn't everything in the universe just play out like a video running in rewind?

[Valk]

@Kuroneko

I think you are right that I misunderstand what you are trying to say, allthough I still do not understand what you are actually trying to say.
Let me think on it for a day

[Cykeisme]

..and just to extrapolate from what I assumed on the previous post (which may be incorrect), since we as the observers are part of the rewinding universe, how would we be able to tell the difference?

In fact, if we look at the entire history and future of events in the universe as a static reel of film, does it even matter which way the reel is "played"? It's still the same, isn't it? Whichever way the reel is played (or if it isn't "played" at all), any observer existing inside the film itself won't be able to tell.

In fact, is it even played at all? If an observer could perceive things from outside time itself, everything is just a static series of events, from the birth to the demise of the universe, a complex work of art in four dimensions.

I realize that probably made no sense, and I probably shouldn't hit the Submit button.

[Valk]

@Kuroneko:
I'm still not getting it, but if we leave 'life' out of it for a moment, and just ask if the universe would be time-symmetrical... would you claim that perception and quantum uncertainty still matters for this question?

..and just to extrapolate from what I assumed on the previous post (which may be incorrect), since we as the observers are part of the rewinding universe, how would we be able to tell the difference?

In fact, if we look at the entire history and future of events in the universe as a static reel of film, does it even matter which way the reel is "played"? It's still the same, isn't it? Whichever way the reel is played (or if it isn't "played" at all), any observer existing inside the film itself won't be able to tell.

In fact, is it even played at all? If an observer could perceive things from outside time itself, everything is just a static series of events, from the birth to the demise of the universe, a complex work of art in four dimensions.

You are entirely correct if the universe is 'time-symmetrical'.

[Winston Blake]

Information is indeed the only thing that could explain a foot being forced off the ground, only to be caught by muscular action bringing it back to the ground (one backwards step).

You're not actually describing a reverse-time universe - you need to be conscious of your own thoughts. Implicit in your imagination is that the heel changes state (in-air vs on-ground) 'before' the foot spends some time in the air, which happens 'before' the toe changes state. But by saying 'let's reverse time in the universe', you're saying that in any sequence of events, all 'befores' are changed to 'afters', and all 'afters' are changed to 'befores'. So the correct description of a 'reverse-time universe' step is that the heel changes state (in-air vs on-ground) [AFTER] the foot spends some time in the air, which happens [AFTER] the toe 'changes state'. This is exactly equivalent to the forward-time description.

Rather than describing a reverse-time universe, you seem to be describing a universe in which time flows forward, but all physical processes happen in the reverse direction. In this universe, all 'usual forces' have been made zero, and all processes are instead propelled by fictitious forces pointing in the opposite direction to that which the 'normal' forces would have had. This applies to the motions of subatomic particles and molecules that compose all electrical and chemical processes, including those of the human brain.

This doesn't really make any sense, i.e. it is easy to find inconsistent cases. E.g. Let's measure time by a clock based on a lead ball falling through honey and passing tick marks - that's a description of what happens in our forward-time universe. In your 'forward-time reverse-process' universe, the ball is 'pushed upwards' through the honey. Now, this looks like the process in our universe 'in reverse'. Yet if the Earth's gravitational force was reversed like that, everything on Earth, including the ball & honey apparatus, would go flying off into space, which is NOT simply the reverse of what happens in our universe. So although in special imaginary cases it RESEMBLES our universe with reversed time, it actually isn't in general. What you're describing is really a forward-time reversed-process universe.

[Cykeisme]

You're probably right, Winston, a simple reversal of fundamental forces wouldn't result in a reversal of the normal flow of events.

We have the clock jar apparatus sitting on the table, which is sitting on the surface of the planet. A reaction force (would that be due to Pauli exclusion or something?) counteracts gravity, and prevents objects from falling through each other into the center of the planet.
If all the forces were reversed, not only would gravity repulse objects, but the reaction force would instead become attraction, and hold the jar to the table. The lead ball, being slowed by honey, would be repulsed upward, though more slowly due to resistance from the honey.

I agree there's probably fundamental problems with just reversing all the forces and the way energy behaves, but I'm sure there's probably a better example that we can come up with than the lead-ball-in-honey clock, isn't there?

[Winston Blake]

Hmm, I admit that I didn't consider reaction forces. However, a jar held to a table by gravity, with the table pushing up against its base, has walls in compression, because the reaction force is not applied 'throughout' the material, like gravity is. A reversed version would have walls in tension, since gravity is 'pushing it up uniformly', while the table 'pulls it down only at the base'.

So if the same apparatus was suspended from a cord, instead of sitting on a table, then in the reversed-force scenario, the cord would buckle. In our world, the cord stays taut. I suppose the buckling would go all the way down to the molecular level - a plastic cord is made of flexible polymer molecules. So rather than 'on ground vs off ground', the difference is now 'tension vs compression'. Further, although I cannot realistically imagine the cord becoming 'rod-like', even if it did, it would be an unstable inverted pendulum, rigid or not. In our world the system is stable under perturbation - so 'the stability of the system' is another difference.

I guess the big thing is that entropy would increase as normal.

EDIT: so I guess my answer to the OP is that if you reverse time and watch it, entropy appears to decrease, but since time is reversed, its actually increasing. Now, conflating reversal of time with reversal of all processes creates a contradiction. But if the concepts are separated and defined, entropy still increases in both scenarios, and there is no 'problem'. I am a little confused but I think this sounds right.

[Valk]

A reversed-physical-processes universe could be closer to what I mean yes. Changing the 't' to 'minus t' will do that in some cases.

However, earlier it was established that it would not cause mass to spread out - at least, not according to Newton:
- A ball some distance above earth's surface
- speed: v = a * t
- change in location: x = v * t = a * t * t = a * t^2
- t^2 is the same for '-t' and for 't', so it would appear that mass is still attracted by gravity.

I would interpret 'reverse time' as 'reverse-physical processes'; but that doesn't matter.
If we imagine a universe where a 'minus' is placed in front of every 't' - (to get around the confusing forwards/backwards semantic part)
Entropy would decrease, and life, once it evolved, would function differently than here. That's what seemed to me like an interesting subject to explore.

[Winston Blake]

If we imagine a universe where a 'minus' is placed in front of every 't' - (to get around the confusing forwards/backwards semantic part)
Entropy would decrease, and life, once it evolved, would function differently than here. That's what seemed to me like an interesting subject to explore.

The fact that the timestep is now negative means that entropy doesn't decrease - the idea of 'decreasing' is implicitly 'forward-time'. When you say 'I'm going to negativise/reverse time', that means everything that looks like it's decreasing is actually increasing, and vice versa. I tried to explain this earlier.

I think you can 'make everything run backwards' two ways. By changing v = a*t to v = a*(-t), or to v = (-a)*t.

In both, all velocities 'go backwards', but the difference is that in the (-t) version, time is counting backwards because the 'infinitesimal timestep' is now negative. In the second one, it's like all forces are reversed, but the timestep is still positive. The integrated versions are different because you need to integrate with respect to t, but for reverse-time, t is now (-t). Thus, x = v*t' + c for the reverse-time universe (t' = -t), so it looks the same as the forward time universe, but x = -v*t' + c for the reverse-process universe (t' = t).

Frankly I'm a little confused but I can't be bothered thinking about it any more - I must admit that I have lost interest.