Time Travel , possible or impossible ?

[IdEatYouAlive]

Yes, what you say is true, but coming back to my original point, the second law of thermodynamics does not, regardless of entropy sinks, display T-symmetry in macroscopic scales, while in smaller microscopic scales, particles can display T-symmetry. As such there cannot be a T-reversed sun, or a T-reversed anything on a macroscopic scale.

[Taylor_Bryant]

Yes, what you say is true, but coming back to my original point, the second law of thermodynamics does not, regardless of entropy sinks, display T-symmetry in macroscopic scales, while in smaller microscopic scales, particles can display T-symmetry. As such there cannot be a T-reversed sun, or a T-reversed anything on a macroscopic scale.

I would disagree with the bolded part. Without a source of energy to turn the clock in a particular direction, figuratively speaking, we would expect entropy to increase bidirectionally.

In considering this matter, I thought about the incident where Comet Shoemaker-Levy 9 was torn apart by Jupiter, which at first does appear to suggest non-T-symmetry. However, the energy source in this case is one which lasts for much longer: gravitational potential. The comet was formed from an extremely low-entropy state in which all of its parts were scattered throughout the cosmos, having near zero gravitational potential (which is always negative; so zero is the most gravitational potential one can have).

So naturally, when the universe as a whole has low entropy, it is going to tend towards a state of high entropy in accordance with the second law of themodynamics. What I am suggesting is that it tends towards a state of high entropy in both directions through time, and the anthropic principle explains why the observed history of the universe is an exception to this rule; a consistent arrow of time is necessary for our existence.

Your point has got me thinking about these things, and I think that the source of our local measurement bias is not in fact the Sun, but rather the low-entropy state of the universe in the past.

[IdEatYouAlive]

But the Comet does not display T-symmetry, because the Comet itself was in a low-entropy state in the beginning (near solid chunk of stone) and then disintegrated in Jupiter, increasing the entropy of the universe as a whole.

The universe was in a very very low-entropy state in the beginning, and after the Big Bang it expanded, and the universe as we know it was created, roughly speaking. The creation of the comet itself can be described as it taking a path from a low-entropy state to a high-entropy state, as whatever formed the comet, crashing moons or destroyed planets or whatever, displays, in accordance to the second law of thermodynamics, a low-entropy object, splitting up into a high-entropy state.

Then as the comet crashes into Jupiter, the already high-entropy state it was in (in context with what it used to be), became an even higher-entropy state.

The second law of thermodynamics in T-reverse would negate the whole law itself, creating abit of a paradox.
In T-reverse, the law would state that high-entropy objects will always, in a closed system, become low-entropy, and this is something that has never been observed, never been proven.

Thermodynamics is a theory of macroscopic systems and therefore the second law applies only to macroscopic systems with well-defined temperatures. For example, in a system of two molecules, there is a non-trivial probability that the slower-moving ("cold") molecule transfers energy to the faster-moving ("hot") molecule. Such tiny systems are not part of classical thermodynamics, but they can be investigated by quantum thermodynamics by using statistical mechanics. For any isolated system with a mass of more than a few picograms[10−12 g], probabilities of observing a decrease in entropy approach zero.

[Taylor_Bryant]

But the Comet does not display T-symmetry, because the Comet itself was in a low-entropy state in the beginning (near solid chunk of stone) and then disintegrated in Jupiter, increasing the entropy of the universe as a whole.

The universe was in a very very low-entropy state in the beginning, and after the Big Bang it expanded, and the universe as we know it was created, roughly speaking. The creation of the comet itself can be described as it taking a path from a low-entropy state to a high-entropy state, as whatever formed the comet, crashing moons or destroyed planets or whatever, displays, in accordance to the second law of thermodynamics, a low-entropy object, splitting up into a high-entropy state.

Then as the comet crashes into Jupiter, the already high-entropy state it was in (in context with what it used to be), became an even higher-entropy state.

The second law of thermodynamics in T-reverse would negate the whole law itself, creating abit of a paradox.
In T-reverse, the law would state that high-entropy objects will always, in a closed system, become low-entropy, and this is something that has never been observed, never been proven.

I agree with all this, except that I also think observing a low-entropy state in the absence of all other knowledge does not imply a lower-entropy state in the past; rather, it implies a higher-entropy state in the past. To illustrate my point with a practical example, assume for a moment that the Big Bang hypothesis is false and that the universe is infinitely old. Should this mean that entropy decreases indefinitely as one goes further back in time? According to an asymmetric interpretation of the second law of thermodynamics, the answer would be "yes", but I don't think that's true.

We can observe that the early universe was in a very low entropy state, but we cannot observe what came before it. If, as I suppose, the second law of thermodynamics is T-symmetric, then we should expect that the universe had much higher entropy before those early observations; in fact, it may even have had a history which mirrors its future (our present) to some degree. In this interpretation, the history of the universe (an observed decrease in entropy as you look back in time) is an anomaly, but it is an anomaly in the same sense that us living on a planet with liquid water is an anomaly; we know there are many planets without liquid water, but it's obvious why we live on one that does have it.

I think our disagreement arises in our interpretation of what it means to be T-symmetric. Do I suggest that a collection of rocks is likely to materialise out of Jupiter's atmosphere, join to form a comet and fly away? No, but it's possible, and the second law of thermodynamics is a probabilistic law; it doesn't say what will happen, only what is likely to happen. Because living in the extremely rare exception to T-symmetry in which we are close to a period of time where the universe existed in a low entropy state is the only way we can exist as living beings, it happens to be where we do exist, just as we live on the extremely rare exception to planets being permanently frozen or gaseous.

This being the exception, I should clarify that the "rule", so to speak, would be a universe in which nothing happens at all, and everything is in thermal equilibrium.

[IdEatYouAlive]

I agree with all this, except that I also think observing a low-entropy state in the absence of all other knowledge does not imply a lower-entropy state in the past; rather, it implies a higher-entropy state in the past. To illustrate my point with a practical example, assume for a moment that the Big Bang hypothesis is false and that the universe is infinitely old. Should this mean that entropy decreases indefinitely as one goes further back in time? According to an asymmetric interpretation of the second law of thermodynamics, the answer would be "yes", but I don't think that's true.

The second law of thermodynamics is not applicable in the case of an infinite universe, nor if the Big Bang theory is wrong, because there would not exist any closed systems. Without any closed systems (the one in interest here, the universe) entropy would not change without any outside force. The very fact that the second law does exist implies that there was a beginning, that the big bang did happen. I do agree with your example though.

We can observe that the early universe was in a very low entropy state, but we cannot observe what came before it. If, as I suppose, the second law of thermodynamics is T-symmetric, then we should expect that the universe had much higher entropy before those early observations; in fact, it may even have had a history which mirrors its future (our present) to some degree. In this interpretation, the history of the universe (an observed decrease in entropy as you look back in time) is an anomaly, but it is an anomaly in the same sense that us living on a planet with liquid water is an anomaly; we know there are many planets without liquid water, but it's obvious why we live on one that does have it.

Yes, this sounds reasonable. However, if you figure out what happened before that Planck second we actually are able to observe, you wouldnt just get the Nobel prize, you'd be the president of the Earth.

I think our disagreement arises in our interpretation of what it means to be T-symmetric. Do I suggest that a collection of rocks is likely to materialise out of Jupiter's atmosphere, join to form a comet and fly away? No, but it's possible, and the second law of thermodynamics is a probabilistic law; it doesn't say what will happen, only what is likely to happen. Because living in the extremely rare exception to T-symmetry in which we are close to a period of time where the universe existed in a low entropy state is the only way we can exist as living beings, it happens to be where we do exist, just as we live on the extremely rare exception to planets being permanently frozen or gaseous.

This being the exception, I should clarify that the "rule", so to speak, would be a universe in which nothing happens at all, and everything is in thermal equilibrium.[/QUOTE]
But the thing is here, that I still think that you are ignoring the part about the second law where it says "closed system" and "without an outside force". In a closed system, then yes, a bunch of rocks would very well be able to 'materialise' into a comet and fly away. But since there are too many other factors that would stop these particles from doing so, gravity, pressure, etc, the particles cannot do that. The second law is still not applicable to micropscopic objects.

It feels like you are talking about the Medioctriy principle and the Rare Earth hypothesis now. It feels relatively needless to say that the Rare Earth hypothesis is just bogus.

[RockoTheWallaby]

Ìf you travel faster than the speed of light you will be [effectively] going back in time. Example being if you wanted to reach the theoretical edge of the universe, you need to travel fast enough to see it(because the universe is expanding at the speed of light). Since it'd be the big bang you'd see, that would be going back in time. I suppose

Since when is the universe expanding at the speed of light? And wouldn't you actually go forward in time?

Correct and explain this to me if I'm wrong folks.

[RockoTheWallaby]

Only for men , women have menopause.

Thats why Christianity is so patriarchal.

[IdEatYouAlive]

Since when is the universe expanding at the speed of light? .

Since the beginning of time.

[The_Disasterpiece]

Since when is the universe expanding at the speed of light? And wouldn't you actually go forward in time?

Correct and explain this to me if I'm wrong folks.

You're right, it would be slightly less due to the fact that everything that moves eventually loses kinetic energy. However, due to the fact it's been expanding for billions of years, you'd need to travel at the speed of light for billions of years in order to see the edge of the universe.

[You_Are_A_Super_Player]

The universe and time is not separate things. Spacetime. To think that every second creates universes is kinda stupid.

Don't mock my many-world hypothesis.

Miko Kaku explains a couple of possible ways to time travel in Physics of the Impossible. However, they are all pretty much impossible, such as one where you go around the universe (if the curvature permit). Also, if M-theory turns out to be right, there may be certain foldings of dimensions that allow a connection between very separated points in spacetime.

[Taylor_Bryant]

The second law of thermodynamics is not applicable in the case of an infinite universe, nor if the Big Bang theory is wrong, because there would not exist any closed systems. Without any closed systems (the one in interest here, the universe) entropy would not change without any outside force. The very fact that the second law does exist implies that there was a beginning, that the big bang did happen. I do agree with your example though.

I don't agree that an infinite universe precludes the second law of thermodynamics from applying. Look at the universe on large scales; the distribution of matter, as well as the transfer of energy between regions, is more or less homogeneous. Thus, we can consider any sufficiently large region of the universe to be representative of its entirety, and if we consider the entire infinite universe as a single closed system, then by that reasoning the second law of thermodynamics applies in any sufficiently large space.

Yes, this sounds reasonable. However, if you figure out what happened before that Planck second we actually are able to observe, you wouldnt just get the Nobel prize, you'd be the president of the Earth.

I'm not saying it's reasonable or possible to determine what happened before that point, just hypothesising about what might have been.

But the thing is here, that I still think that you are ignoring the part about the second law where it says "closed system" and "without an outside force". In a closed system, then yes, a bunch of rocks would very well be able to 'materialise' into a comet and fly away. But since there are too many other factors that would stop these particles from doing so, gravity, pressure, etc, the particles cannot do that. The second law is still not applicable to micropscopic objects.

My point is that those rocks can materialise into a comet, even with all the other factors involved. Take Saturn's rings, for instance, which are thought to have once been a single moon; if the individual rocks in those rings all had just the right velocities, and approached each other in just the right way, they could well spontaneously form a moon again, but the chances of this happening are so vanishingly small that the possibility is generally discarded altogether.

What I am suggesting is that the probability of this event is equally tiny no matter which way the clock turns. The event of a moon existing and then smashing into pieces is just as unlikely as a collection of fragments spontaneously forming a moon. It just doesn't appear to us that way because we live in an extremely improbable environment in which moons do exist.

It feels like you are talking about the Medioctriy principle and the Rare Earth hypothesis now. It feels relatively needless to say that the Rare Earth hypothesis is just bogus.

I haven't actually heard of the Rare Earth hypothesis until now; however, I am rather fond of the anthropic principle, which applies to what I've been saying here.

You're right, it would be slightly less due to the fact that everything that moves eventually loses kinetic energy. However, due to the fact it's been expanding for billions of years, you'd need to travel at the speed of light for billions of years in order to see the edge of the universe.

This doesn't make any sense at all. There is no edge of the universe, and the universe itself doesn't have any kinetic energy because it's not made of matter. The edge of the observable universe is expanding at the speed of light, because light from further away hasn't had time to reach us yet since the beginning of time, but even if you could catch up to it you would simply be at the centre of another observable universe.

[IdEatYouAlive]

I don't agree that an infinite universe precludes the second law of thermodynamics from applying. Look at the universe on large scales; the distribution of matter, as well as the transfer of energy between regions, is more or less homogeneous. Thus, we can consider any sufficiently large region of the universe to be representative of its entirety, and if we consider the entire infinite universe as a single closed system, then by that reasoning the second law of thermodynamics applies in any sufficiently large space.

Yes, the universe as we know it is pretty much homogeneous. However, I cant agree with you saying that an infinite universe can be considered a single closed system. Thats the whole thing with infinity. And the universe as we know it, being finite, will not and can not become infinite. Because of Heat Death. This is assuming that dark energy and matter are bound by the same physical laws as regular energy and matter, and while not proven, it has not been observed to be otherwise.

My point is that those rocks can materialise into a comet, even with all the other factors involved. Take Saturn's rings, for instance, which are thought to have once been a single moon; if the individual rocks in those rings all had just the right velocities, and approached each other in just the right way, they could well spontaneously form a moon again, but the chances of this happening are so vanishingly small that the possibility is generally discarded altogether.

What I am suggesting is that the probability of this event is equally tiny no matter which way the clock turns. The event of a moon existing and then smashing into pieces is just as unlikely as a collection of fragments spontaneously forming a moon. It just doesn't appear to us that way because we live in an extremely improbable environment in which moons do exist.

Alright, I can agree with that.

This doesn't make any sense at all. There is no edge of the universe, and the universe itself doesn't have any kinetic energy because it's not made of matter. The edge of the observable universe is expanding at the speed of light, because light from further away hasn't had time to reach us yet since the beginning of time, but even if you could catch up to it you would simply be at the centre of another observable universe.

Well, regardless of where you go, how fast you go, you will still be at the center of this universe. Big Bang happened where we are now in three-space, and it happened in Alpha Centauri in three-space, it happened at any given point in the universe, in three-space.

Lets take, again for the sake of the argument, a balloon as an example. At the beginning of time, the Big Bang, the balloon was infinitesimally small. Then, the balloon started to expand, in all directions (I think, I'm not well-versed in the shape of the current universe) and matter was/is still inside the balloon. So you are always at the center of the balloon, always at the point of big bang.

[You_Are_A_Super_Player]

A finite universe can become infinite if it is flat or negative.

[IdEatYouAlive]

And what do you base that on?

[XsrdX22]

Magic

[Taylor_Bryant]

Yes, the universe as we know it is pretty much homogeneous. However, I cant agree with you saying that an infinite universe can be considered a single closed system. Thats the whole thing with infinity. And the universe as we know it, being finite, will not and can not become infinite. Because of Heat Death. This is assuming that dark energy and matter are bound by the same physical laws as regular energy and matter, and while not proven, it has not been observed to be otherwise.

I don't agree with the universe as we know it being finite. The observable universe - that part of it which we can see (which is possibly what you meant, I'm not sure) - is finite, because it's only 13 billion years old, and therefore light from more than 13 billion light years away hasn't had time to reach us yet. It's possible that there is nothing beyond that point, and it's possible that it extends infinitely beyond that point, and we have no way of knowing.

What I meant by an infinite universe being a single closed system is that nothing happens to energy on large scales except that it gets transferred between parts of the universe (i.e. there's no way for it to enter or exit the universe, by definition of what a universe is), which are in thermal equilibrium because the universe is homogeneous, so the net energy transfer is zero. You could enclose any large region with a perfectly reflective bubble for all forms of energy, and because equal amounts of energy are transferred in both directions its behaviour would not change by having its own energy reflected back into it, so while it's not technically a closed system it would behave as one.

Well, regardless of where you go, how fast you go, you will still be at the center of this universe. Big Bang happened where we are now in three-space, and it happened in Alpha Centauri in three-space, it happened at any given point in the universe, in three-space.

Lets take, again for the sake of the argument, a balloon as an example. At the beginning of time, the Big Bang, the balloon was infinitesimally small. Then, the balloon started to expand, in all directions (I think, I'm not well-versed in the shape of the current universe) and matter was/is still inside the balloon. So you are always at the center of the balloon, always at the point of big bang.

That's assuming a closed, positive curvature, finite universe model, which is but one possibility. It's also possible that the universe is more like an infinite rubber sheet that's expanding outward, in which case the observable universe around any point is a circle with radius equal to the age of the universe (distance and time being fundamentally equal quantities related by the speed of light). It's also possible that it has negative curvature, which is a phenomenon that only exists for 3-space curving within 4-space and can't be represented by a rubber sheet, but which would also result in an infinite universe. In all three models, the Big Bang happens everywhere at the same time, but none of them have a centre. As I said, the only reason we appear at the centre is because light from further than 13 billion light years away hasn't had time to reach us, so in every direction visibility extends for 13 billion light years.

You_Are_A_Super_Player: A finite universe cannot become infinite. The universe either started out finite and is still finite, or it started out infinite and is still infinite.

[ugly_bar_stard]

The Idea that you could go back in time and change History is cool ,Traveling to the Future could be beneficial it would certainly be an interesting chess game with deadly consequences HaHaHa.

[You_Are_A_Super_Player]

And what do you base that on?

You_Are_A_Super_Player: A finite universe cannot become infinite. The universe either started out finite and is still finite, or it started out infinite and is still infinite.

I was wrong. But finding that out just made my head explode.

[IdEatYouAlive]

Time is a continuum, sillyheads.

No shit sherlock, the fact that space and time is effectively the same thing has been known since 1905 with the introduction of special relativity, though ideas and notions that this has been the case has been around since 1750.

[goldenboyz]

Time travel is completely impossible if you cannot build the space vehicle that is able to move faster than the speed of light. To think about a visible time machine, I think, is now just like building a beautiful castle in the air. You can merely dream about it, it never comes true. However,an invisible time machine may rock the whole world, unexpectedly making Star Trek a reality.