Time travel and the second law of thermodynamics

[Space_Invaders]

I know that many users in this forum do not believe backwards time travel is possible, and indeed I do not think so myself. Nevertheless, some scientists do think it's possible and I think it's worth considering its implications.

I'm not talking about the grandfather paradox or similar effects, but of the relation between backwards time travel and entropy. Imagine a time machine inside a universe which is about to suffer heat death, very near to maximum entropy. The time machine goes back in time to present day and, voilà, it is saved. It has a few more billion years to live. It repeats this process every time heat death is nigh, and thus, it can exist forever.

Nevertheless, this would mean that the time machine itself has defeated entropy: it can keep on using energy and producing work forever, since every time it is about to reach maximum entropy, it sends itself back in time, where it can pick up more energy.

Now, I am not an expert in physics, rather in biochemisty, so I guess there might be a few wrong assumptions in what I just said. But, what other issues do you see in the relationship time travel-entropy?

 

[Fallingstar1971]

Ahhhhhhhh but the second law is saved...............

You can send the time machine back a billion times...................

But the machine never gets younger

So it will still age and fall apart

Entropy is saved, the second law is safe from you meddling time travelers......

Star

 

[BoJangles2]

Interesting thought, another paradox that can arise (one of my own pet conjectures) is conservation of energy.
If you send any matter back in time, you increase the gross amount of energy at the particular point in time. I.e. if you send a person back in time who weights 70kg (skinny scientist type) the universe actually weighs a little bit heavier than it would have, breaking the golden rule of conservation of energy.

Actually this gets worse, if the person (or thing) you sent back in time couldn’t stop the time travel event from originally happening (in the future) somehow, you would cause a feedback loop, i.e. the time travel event would keep happening again and again. To an observer at the original time and place when the time traveler presses his go button; the universe implodes into a singularity due to the feedback of mass and energy.

Though this does raise an issue though, even if the time travelling event can’t be stopped on purpose, there are certain situations where nature would stop it from happening anyway. I.e. there’s a probability after the loop runs a certain amount of times, something stops the event from taking place in the first-place, i.e. a mountain of dead scientists tells the original scientists this isn’t a good idea, and the loop stops.

Personally, there are too many paradoxes with time travel; it doesn’t work on many levels mathematically, even philosophically, and it breaks just about every staple of physics it can lay its hands on (IMO), therefore it can’t be done (well backwards in time anyway).

 

[Fallingstar1971]

I dunno, your moving mass from one point to another

If space and time are really spacetime, then any change in one would require an effect in the other.

If I sent you back in time 1 year, the Universe would still weigh the same. This is because you no longer exist in my "when" so the doubling of mass in your "when" balances out. The Universe, containing ALL whens, stays the same.

If I chop off my finger and attach it to my toe, my weight stays the same (morbid yes, but you get my point)

If space and time are part of the same thing, if I move you in time I should have to move you in space as well.

If space and time are part of the same thing, then by moving you in space, I should be moving you in time (if I instantaneously moved you 1 light year away, and you looked back at Earth with a strong enough Telescope, you could conceivably see yourself getting ready for your trip, for you will be seeing one year into the past)

If space and time are part of the same thing, moving an object from one point inside the Universe to another point inside the Universe will not change its overall weight. (For at point A the object no longer exists, it has moved to point B.)

If space and time are part of the same thing, then by moving an object from one "when A" in the Universe to another "when B" in the Universe should not change its overall weight, for it ceases to exist in one of the "when A", it has moved to "When B"

The Universe contains both time and space. They are both parts of the Universe. The Universe as a whole. All of time, All of space, contained in one Universe. Again, changing locations in this model of the Universe should not change its weight

But that brings up another interesting question. Two objects cannot occupy the same space at the same time.

As silly as it sounds, it also means that two objects cannot occupy the same space (mass would indeed double)

And it means that no two objects can exist at the same point of time. (you cannot overlay a future you over a past you.
There would have to be two of you. You exist in both time and space, and your true location in the Universe is based on this description, so two of you would not be able to exist in the same time, because since space and time are part of the same thing, the same time means the same space.)

If space and time are part of the same thing, then that right there could stop you in your tracks (at least from traveling within your own lifetime)


I guess I will close by saying that any instantaneous movement in space causes an equally instantaneous movement in time.

But in order for that you need to be FTL

Star

 

[mabus]

I think the problem has more to do with how our brains are hardwired. The very hard wiring which causes us to experience conciousness also makes us percieve the passage of time as if it were an object you travelled through, something like a tunnel you go into and out of.

Perhaps a helpful analogy is to consider the difference in perspective between walking up and down a staircase and remaining stationary on a moving escalator. As the universe expands we are in a sense, stationary in time, stuck in the ever present "NOW", but the apparent motion we percieve because of our sense organs, gives us the illusion that we are somehow travelling through time as if time itself was stationary and it was we who were travelling backwards and forwards. Something to consider.

 

[Fallingstar1971]

OK Lets let logic run a bit farther...........

If I instantaneously transport you 1 ly away, you will see one ly into the past when you look back at Earth. (I will use my Einsteinian Compensator to cancel out relativity for your transport. Fictional yes, but I need to get you out there FAST)

NOW how could I beam you back? You, looking at me, are seeing one year into the past. From your new POV. I havent perfected my transporter yet and the only way home for you is up to you.

So lets say, that you has a light drive that can go c. (Invented and tested before Einsteinian Compensator, unable to break the light barrier, but it can go c, but thats ALL it can do)

It will still take you one year to get home. You could conceivably arrive right after I transported you (from my POV) Even though you just traveled for an entire year to get back.

So even if I perfected the transporter, the Universe as a whole entity (1 universe=1 universe) would balance itself out

But the above scenario seems to perfectly demonstrate why FTL is so unrealizable. Once you started you trip, you could never return. Every jump takes you farther and farther into the past. Or does it?

So you jump out, you are one year into the past. You turn around and jump back. When you arrive, is it your "time" (from your POV) or are you still one year into the past?

Every light year you jump away is one more year into the past you are going

Reverse logic says that for every light year closer I get to Earth, I should be traveling forward in time (If I stop every light year for course corrections or whatever, I will progressively see another year pass)

Ok. SO now I can get you out there, and you can get back, and Time SEEMS to be behaving, so let me add just one more thing.

If we set a course with our Supership, (say, to Proxima Centauri) As you travel away from Earth, going into the past, for every year you go back in time on Earth, you are going FORWARD in time one year on Proxima.)

So even though your going backward in time to one place, your also going forward in time according to another place, and this is all happening AT THE SAME TIME.

So again, the Universe balances itself out.

I really dont think we could destabilize the Universe. Whatever you invent, whatever you do.

We are products of the Universe. All of us. Energy cannot be created or destroyed, only transformed. Our energy came from the big bang. The energy for our space ship came from the big bang. All of this energy was accounted for in the beginning.

Every power source, all of it is but a tiny fraction of this accounted energy.
We ourselves are but a tiny fraction of that same energy

So no matter how much energy we release, the Universe as a whole can and will balance itself out. Only another Universe feeding energy to or taking energy from us could upset that balance.

Not even time travel itself can break it. As I just described, the Universe will always arrange itself so that it will balance.

Or would it be more accurate to say that no matter what we do, the Universe will adjust Reality so that everything balances out and all laws are obeyed?

Star

 

[mabus]

OK Lets let logic run a bit farther...........

If I instantaneously transport you 1 ly away, you will see one ly into the past when you look back at Earth. (I will use my Einsteinian Compensator to cancel out relativity for your transport. Fictional yes, but I need to get you out there FAST)

NOW how could I beam you back? You, looking at me, are seeing one year into the past. From your new POV. I havent perfected my transporter yet and the only way home for you is up to you.

So lets say, that you has a light drive that can go c. (Invented and tested before Einsteinian Compensator, unable to break the light barrier, but it can go c, but thats ALL it can do)

It will still take you one year to get home. You could conceivably arrive right after I transported you (from my POV) Even though you just traveled for an entire year to get back.

I do not believe this is correct. Let's replace "light" with a tennis ball.

Say you somehow magically transported me 1 light year away. If I had a light drive I would travel at C and left there right now, it would take me 1 full year to get here. If I left right now, I would not get back until next year, not "right after" you transported me. Remember to travel 1 Light year at C, it takes 1 full year.

But the above scenario seems to perfectly demonstrate why FTL is so unrealizable. Once you started you trip, you could never return. Every jump takes you farther and farther into the past. Or does it?

It doesn't. You don't really go into the past. Let me try to demonstrate what I mean. Instead of light, let's deal with tennis balls which we are more familiar with intuitively.

Let's assume I am 1 light year away from here and I remain there. I throw a tennis ball at you at 186,000 miles per second and because of the vaccum of space, let's assume it never slows down. The ball will take 1 year to reach you. It has not travelled into the past in any way, and you will not recieve the ball for a full year. There is no time travel in this.

In a similar way, if I am 1 Light year away from you and travel towards you, I am not time travelling either.

We're taught to think of distant light as a bit of a time machine, in that it allows us to see into the past. We see the light as it was when it left a distant object, in the past, but reference to a time machine is more of in the poetic sense, and in an effort to help the uninitiated grasp the usefulness of viewing time that comes to us from distant objects and how it shows us how the objects once were. Don't confuse that for time travel in the ordinary sense of time machines.

 

[darkmatter4brains]

Say you somehow magically transported me 1 light year away. If I had a light drive I would travel at C and left there right now, it would take me 1 full year to get here. If I left right now, I would not get back until next year, not "right after" you transported me. Remember to travel 1 Light year at C, it takes 1 full year.

I think there's some confusion on this if I am reading you guys correctly. It would take ONE full year in the Earth frame, but it would take ZERO years in the frame moving at C.

You can get anywhere you want in the Universe as fast as you want to get there (IN YOUR FRAME) by getting your speed closer and closer to C. Good luck achieving the energy requirements though.

I mentioned this over in the Unexplained earlier today - an equation that is often called the "rocket equation" in Special Relativity, will give you the speed (measured in fractions of C) you need to travel at, to go any distance in the Universe, in whatever amount of time you desire, once again, as measured in your frame.

In fact, the Lorentz transformation equations "hint at" ZERO distance and ZERO time between any two events for a frame traveling at C. But, really, the equations kind of break down at C, so you can only legitimately look at them in the limit of approaching C, which does trend towards zero.

This is why they say a photon ( or any massless particle ) does not "sense" the passage of time. This is basically the logic that was used to determine that neutrinos do in fact have a small mass.

 

[Fallingstar1971]

OK let me try this another way

Proxima 4.7 ly away
Earth 0 ly away

OK we see Earth as it is now and Proxima as it was 4.7 years ago.

I teleport you 1 ly away. You take some readings.

Earth appears as it did 1 year ago
Proxima appears as it did 3.7 years ago

I teleport you 2 ly away. You take some readings

Earth appears as is did 2 years ago
Proxima 2.7 years ago.

As you teleport away, Each time you re-appear 1 ly farther away and one year back in time (looking back at Earth)
Now I teleport you straight to Proxima, and send another person out into space on the same course.

From Proxima, Earth appears as it did 4.7 years ago.
I teleport new guy one light year away

You look at new guy, he just moved forward in time one year. (from your point of view) I look at the new guy, he has moved backward in time (I see him one year older from earth, his light is a year old) Now you see him 3.7 years into the past. Once he arrives, he will catch up to your present.

Now, I teleport him out one ly. It took no time for him to get there at all. He turns around and turns on his light drive.
Now remember, from his point of view, he is one year into the past. After all, he is seeing Earth as it was one year ago. If he (not me) teleported to Earth from his location,(not mine) he would arrive one year in the past. But since he has to use his light drive, he wont arrive for another year, theraby catching him up to the time he left.

My point being is that I never sent him through time at all. I changed his spatial co-ords and by doing so, moved him through time. The trick is how quickly you can change your location in space

Star

 

[Kessy]

I think the problem has more to do with how our brains are hardwired. The very hard wiring which causes us to experience conciousness also makes us percieve the passage of time as if it were an object you travelled through, something like a tunnel you go into and out of.

I think Mabus is right, the problem with this idea is that our hard wired perception of time is simply not applicable in situations like this. I think when most people try to think about space time as a unified thing, we think of it as if we were an observer outrside of space time looking at the whole of it at once. But that kind of implies that the observer has their own flow of time, independent of our universe's. Now it's entirely possible that such a "hypertime" does exist, but we currently have no reason to think that it does, and I don't think it should be necessary to understand our spacetime.

But this is getting a bit off topic. On the original question of entropy and time travel, my understanding is that entropy and the progression of time are generally considered equivalent to each other, so I suspect that time travel would of necessity break the second law. Regardless, if time travel is possible, it would require a big rewrite of physics as we currently understand it

As for the question about FTL and time travel, in special relativity, the two are the same thing. Anything traveling faster then light would appear to be moving back in time to some observers. Strangely enough, it would also appear to have an imaginary mass (as in the square root of a negative number) What that means is anyone's guess...

 

[darkmatter4brains]

OK let me try this another way

Proxima 4.7 ly away
Earth 0 ly away

OK we see Earth as it is now and Proxima as it was 4.7 years ago.

I teleport you 1 ly away. You take some readings.

Earth appears as it did 1 year ago
Proxima appears as it did 3.7 years ago

I teleport you 2 ly away. You take some readings

Earth appears as is did 2 years ago
Proxima 2.7 years ago.

As you teleport away, Each time you re-appear 1 ly farther away and one year back in time (looking back at Earth)
Now I teleport you straight to Proxima, and send another person out into space on the same course.

From Proxima, Earth appears as it did 4.7 years ago.
I teleport new guy one light year away

You look at new guy, he just moved forward in time one year. (from your point of view) I look at the new guy, he has moved backward in time (I see him one year older from earth, his light is a year old) Now you see him 3.7 years into the past. Once he arrives, he will catch up to your present.

Now, I teleport him out one ly. It took no time for him to get there at all. He turns around and turns on his light drive.
Now remember, from his point of view, he is one year into the past. After all, he is seeing Earth as it was one year ago. If he (not me) teleported to Earth from his location,(not mine) he would arrive one year in the past. But since he has to use his light drive, he wont arrive for another year, theraby catching him up to the time he left.

My point being is that I never sent him through time at all. I changed his spatial co-ords and by doing so, moved him through time. The trick is how quickly you can change your location in space

Star

Star,

The only problem I see here - assuming I understand you correctly, which I may not be - is that your scenarios seem to depend on visualization based on the finite speed of light. For example, the Earth appears one year in the past because light takes one year to get to your new position.

Relativistic effects, as far as time dilation and lorentz contraction, have nothing to do with that. They don't come about from "when light hits your eyes". Just because the sun appears 8 minutes in the past, as viewed from Earth, doesn't mean it's really 8 minutes into our past. That's just a visual effect, due to the finite speed of light, and really has nothing to do with relativity or time dilation.

The true relativistic observer, as taught in most classes, isn't even a human. It's an infinite lattice work of rods and clocks that are synchronized in such a way to take out any such effects as mentioned above.

 

[LKD]

I would like to add something if I could on my own beliefs and understandings which is slightly off topic but inline with some of the examples. At least I hope. Forgive me for posting this little bit of an absurd idea.

I think that we already time travel backwards. Through memories. Nature is amazingly efficient, and being able to re-live events is the best way to use information instead of storing it by some physical means. I could think of quite a few examples of this, but it is a side thought to this topic.

In regards to entropy (ignoring my questionable thought processes) is it not unreasonable that you need to expend energy in order to receive energy, and vice versa, to maintain the balance? That assuming time travel is possible, you would need to loose as much, or more energy to entropy, as you would gaining that from another future or past source? That if the target time and space is not capable of expending the energy to travel, that you can not move through time in any way.

 

[bouchard]

ok I have a ? regarding this subject. I have had a heated debate w/ my husband regarding time travel & entropy & I am far from an expert on any science subject, which admittedly is my husbands strong suit, but I am having a hard time wrapping my mind around the easy dismissal he keeps giving my time travel argument. Isn't matter, or lack there of, a major issue with the theory of time travel. For instance if I burn a fire log, thereby changing matter/ majority of the log adds to universal entropy (which is no longer "usable" matter therefore no longer "transformable" matter). Then if time travel were possible and I went back to the moments before I began to burn the log then that log (having been changed to "heat") would no longer be there? Right? If the matter at any time has been transformed into unusable(un-transformational) matter then it is impossible for that matter to return to the state in which it was??? Meaning a time traveler who did travel back in time would never "encounter" anything because all lost matter could not transform back or rather each and every minute change (which all cause some amount of matter to become heat) would void out the existence of "the past" to which the traveler may want to arrive at? I realize I am not scientifically inclined but I am having a hard time grasping how anything that has been "used" is suppose to exists as it was before it was "used". My husband has told me one is not related to the other but has not been able to sufficiently explain to me how they are not codependent. So can someone please tell me where I am wrong or what I am missing?? If I am way off -be kind- I realize my grasp of science is sketchy.


Thanks

J Bouchard

 

[Jerromy]

how anything that has been "used" is suppose to exists as it was before it was "used".

Nothing as far as scientific fact is concerned is ever "used", it is merely changed. When matter is changed to energy the same amount of energy con concievably be returned to the state of matter. Far more difficult to understand that a log of wood could be "un-burned" but simpler examples of chemical reactions show that when a reaction occurs creating energy, the same amount of energy can be used to return the chemicals to their prior states. I can't think of a specific example at the moment but it isn't the same as the theoretical time travel situation.

As a thought experiment let's assume we have a bubble which is isolated from the normal flow of spacetime. As you sit watching your fireplace with the burning log, then click your magical rewind button, you can witness the smoke and flames flowing back into the log as it absorbs heat from the room. Nothing is gained or lost at any given moment only changed. The conflict which occurs in the idea of time travel within your bubble is how can you go backward in time and witness a log "un-burn" when you have some of the heat it produced inside of your bubble? The universe NEVER creates anything from nothing and nothing is ever lost. If you tried to go back in time something from your destination would have to be exchanged that has the same amount of energy to keep the balance.

 

[bouchard]

Ok well I guess I have a serious misunderstanding. I thought once energy has been converted(used, transformed...) into heat that such a transformation was the final one or rather that the state of energy as heat was no longer transformable. Under such a misunderstanding it is clear I would not get the time travel concept correctly.