Time travel

  • Thread starter Tony.K
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Hi, I want to know from proffesionals if time travel is physically possible.

Thanks,
Tony
 

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  • #2
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I'm not a physicist, but I am going to say no. In my opinion, time is an unchangeable constant and its only because we have found a way to quantify it, that some people believe we can manipulate it.
 
  • #3
WarPhalange
As far as people can tell now, no, it's not possible.
 
  • #4
Nabeshin
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The fact that our world is not flooded by time travelers or otherwise destroyed by their presence seems to suggest that it is impossible. Similar to fermi's paradox, if you ask me.
 
  • #5
rbj
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time travel in a single direction (toward the future) at a rate of 1 second into the future per second of travel time, that is possible since it is what we commonly do now ("Time keeps on slipping, slipping, slipping into the future..."). if you get a fast enough spaceship and a helluva lot of energy, you can travel (one way) even farther into the future by use of time dilation established in special relativity.

travel to the past? not possible.
 
  • #6
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Oh, you mean the THEORY of special relativity?
 
  • #7
cristo
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Oh, you mean the THEORY of special relativity?
Please don't start discussing what I think you're hinting towards by bolding the word "theory."
 
  • #8
There are many ideas on how to time travel.

The simplist one allows you only to travel forward, into the future. It involves you travelling in a spaceship at speeds approaching but never reaching C. The closer your speed is to C, the more time dilates. In layman's terms, the faster you go, the slow time goes for you inside your spaceship. So you could go for a leisurely spaceflight for a year, and come back to Earth to find out that 30 years have passed. This same effect can be achieved if you use your spaceship to hover outside of a massive blackhole.
 
  • #9
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I'd argue that time doesn't really go forwards or backwards at all, it just is. Mass/energy and movement is all that matters. To 'go back in time' you need to move everything back to exactly the way it was, which could take some work.

About 'time dilation', could someone explain why we even call it that? To my knowledge the phenomena could be explained by simply saying that the faster an object goes, the slower it does its atomic processes. I don't see a need to actually take the 'time is relative' part into the equation. A process that might take 1 second at 'normal speed' could take 2 seconds at a higher speed; just means the process went slower, not that *time* did. Simple, and less confusing than mixing in time as a non-constant.
 
  • #11
russ_watters
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I don't see a need to actually take the 'time is relative' part into the equation. A process that might take 1 second at 'normal speed' could take 2 seconds at a higher speed; just means the process went slower, not that *time* did.
If every physical process experienced exactly the same time dilation, how would you differentiate between physical processes slowing down and time itself slowing down? And why would it not be easier to unify it under the heading of "time dilation" and consider it to be a single concept?

In any case, it is a necessary part of the equation because the equation doesn't work without it. If we concluded that clocks run slow but time doesn't, that would mean our measurements of the speed of light are wrong - but our measurments agree with SRT.
 
  • #12
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My understanding is that closed timelike curves (travel to your past) are compatible with General Relativity, but the solutions usually require exotic matter.
 
  • #13
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If every physical process experienced exactly the same time dilation, how would you differentiate between physical processes slowing down and time itself slowing down? And why would it not be easier to unify it under the heading of "time dilation" and consider it to be a single concept?
Well, maybe it would, I just feel it's sort of misleading. I'd rather we kept the term 'time' as a global constant, i.e. always from the observer's pov, instead of making it a local variable. Maybe just renaming it to something which doesn't include the word 'time'? *shrug*

russ_watters;1955086[/quote said:
If we concluded that clocks run slow but time doesn't, that would mean our measurements of the speed of light are wrong - but our measurments agree with SRT.
I don't follow. What would this change? The speed of light is correct in relation to how we measure it, taking into account the movement our planet( not to mention solar system and galaxy ). But since it's all relative, moving to another galaxy would make the speed of light faster or slower since the clocks we measured it with over there would experience a different 'time dilation'. Here it might be 300,000,000'ish meters a second, there it might be 310,000,000'ish. Not because the light moved faster, but because the atomic processes in the clock moved faster, causing the 'second' to be shorter.

I'm not sure why concluding that 'clocks run slower but time doesn't' would change anything.
 
  • #14
ZapperZ
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Well, maybe it would, I just feel it's sort of misleading. I'd rather we kept the term 'time' as a global constant, i.e. always from the observer's pov, instead of making it a local variable. Maybe just renaming it to something which doesn't include the word 'time'? *shrug*

If we concluded that clocks run slow but time doesn't, that would mean our measurements of the speed of light are wrong - but our measurments agree with SRT.
I don't follow. What would this change? The speed of light is correct in relation to how we measure it, taking into account the movement our planet( not to mention solar system and galaxy ). But since it's all relative, moving to another galaxy would make the speed of light faster or slower since the clocks we measured it with over there would experience a different 'time dilation'. Here it might be 300,000,000'ish meters a second, there it might be 310,000,000'ish. Not because the light moved faster, but because the atomic processes in the clock moved faster, causing the 'second' to be shorter.

I'm not sure why concluding that 'clocks run slower but time doesn't' would change anything.
You have a severe misunderstanding of Special Relativity and time dilation.

You can move to anywhere, and the speed of light would STILL be measured the same! That's the main postulate of SR, that in ANY inertial reference frame, each observer in that frame will ALL measure light be moving at c. No exception! In fact, two observers in two different frames sees the SAME light moving at c in each of their own frame!

What this means is that the "time dilation" that is being seen is what YOU see of the time in the OTHER frame. You see no change in your time, no matter where you are, and no matter in which frame you are in! It is only from the point of view of an observer in a DIFFERENT frame is your time slower, not from your own observation of your own time!

You are seeing the clock on alpha centauri to run slower, while the creature on alpha centauri sees your clock as moving slower. That's it.

Zz.
 
  • #15
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If we concluded that clocks run slow but time doesn't, that would mean our measurements of the speed of light are wrong - but our measurments agree with SRT.
I don't see how? Whether time becomes dilated or the clock itself actually operates slower is only relative.

I'd argue that time doesn't really go forwards or backwards at all, it just is. Mass/energy and movement is all that matters.
I completely agree. In my mind we have absolutely no way of measuring "time" itself just the processes that happen in a chronotomic (yeah, I made that word up) order. The fact that time can change by any means is to me highly illogical, although I do understand the concept of time dilation and the reasoning behind it. While I do comprhend time dilation and how it relates to the speed of light, I can not physically comprehend how time can not be constant given any frames of reference.

You are seeing the clock on alpha centauri to run slower, while the creature on alpha centauri sees your clock as moving slower. That's it.
EDIT: Never mind, I figured it out.
 
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  • #16
DaveC426913
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I completely agree. In my mind we have absolutely no way of measuring "time" itself just the processes that happen in a chronotomic (yeah, I made that word up) order. The fact that time can change by any means is to me highly illogical, although I do understand the concept of time dilation and the reasoning behind it. While I do comprhend time dilation and how it relates to the speed of light, I can not physically comprehend how time can not be constant given any frames of reference.
It is important to understand that there is no "absolute frame of reference" whereby we could ever, even in principle, determine whether time is slow or fast. Frames of reference and their measurements of time are only relative to each other.
 
  • #17
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You have a severe misunderstanding of Special Relativity and time dilation.

You can move to anywhere, and the speed of light would STILL be measured the same! That's the main postulate of SR, that in ANY inertial reference frame, each observer in that frame will ALL measure light be moving at c. No exception! In fact, two observers in two different frames sees the SAME light moving at c in each of their own frame!

What this means is that the "time dilation" that is being seen is what YOU see of the time in the OTHER frame. You see no change in your time, no matter where you are, and no matter in which frame you are in! It is only from the point of view of an observer in a DIFFERENT frame is your time slower, not from your own observation of your own time!

You are seeing the clock on alpha centauri to run slower, while the creature on alpha centauri sees your clock as moving slower. That's it.

Zz.
I realize you don't notice the dilation in your own time, but let me try an example so I can see where my understanding is wrong. If you're in a spaceship doing 99% lightspeed for 1 second ( earth-observer time ), you, as a passenger in the ship, will only register a fraction of that time( though to the passengers everything appeared normal ), correct so far? The earth clock goes 1 second, while the ship's clock goes, let's say 0.1 second to make it simple.

Then, if both the Earth and the Ship started measuring the same photon immediately after the ship went to near-lightspeed, they both would have registered that the photon travelled 299,800,000'ish meters. However, the earth-clock would say the photon travelled that distance in 1 second, while the ship clock would say it travelled the distance in 0.1 second.

This is incorrect?
 
  • #18
DaveC426913
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I realize you don't notice the dilation in your own time, but let me try an example so I can see where my understanding is wrong. If you're in a spaceship doing 99% lightspeed for 1 second ( earth-observer time ), you, as a passenger in the ship, will only register a fraction of that time( though to the passengers everything appeared normal ), correct so far? The earth clock goes 1 second, while the ship's clock goes, let's say 0.1 second to make it simple.

Then, if both the Earth and the Ship started measuring the same photon immediately after the ship went to near-lightspeed, they both would have registered that the photon travelled 299,800,000'ish meters. However, the earth-clock would say the photon travelled that distance in 1 second, while the ship clock would say it travelled the distance in 0.1 second.

This is incorrect?
It is.


For one, you must recognize that BOTH frames of reference are EQUALLY VALID. The Earth-bound measurement is no more "right" than the spaceship-bound measurement. Realize that the Earth is just a giant spaceship and, like the little spaceship, it has no "absolute velocity". You cannot say that "the Earth is stopped while the spaceship is moving". This is simply not true.


As for your experiment, distances in the spaceship's frame of reference are compressed. The photon will not need to travel 300,000 metres, its emission point and its absorption point will be measured at only 30,000 metres, thus it only takes 0.1 second to travel that far.
 
  • #19
WarPhalange
It is incorrect. They would both measure it going at exactly the speed of light.

But, this is exactly why time dilation happens. In order for both people to measure the photon going at the same speed, despite both of the people going at different speeds, means that time has to be experienced differently for them.

So the guy going standing still sees the photon moving at c and sees the other guy moving at 0.99c After 1 second, the photon moved a distance of c*1s and the ship moved a distance of 0.99c*1s. All is well so far. Now, the guy in the moving ship should therefore see a slower photon, right? Just like how you and another car are driving at 60mph it looks like you're both just standing still from your point of view? But the postulate (or whatever you want to call it) is that the photon moves at c always. So what else can change? Well, if the moving guy measures the photon for a second, he'd normally see it only move a distance of 0.01c*1s, right? So, what has to happen is that instead of measuring it for 1 second from the point of view of the guy standing still, it would only be 0.01 second, therefore the guy on the ship would still measure c.

You see? Time dilation comes from a constant speed of light, not the other way around. Here's something that can explain it a bit better:
http://en.wikipedia.org/wiki/Time_dilation#Simple_inference_of_time_dilation

One thing I haven't been able to resolve is that if you have a guy standing still and another guy going 0.5c, time is experienced slower by the guy going 0.5c, right? But in that guy's point of view, you are going at 0.5c, so you should have the slower time. What gives? Is it because one of them accelerated more than the other some time in the past, ergo that is the one who is "moving" relative to the other one?
 
  • #20
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One thing I haven't been able to resolve is that if you have a guy standing still and another guy going 0.5c, time is experienced slower by the guy going 0.5c, right? But in that guy's point of view, you are going at 0.5c, so you should have the slower time. What gives? Is it because one of them accelerated more than the other some time in the past, ergo that is the one who is "moving" relative to the other one?
I believe SRT states that you and the other guy would both percieve each others times as going slower than the ones measured in your own reference frames (which you have "normal" time according to you).

Back to the OP's original question, how is it that according to SRT, that you can actually give time a positive time dilation?
 
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  • #21
DaveC426913
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One thing I haven't been able to resolve is that if you have a guy standing still and another guy going 0.5c, time is experienced slower by the guy going 0.5c, right? But in that guy's point of view, you are going at 0.5c, so you should have the slower time. What gives? Is it because one of them accelerated more than the other some time in the past, ergo that is the one who is "moving" relative to the other one?
No, what happened in their past is not relevant. Yes, they both observe the other one moving slower. The apparent paradox is resolved when you realize that simultaneity of events is also relative.

For one, they cannot directly compare notes. First, they would have to get together, meaning one or both of them would have to undergo acceleration to match speeds.

For two, they would disagree on the timing of events. Having both observed some objective experiment (such as turning on a flashlight aimed at a target) they would have different accounts of how long certain events took and what distances were measured.
 
  • #22
George Jones
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Hi, I want to know from proffesionals if time travel is physically possible.

Thanks,
Tony
For an excellent, non-technical reference, have a look at the second edition of Time Machines: Time Travel in Physics, Metaphysics, and Science Fiction by Paul Nahin. This is a wonderful book that is written for the educated layperson.

Physicist (and relativist) Kip Thorne wrote a foreword for the second edition of this book, and here's a quote from this foreword: "It now is not only the most complete documentation of time travel in science fiction; it is also the most thorough review of serious scientific literature on the subject - a review that, remarkably, is scientifically accurate and at the same time largely accessible to a broad audience of nonspecialists."
DaleSpam said:
My understanding is that closed timelike curves (travel to your past) are compatible with General Relativity, but the solutions usually require exotic matter.
While exotic matter can used as a source for spacetimes that have closed timelike curves, exotic matter is not required. Examples that don't use exotic matter include those given by Godel, Tipler, and rotating black holes.
Topher925 said:
Back to the OP's original question, how is it that according to SRT, that you can actually give time a positive time dilation?
The OP's original question is not restricted to the framework of the the special theory of relativity.
 
  • #23
WarPhalange
For two, they would disagree on the timing of events. Having both observed some objective experiment (such as turning on a flashlight aimed at a target) they would have different accounts of how long certain events took and what distances were measured.
I see, so if both of them were somehow looking at the other guy (say actually just a projected image towards the observer) to see who dies first of old age, and they both happened to kick the bucket at the same time, they would both see themselves die first because light from the other guy hasn't reached them yet. Is that right?
 
  • #24
DaveC426913
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I see, so if both of them were somehow looking at the other guy (say actually just a projected image towards the observer) to see who dies first of old age, and they both happened to kick the bucket at the same time, they would both see themselves die first because light from the other guy hasn't reached them yet. Is that right?
It's a bit more complex than that; it's not simply the light lag; you could likely revise the experiment to factor that out (for example, by having the two spaceship fly past each other with little distance).

There isn't really a simple answer. But ultimately, yes it has to do with the fact that each observer's observance of events is unique to his frame of reference, and will appear to conflict with other FoRs (before SR is factored in).
 
  • #25
WarPhalange
Okay, thanks. Lastly, I've seen this in my own Modern Physics textbook, the whole ladder and barn paradox. It says there is no unique solution. I call BS on that. I can imagine that there will be some sort of experiment for that in the future, since it's not complicated, just takes a lot of resources. So what would happen?
 

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