Time Travel: What are the Theories & Factors?

[Theseus]

There has been much discussion of time travel, but I haven't yet found an answer to a question I have. That is, what are the specific theories or factors that allow for the possibility of time travel?

The issue has been discussed and debated many times on this forum and I'm not so much interested in rehashing that here. I would like to know what are the basic scientific facts or principles that allow for this topic to even be considered as a possibility.

Can someone provide a succinct answer?

 

[rede96]

There has been much discussion of time travel, but I haven't yet found an answer to a question I have. That is, what are the specific theories or factors that allow for the possibility of time travel?

The issue has been discussed and debated many times on this forum and I'm not so much interested in rehashing that here. I would like to know what are the basic scientific facts or principles that allow for this topic to even be considered as a possibility.

Can someone provide a succinct answer?

Time travel into the future - This has been proven. Basicalyly if are moving realtive to someone else, you will age slower then those you leave behind. If you travel near the spead of light, a year for you could be many years for those not traveling relative to you.

Time travel into the past - No proof, just many theories.

 

[ract]

Time travel into the future - This has been proven. Basicalyly if are moving realtive to someone else, you will age slower then those you leave behind. If you travel near the spead of light, a year for you could be many years for those not traveling relative to you.

If I travel in a straight line I would arrive at a point quicker than if I traveled in a direction other than that straight line. By the time I arrived from taking the scenic route my other self would be the same age, but could be justified in saying I was younger than you when I arrived, however ageing happens at the same rate.

 

[rede96]

ageing happens at the same rate.

No, not necessarily.

If humans only lived to the age of 70 and then died, we would all experience 70 years of life.

However, if you and I are the same age, say 40, and you stay here on Earth while I travel around the galaxy for about 10 years at 0.95 of the speed of light, when I returned I would 50, but you would be dead! (by about 2 years)

 

[ract]

I am sorry but this is inaccurate

All you are saying is that during the time I was 'stationary' you traveled vast distances, and because your concept of time is associated with the distance light travels through space in a given period, you confuse distance traveled with ageing.

How could you be sure that I had not moved through space quicker than your 0.95 the speed of light? Firstly there is the rotation of the Earth on its axis at x speed, then the rotation of the Earth around the sun, added to that is the rotation of our solar system within the milkyway, plus the rotation of the milkway itself, furthermore our local group of galaxies is moving through space... I could go on

So, because you traveled in a straight line close to the speed of light, and as light, for you, is associated with time, you have ended up where you are. My concept of ageing is associated with the average number of beats of the mammal heart, and so distance traveled has nothing to do with ageing.

 

[phinds]

I am sorry but this is inaccurate

You should read up on this before you make firm statements that are not correct. Read about the twin paradox.

 

[bcrowell]

Here is a nice article that is not overly technical: http://plato.stanford.edu/entries/time-travel-phys/

There is a good popular-level treatment in Thorne, Black Holes and Time Warps, ch. 14.

Here is a recent and detailed, but pretty technical, article: http://philsci-archive.pitt.edu/4240/

This may be helpful: http://en.wikipedia.org/wiki/Chronology_protection_conjecture

Theseus, the most effective answer for you is going to depend on your background. Could you tell us about your background in math and physics? Have you had calculus? What books on relativity have you read?

 

[nitsuj]

My concept of ageing is associated with the average number of beats of the mammal heart, ...

That's just bizarre. (i'd guess it is more literally cells in your body becoming a copy of a copy of a copy of a copy...peak balance of freshness/coolness is in your 20's)

I'd guess it's not remotely about "traveling vast distances". It's the speed of travel relative to whatever you measure speed of travel from. the near c speeds the time traveler is going would have to be measured relative to your velocity, which would be the case if he leaves umm, the same planet you are on. Otherwise as you noted, that pesky relativity principle thing pops up and then who knows who's coordinates? (less near c speeds)

 

[DaveC426913]

ract, your claims are odd at best.

Relativistic time dilation is quite real. If you took a relativistic trip around the galaxy and returned, you would indeed be much younger than anyone on Earth.

But that is an extreme. Relativistic time dilation happens on any distance and time scale down to subatomic particles in accelerators here on Earth.

 

[edgepflow]

In my opinion the central test of backward time travel for physics would be if different times coexist. For example, are the events of 1988 still in existence? This is the "block" model of the universe in which all events past, present, and future coexist. I don't think anyone knows a practical way to test this at this time.

So, in principle, if you traveled back 20 years, met your younger self, and both of you later stepped back in time, you could accumulate an arbitrarily large number of copies of yourself. If this number is 2, 10, 1000, or 10^34, they always existed at this slice of time.

My gut feeling is this is not how the universe works. But there is no conclusive proof it is impossible. There are some GR solutions that allow backward time travel (spinning cosmic string, Tipler Cylinder, worm hole) but many think they are not physical.

I did not mention parallel universes and alternate realities in this post - and I will this idea fade into the vacuum of space for now!

Here is a reference for the idea of block model of the universe:

Black Holes, Wormholes & Time Machines by Jim Al-Khalili

And here is one for issues of making copies of yourself:

How to Build a Time Machine by Paul Davies

 

[WannabeNewton]

Now you are completely arguing semantics. Do you understand the concept of a reference frame? You seem to keep arguing 'ageing' as some absolute entity. A statement like "...where one is able to traverse that distance at a speed that will come close to catching up with time..." makes no sense at all. What does it mean for speed to catch up with time?

 

[bcrowell]

This thread has drifted toward baseless speculation. I've deleted the last 6 posts. Ract, please pay attention to the messages that show up at the top of your screen telling you why your posts are being deleted.

Folks, this is not a topic on which nothing is known. This is a topic in general relativity that has been intensively studied and about which a great deal is known. Don't post opinions unless you have the relevant knowledge, in accord with PF's rules prohibiting overly speculative posts https://www.physicsforums.com/showthread.php?t=414380 .

If this doesn't clean itself up, I will close the thread.

 

[Cosmo Novice]

Ok so let's be realistic.

Time dilation (Future time travel) would theoretically allow a relativistic traveller to "travel" into the future - I think this has been well established in this post and the concept is clearly defined in SR with regards to the invariance of the speed of light.

Time traveling backwards is an entirely different story. I think any sort of "time travel" related to structure of the manifold of the internal U and physical light cones can only ever be theoretical anyway - we won't be building light speed ships soon! That being said if we assumed wormholes did exist (spacetime wormholes not just spatial) then the Novikov self consistency principle has been proven to hold to prevent causal violations. Also as I understand it, FTL is theorized to create causal violations.

All highly speculative but I recommend the original poster looks into CTCs Godel metric and the Novikov self consistency principle.

 

[phinds]

... I recommend the original poster looks into ... the Novikov self consistency principle.

Thanks ... I had not seen that before and find it very interesting. Nice to know there's some scientific basis for my gut belief, especially since GR and QM have taught me that my gut beliefs often are not worth anything.

 

[Tanelorn]

I have trouble believing that past and future versions of the universe exist somewhere permanently.
The amount of information would be incredible if there is a version of the universe in perpetual exsitance for every quantised step in time.
Are there time equivalents to the Planck length?

So if a past version of the universe no longer exists I cannot see how it is possible to visit it through wormholes or any other equally speculative method

The only way around it would be a cyclic universe which precisely repeats our own universe but that doesn't seem plausible either.

 

[DaveC426913]

I have trouble believing that past and the future versions of the universe exist somewhere permanently.
The amount of information would be incredible if there is a version of the universe in perpetual exsitance for every quantised step in time.

Yes but your logic doesn't make sense. That information is distributed evenly across a continuous dimension - there's plenty of room for it.

Look at this analogy by removing one dimension. You're essentially saying that you have trouble believing that a brick can have more than width and height. To expect it also has a continuous existence over its entire length is too much.

Not only does it have all the information contained in its width and height, but there is an incredible amount of information for every single width/height point along the brick's entire length. That's a lot information. Surely, only a slice of the brick along its length would be in existence.


No, a continuous dimension - such as length in this case - or time in the case above - can contain as many points in it as are necessary to describe all other dimensions. In a way, that's kind of the definition of a dimension. The fact that the dimension exists at all means it can "hold" a point for all other dimensions (length, width and height, throughout time).

 

[phinds]

So if a past version of the universe no longer exists I cannot see how it is possible to visit it through wormholes or any other equally speculative method
.

Yes, but the POINT of time travel (which I don't believe in) is that you are going back to the only existence that the past had at the time you arrive. You're not recreating it, you're visiting its single existence.

 

[Tanelorn]

Dave, I see your point about time being just another dimension.
You just witnessed my mind in the process of boggling at the thought of one psuedo infinite quantity being multiplied by another psuedo infinite quantity.

I sometimes imagine that the only way such a big universe can exist at all is for it to only exist for the briefest of quantized moments, that way averages remain small. I used to use very short, extremely high power radar pulses :)

 

[Tanelorn]

phinds, I was just sayiing that the past has to still be in existence for someone to visit it, or as you say, the past has to be recreated when one visits it.

 

[DaveC426913]

phinds, I was just sayiing that is has to still be in existence for someone to go to it.

But what does it mean to be "still in existence"? Time is a dimension, like length. Does the left end of a brick case to exist when we are looking at the right end?

 

[rede96]

But what does it mean to be "still in existence"? Time is a dimension, like length. Does the left end of a brick case to exist when we are looking at the right end?

A good point, but I wouldn't say that time is like the other 3 dimensions of L,W and H. For example I can walk up and down the length of a brick without there being any issue with causality. I can't do that with time.

 

[phinds]

A good point, but I wouldn't say that time is like the other 3 dimensions of L,W and H. For example I can walk up and down the length of a brick without there being any issue with causality. I can't do that with time.

Yes, I agree that you can't because I don't believe in time travel, BUT once again, the POINT of the concept of time travel is that you CAN move in time.

Because, apparently, you share my disbelief in time travel, you seem to simply refuse to consider the CONCEPT of time travel. The concept of time travel is that you DO travel in time and you visit the past back when it existed, you do not recreate it and it doesn't have to stay in existence in order for you to revisit it.

EDIT: OOPS ... I seem to have meshed two different posts and responded as though they were from the same poster. Sorry about that.

 

[Cosmo Novice]

A good point, but I wouldn't say that time is like the other 3 dimensions of L,W and H. For example I can walk up and down the length of a brick without there being any issue with causality. I can't do that with time.

According to the self constincency principle (at least mathematically) is is impossible to violate causality, as the causality has already proven to be self consistent with observation.

...information is distributed evenly across a continuous dimension - there's plenty of room for it.

Would I be correct in saying that when we are talking about an extended continuum with regards to "points" in time that we are actually discussing frames of reference? The visitation to a "point" in time must be a point valid in another present reference frame?

As photons are the universes information carriers, and all frames of reference depend explicitly (in the physical sense) on c for infomation propagation, then time travel is entirely dependent on frame of reference. Faster than light travel in theory can be used for time travel - however this is entirely a thought process and has no bearing on the physical world, in fact it would cause causality violations.

 

[rede96]

Yes, I agree that you can't because I don't believe in time travel, BUT once again, the POINT of the concept of time travel is that you CAN move in time.

I agree but with one slight difference. I believe that we can experience time passing at different rates, but not in 'Time Travel' per se. I mean in the sense that there is a way I can instantly jump to a future or past.

Because, apparently, you share my disbelief in time travel, you seem to simply refuse to consider the CONCEPT of time travel.

I guess that wasn't aimed at me, but just to clarify, I don't believe in time travel as stated above. There is obviously no proof that one can jump to a past or future yet and I haven’t been able to reconcile the causality issue in my own mind either. However I am still open to it.

 

[phinds]

I agree but with one slight difference. I believe that we can experience time passing at different rates, but not in 'Time Travel' per se. I mean in the sense that there is a way I can instantly jump to a future or past.

OK, I guess I see the distinction. You are distinguishing out "time travel" as not being what you mean because you mean instantaneous movement which, I guess, you don't define as travel. Since I don't believe in instantaneous travel OR time travel, I'm good either way.

I guess that wasn't aimed at me ...

Yes, thus my OOPS at the end of my post

 

[DaveC426913]

A good point, but I wouldn't say that time is like the other 3 dimensions of L,W and H. For example I can walk up and down the length of a brick without there being any issue with causality. I can't do that with time.

Causality in space is the same as causality in time.

You cannot have cause/effect events occur at opposite ends of the brick simultaneously - that would be a causality violation.

 

[phinds]

You cannot have cause/effect events occur at opposite ends of the brick simultaneously - that would be a causality violation.

Dave, I'm still muddy on non-locality so I have a question here. I thought that non-locality said that you not only can but do have simultaneous cause/effect events separated by space, just that you can't send information that way since the "cause" is that you measure a property of one particle and the "effect" is that the property of another particle then instantaneously takes on its characteristics which are determined by the results of your measurement on the first particle. What am I missing here?

Thanks

 

[rede96]

Causality in space is the same as causality in time.

You cannot have cause/effect events occur at opposite ends of the brick simultaneously - that would be a causality violation.

Oh, ok. Sure. You can have causality in time and space. However that doesn't mean they are the same.

I can walk back and forth on the brick for as long as I care without meeting myself. But I could potentially meet myself if I could go back and forth in the time dimension.

So I guess my point was I can't create any causality violation by me traveling back and forth in any of the 3 space dimensions, but I could in the time dimension.

So the time dimension is significantly different than the 3 spatial dimensions and therefore we shouldn’t compare them in order to demonstrate that just because something happens in the spatial dimension, it follows that this may be the case in the time dimension.

 

[DaveC426913]

Oh, ok. Sure. You can have causality in time and space. However that doesn't mean they are the same.

I can walk back and forth on the brick for as long as I care without meeting myself. But I could potentially meet myself if I could go back and forth in the time dimension.

So I guess my point was I can't create any causality violation by me traveling back and forth in any of the 3 space dimensions, but I could in the time dimension.

I think you're missing the point. What I'm saying is that it is just as valid to have a causality violation by linking xyzt with x'yzt as it is linking xyzt with xyzt'.

You keep talking about "walking back and forth" from one end of the brick to the other. But in doing so, you are describing a movement in two dimensions: x and t.

The point is, moving in only one dimension (x) without moving in another dimension (t) can create violations.

 

[ract]

Nonesense!

What like ageing less just because I am traveling through space, that's nonesense!

Expose my point if its nonesense and let's get this over with now!

I am not running away, deleting points that I don't agree with I am willing to discuss!