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 propogation, then time travel is entirely dependant 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!

 

[DaveC426913]

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

Relativistic time dilation has been observed in every test in every particle accelerator in the last half-century. It has been measured to many decimal places. It is one of the most tested and confirmed experiments in all of science.

If you were to disagree, you would have to do better than "nonsense"; you would have to refute an overwhelming preponderance of established test results.
If you wish to continue discussion, phrase your posts as questions, asking why your preconceptions do not match observations. Otherwise, expect infractions and banning. I'm trying to save you here. It is probably already too late. Last chance.

 

[ract]

I am not contending any of the results you mention.

The point here is thus...

The context with which I am using the word 'ageing' refers to the human condition, hence why I keep mentioning 'heart beats'. You merely point toward a concept of time developed only within the field of physics... One example of which is; A light year = The distance light has traveled in the time it takes for the Earth to orbit the sun.

I am stating that when Biology is introduced into this concept it collapses as a valid interpretation of time.

 

[phinds]

Dave,

Don't know if you got caught up in the frustration of trying to deal with ract (I sent Ben Crowell a PM praising his tolerance and patience with this kind of poster and I have to say I admire you equally for it) but at any rate, I asked in several posts back (#27) if you could help me w/ non-locality vis-a-vie one of your statements.

Thanks

 

[DaveC426913]

[deleted off-topic material about the definition of a light year - bcrowell]

So no answer to my Biology point then!

Biology is comprised of atoms. Atoms are subject to time dilation, as are all chemical processes and forms of energy. It is important to understand that there is nothing that is not subject to time dilation. It is indeed, the passage of time itself that is dilated.

 

[DaveC426913]

Dave,

Don't know if you got caught up in the frustration of trying to deal with ract (I sent Ben Crowell a PM praising his tolerance and patience with this kind of poster and I have to say I admire you equally for it) but at any rate, I asked in several posts back (#27) if you could help me w/ non-locality vis-a-vie one of your statements.

Thanks

I saw it. I haven't composed a reply yet. Hopefully, we can get the thread back on track before
a] the thread gets locked and cleaned up
b] ract gets banned
c] both

P.S I am not frustrated. Quite the contrary, I feel like a missionary with a lost soul that might, just might, be saved.

 

[DaveC426913]

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

I'm not really sure what you're getting at.

Two events happening simultaneously at opposite ends of a brick cannot have a cause/effect relationship.

Non-locality does not violate this cause-effect relationship because one event is not the cause of the other.

 

[DaveC426913]

A light-year is the distance light travels in one Earth year

Isn't that exactly what I said?

You seem to have twisted my words to say that I said; a light year is equally in distance to the distance Earth covers in its orbital circufrence.

ract/actr is correct. I misread what he said in post 26:

A light year = The distance light has traveled in the time it takes for the Earth to orbit the sun.

I retract (or should I say aetrctr) the assertion of his ignorance and apologize for that.

Unfortunately, it would not have prevented the banning. ract was banned for breaking the rules many, many times with unfounded assertions and zany ideas. As I said, it was only a matter of time.

 

[phinds]

Non-locality does not violate this cause-effect relationship because one event is not the cause of the other.

OK, that's where I'm confused. I was under the impression that the measurement on the first particle causes the instantaneous setting of the characteristic of the entangled particle. Is that not cause and effect? I AM fuzzy on all this so perhaps it's just the terminology that has me confused. I find non-locality mind-boggling.

 

[phinds]

I saw it. I haven't composed a reply yet. Hopefully, we can get the thread back on track before
a] the thread gets locked and cleaned up
b] ract gets banned
c] both

P.S I am not frustrated. Quite the contrary, I feel like a missionary with a lost soul that might, just might, be saved.

Well, I see ract finally got banned. Gosh I'm shocked, shocked I say. I would never have seen that coming.

I admire your abiilty to take that point of view in the face of someone like ract. I understand it and admire it, but can't really manage it myself. I just get swamped by the frustration and I have a deepseated dislike of people who just seem to not WANT to learn. I'm good with working all day with someone who seems to want to learn but in the face of that kind of willfull disregard for all evidence, I just can't maintain the equinaminity that you and Ben have.

I appreciate any help you can offer on non-locality

 

[DaveC426913]

OK, that's where I'm confused. I was under the impression that the measurement on the first particle causes the instantaneous setting of the characteristic of the entangled particle. Is that not cause and effect? I AM fuzzy on all this so perhaps it's just the terminology that has me confused. I find non-locality mind-boggling.

There is no cause and effect.

If there were, we could use it to signal.

We don't really know how it works (certainly I don't).

 

[DaveC426913]

I admire your abiilty to take that point of view in the face of someone like ract. I understand it and admire it, but can't really manage it myself. I just get swamped by the frustration and I have a deepseated dislike of people who just seem to not WANT to learn. I'm good with working all day with someone who seems to want to learn but in the face of that kind of willfull disregard for all evidence, I just can't maintain the equinaminity that you and Ben have.

He doesn't have access to evidence because he hasn't read up on anything. If he were science-aware yet still willfully ignorant, that would be one thing, but he's not. So all he's ever known are shadows on the cave wall - his whole world. But if someone unshackled him and let him out of the cave, he'd burst into tears at how vast and wondrous the real world is.

 

[phinds]

He doesn't have access to evidence because he hasn't read up on anything. If he were science-aware yet still willfully ignorant, that would be one thing, but he's not. So all he's ever known are shadows on the cave wall - his whole world. But if someone unshackled him and let him out of the cave, he'd burst into tears at how vast and wondrous the real world is.

Again ... admirable.

Perhaps my problem is that I'm too willing to believe that folks like that HAVE made some effort to understand things but have become set in their point of view early on and just aren't willing to change it even in the face of evidence. He certainly didn't seem at all interested in listening to any of us. You and Ben are great role models so I'll try to do better, but at my age, I am myself unfortunately somewhat set in MY ways. I spent a lot of time during my career mentoring programmers and engineers but they all WANTED to learn so it was relatively easy.

 

[rede96]

Well, I see ract finally got banned. Gosh I'm shocked, shocked I say. I would never have seen that coming.

I miss all the fun! :) Seems like I need to stay up later or move to a different time zone lol. But just in case Ract is still reading I thought I’d have stab at answering.

I didn't see all the posts, but I guess what Ract was saying is we all age, at the age we age, in our own frame. This is a biological process and 'moving' doesn't change that.

Our hearts will beat the same number of beats they were intended to beat despite 'moving around' (As an aside, there is an interesting theory that all animal’s hearts beat about the same number of beats. It is just that some beat a lot faster, hence they die sooner.)

What he didn't seem to get was the concept that moving relative to someone else means that the two people will age a different rates relative to each other, (i.e. the time between heart beats will seem to pass differently between the two frames.) but they will age the same in their own frame.

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'.

(Forgive my ignorance Dave, does x' just signify another point on the x-axis or is it also directional, as in -x?. Also, what do youn mean by 'linking'?)

So I may be missing the point, it depends on how I interpret your response. If you were saying that any two events in 4D space time could have a causality violation, regardless of coordinates, then yeah I get that.

In any case, what I was trying to say is that we have to treat the time axis differently the spatial ones and not assume the axis share properties.

EG: I can arbitrarily set direction for xyz. The direction for t is already set. I need to do something to move through xyz, I move through t without doing anything. etc.

So for me I don’t see us living in a 4D universe, as all the dimensions aren’t symmetrical. (Like a 1D, 2D and 3D universe would be without the time element.) I would say we live in a 3D universe with a time element. But that’s just me.

As regards time travel:

If I pick a point in time and space where I am, I can put everything I see in some spatial geometry, where other frames have a value along the xyz relative to me. However, other frames will not have a time value relative to me, t will always be 0.

And it is that last point that I get hung up on. I see how I can move between other frames along the spatial axis, as they have a value relative to me. But I can't see how I can move in time relative to another frame, as the time value is 0.

As all frames are valid, then no frame can move along the time axis relative to another frame.

So I conclude that time travel can't happen.

 

[bcrowell]

Folks, let's try to restore a decent signal-to-noise ratio to this thread by confining discussions to physics. If you see more posts that appear to be sockpuppets of ract, just report the posts and let the mentors deal with it.

As all frames are valid, then no frame can move along the time axis relative to another frame.

So I conclude that time travel can't happen.

You seem to be concluding that just based on mathematics, there can't be any such thing as time travel. But that is isn't so. GR allows spacetimes such as the Godel metric http://en.wikipedia.org/wiki/Gödel_metric that contain closed timelike curves (CTCs) http://en.wikipedia.org/wiki/Closed_timelike_curve . "CTC" is basically a relativist's euphemism for "time travel." One problem with your reasoning is that you're talking about frames, but GR doesn't have global frames of reference, only local ones.

In addition to the resources that I listed in #7, there is a very nice popular-level book, Time Travel in Einstein's Universe: The Physical Possibilities of Travel Through Time, by Gott https://www.amazon.com/dp/0618257357/?tag=pfamazon01-20 .

I think part of the reason for the poor initial quality of this thread was that the loudest voices were the voices of those who hadn't taken any time to learn about the topic. Since they hadn't taken time out to study up on it, they were able to post immediately and copiously. A constructive thing for folks to do at this point would be to look at one of the print or online resources, find one that is at the right level for them, read it, and ask questions about things they don't understand.

 

[DaveC426913]

I didn't see all the posts, but I guess what Ract was saying is we all age, at the age we age, in our own frame. This is a biological process and 'moving' doesn't change that.

Our hearts will beat the same number of beats they were intended to beat despite 'moving around'

This is captured by saying in one's own frame of reference, one is always at rest. Thus, proper time is experienced.

What he didn't seem to get was the concept that moving relative to someone else means that the two people will age a different rates relative to each other, (i.e. the time between heart beats will seem to pass differently between the two frames.) but they will age the same in their own frame.

Perhaps. We are left to guess, since he insisted on (incorrect) assertions rather than questions.

(Forgive my ignorance Dave, does x' just signify another point on the x-axis or is it also directional, as in -x?. Also, what do youn mean by 'linking'?)

Yes. x' simply means some other point along the x axis.

A cause/effect link.


Poking an atom at x at time t cannot be a cause for the movement of an atom at x' at time t. The fact that they ar both at time t means they are separeted in space but not time, so for one to cause the other would require superluminal interaction. As soon as you have that, you can make causality violations.

So I may be missing the point, it depends on how I interpret your response. If you were saying that any two events in 4D space time could have a causality violation, regardless of coordinates, then yeah I get that.

Yes. If you set them up right. (i.e. farther apart in space (at c) than in time).

In any case, what I was trying to say is that we have to treat the time axis differently the spatial ones and not assume the axis share properties.

They are not identical, but for many purposes, they can be treated the same.

EG: I can arbitrarily set direction for xyz. The direction for t is already set. I need to do something to move through xyz, I move through t without doing anything. etc.

Yes. There are three space-like dimensions and one time like dimension. A timelike dimension is one that, for us, we can only move in one direction and we are doomed to move through it at a speed not in our control.

So for me I don’t see us living in a 4D universe, as all the dimensions aren’t symmetrical. (Like a 1D, 2D and 3D universe would be without the time element.) I would say we live in a 3D universe with a time element. But that’s just me.

The point is to look at the similarities rather than the differences. You wouldn't try to measure a brick using feet for width, light years for height and furlongs for length.

As regards time travel:

If I pick a point in time and space where I am, I can put everything I see in some spatial geometry, where other frames have a value along the xyz relative to me. However, other frames will not have a time value relative to me, t will always be 0.

Why? If you are moving relative to them at all, you're moving through spacetime at a different rate. One of the vlaues that will be in flux is time, just like any other dimension might be.

And it is that last point that I get hung up on. I see how I can move between other frames along the spatial axis, as they have a value relative to me. But I can't see how I can move in time relative to another frame, as the time value is 0.

As all frames are valid, then no frame can move along the time axis relative to another frame.

But we do it all the time when objects move relative to each other. Their passage through time diverges.

 

[rede96]

A constructive thing for folks to do at this point would be to look at one of the print or online resources, find one that is at the right level for them, read it, and ask questions about things they don't understand.

I agree totally, but unfortunately I don't always get time during the week for any in depth reading. But I can usually manage a post or three. (BTY I am looking for a good distance learning course. When I work away from home, I have plenty of time on my hands and would love to be able to appreciate this topic fully.)

So my approach will be to read and absorb what I see in the threads here and ask questions based on that. It isn't always the best way but it has helped my understanding greatly.

When I do get time, I do read up on things. But the best way I learn is to question and challenge. That often means thinking about things in a different way in order to check out my thought process and the theories. Particularly on matters that have not been proven conclusively, such as time travel.

Look Ben, with the greatest of respect, I think you should allow people a bit of slack from time to time. I know how frustrating it must be for you, but you've got the full range of people here and we all learn in different ways. As a trainer, that is one thing I do know from experience. As long as people are being respectful, there shouldn't be a problem with some slightly speculative posting, as long as it is in the spirit of learning and they are not asserting that they are right.

You seem to be concluding that just based on mathematics, there can't be any such thing as time travel. But that is isn't so. GR allows spacetimes such as the Godel metric http://en.wikipedia.org/wiki/Gödel_metric that contain closed timelike curves (CTCs) http://en.wikipedia.org/wiki/Closed_timelike_curve . "CTC" is basically a relativist's euphemism for "time travel." One problem with your reasoning is that you're talking about frames, but GR doesn't have global frames of reference, only local ones.

No I wasn't concluding anything, just explaining my thought process as mentioned above. I have read and watched a lot on time travel, albeit pitched at a very 'layman' level. So I am aware and very open to it. There just isn't any empirical evidence to back it up, so it is all just theory. Therefore I see it as fair game to challenge, even at my elementary level. I would never assume to be correct, but the challenging part is the most fun for me and learning should be fun!

 

[nitsuj]

So for me I don’t see us living in a 4D universe, as all the dimensions aren’t symmetrical. (Like a 1D, 2D and 3D universe would be without the time element.) I would say we live in a 3D universe with a time element. But that’s just me.

Not just you rede96. If I understand your interpretation that it's a "3D universe with a time component", than I agree as well that it is a better description of spacetime then 4D from a conceptual perspective.

The time "element" encompasses the other 3D's [STRIKE]as you pointed out[/STRIKE]. (personally, I feel time is merely an observation/derivative of the 3Ds)

I am unsure how you can "conclude" time travel via time dilation is not possible when considering your preferred description of "3D universe with a time element".

Your comment "...as all the dimensions aren’t symmetrical." implies you know that time dilation not only occurs, but is a typical quality of spacetime. Add to that, people can travel safely (proven) through slightly asymmetric spacetime and viola, time travel.

I don't find it a too great a leap to assume people could travel through remarkably distorted spacetime, however that may cause cancer so I don't recommend it be abused .

 

[rede96]

Yes. x' simply means some other point along the x axis.

Great, thanks for that.

Poking an atom at x at time t cannot be a cause for the movement of an atom at x' at time t. The fact that they ar both at time t means they are separeted in space but not time, so for one to cause the other would require superluminal interaction. As soon as you have that, you can make causality violations.

Cool, got it thanks. Although I am not sure how quantum entanglement fits into all this.


As regards time travel:

Why? If you are moving relative to them at all, you're moving through spacetime at a different rate. One of the vlaues that will be in flux is time, just like any other dimension might be.

But we do it all the time when objects move relative to each other. Their passage through time diverges.

Ok, let me try and explain my thought process here.

First off, I agree that if I am moving relative to another frame then time dilates occurs. I think I've got a good handle on that now, and it was not my intention to challenge it. I whole heartedly support it.

I do still try and visualise just 'how' it happens. Which although is good fun, can get me into trouble leading me down the wrong path. (Such as this post!)

Whilst doing that (Trying to visualise) I was wondering if time or space time was quantised. I read some stuff on wiki about Plank time / length etc.

So it got me thinking that if time (or space time maybe) may be quantised, then we move through our world lines in discrete chunks, one after the other.

So what I was doing was taking a snapshot of the universe from my frame. Trying to visualise what one of the chunks was like.

So as it is a snapshot, nothing is moving, don't have to worry about the differences in time etc., just mapping where everything is in space and time relative to me at that moment.

As I was looking at it from my frame, all time would be the same. Hence t = 0. Now obviously, if we keep indexing the chunks we would observe time dilation, sure. But at this snap shot I took, time would = 0 wrt my FoR.

So next I imagined that I wanted to move through space time to the next 'chunk' as it were.

I can see that there are lots of options for me to choose in the spatial directions. But there are no options for me to move anywhere in time.

All I can do is move to the next chunk, where time resets to 0 again from my frame.

So in anyone plank unit of time, there is no future or past to go to for my FoR. The best I could do maybe is to move about in 1 plank unit of time.

So that lead me to think maybe that is why we can't travel in the time dimension. Although we still would experience differences in time due to time dilation, but the two are different in my mind.

And, looking at it this way, even if I could move backwards, chunk by chunk, all I would do is put the universe and everything in it in reverse. So even if I was to travel back with it, it would be pointless, as my thoughts would be running backwards too. (If that makes sense!)

What I mean by that is maybe the way we think requires forward movement in time, and maybe we can't go back and forward in time simultaneously.

Also, as an aside, I also wondered if this was how a photon would see life. (Bad terminology I know, photons don't have a frame a reference!) As they travel at the speed of light they are not bound to move through time as we do, so they are free to move through space. Hence from the photons point of view, instant travel. A photon would move spatially in this chunk of space time until it was absorbed.

Anyway, as I said, this is just my way of thinking about.

Now for the fun bit! Where all the people that really understand this tell me what a crock of it is. Which is fine, because that actually really helps me.

So thanks in advance!

 

[rede96]

Not just you rede96. If I understand your interpretation that it's a "3D universe with a time component", than I agree as well that it is a better description of spacetime then 4D from a conceptual perspective.

The time "element" encompasses the other 3D's [STRIKE]as you pointed out[/STRIKE]. (personally, I feel time is merely an observation/derivative of the 3Ds)

Cool! Thanks

I am unsure how you can "conclude" time travel via time dilation is not possible when considering your preferred description of "3D universe with a time element".

Sorry, my bad. That wasn't what I was trying to say at all. Maybe I explained myself a bit better in my last post.

 

[Dale]

First off, I agree that if I am moving relative to another frame then time dilates occurs. I think I've got a good handle on that now, and it was not my intention to challenge it. I whole heartedly support it.

I do still try and visualise just 'how' it happens. Which although is good fun, can get me into trouble leading me down the wrong path. (Such as this post!)

Whilst doing that (Trying to visualise) I was wondering if time or space time was quantised. I read some stuff on wiki about Plank time / length etc.

While there are some theories that have quantized time or space there are to date no experimental observations that support them, so the effects are too small to matter at any scales that we can reach currently. So basically, although they exist, they don't actually explain anything that isn't already explained, including time dilation.

So that lead me to think maybe that is why we can't travel in the time dimension.

In this post and your earlier posts in this thread I think you are over-emphasizing this point quite a bit. There are only two important differences between time and space, first they have different signs in the metric, and second there is only one dimension of time while there are three of space. This is completely summarized by the fact that the signature is (-+++).

If there were three dimensions of time and one of space (---+) then you could have closed timelike curves in flat spacetime and you could not have closed spacelike curves. In a 1+1D spacetime with one dimension of time and one of space you cannot have closed curves of either natures, a timelike path can never "return" to the same time and a spacelike path can never "return" to the same position.

So yes, time is different from space, and that is completely and fully recognized by describing spacetime as Minkowski, or pseudo-Riemannian, or (-+++) signature, or 1+3D spacetime. But that does not mean that they are not otherwise equivalent.