Is Time Travel Possible and What Are Its Implications?

[mlw]

Hello everyone,
I am new to this forum and would greatly appreciate your expertise in answering (or attempting to) a few questions for me. I don't care if you focus on one or all of them, but any answers are appreciated! These are about time travel, with a focus on traveling BACK in time...

1. Will time travel ever be possible?
2. If so, how soon? Also, how far back would we be able to travel?
3. Would our bodies be able to survive time travel?
4. Would time travellers be able to communicate with other humans, or would they be on a separate plane of time/dimension?
5. Is it possible that spaceship sitings are time travel machines from the future?

Also, if anyone can answer #4 (at least in theory), I'd be grateful.

 

[PhY]

Hello everyone,
I am new to this forum and would greatly appreciate your expertise in answering (or attempting to) a few questions for me. I don't care if you focus on one or all of them, but any answers are appreciated! These are about time travel, with a focus on traveling BACK in time...

1. Will time travel ever be possible?
2. If so, how soon? Also, how far back would we be able to travel?
3. Would our bodies be able to survive time travel?
4. Would time travellers be able to communicate with other humans, or would they be on a separate plane of time/dimension?
5. Is it possible that spaceship sitings are time travel machines from the future?

Also, if anyone can answer #4 (at least in theory), I'd be grateful.

1. Hopefully not (too much power corrupts). Imagine Neo-Nazi traveling back through time & warning Hitler not to attack Russia.
5. Spaceship Sightings are Fake and no, Humans tend the screw up and make mistakes and we would know about it.

 

[pervect]

1) It is currently unknown if time travel is theoretically possible within the framework of General Relativity. This is currently an active research topic.

2) and 3) can't be answered at all given that we don't have the theoretical issues fully resolved. (This is like asking someone who is trying to determine if something is even possible in principle "When is the product based on this idea going to ship to customers?).

4) cannot be given a definite answer, but it seems highly unlikely.

If the OP is primarily interested in 5), the best forum would be 'skepticism/debunking', and I'll move the thread there if the OP requests it. However, there will tend to be more information of UFO's and not so much information on time travel there, I think.

 

[mlw]

I was also wondering how certain most physicists are that wormholes do exist.

I'm assuming if time travel were ever possible, and someone did visit this century, we of course wouln't know about it. I would think by that time the human race would have evolved at least enough to know that that would have an effect on their own place in time. If they were to announce, "Hey, we're from the future!" it would only serve to endanger their own lives.

My main concern is the question as to whether time has already been altered (in other words, has any living creature already traveled back in time?). To me, the only way to be sure is to assume that time travel (no matter how far back) will one day (no matter how far in the future) be possible. Also, is it true that the future has already occurred? I assume this would mean that the past, present, and future are occurring simultaneously (correct me if I'm wrong)?

 

[mlw]

My main concern is whether time travelers would be on a separate plane or dimension of time if traveling back. Thanks!

 

[vsandel]

I'm not a physicist but I will add my two cents worth. Time is nothing but change. In a
static universe (without change) there would be no such thing as time. We measure time by comparing the rate of change of one process to another.

As for time travel, to go back in time every change would have to be reversed. Since changes in the positive direction follow the principles of thermodynamics, going from high energy to low energy and from order to disorder, where would the energy come from to reverse all the changes? To go back in time every change in the whole universe would have to be reversed. It would take an all powerful God could accomplish that. Time travel sounds pretty far fetched to me.

There are some theories of how the universe works that would allow time travel, the one which specifically comes to mind is the proposal that there is no change in the universe, motion is an illusion. The author (whose name I don't remember) states that the universe is controlled by a gigantic wave function that describes the position of every particle in the universe, and the "now" runs through this wave function taking the most probable course. In this view of the universe it is only the "now" that moves.

There would still be the problem of energy. The most probable course would still follow the laws of thermodynamics, so for the "now" to retreat from one spot in the wave function to an earlier state, it would still take energy to reduce the entropy to the earlier state.

 

[mlw]

If time is an illusion and is a function of distance, and is relative, why would everything in the universe need to be revsered for one time traveller?

 

[russ_watters]

Where do you get the idea that time is an illusion? It most certainly is not.

 

[Ricardo19]

If time is an illusion and is a function of distance, and is relative, why would everything in the universe need to be revsered for one time traveller?

time is not an illusion einstein said that gravity is described as the wraping of space and time near an object with mass the more gravity the more wraping of space and time
so you could say that that gravity and time are pretty much related

 

[peter0302]

Time, space, energy, velocity, mass, force, wave function, etc...

All these concepts are variables in equations. You can take one of several subsets of these and call them "independent" or "real" and the rest will be "dependent" on that subset, or "illusions." So which ones are "real" and which ones are "illusions?" Who's to say? Maybe none are real...

 

[russ_watters]

No. You are misusing those words. Just because something is variable or relative does not mean it is not real.

 

[Hernik]

My main concern is whether time travelers would be on a separate plane or dimension of time if traveling back. Thanks!

I don't understand what you mean... The concept of traveling back in time is to travel to a time that has already been in the timetravelers life (or before that). So why should it be another dimension of time (Whatever that is..) or a separate plane? Or are you asking whether one will be in the past "at the same time" as the dear beloved ones left i the "now" - like there being two sets of time or something?

If that's the case: The discriptions of time machines which I've read about are dealing with the same spacetime as the one we are in now - just different places at a different time.

These time travel machines were based on theories on black holes or tunnels between different parts of space/time. They needed so enormeous amounts of energy in order to create the right circumstances that it is unlikely it'll ever work and also unlikely anybody will be able to experiment with such a construction in our lifetime.

 

[peter0302]

No. You are misusing those words. Just because something is variable or relative does not mean it is not real.

Ok,so what's your definition, Morpheus? :)

 

[TomMac321]

Ok if you could go back in time I don’t think you could change anything think about it if you wanted to change something so bad that you went back in time to do then in the future it would be fixed and you wouldn’t have to go back in fix it there for ether not altering the future of undoing existence as we know it

 

[TomMac321]

time

Ok if you could go back in time I don’t think you could change anything think about it if you wanted to change something so bad that you went back in time to do then in the future it would be fixed and you wouldn’t have to go back in fix it there for ether not altering the future of undoing existence as we know it


and time is a Time is a diminution but also just a word

I have more info that is more scientific but may I ask why you want to know this

 

[Hernik]

Ok if you could go back in time I don’t think you could change anything think about it if you wanted to change something so bad that you went back in time to do then in the future it would be fixed and you wouldn’t have to go back in fix it there for ether not altering the future of undoing existence as we know it

I agree. If you come back to now from the future, anything you do now "was" part of the history in the future you left.

I find it very strange what the nature of time is. I never much believed the idea of time being a result of change. I think time must have to do with interaction between energies. Otherwise why would it pass differently depending on the field of gravity and the velocity.

 

[basePARTICLE]

E = m * c^2.

Did any of you read The future of time(2006). A good place to start. Many questions were approached realistically, by a number of the world's leading physicists!

Hawking gave a decent description of what would have been entailed to make time travel a realism. I think some Mathmatics was used, but I cannot remember exactly. Sorry if I am off topic, just thought that book containing relevant material might help.

 

[asymmetric]

you say that to be able to go back in time all changes must be reversed...if this could be done or if this actually happens naturally then, or rather, therefor they are also changes ...these new ''reversed'' changes are also processes with a rate of change one respect to another...so I'm sure this would not conclude in traveling backwards in time.
your idea of all movement , which is a change from one state to another and the time this change takes to become something else is i understand being studied here in spain...i saw this on a science programm a few months after i was working on this issue myself...
the idea being that ''time is unity or of value one'' always for the whole cosmos.
this means that all processes will always sum the same one for the whole cosmos.
its a bit dificult to explain...maybe somewhere i can get some more info...

I'm not a physicist but I will add my two cents worth. Time is nothing but change. In a
static universe (without change) there would be no such thing as time. We measure time by comparing the rate of change of one process to another.

As for time travel, to go back in time every change would have to be reversed. Since changes in the positive direction follow the principles of thermodynamics, going from high energy to low energy and from order to disorder, where would the energy come from to reverse all the changes? To go back in time every change in the whole universe would have to be reversed. It would take an all powerful God could accomplish that. Time travel sounds pretty far fetched to me.

There are some theories of how the universe works that would allow time travel, the one which specifically comes to mind is the proposal that there is no change in the universe, motion is an illusion. The author (whose name I don't remember) states that the universe is controlled by a gigantic wave function that describes the position of every particle in the universe, and the "now" runs through this wave function taking the most probable course. In this view of the universe it is only the "now" that moves.

There would still be the problem of energy. The most probable course would still follow the laws of thermodynamics, so for the "now" to retreat from one spot in the wave function to an earlier state, it would still take energy to reduce the entropy to the earlier state.

 

[quantify]

hypothesis that one where to somehow travel between two different planes of time/space. If all planes are relative to each other but not necessarily traveling at the same speed, then after making this transition and say waiting a week and then traveling back to the original plane you would enter at a time relative to the plane you just left. Unless both planes in this equation were traveling at the same precise speed then you would have gone back in time relative to both planes.

 

[vsandel]

Would every change need to be reversed?

If time is an illusion and is a function of distance, and is relative, why would everything in the universe need to be revsered for one time traveller?

I haven't been on the forum until now, but your question of why reversal of time requires every change to be reversed requires an answer.

If you assume that only certain changes would have to be reversed, who would decide which ones to be reversed? To truly go back in time, you would have to find everything as it was at the earlier date. If it were not, then how could one really say that they had gone back in time?

To go back in time the past must still exist. We cannot experience anything that doesn't exist except in our minds (memories?). Time travel necessarily assumes a theory of the universe (or infinite universes) that allows the past to exist. If it still exists, all changes would of necessity be reversed.

To travel to the future would require a deterministic view of all change. By that I mean any future state could be determined if the present state were known. While classical mechanics would allow this, quantum mechanics has eliminated the deterministic view of change.

 

[peter0302]

To travel to the future would require a deterministic view of all change. By that I mean any future state could be determined if the present state were known. While classical mechanics would allow this, quantum mechanics has eliminated the deterministic view of change.

No, to paraphrase Captain Barbossa in Pirates of the Caribbean, it's not getting to the future that's the problem. It's getting back.

What I mean is, you can "easily" travel to the future by traveling close to the speed of light for some time and then decelerating back to 0. You just can't go back.

Anyway, I like your pointing out that time travel assumes the "past" still "exists" in some form. I've also wondered about this.

 

[vsandel]

Whose Future?

No, to paraphrase Captain Barbossa in Pirates of the Caribbean, it's not getting to the future that's the problem. It's getting back.

What I mean is, you can "easily" travel to the future by traveling close to the speed of light for some time and then decelerating back to 0. You just can't go back.

Anyway, I like your pointing out that time travel assumes the "past" still "exists" in some form. I've also wondered about this.

Question is whose future would you return to? Your life would have been continuous, so there is no "break" in time which there would be if you had actual time travel.

Here is a question I have concerning the twin paradox, where one twin goes on a space ride near the speed of light, then comes back to Earth to find his brother much older than himself. Einstein derived the time dilation formula assuming the direction of travel is parallel to the beam of light in the positive x direction. To keep the speed of light constant the time has to slow down relative to time on earth. But on the return voyage you would be going in the -x direction. Measuring the speed of light of that same beam would make the voyagers time speed up by the same amount that it slowed in going away from earth. Therefore, when the traveling twin reached Earth he would be exactly as old as his brother. The term in the time dilation equation:

t-v(-x)/c exp 2​

becomes positive on the return trip. I can't believe that reputable phycists who have dealt with the twin paradox missed this, so where am I wrong?

 

[peter0302]

The Lorentz factor is always 1/sqrt(1-v^2/c^2). So velocity is always squared, and whether it was positive or negative would be irrelevant.

The real issue is that the Earth's time appears to slow down for the traveller as well. This is reconciled when the traveller decelerates back to the Earth's reference frame, both on the "turn around" and the "landing". That's when the Earth twin appears to age from the traveller's persepctive.

 

[vsandel]

The Lorentz factor is always 1/sqrt(1-v^2/c^2). So velocity is always squared, and whether it was positive or negative would be irrelevant.

The real issue is that the Earth's time appears to slow down for the traveller as well. This is reconciled when the traveller decelerates back to the Earth's reference frame, both on the "turn around" and the "landing". That's when the Earth twin appears to age from the traveller's persepctive.

You are neglecting a term in the numerator. Einstein gives the time dilation equation as:

t' = __t-vx/c^2____
sqrt(1-v^2/c^2)​

where t' is the traveler's time,t is Earth time, and x is the coorinate along which both the light beam and the spaceship is traveling. The x is positive on the way out from earth, but negative on the way back. Therefore t' will be slower than t on the way out, but faster than t by an equal amount on the way back. There is no term for accelleration or decelleration in the equasion, only velocity. Thus the time dilation relative to Earth will change during the accelleration and decelleration because the velocity is changing, but that does not cancel out the time differences while traveling at constant velocity.

 

[peter0302]

The x is positive on the way out from earth, but negative on the way back.

Yes, and so is "v". The negatives therefore cancel out to a positive.

 

[JesseM]

You are neglecting a term in the numerator. Einstein gives the time dilation equation as:

t' = __t-vx/c^2____
sqrt(1-v^2/c^2)​

where t' is the traveler's time,t is Earth time, and x is the coorinate along which both the light beam and the spaceship is traveling.

That's not usually referred to as the "time dilation equation", rather it's the part of the Lorentz transformation that translates the time-coordinate of a single event in the unprimed frame to the time-coordinate of the same event in the primed frame. When people talk about the time dilation equation, they're usually referring to an equation that relates a time interval in one frame (like the time elapsed on a clock between two points on its journey) to the corresponding time interval in another. See the discussion on this thread.

 

[ZombieFrog]

New Here

I too am new here, and perhaps i am completely naive, but it seems to me that asking what energy could possibly reverse all the changes in the universe (aka go back in time) is very strange. perhaps as strange as asking what energy could possibly cause all the changes currently happening (aka go forward in time). again perhaps i am just naive.

as well it seem too me that time travel while possible (and i am indulging in speculation here) would be rather limited. Entering a time-space warp/vortex at one location and exiting at another location would result in a shift of time. This would be possible with large time-space warps, perhaps Black Holes, Wormholes, or even some human super-laser/light vortex, but in any case the forces being tampered with border on uncontrollability.

It also seems that any machine built would only be able to shift an object through time to the moment of the machines creation, and not further back, making it useless for traveling to the future, or far into the past (unless the machine has existed for a long time)

I am not a physicist but this is what i have gathered from my readings

 

[JesseM]

I too am new here, and perhaps i am completely naive, but it seems to me that asking what energy could possibly reverse all the changes in the universe (aka go back in time) is very strange. perhaps as strange as asking what energy could possibly cause all the changes currently happening (aka go forward in time). again perhaps i am just naive.

It isn't really "reversing all the changes in the universe". Here's an analogy I posted a long time ago on this thread:

That is not how time travel would work in general relativity. In both special and general relativity, you have to get rid of the idea of a single universal present, since these theories say that different observers have different views of whether two different events happened "at the same time" or not, and each observer's reference frame is equally valid. So instead you have to think of a single static 4-dimensional "spacetime" which contains the entire history of the universe; traveling back in time in this context means that an object's "worldline" curves back on itself and revisits a region of spacetime it already crossed through before.

Think of a block of solid ice with various 1-dimensional strings embedded in it--if you cross-section this block, you will see a collection of 0-dimensional points (the strings in cross-section) arranged in various positions on a 2-dimensional surface, and if you take pictures of successive cross-sections and arrange them into a movie, you will see the points moving around continuously relative to one another (in terms of this metaphor, the idea that there is no single universal present means you have a choice of what angle to slice the ice when you make your series of cross-sections). You shouldn't think of time travel as the points returning to precisely the same configuration they had been in at an earlier frame of the movie; instead, you should just imagine one of the strings curving around into a loop within the 3-dimensional block, what in general relativity is known as a "closed timelike curve".

 

[vsandel]

My assumption is that to visit the past, the past must still exist. If the object's "worldline" curves back on itself to revisit a region of spacetime it already crossed through before, would that space time not have the identical state and energy as when first crossed? If so, wouldn't the changes have to be reversed? It seems that you are assuming an infinite universes model of the cosmos in which the "now" travels from one universe to another. I have many questions about such a model. Occham's Razor suggests that this greatly complicated model of the cosmos is inferior to a model in which time is only change, and once a "now" is changed there is no going back without reversing that change.

One assumption made by people who consider time travel is that in going back in time a person would not change. I believe that if time travel were truly possible, a person going back in time would be the age he was at the earlier date, have the memories and knowledge that he had at the earlier date with no memories of his future life. Therefore his life in the past would play out exactly as it did the first time. He would not even know that he visited the past. This rules out the paradox of changing the past to make the future impossible.

 

[vsandel]

Yes, and so is "v". The negatives therefore cancel out to a positive.

Correct, Peter. I was looking at the situation of a person measuring the speed of light while traveling back to earth. His velocity would add to the speed of light if it were not for relativity. Therefore to keep c constant time would have to go faster for the traveler relative to Earth time. I am not proficient in the math of all of this, but is my reasoning not correct?

 

[bchui]

A very simple "proof" for time travel not possible is that nobody from the future had ever been traveled back to meet us. Further, there is no solid proof ever for any extra terrestial intelligents had ever visited our solar system. I doubt that even space travel is more difficult tahn ever imagined, if not impossible!

 

[JesseM]

My assumption is that to visit the past, the past must still exist. If the object's "worldline" curves back on itself to revisit a region of spacetime it already crossed through before, would that space time not have the identical state and energy as when first crossed? If so, wouldn't the changes have to be reversed?

What do you mean by the changes being reversed? If I travel from Los Angeles to New York, it's not that the Los Angeles around me has to be transformed into New York, it's that I move from one preexisting city to another. Similarly, a time traveler just has a worldline that moves from a future region of spacetime to a past region, but the future region doesn't "become" the past region.

It seems that you are assuming an infinite universes model of the cosmos in which the "now" travels from one universe to another.

Where did I assume anything about infinite universes? I am imagining that there is no objective "now", any more than there's an objective "here". There's just 4D spacetime, which is represented in my analogy by the entire block of ice (in philosophy this idea is known as eternalism, or the B-theory of time, and is opposed to presentism which says that only the present exists). Even without time travel, relativity calls the notion of an objective "now" into question, because different reference frames will disagree about whether two spatially separated events happened "at the same time" or "at different times" (this is a feature known as the relativity of simultaneity), and relativity says that as far as physics goes all these reference frames are on equal footing.

 

[peter0302]

A very simple "proof" for time travel not possible is that nobody from the future had ever been traveled back to meet us.

That assumes that visitors from the future would allow their presence to be known.

Further, there is no solid proof ever for any extra terrestial intelligents had ever visited our solar system.

Michael Jackson.

 

[W3pcq]

A very simple "proof" for time travel not possible is that nobody from the future had ever been traveled back to meet us. Further, there is no solid proof ever for any extra terrestial intelligents had ever visited our solar system. I doubt that even space travel is more difficult tahn ever imagined, if not impossible!

How can you be sure that there is no evidence of either extra-terrestrials or time travelers
from the future. I think what you mean is that you have no evidence. So maybe you have no reason to think so, but that is not proof. That is like saying I haven't seen it with my own eyes, so it can't exist.

Anyone ever seen the south park episode where time travelers from the future are coming back to earn more decent wages and overpopulating the present? It is hilarious.

 

[Chip Orr]

There are those in the science community who fancy the idea of time travel to be a reality. I have a problem with their desciption of what time is as well as their grandios mathimatical theories describing Time travel. First of all let me get this out of the way. I believe in absolute time. Yes I know ,thats a no no to many. What I mean by this is that now is now and there is a thing called simaltanity. Your clock might be ticking at a different rate and we might not agree on what time an event happened but the event happened in the now which has nothing to do with information flow reaching you. Now is now! Some models state that warped space time in conjuction with a worm hole could bring you to the future and then another worm hole could return you to a moment before you disembarked initially. This reduces time to information flow at the speed of light. If that's the case you can beat the flow of info to a spot in the universe and the event has not happened yet. Yes you would know the outcome before it happens but that is not time travel! That is just insider trading. I believe if you traveled back to you starting point after your "insider" experience through a worm hole you would end up back in the now and could never go back to a time before the now. Of course you never would have left the now to begin with. Yes your wrist watch would not agree with others around you. In other words the "now" is an absolute moment in time and you cannot travel back before it. If you want to call traveling at high velocity and coming back to find everyone back at home quite a bit older "time traveling" then yes I can agree with you. If you want to reduce the flow of time to being equal to the flow of information through space I cannot. Simultaneous action between two objects is a reality in the quantum realm. When the "theory of everything" is perfected I do believe it will include a form of absolute time while still honoring SR.

 

[JesseM]

There are those in the science community who fancy the idea of time travel to be a reality.

Not really, they just note that general relativity seems to allow it in certain unusual spacetimes, but I think everyone would also acknowledge that we need to understand more about the relationship between GR and quantum mechanics in order to judge if these scenarios are actually possible.

Simultaneous action between two objects is a reality in the quantum realm.

No, quantum field theory respects relativity, and therefore you can analyze the same physical situation from the perspective of different frames which disagree about simultaneity, and the laws of physics will work exactly the same in each of these frames.

When the "theory of everything" is perfected I do believe it will include a form of absolute time while still honoring SR.

Impossible by definition to have absolute time yet still honor SR, because the only way to establish a preferred definition of simultaneity by physical experiment would be to show that the laws of physics don't work precisely the same way in every frame, that one frame is "preferred" physically, which violates the first postulate of SR.

 

[peter0302]

No, quantum field theory respects relativity, and therefore you can analyze the same physical situation from the perspective of different frames which disagree about simultaneity, and the laws of physics will work exactly the same in each of these frames.

The laws of physics may work the same in every frame but every interpretation of QM other than many-worlds requires faster-than-light influence. You can argue with the semantics but at the interprative level there's no getting around it.

 

[Chip Orr]

No, quantum field theory respects relativity, and therefore you can analyze the same physical situation from the perspective of different frames which disagree about simultaneity, and the laws of physics will work exactly the same in each of these frames.
You might disagree as an observer about simultaneity from the perspective of different frames but after analyzing the event properly by compensating for info travel distance, velocity of observer in said frame etc. you would agree on the simultaneity of the event. On the other point there is not a "preffered frame" where physics doesn't work the same as in other frames. Absolute time would be a moment shared by all frames but not bound by the same apparent time.

 

[JesseM]

You might disagree as an observer about simultaneity from the perspective of different frames but after analyzing the event properly by compensating for info travel distance, velocity of observer in said frame etc. you would agree on the simultaneity of the event.

Simultaneity doesn't refer to seeing events at the same time, it is already based on "compensating for info travel distance" (with each observer assuming that info travels at c in their own rest frame). For example, if in the year 2010 as measured by my clocks I see the light from an event which is 10 light-years away as measured by my rulers, and then in the year 2020 I see the light from an event which is 20 light years away, I will label these two events as "simultaneous" in my frame. But if each observer follows a similar procedure for "compensating for info travel distance", they will disagree about whether events are simultaneously. Take a look at the thought-experiment Einstein uses to illustrate this here and here, which is also illustrated in a little animation http://www.cord.edu/dept/physics/credo/etrain_credo.html , and discussed on pp. 62-63 of Taylor and Wheeler's book spacetime physics as follows:

The Principle of Relativity directly predicts effects that initially seem strange--even weird. Strange or not, weird or not; logical argument demonstrates them and experiment verifies them. One effect has to do with simultaneity: Let two events occur separated in space along the direction of relative motion between laboratory and rocket frames. These two events, even if simultaneous as measured by one observer, cannot be simultaneous as measured by both observers.

Einstein demonstrated the relativity of simultaneity with his famous Train Paradox. (When Einstein developed the theory of special relativity, the train was the fastest common carrier.) Lightning strikes the front and back ends of a rapidly moving train, leaving char marks on the train and on the track and emitting flashes of light that travel forward and backward along the train (Figure 3-1). An observer standing on the ground halfway between the two char marks on the track receives the two light flashes at the same time. He therefore concludes that the two lightning bolts struck the track at the same time--with respect to him they fell simultaneously.

A second observer rides in the middle of the train. From the viewpoint of the observer on the ground, the train observer moves toward the flash coming from the front of the train and moves away from the flash coming from the rear. Therefore the train observre receives the flash from the front of the train first.

This is just what the train observer finds: The flash from the front of the train arrives at her position first, the flash from the rear of the train arrives later. But she can verify that she stands equidistant from the front and rear of the train, where she sees char marks left by the lightning. Moreover, using the Principle of Relativity, she knows that the speed of light has the same value in her train frame as for the ground observer (Sectin 3.3 and Box 3-2), and is the same for light traveling in both directions in her frame. Therefore the arrival of the flash first from the front of the train leads her to conclude that the lightning fell first on the front end of the train. For her the lightning bolts did not fall simultaneously. (To allow the train observer to make only measurements with respect to the train, forcing her to ignore Earth, let the train be a cylinder without windows--in other words a spaceship!)

Did the two lightning bolts strike the front and the back of the train simultaneously? Or did they strike at different times? Decide!

Strange as it seems, there is no unique answer to this question. For the situation described above, the two events are simultaneous as measured in the Earth frame; they are not simultaneous as measured in the train frame. We say that the simultaneity of events is, in general, relative, different for different frames. Only in the special case of two or more events that occur at the same point (or in a plane perpendicular to the line of relative motion at that point--see Section 3.6) does simultaneity in the laboratory frame mean simultaneity in the rocket frame. When the events occur at different locations along the direction of relative motion, they cannot be simultaneous in both frames. This conclusion is called the relativity of simultaneity.

The relativity of simultaneity is a difficult concept to understand. Almost without exception, every puzzle and apparent paradox used to "disprove" the theory of relativity hinges on some misconception about the relativity of simultaneity.

On the other point there is not a "preffered frame" where physics doesn't work the same as in other frames.

I agree, but if there was a preferred definition of simultaneity, that would require a preferred frame. If we assume there are no preferred frames, there can be no preferred definition of simultaneity.

Absolute time would be a moment shared by all frames but not bound by the same apparent time.

What do you mean by "shared by all frames" and "not bound by the same apparent time"? Can you give an example?

 

[Chip Orr]

Simultaneity doesn't refer to seeing events at the same time, it is already based on "compensating for info travel distance" (with each observer assuming that info travels at c in their own rest frame). For example, if in the year 2010 as measured by my clocks I see the light from an event which is 10 light-years away as measured by my rulers, and then in the year 2020 I see the light from an event which is 20 light years away, I will label these two events as "simultaneous" in my frame. But if each observer follows a similar procedure for "compensating for info travel distance", they will disagree about whether events are simultaneously. Take a look at the thought-experiment Einstein uses to illustrate this here and here, which is also illustrated in a little animation http://www.cord.edu/dept/physics/credo/etrain_credo.html , and discussed on pp. 62-63 of Taylor and Wheeler's book spacetime physics as follows:


I agree, but if there was a preferred definition of simultaneity, that would require a preferred frame. If we assume there are no preferred frames, there can be no preferred definition of simultaneity.

What do you mean by "shared by all frames" and "not bound by the same apparent time"? Can you give an example?

The world map of the universe is the now, the world picture is the universe as it appears to be but isn't. The simultaneity of an event has nothing to do with the time an observer receives information or the amount of time dilation they are experiencing. Most events that seem to be simultaneous are not and those that don't seem to be might be. The now is the same for all frames regardless of perspective. It seems to me that man has arrogantly put himself at the center of a system once again by thinking his detection of an event has any bearing or relivance on the actual now of the event. Thank you for your time!

 

[vsandel]

What do you mean by the changes being reversed? If I travel from Los Angeles to New York, it's not that the Los Angeles around me has to be transformed into New York, it's that I move from one preexisting city to another. Similarly, a time traveler just has a worldline that moves from a future region of spacetime to a past region, but the future region doesn't "become" the past region.

Two assumptions I make are that time is just the rate at which things change, and that to visit the past the past or future they must both exist. The definition of time travel as popularly conceived is that a person visits either an earlier or later "now". If my assumptions are correct, either the earlier and later "now" has to exist, or if they don't, then to go back to an earlier "now" the changes that have taken place between the present "now" and the earlier "now" would have to be reversed. The latter possibility is ridiculous, so that leaves a view of the cosmos that physically preserves intact all previous "nows" The proposed cosmological models that I have read about that preserve the past "nows" are the infinite universes models. I reject them on the basis of Occam's Razor. Therefore I do not believe time travel as popularly conceived is possible.

 

[JesseM]

Two assumptions I make are that time is just the rate at which things change, and that to visit the past the past or future they must both exist. The definition of time travel as popularly conceived is that a person visits either an earlier or later "now". If my assumptions are correct, either the earlier and later "now" has to exist. If they don't, then to go back to an earlier "now" the changes that have taken place between the present "now" and the earlier "now" would have to be reversed. The latter possibility is ridiculous, so that leaves a view of the cosmos that physically preserves intact all previous "nows" The proposed cosmological models that I have read about that preserve the past "nows" are the infinite universes models. I reject them on the basis of Occam's Razor. Therefore I do not believe time travel as popularly conceived is possible.

But in relativity there isn't even a single "now", each frame has a different definition of now (two events that happen at the same time in one frame happen at different times in another), and all frames are equally valid physically. This is what leads you to the "spacetime" picture where what really exists is a 4-dimensional structure containing every event in every time, and different frame's "nows" represent different ways of slicing this structure up into 3-dimensional slices (sort of like if you sliced up the block of ice in my analogy into 2D slices, with different frames slicing it up at different angles). I guess this is somewhat similar to your notion of "physically preserving intact all previous nows", except "preserving" suggests that you are imagining some genuine "flow of time" with past states having to be "preserved" after they are no longer in the now, whereas the 4-dimensional view just suggests all events in history coexist in a single static 4D spacetime, and there is no objective "now" moving along it in the first place. And this 4D spacetime perspective has nothing to do with any cosmological model of infinite universes as you suggest, it's just the standard way of imagining a single universe in general relativity...the very words "the universe" in GR are usually taken to mean a 4D spacetime rather than a 3D space which changes over time.

 

[vsandel]

But in relativity there isn't even a single "now", each frame has a different definition of now (two events that happen at the same time in one frame happen at different times in another), and all frames are equally valid physically. This is what leads you to the "spacetime" picture where what really exists is a 4-dimensional structure containing every event in every time, and different frame's "nows" represent different ways of slicing this structure up into 3-dimensional slices (sort of like if you sliced up the block of ice in my analogy into 2D slices, with different frames slicing it up at different angles). I guess this is somewhat similar to your notion of "physically preserving intact all previous nows", except "preserving" suggests that you are imagining some genuine "flow of time" with past states having to be "preserved" after they are no longer in the now, whereas the 4-dimensional view just suggests all events in history coexist in a single static 4D spacetime, and there is no objective "now" moving along it in the first place. And this 4D spacetime perspective has nothing to do with any cosmological model of infinite universes as you suggest, it's just the standard way of imagining a single universe in general relativity...the very words "the universe" in GR are usually taken to mean a 4D spacetime rather than a 3D space which changes over time.

If there is no "now", only 4D spacetime, then would the future not be fixed as well as the past? This would be a predetermined universe just playing out as a movie.

I am not acquainted with relativity mathematically, but in my way of thinking the relativity of simultaneity does not rule out an absolute "now". I look at it as a measurement problem much like the Heisenberg uncertainty principle. The accuracy of simultaneous measurement of the momentum and position of a particle is limited because measuring the momentum disturbs the position and visa verse. That does not imply that the particle does not have momentum and position simultaneously, just that we can't measure it. Looking at time as change, the "now" would be the state and position of every particle in the universe at a given instant. Although we can't measure it, who is to say it doesn't exist?

Furthermore, general relativity is just a mathematical model of reality, not the reality itself. It correctly predicts some things and is the best current model of reality, but as soon as it makes a wrong prediction it will be "back to the drawing board". To completely dismiss ideas that are not based upon GR seems to me to be to be unwise until there is a lot more evidence that the theory truly predicts reality accurately.

 

[JesseM]

If there is no "now", only 4D spacetime, then would the future not be fixed as well as the past? This would be a predetermined universe just playing out as a movie.

Sure, all events would be fixed in spacetime (though this is not the same as 'determinism' which assumes that complete knowledge of the present is enough to uniquely predict the future). In the 4D view it wouldn't even really be "playing" out, since that would imply some notion of a "moving present" passing through spacetime...instead all of spacetime would just exist like a static geometric structure.

I am not acquainted with relativity mathematically, but in my way of thinking the relativity of simultaneity does not rule out an absolute "now".

You're right that it doesn't rule it out in a metaphysical sense, but it rules it out as a notion that could have any physical correlates whatsoever--if relativity is correct there would be absolutely no experiment that could distinguish the "correct" definition of simultaneity from the "false" definitions of other frames, so true simultaneity would be just a sort of phantom, a bit like the idea of entities "outside" the physical universe.

I look at it as a measurement problem much like the Heisenberg uncertainty principle. The accuracy of simultaneous measurement of the momentum and position of a particle is limited because measuring the momentum disturbs the position and visa verse. That does not imply that the particle does not have momentum and position simultaneously, just that we can't measure it.

Sure, this is a decent analogy, as long as you accept that quantum mechanics means that it is impossible in principle to ever measure them simultaneously, not just that it's something we can't do at present with existing experimental techniques (of course quantum mechanics could be wrong, but the same is true of relativity). And there are some serious problems with imagining that values for noncommuting variables exist simultaneously, it seems that you'd have to accept the possibility of hidden FTL effects as well--see this thread.

Furthermore, general relativity is just a mathematical model of reality, not the reality itself. It correctly predicts some things and is the best current model of reality, but as soon as it makes a wrong prediction it will be "back to the drawing board". To completely dismiss ideas that are not based upon GR seems to me to be to be unwise until there is a lot more evidence that the theory truly predicts reality accurately.

General relativity the physical theory will have to be replaced by a theory of quantum gravity, but the relativity of simultaneity is a consequence of Lorentz-invariance, which is not a specific physical theory but rather a symmetry in the local laws of physics, like translational symmetry, which can be true of many possible theories (for example, although quantum field theory was discovered long after special relativity, it is compatible with SR because it is a Lorentz-invariant theory). This post from sci.physics.relativity argues for why "aether theories" which postulate a single preferred frame should be considered very unlikely, and some of the given reasons would also make good arguments against the idea that some future theory will end up violating Lorentz-symmetry (especially reason #6). The basic question is, if fundamentally the laws of physics don't respect this symmetry, why have all observed phenomena so far respected it? Is it just a gigantic coincidence?

 

[vsandel]

The Twin Paradox

Yes, and so is "v". The negatives therefore cancel out to a positive.

In an earlier submission I stated that concerning the twin paradox, when a spaceship travels away from the Earth at a velocity near the speed of light time in the spaceship slows down relative to the time on earth, but on the return trip time in the spaceship speeds up relative to Earth time. Therefore the returning twin has aged exactly as much as the twin who stayed on earth. I stand by my conclusion in spite of the fact that I wrongly stated that x in the equation is negative on the way back:

t'=(t-vx/c^2)/sqrt(1-v^2/c^2)​

In Einstein's train illustration, the origin is placed on the embankment (earth) and both the train and light beam travel along the x axis. When the train (or space ship) is traveling in the positive x direction, x is positive and so is v. However, if at some point far from Earth the train (space ship) is turned around and now travels toward the light source, x is still positive but v is now negative as Peter points out. Therefore the traveler's time speeds up relative to earth.

Using Einstein's reasoning in explaining relativity, traveling toward the light source would cause the expected speed of light measured on the train (space ship) to be the sum of c+v where v is the velocity toward the light source. Since c must be constant, time must speed up in the train (spaceship) relative to the embankment (earth). Thus the slowing of time in a spaceship going away from Earth would be compensated by the speeding up of time on the way back. Therefore the twin paradox is false, and you cannot travel to the future by going on a space journey at near the speed of light. Is my reasoning correct?

 

[peter0302]

I stand by my conclusion in spite of the fact that I wrongly stated that x in the equation is negative on the way back:

t'=(t-vx/c^2)/sqrt(1-v^2/c^2)​

I should have pointed out in the other thread that that formula really is about coordinate transformations, not figuring out propert time. The formula you should be using is t'=t/gamma, which is independent of direction.

 

[vsandel]

I should have pointed out in the other thread that that formula really is about coordinate transformations, not figuring out propert time. The formula you should be using is t'=t/gamma, which is independent of direction.

Like I said earlier, I am not a physicist. What is gamma? The formula I mention is taken from Einstein's book "Relativity", which is high school level reading.

 

[JesseM]

Like I said earlier, I am not a physicist. What is gamma? The formula I mention is taken from Einstein's book "Relativity", which is high school level reading.

"gamma" (also written as the greek letter \gamma) is used as shorthand for the factor \frac{1}{\sqrt{1 - v^2/c^2}} which appears in a lot of different equations in relativity.

For example, for v=0.6c, gamma would be 1.25.

 

[vsandel]

Thanks, JesseM. I appreciate your comments to my posts. Is my reasoning correct that on the return trip from a space journey time relative to Earth time speeds up?

 

[JesseM]

Thanks, JesseM. I appreciate your comments to my posts. Is my reasoning correct that on the return trip from a space journey time relative to Earth time speeds up?

No, as long as the speed relative to Earth on both legs of the journey is the same, the time dilation factor (which depends only on the value of the speed v) will be the same on both legs too. Your argument seems to be similar to the one made by joey_m on this thread, so have a look at my comments there, especially the numerical example I gave in post #6 (and elaborated on in later posts).