What are the disadvantages of traveling into the future?

[Art]

You can't travel into the future it does not exist, that would violate causality.

Actually to be frank current thinking says that even with traveling back in time it can't be possible because we'd of seen people traveling back in time to the point where time travel became possible, the reasons are mathematical. Since it's not possible it's currently impossible. It's all a little complicated.

Maybe not necessarily so.

Okay it can't be done by traveling ever faster but if there was a way to time travel by jumping to a different set of spacetime coordinates then whether you moved forward or backward in time you would also change position in space to fit the physical space coordinates appropriate to your time destination. So perhaps the rule is simply you can't occupy the same space at a different time which helpfully avoids all those nasty paradoxes. In fact the moving in time part would be pretty irrelevant for the person doing it but it would make a great way to achieve long distance space travel.

 

[Evo]

Maybe not necessarily so.

Okay it can't be done by traveling ever faster but if there was a way to time travel by jumping to a different set of spacetime coordinates then whether you moved forward or backward in time you would also change position in space to fit the physical space coordinates appropriate to your time destination. So perhaps the rule is simply you can't occupy the same space at a different time which helpfully avoids all those nasty paradoxes. In fact the moving in time part would be pretty irrelevant for the person doing it but it would make a great way to achieve long distance space travel.

Isn't that movie physics? How exactly would you accomplish that?

 

[Gokul43201]

Maybe not necessarily so.

Okay it can't be done by traveling ever faster but if there was a way to time travel by jumping to a different set of spacetime coordinates then whether you moved forward or backward in time you would also change position in space to fit the physical space coordinates appropriate to your time destination.

I don't understand this part at all.

So perhaps the rule is simply you can't occupy the same space at a different time which helpfully avoids all those nasty paradoxes.

Actually, that is a perfectly legal thing to do, and we do it all the time - by not moving. Did you mean to say that you can not uccupy the same time co-ordinate at different space co-ordinates? Because that will give you paradoxes. More generally, traveling through any spacelike interval is a problem. Traveling across timelike intervals, however, is not only allowed, it is all that's possible.

 

[Art]

Isn't that movie physics? How exactly would you accomplish that?

Unfortunately I have no idea how you would do it.

I guess my point is time travel is normally dismissed as an unrealistic concept because it requires impossibly fast space ships or if done in situ for inducing impossible paradoxes. I'm just remarking that if by moving in time you also move in space you avoid the definite no no's and at least make it hypothetically possible.

 

[Art]

I don't understand this part at all.
Actually, that is a perfectly legal thing to do, and we do it all the time - by not moving. Did you mean to say that you can not uccupy the same time co-ordinate at different space co-ordinates? Because that will give you paradoxes. More generally, traveling through any spacelike interval is a problem. Traveling across timelike intervals, however, is not only allowed, it is all that's possible.

We do it all the time?? How do you figure that??

We're on a planet spinning on it's axis orbiting a sun which orbits the galaxy which itself moves through an expanding universe. If you got even one second out of synch you would find yourself a long long way from planet Earth and so we have certainly never occupied the same space at two different moments much less all the time.

For instance if you went back in time a million years unless you dragged the entire planet/sun/solar system/galaxy/universe with you then you would appear at your point in space as it was back then, 1 million years before planet Earth reached this spot which is why I said time travel could equate to serious long distance space travel.

 

[Gokul43201]

We do it all the time??

We're on a planet spinning on it's axis orbiting a sun which orbits the galaxy which itself moves through the universe. If you got even one second out of synch you would find yourself a long way from planet Earth.

I just did a calculation and I find myself about a foot away from where I was, for a 1 second error. Remember, there is no such thing as an absolute rest frame. The only frame meaningful to me is the inertial frame in which I'm at rest at any moment. So it doesn't matter how fast the Earth or the Solar System or the Galaxy are moving, what matters is their accelerations. The greatest of these accelerations is the Earth's rotational acceleration about its polar axis - which is about a foot/s^2 (the others are several orders of magnitude smaller).

If I vanished at some instant for a period of about 1 second, and then in the inertial frame that was attached to me when I disappeared, I reappear at the same spatial co-ordinates that I had before disappearing, I'll find myself about a foot up in the air from the spot I was on before I left.

For instance if you went back in time a million years unless you dragged the entire planet/sun/solar system/galaxy/universe with you then you would appear at your point in space as it was back then, 1 million years before planet Earth reached this spot which is why I said time travel could equate to long distance space travel.

You can't talk about time (and hence, time travel) without specifying a frame. And your rest frame is, like it or not, being dragged along with the planet/sun/solar system/galaxy/universe. And rest frames which are inertial are only gradually spinning away from the Earth.

 

[Art]

I just did a calculation and I find myself about a foot away from where I was, for a 1 second error. Remember, there is no such thing as an absolute rest frame. The only frame meaningful to me is the inertial frame in which I'm at rest at any moment. So it doesn't matter how fast the Earth or the Solar System or the Galaxy are moving, what matters is their accelerations. The greatest of these accelerations is the Earth's rotational acceleration about its polar axis - which is about a foot/s^2 (the others are several orders of magnitude smaller).

If I vanished at some instant for a period of about 1 second, and then in the inertial frame that was attached to me when I disappeared, I reappear at the same spatial co-ordinates that I had before disappearing, I'll find myself about a foot up in the air from the spot I was on before I left.

You can't talk about time (and hence, time travel) without specifying a frame. And your rest frame is, like it or not, being along with the planet/sun/solar system/galaxy/universe.

Your reference to your inertial frame is what I am contesting. We can't measure our absolute rest frame unless we traveled in time and then of course we could and with time travel it is the absolute rest frame that becomes relevant although simply realising that we are traveling in many different ways simultaneously even without knowing the actual speeds is enough to show your calculation provides an erroneous answer. Just looking at the distance the Earth travels in it's orbit of the sun yields a displacement of 9 miles in that 1 second and the sun's motion around the galaxy yields a further displacement of 138 miles in that second.

 

[Gokul43201]

Your reference to your inertial frame is what I am contesting. We can't measure our absolute rest frame

You can't measure any frame. It is meaningless to speak of measuring a frame. You possibly mean something else, but I'm not getting it.

unless we traveled in time and then of course we could

No, and no. First of all, to even talk about time, you need to first specify a frame.

and with time travel it is the absolute rest frame that becomes relevant

There's no such thing as an "absolute rest frame"...unless you wish to contest Relativity. Do you?

 

[Art]

You can't measure any frame. It is meaningless to speak of measuring a frame. You possibly mean something else, but I'm not getting it.

No, and no. First of all, to even talk about time, you need to first specify a frame.

There's no such thing as an "absolute rest frame"...unless you wish to contest Relativity. Do you?

To clarify I am saying IF you could travel in time in the way I described, by knowing how much you traveled in time and measuring the distance you now are from the Earth you would be able to calculate the true velocity of the Earth as you have now decoupled yourself from the uniform motion of your initial inertial frame. That is what I meant by having an absolute rest frame though perhaps that's the wrong terminology??

Still waiting for your response to the displacements I gave for 1 second using our known velocities. Do you contest them? If so I think you are missing my point entirely.

 

[Gokul43201]

To clarify I am saying IF you could travel in time in the way I described, by knowing how much you traveled in time and measuring the distance you now are from the Earth you would be able to calculate the true velocity of the Earth. That is what I meant by having an absolute rest frame though perhaps that's the wrong terminology??

I may be misunderstanding everything you're saying, but again, there is no such thing as "the true velocity of the Earth". It is meaningless in the exact same way that an absolute rest frame is. You can not "decouple" from velocities, only from accelerations.

Still waiting for your response to the displacements I gave for 1 second using our known velocities. Do you contest them? If so I think you are missing my point entirely.

Perhaps I am. Using the velocity of the Earth relative to the Sun is meaningless. It is the accelerations that matter, not velocities. There is no such thing as a true velocity. But there is a true acceleration.

 

[Art]

I may be misunderstanding everything you're saying, but again, there is no such thing as "the true velocity of the Earth". It is meaningless in the exact same way that an absolute rest frame is. You can not "decouple" from velocities, only from accelerations.

Perhaps I am. Using the velocity of the Earth relative to the Sun is meaningless. It is the accelerations that matter, not velocities. There is no such thing as a true velocity. But there is a true acceleration.

We currently do not know the Earth's velocity within the universe because we have no point of reference from which to measure it. IF (and a BIG if) we could travel through time under the conditions I specified then we would have our reference point and so the true velocity of the Earth could be calculated (distance traveled/time)

I honestly do not see what is difficult to understand in the concept of displacement through time travel. On a simple local basis we know how far the Earth travels in a second and so ignoring all other motions that is the distance you would be from the Earth if you moved back or forward 1 second.

 

[Gokul43201]

We currently do not know the Earth's velocity within the universe because we have no point of reference from which to measure it. IF (and a BIG if) we could travel through time under the conditions I specified then we would have our reference point and so the true velocity of the Earth could be calculated (distance traveled/time)

No, Art, that's not right. The problem with talking about a "true velocity" is not that it can not be measured in practice but that it does not exist even in theory (or in a thought experiment). To describe the "conditions" for your form of time travel, you postulate the existence of an absolute rest frame. Then you deduce that by measuring the displacement in this frame you can find a true velocity.

The reason there is no true velocity is that there is no absolute rest frame in the first place.

I honestly do not see what is difficult to understand in the concept of displacement through time travel. On a simple local basis we know how far the Earth travels in a second and so that is the distance you would be from the Earth if you moved back or forward 1 second.

No, Art, that's wrong. A correct statement would look like this: "We know how far the Earth travels in a second, relative to some chosen inertial frame, and so that is the distance you would be from the Earth if you moved back or forward 1 second, in that same frame."

 

[Art]

No, Art, that's not right. The problem with talking about a "true velocity" is not that it can not be measured in practice but that it does not exist even in theory (or in a thought experiment). To describe the "conditions" for your form of time travel, you postulate the existence of an absolute rest frame. Then you deduce that by measuring the displacement in this frame you can find a true velocity.

The reason there is no true velocity is that there is no absolute rest frame in the first place.

Gokul call it whatever you want, inertial frame absolute frame or even a cold frame. My very simple point remains; if you knew precisely how far the Earth traveled in 1 second ie X miles from t-0 to t-1 then by definition you would know the speed of the Earth.

No, Art, that's wrong. A correct statement would look like this: "We know how far the Earth travels in a second, relative to some chosen inertial frame, and so that is the distance you would be from the Earth if you moved back or forward 1 second, in that same frame."

I don't know if you are obfuscating deliberately but to keep it really simple I specifically limited my example to a local level using only known velocities so obviously this means they are all in the same frame.

I suspect you no longer want to hang onto your 1 foot displacement? Does that mean you now accept the local displacements of 9 miles and 138 miles I provided?

 

[DaveC426913]

You can't travel into the future, for the reasons given above. That is a scientific answer.

*scratches head* Why is everyone claiming he can't travel into the future?

If he got in a ship and did a quick trip around the centre of the galaxy, he'd arrive home
millions of years in the future.

 

[Gokul43201]

Gokul call it whatever you want, inertial frame absolute frame or even a cold frame. My very simple point remains; if you knew precisely how far the Earth traveled in 1 second ie X miles from t-0 to t-1 then by definition you would know the speed of the Earth.

No, you do not know any kind of true speed and you do not have any kind of absolute frame. You are again making meaningless statements. I can just as well choose a frame where the Earth travels 0 miles in 1 second.

I don't know if you are obfuscating deliberately but to keep it really simple I specifically limited my example to a local level using only known velocities so obviously this means they are all in the same frame.

And by the particular choice of frame you can get a distance of a light year or a millimeter. There is no fixed distance, independent of frame.

I suspect you no longer want to hang onto your 1 foot displacement? Does that mean you now accept the local displacements of 9 miles and 138 miles I provided?

You suspect wrong. Your number is meaningless. I stand by my estimate of about a foot. It is correct. But I no longer have the energy to go on with this exercise.

 

[Schrodinger's Dog]

*scratches head* Why is everyone claiming he can't travel into the future?

If he got in a ship and did a quick trip around the centre of the galaxy, he'd arrive home
millions of years in the future.

Er OK...

 

[Art]

You suspect wrong. Your number is meaningless. I stand by my estimate of about a foot. It is correct. But I no longer have the energy to go on with this exercise.

If an object traveling perpendicular to the Earth's orbit around the sun intersects Earth's orbital plane at a point 'A' 1 second after the trailing edge of the Earth passes that same point 'A' how much will it miss the surface of the Earth by, 1 foot or 9 miles?

To keep it simple let's make the object massless and ignore the angular component of the Earth's motion.

 

[vanesch]

If an object traveling perpendicular to the Earth's orbit around the sun intersects Earth's orbital plane at a point 'A' 1 second after the trailing edge of the Earth passes that same point 'A' how much will it miss the surface of the Earth by, 1 foot or 9 miles?

To keep it simple let's make the object massless and ignore the angular component of the Earth's motion.

The point, Art, that Gokul is making, is that in order to say "travels perpendicular to the Earth's orbit", you already need to fix a reference frame. I guess you are talking about a frame fixed to the sun. But a frame fixed to the Earth would see different objects "travelling perpendicular to its orbit". As seen from the moon, this direction is different than as seen from the sun. As seen from the earth, *every* direction is going to be perpendicular to the Earth's orbit.

As Gokul pointed out (it was Galileo's major discovery), there is no such thing as "absolute velocity", simply because velocity is a concept that is only defined with respect to a frame, and there is no way to fix a frame, unless you fix it with respect to some objects, or with respect to hypothetical motions with respect to some objects.

It is not a matter of "not knowing practically how to do it". It is an impossibility of principle, which is due to a symmetry of the laws of nature. The reason is that there is no way to distinguish, by any physical experiment, a frame, and a frame moving in uniform motion wrt to the former. All laws of nature are the same in both. Note that this is not true for rotation!

You could just as well talk about the "absolute angle of a certain direction". You can't. You can only define an angle between two directions, and there's no way to find a "preferred direction" without referring to a material object.

 

[Art]

The point, Art, that Gokul is making, is that in order to say "travels perpendicular to the Earth's orbit", you already need to fix a reference frame. I guess you are talking about a frame fixed to the sun. But a frame fixed to the Earth would see different objects "travelling perpendicular to its orbit". As seen from the moon, this direction is different than as seen from the sun. As seen from the earth, *every* direction is going to be perpendicular to the Earth's orbit.

Yes I used the sun as the reference point and the Earth's orbit around it. I also quoted figures earlier using the galaxies centre as the reference point. Extrapolating further one could use the local group of galaxies as the reference point and so on until the universe itself becomes the reference point.

As Gokul pointed out (it was Galileo's major discovery), there is no such thing as "absolute velocity", simply because velocity is a concept that is only defined with respect to a frame, and there is no way to fix a frame, unless you fix it with respect to some objects, or with respect to hypothetical motions with respect to some objects.

My suggestion is if time travel were possible the fabric of the universe itself would be the frame which would be the ultimate baseline to measure all velocities.

It's hard to think of a good analogy but I'll try.

Imagine if one could take a time series of snapshots of the universe (ignore expansion for the moment for the sake of simplicity) all from the same viewpoint you would see spatial changes from 1 picture to the next over time. Now if you were represented by a dot on these photos joined (by gravity) to a bigger dot (Earth) you would see your position (and your connected bigger dot) change in concert with the spatial changes in the photos with a consequent change in coordinate position relative to all the other dots on the picture and the frame of the picture itself. If you were then to lift the dot representing you from the last picture and superimpose it in the same coordinate position as the frame you lifted it from back onto the first picture you would no longer be connected to the larger dot (unless you and the Earth are at the centre of the universe). You could then measure how far away the larger dot is from you and as you know your temporal displacement (and to keep it simple ignore angular momentum) calculate it's velocity. No??

Anyway regardless of velocities my central point was if one traveled in time it would equate to traveling a distance through space as you now occupy a position in space that either a) the Earth hasn't yet reached or b) the Earth has already passed through.

To try another analogy if you were standing on a travelator with a friend and equate it's movement to time and the view you see on the walls either side from your position as the universe locked to that time and then you were to jump either forward or backward on the travelator your friend would be nowhere to be seen as he is now in the future or the past and your view of the universe would change to reflect your new position in time.

 

[ZapperZ]

Er.. spacelike travel is not identical to timelike travel. In SR, spacelike events are not identical to timelike events. I'm sure you know that (to be able to make the claims that you have posted so far), so I'm not sure why you are equating ".. traveled in time it would equate to traveling a distance through space .."

Please note that while this may be in GD, the PF rules still apply. What you have posted contradicts SR (as stated by Gokul earlier). If you wish to do that, then this cannot be done here and only in the IR forum.

Zz.

 

[Evo]

I'm closing the thread as it seems to be getting too speculative.