# Time travel without breaking any laws of physics

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This idea popped into my head while I was taking my nighly swim. It is a way to travel back into time, without violating any laws of physics, using only well established principles of physics. It ends with a twist.

Picture this scenario, you share a large house with a group of closest friends, both male and female. One day you leave the house to get some adult refreshments and NASA scoops the house up and send it into space at near light speed. In their reference, say, only a day goes by, due to their relativistic velocity. But in your reference, say 20 years go by.

After twenty years, NASA puts the house back. When you go to see your old friends, they have not aged but a day. Essentially by visiting them you go back in time twenty years. After one day nothing has change among your friends. There are still expecting the adult refreshments. You friend buzzard is still has the same pizza stain on his shirt. The house becomes a relativistic time capsule, which allows you to go back into time, without violating any of the laws of physics.

This is a two for one deal. Your friends get to see someone from the future. You tell then how the Red Sox won the world series in 2015.

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but, you've still aged 20 years. It's time travel, but it's irreversible.

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I actually made a very similar point just the other day... here:

I agree with your reasoning, but remain open to correction. To argue that "time dilation" is not "time travel" seems to ignore the relativity of observers.

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technically you never saw a spec of the past of your house. you just saw the 10 seconds in the future after your house was taken much later than 10 seconds. you could do it with the whole planet. and then just be shortly in the future but much older, but still you never saw the past. the future arrived more slowly is all. but you could do it to preserve the present for enjoyment in the future, like a time capsule. and actually that would be pretty cool to send stuff into space like that and get it back in the future in mint condition. mail your great great great great grandson some baseball cards by spacemail. or forget that, just take a bunch of stuff that will appreciate crazy over time with you into space moving really fast come back to earth much much later and now you're rich. you could probably have surplus after having bought the equipment you needed to do it too including interest on the loan you had to take out. especially since it would have become vintage and could be worth a few bucks.

but knowing my luck i would come back to the planet of the apes or something.

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Typically time travel has you going back into time. I simply change the reference and had the past frozen in time, and you moving forward. The past in the house never moved to your future, so it is still the past. Again the ideas was not to violate any of the laws of known physics but to create the same affect one would see if you went back in time 20 years to the house you once lived in with your friends.

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pioneer---

What you have described is more or less the twins paradox.

Presumably NASA must accelerate the house to light speed? Time dialation only works at constant velocity. So as your friends accelerate, they age more quickly. They are as old as you are when they return to Earth.

Do a google search if you don't believe me.

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What you have described is more or less the twins paradox.

Yep. And personally I don't really see the point of this thread. In the end, it just seems to use a different meaning of time travel than I'd expect.

Presumably NASA must accelerate the house to light speed?

No, but you probably know that yourself and just used sloppy language.

Time dialation only works at constant velocity.

Not exactly sure what you mean. Yet, I think a fitting reply is "no".

So as your friends accelerate, they age more quickly.

I think here I can safely say "no". It's a frame-dependent statement at best.

They are as old as you are when they return to Earth.

I'm even more convinced of a "no", here.

Do a google search if you don't believe me.

I'll pass on this one. In the inertial frame you stay in, $d\tau = dt/\gamma(v), \gamma \geq 1.$ No acceleration term in there. For me, that's more convincing than the next-best google hit.

The idea works, except for the badly chosen numbers (your friends will most likely not want a beer after being smashed by the enourmous acceleration required for these numbers) and NASA not having the technology to do it - but that's nitpicking. I just don't really see the point of the example. The result seems pretty much a triviality (like said, it's just the twin's paradoxon).

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You know, I thought I had something to say, but really I have nothing. Uhh, wow, hooray for relativity?

In that respect we're constantly traveling forward in time without violating the laws of physics. What's so exciting about that?

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Shit---

You are correct.

Perhaps I should have done a google search myself

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actually the time dilation equation you posted doesn't provide a complete answer to the paradox, in the frame of the earth you could run that equation and you would get the answer that the twin has aged x number of years while you aged y number of years. however you could just as easily state that the reverse were true for twin in the space ship. the question really becomes what are we missing which decides which equation is true.

you might be tempted to simply state that the ship is an accelerated frame and thus you can't use it as a frame of reference. however if the ship accelerated in such a way that every object on board accelerated at the same speed, so no measurement could show that it was in an accelerating frame, then the space ship could just as easily say that the earth was accelerating away from it.

to make it simpler imagine two particles which existed in a universe devoid of everything but the particles and a constant gravitational field that started at a point in between the particles and drew one of them off along the x axis. now say that that field were to reverse itself and send the particle back in such a way that it would eventually be at rest with respect to the stationary particle. which particles frame was the proper one?

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Sometimes, the words around the equations are important, too. "In the inertial frame you stay in" meant earth's frame. Tau is the time measured by the ppl in the house, t is the time measured on earth, v is the velocity of the house as seen from earth. I did not claim the equation to be true in the other frame. I have not stated the integration boundaries (because they were irrelevant for my statement). Certainly, both objects (house and you) experience a certain time between the NASA theft and the reunition. These times cannot depend on a chosen frame of reference. So it is possible to explicitely calculate the in one frame (like the one for which the equations are true) and know other possibly more complicated results would give the same. Assuming a specific acceleration profile, the results could look something like this:

,

with T being the total time measured in earth's frame.

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however if there is no way of choosing a proper inertial frame, and the equations in both frames are inconsistent. Then you still have a paradox that prevents you from stating absolutely that the frame of the earth is the one from which the equations should be run.

forgive me I'm no expert in this subject, I'm just going off what my modern physics professor said, and the few general relativity texts that I've read the intro to (I'm taking my first course in GR this semester ). However from what i've seen the time dilation equation of special relativity is missing an extra term which shows the time dilation to to acceleration effects (usually gravity).

For instance the global positioning system has to be correct something like 19 nanoseconds of lag due to SR and 33 nanoseconds worth of boost due to GR.

however as the twin perodox shows SR is only consisted with non-accelerating frames of reference, and once you have any accelerating reference point, the equations are inconsistent.

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however if there is no way of choosing a proper inertial frame, ...

Earth's surface is a good approximation to an inertial frame in this case. The correction due to the gravitational field that you stay in is in the order of $10^{-8}$. Correction due to earth's rotation is probably in the same order (too lazy to calculate it). Corrections for the house are even smaller (and counter the corrections for the earth frame, anyways).

...and the equations in both frames are inconsistent.

That depends on the point of view. From my point of view, the equations are the same in every frame: The eigentime tau an objects experiences during its path is:

$\tau = \int_0^1 \| \frac{d\gamma (x)}{dx}\| dx$,

where $\gamma(x)$ is the curve the object takes through spacetime (not the Gamov factor!), x is the curve parameter running from 0 to 1 and $\| v \| := \sqrt{g_{\mu \nu} v^{\mu} v^{\nu}}$ (I know ppl who would kill me for this notation ). You might want to keep that formula in mind for your lecture for that it plays a very fundamental rule (of being the definition of eigentime and also the starting point to derive the GR equations of motion - we even had an article on WiSci about that).

What I wrote was just the specialization of this for a certain choice of spacetime geometry (approximately flat) and coordinate system (time-orthogonal cartesian, resting with the guy on earth).

Then you still have a paradox that prevents you from stating absolutely that the frame of the earth is the one from which the equations should be run.

As said, you can chose any coordinate system in principle. If you wanted to take into account earth's gravitational field, the rotation of earth and make the simplification that there are no other spacetime-bending bodies, then you could take the Schwarzschild metric and have both bodies moving in that CS accordingly (that's the assumption I have approximated the $10^{-8}$-correction from). The big point in GR is: You can chose any coordinate system you want, the result for certain types of values (scalars like lengths of trajectories, depending on point of view possibly also vectors and tensors) will, and must, always result in the same value.

...I'm just going off what my modern physics professor said,

I don't really understand what he said.

...and the few general relativity texts that I've read the intro to (I'm taking my first course in GR this semester ).

Perhaps if you quoted some passages it would be easier for me to understand what they/you mean. Have fun with your GR course. Personally, GR was by far the most interesting course (topic-wise, the lecturer was no too good) I took in university.

However from what i've seen the time dilation equation of special relativity is missing an extra term which shows the time dilation to to acceleration effects (usually gravity).

Yes and no. SR is just an approximation. I'm not sure if I agree with the statement "extra term" for that the calculation approach is simply a bit different. But of course you can always define "extra term" = "GR result" - "SR result".

however as the twin perodox shows SR is only consisted with non-accelerating frames of reference, and once you have any accelerating reference point, the equations are inconsistent.

That depends on where you draw the line between SR and GR. Some people say SR is flat spacetime and non-accelerating frames. Others count accelerating frames to SR (the step to accelerating frames is not much of a deal) and draw the distinction to GR when non-flat geometries are involved. Personally, I tend to go one step further and make the distinction at the point where the influence of energy on spacetime is considered (that is, when the equations of motion of objects within spacetime are completed by the field equations that describe the "reaction" of spacetime on the particle content).

Either way, and that probably clears things up a bit: The equation I posted was derived by GR methods -using some approximations- in the first place. I just (and not too surprisingly) turned out to approximately result in the equations you'd expect from SR in the first place. I barely think about relativistic problems in the SR context (quantum theories being the big class of exceptions) because conceptually I find GR easier to understand.

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The SR affect has nothing to do with the reference you chose. It only works on the reference that is in motion. The reason this is so is that only that reference had the energy input needed to achieve the final state. The difference is the difference between 2-parameter and 3-parameter SR. With 2-parameter SR, the references appears relative. But if you add the third or mass parameter, only the one with real velocity and energy is able to generate relativistic mass. The stationary reference never changes relative to a 3-parameter analysis.

I did this example before, but I will do it again. It is called the SR workout. You go to a track and sit yourself down in a chair. You then focus on the fastest runner and using relative reference, pretend he is stationary and you are the one in motion. Yet get to burn calories without ever having to move or bust a sweat.

Obviously, common sense would say, inspite of the relative reference, it is only the one with absolute motion, who is using energy. With 2-parameter SR we can pretend. But with 3-parameter SR, you need to do an energy balance. The result is that relative reference does not always express the reality of what is going on.

Relative to the house and occupants, we had to pump a lot of energy into it to be able to get those velocities. This energy is the basis of the time capsule or the time dilation. The person left on earth didn't get that same energy. So his cycles will stay the same as they have always been.

In this example, one does not technically travel back in time in the normal sense. But it does create an affect that would look the same. It would be a living flashback to younger days when life was about to unfold.

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• 3 weeks later...

Atheist we just talked out this problem in general relativity (it is my favorite course as well) I found the reason we were disagreeing.

you were speaking from thepoint of view as if the space ship knew it was accelerating thus making it an invalid reference frame, while I was speaking from one where it was not and thus geodesic.

this was the version of the problem I putto my professor for clarification on.

imagine a gravitational potential well, where the earth is situated at the top of the well at a point where dv/dt is zero. and the ship is an infinitesimal discplacement closer to the well where dv/dt is not zero.

the ship thus falls into the well, and begins a harmonic oscilation where it eventually comes back to that infinitesimal distance from the earth, if you were to compare times between these two reference frames they would come back to have an infinitesimal discrepancy as they were both geodesic, and as such the they are both valid reference frames.

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I don't understand what you say. There is no reason the rocket should be travelling on a geodesic. It's a rocket, not a rock. There is no reason that a coordinate system should lead to inertial frames in every spacetime point (in fact it is, iirc, possible to prove that for arbitrary spacetime structure this is impossible) -> there'd better not be a need for GR to only work in inertial frames or in frames where (t,0,0,0) is a geodesic.

Could it be that you're talking about a different problem than the original one?

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No one said Time Travel was impossible. In fact were all travelling through time as we speak...

Think of time as a series of one way streets going in the same direction... Some of these streets might be moving faster or slower but essentially they're all going the same way.

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Reminds me of Rip Van Winkle.....

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I said before that what me and bentheman were talking about was the case where the rocket was in a situation where it was accelerated in such a way as it could be considered an inertial frame of reference (say a gravity well) I still havent done the gerneal relativistic calculation on a rocket however I do know that if the rocket can feel the acceleration then the twin comes back younger

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