# Can I turn my teleporter into a time machine?

Imagine that I invent a teleporter that lets me instantaneously transport myself across any distance. It consists of two manhole-cover-sized plates. Step on one, press a button, and … POP … I instantly appear on the other plate no matter where it is in the universe.

At first I amuse myself by putting one plate on Mars and the other on Earth. Since Mars and Earth are several light minutes apart, I can wave up at the sky, step on the plate, pop to the other planet, and then look through my super-powerful telescope to see myself waving.

But after a while waving to myself gets boring. What I’d really like to do is shake hands with myself! Is there any way I can use my instantaneous teleporter as a time machine? Maybe something involving very fast rockets or black holes? But, please, nothing that violates the laws of physics. I’ve already done that once by inventing the teleportation plates, and I’m afraid if I do it again it might destroy the universe.

I once read a story with this as the concept. It involved a teleporter that requires two gates that have been linked together. The time travel part worked by accelerating one of the gates around for a while, then bringing it back. Since the stationary gate is now older than the accelerated gate, the stationary gate is now synchronised to the future accelerated gate.

It’s your imagination. Do whatever you want.

Well, I could have just asked “Does an instantaneous teleporter violate causality?”, but where’s the fun in that?

Perhaps you could rewire it to teleport this thread into the proper forum? Or maybe you need to just snip the red wire? You do realize that use of duct tape reduces the frequency of the flux capacitor, don’t you?

Y’know, you guys could wait for a few more serious answers before making fun of the OP.

The short answer is, assuming Special Relativity is true (and we have every reason to believe that it is), then any device that can be used to get you from Point A to Point B before light could get there traveling through a vacuum, can also be used as a time machine. It doesn’t matter whether it’s a wormhole, a warp drive, or a magic carpet. In the specific case of a linked-gate sort of FTL device, like your teleporter or a wormhole, the most straightforward method is to either fly one of them around at high speed for a while, as Grumman suggests, or to leave one in a strong gravitational field for a while.

Of course, the problematic question is “What does it mean for two distinct points in spacetime to be instantaneous?”. According to special relativity, different frames of reference will consider “simultaneously to right now, but over there instead of here!” to be at different times. Assuming your machine could take advantage of this, two teleportations using different frames of reference could easily pull you unambiguously backwards in time (or forwards if you like).

Sure. Just turn the cardboard box around and climb in. But don’t flip it over and climb underneath—that’s how you turn it into a transmogrifier.

So it sounds like if I took plate A with me on a high-speed jaunt to Alpha Centauri and back I could get a temporal separation of 8 years or so between it and plate B. (How much separation would depends on the different elapsed times of the moving and stationary plates, I assume.) So when I got back to Earth I could set the two plates side by side and use them to hop backwards or forwards in eight year jumps.

Does teleporting A -> B take me back in time? Or is it B -> A?

A single teleportation in a manner which keeps you at the same time according to some frames of reference will also take you forward in time according to some other frames of reference and will take you backward in time according to some other frames of reference. So neither teleportation in itself is taking you back in time; it’s just that you can arrange it, by having the frame of reference in which the one is considered “instantaneous” different from that for the other, so that the two teleportations in a row coordinate to ultimately take you backwards in time according to every frame of reference.

(I think perhaps the best thing to do is to illustrate this visually, drawing out the light cones and so on. Unfortunately, I am lazy…)

I don’t believe this is true. Wouldn’t the forwards and backwards movements cancel each other out?

Here’s a picture:

One frame of reference:

``````

\ /   \  \ /  /
B     \  A  /
/ \     \/ \/
/   \    /\ /\
/     \  /  C  \
\/  / \  \

``````

The same situation, but drawn according to another frame of reference:

``````

/\  \ /  /
\     /  \  A  /
\   /    \/ \/
\ /     /\ /\
B     /  C  \
/ \   /  / \  \

``````

Read the horizontal axis on this picture as one dimension of space, and the vertical axis as time. A, B, and C are three particular locations in spacetime, and the lines indicate the paths that light takes when emitted from various points (I wish I could combine the monospacing of the “code” tag with colors. Perhaps there is a way?).

Note that in the first diagram, A is at the same time as B. Accordingly, one could presumably teleport from A to B (that being instantaneous motion across space, according to this frame of reference). However, in the second diagram, B is at the same time as C. Accordingly, one could presumably teleport from B to C (that being instantaneous motion across space, according to this frame of reference). The cumulative effect is to teleport from A to C, which is unambiguously teleportation backwards in time (as C is within the past light cone of A).

Yes you can change a transporter into a time machine. People do it all the time. In fact I do know you from Adam, as I met him.

What is your reasoning for believing this is a plausible concept? If I had a teleporter and wanted to go 1 year back in time (points A and C), using Alpha Centauri as point B, what two frames of reference would be used to create the situation you believe exists, that would allow me to teleport to Alpha Centauri and then back to Earth, 2009?

I don’t think the “accelerate the teleporter, and then bring it back”-method would necessarily turn it into a time machine. It would of course be easy to test once we had the machine, but for now we have to a couple of things to figure out first.

With special relativity there’s no longer an agreed upon simultaneous, so there’s no longer an obvious definition for instantaneous. I see no reason for a definition that would warp simultaneity permanently for the two teleporter units.

General relativity is a better bet. Put one unit in a really tall tower, and keep the other one at ground level. Wait for a really long time… No, on second thought I don’t think that’d make them time machines either.

If the teleportation is instantaneous in a frame of reference determined by the two ends of the teleporter, then you’d be able to make a time machine by teleporting, accelerating the two teleporters to change their frame of reference, then teleporting back.

If the teleportation was instantaneous in some fixed special frame of reference, say one aligned with the cosmic microwave background, then you wouldn’t be able to use this as a time machine. You’d never go backwards in time in that special frame, and you’d never be able to end up backwards in time in the same location in any other frame of reference.

The first frame of reference would be one moving in the direction from Alpha Centauri to Earth, and the second one would be moving in the opposite direction.

Sure, but the existence of such a fixed reference frame would violate Special Relativity. The CMB does define an interesting and convenient reference frame, but it’s not a special one with regards to the laws of physics, any more than the Earth’s reference frame (also interesting and convenient) is special.

I don’t actually know nearly as much about this as Chronos does, who already answered your question, but in case you care what my particular reasoning was, it was this: “the temporal relation between X and Y is the same according to all frames of reference if and only if they are within each other’s light cones; otherwise, different frames of reference will have X before Y, X at the same time as Y, and X after Y”. Using this, one can easily see that the second picture must be the same as the first from some other frame of reference, since A and C are both outside of B’s light cone, albeit within each other’s.

Wasn’t this a topic of one of the essays in Larry Niven’s All The Myriad Ways?

It wouldn’t violate Special Relativity in the sense of contradicting any experiment that’s ever been performed, or invalidating any equations of SR. While the CMB isn’t special WRT the laws of physics as we currently understand them, we don’t know how to perform instantaneous teleportation using those laws of physics either, do we? So instantaneous teleportation will require some addition to the known laws of physics. Perhaps that addition will reveal that there is a special reference frame, even though the subset of physics we currently know of is the same for all reference frames.