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Time Travel: Where Does The Energy Go?

Hey Everyone, I’m still polishing off my top 5 fictional technology post. It will be ready soon but in the meantime…..

We recently received the following question:

First of all, of course, I love the podcast.  It was the very first one I ever subscribed to, way back in 2007.

My first question is one I’ve wondered about for as long as I can remember, and I’m certain the answer stems from a lack of understanding of the fundamental nature of the universe, but I’m at a loss.  So here it is: How would time travel not violate the first law of thermodynamics?  If I travel back in time three years, suddenly three years ago there came to be more energy in the universe than there was the moment before I arrived.  The same problem comes from jumping forward in time.  What am I missing?

Thanks for the question Geoff,

I think I can tackle your time travel question.

I’m not sure that trying to answer this question specifically concerning traveling back in time would be meaningful since this type of travel is likely impossible and any supposed support for it is scientifically tenuous at best. Because of this and the fact that this scenario wreaks havoc with the very notions of cause and effect itself, it would seem weird to complain about other potential scientific inconsistencies.

If pressed though I’d ask how traveling back in time is generally achieved. If you’re just popping into the past then yes, thermodynamics could have an issue. I can imagine scenarios however in which the extra energy popping into the past is offset in some way by what disappears from the future.

Traveling forward in time (faster then 1 sec per sec) does not have any of the problems just described. This has the full support of not only scientific theory but experiment and observation as well. The energy to send a human into the future at a good clip would be gargantuan but it is totally possible and inevitable barring any extreme bits of nastiness happening to us.

The key point here, I believe, is that any real time-traveler will not be going into the future by stepping through a portal or turning a dial to suddenly pop into the future. Problems with thermodynamics may indeed arise if that were the case but nothing in physics says we can time-travel in the magical ways we see and love in fiction. If you want to get to the future fast you have 2 options; travel at a significant fraction of the speed of light or get close to an intense gravitational source.

Therefore, you and your energy never disappear from the universe at any point in your journey. There is never any discontinuity or abrupt unexplainable change in your mass/energy caused by your time-travel.  Your mass\energy during this whole process follows predictable and explainable paths for your entire time traveling journey.

Think of an object in a very fast but stable orbit close to a black hole. That object is traveling into the future in a real sense. If it has been orbiting for 100 years (earth time) and we were able to question an alien on that object, he may say he’s been orbiting for only a few weeks of subjective time and from his perspective we would be from the future. There would be no need to explain where the extra mass/energy came from because from the universe’s perspective there is no “extra”.

To beat this dead horse even more, consider this: the entire universe consists of countless objects moving at different velocities relative to each other. This means everything is going into the future at different speeds, even different parts of your body.

5 comments to Time Travel: Where Does The Energy Go?

  • This means everything is going into the future at different speeds, even different part of your body.

    There’s an awesome joke in there somewhere. I’m sad that I can’t find it. :(

  • richardhesketh

    Ok.. as I understand it(loosely!).. if String Theory and Multiple Universe theory are to work, then any future retrograde time travel will depend on delivering the subjects atoms into an alternate past.. therefore, no change to the state. It may be happening. No, strike that.. IS happening. Or not. Or … something.

    I should have listened to Brian Cox more closely.. ;-)

  • Interestingly future time travel is in our grasp!

    To travel to the future at 2 sec/sec relative to a reference point, you must travel relative to that point at ~sqrt2*c. To accelerate a 100 kg man to this speed, you need ~9 exajoules, or about 2.5 PWh (minimum, poured exclusively into acceleration, and not correctly calculated for relativistic velocities).

  • The Prof

    I have been telling my students that time travel into the past will never be done, but not for any energy problem as the amount of energy/matter in the universe in unchanging (there are some caveats here. The reason I give is “Where are the travelers from the future?”. This is simplistic but nevertheless a strong argument. We may have travelers from a parallel universe here but they haven’t announced themselves yet.

  • Prof,

    Your are absolutely right, I think it’s Hawking who may have first popularized that notion. It’s a version of the Fermi paradox.

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