time travel

i have a question - is time travel theoretically possible? What does Einstein’s theory of relativity tell us about traveling backward and forward in time? thanks.
emma

The Master speaks.

http://www.physics.hku.hk/~tboyce/sf/topics/wormhole/wormhole.html

http://www.anu.edu.au/Physics/courses/A07/studentsites/studentsites2001/WAG/Wormholes.html

http://www.dd.chalmers.se/~f93jojo/sidan2.htm

I’ve tried to find a few sites about wormholes and negative energy, which seems to make time travel possible (if only in an extremely theoretical way). Of course, all the methods proposed are completely untested.

Every night I travel 8 hours into the future.

Does that count?

As Cecil’s column mentions the jury is still out on this one. Time travel causes so many problems that seem unresolvable that many figure it must be impossible.

However, I think it was Kip Thorne (eminent physicist) who showed that as far as we know today there isn’t anything actually prohibiting time travel. Apparently Mr. Thorne had gotten fed up with ceaseless questions about time travel so he actually set out to prove it is an impossibility. Much to his surprise he could find nothing in physics as we know it today (mostly Relativity for this) that absolutely prohibits time travel.

However, as also shown in Cecil’s article, any proposed workable time machine is almost inconceivably difficult to build. Maybe not truly an impossibility but so darn close that it may as well be.

No one has ever pinned down exactly what time is. True, it’s used in physics as a dimension, but all that means is that it is a variable. See Stanford University’s Time, including arguments about whether time had a beginning, whether travel in time is philosophically possible, and the 3D versus 4D views on time.

The faster you go, the slower time is relative to everything else. So it is physically possible to be travelling at high rates of speed for 1 hour and comeback to your origin and 1 year has past.

…you cant go back tho.

Part of the problem is that we can’t even speculate on what time travle is.

If you go back in time, do you get younger? Does your current self replace your older self in past-time? Would you appear as a ghost, unable to interact with the past, only observe it?

These questions lay the foundations for the plethora of paradoxes time travel introduces. A proposed mechanism for time travel would have to address which of the above scenarios would manifest themselves.

Read the link earlier in the thread to Cecil’s column on this to see a proposal for a working time machine.

Your way involves travelling faster than light which is impossible (although it would imply backwards time travel). Unfortunately when you hit lightspeed (not possible) on your way to going faster than lightspeed (not possible) time will stop for you (not possible) so you’ll never go over the speed of light…the Universe will end before that happens.

Not being able to travel faster than the speed of light so far remains inviolate as a law of the Universe near as we can tell. Time travel on the other hand we’re not so clear on.

[sub]If YOU’VE done six impossible things this morning why not round it off with breakfast at Milliways! The Restaurant at the End of the Universe! ~Douglas Adams[/sub]

I’m sure it’s been answered elsewhere but since we’re on the subject…

Why can’t you go faster than light? Other than the fact that I get winded running at 10 mph, of course.

Even if you were able to go back in time, you would have to have a space ship to go back to where the earth was years ago.

The earth, and our galaxy, has traveled hundreds of millions of miles and we are no where near where we were just a few years ago.

It would have to be a very fast space ship, or you would spend all your life getting back to where the earth was in the year that you traveled back to.

Just not worth it.

Would a time machine cause a conflict with the cause/effect principle of science? Would time travel to the future lend credibility to the religious idea of preordination?

A whole bunch of reasons. Here are some of them:

[ul]

[li] Time slows as you approach the speed of light. Imagine you had a space ship that could achieve light speed and you hit the light speed button (which gets you to speed instantly…hypothetically) and as fast as you can you hit the button again to turn off your light speed drive. You will NEVER EVER hit that button the second time. Time has stopped for you. The Universe will cease to exist before you turn off your machine.[/li]
[li] Your mass increases as you approach the speed of light. Once you reach light speed you would have infinite mass which has a few implications…one of which is you’d need infinite energy to move you (more energy than the entire Universe has available).[/li]
[li] Length decreases as you approach light speed. If you hit light speed the Universe, to you, would be an infinitesimal point.[/li]
[li] An implication of the above two reasons would be also having you occupy every point in space simultaneously.[/li][/ul]

That should do for now I think. Understand that these predictions of Relativity have been tested over and over again and the tests match perfectly with the theory (mass increase, time slowing down and so on). There is nothing so far to suggest that this is not an absolutel limit in the Universe.

[sub]NOTE: Some ‘things’ can have apparent motions greater than light speed. However, none of them are tangible things. For instance, you could get a shadow to move faster than lightspeed but a shadow is not a ‘real’ thing.[/sub]

Maybe but you can conceive of ways around this such as parallel universes and time travel merely switches universes for you such that to you it seems like you travelled in time. Of course, parallel universes are untestable (although in a way implied by Quantum Mechanics) and a pretty big pill to swallow in their own right. It would avoid nasty causality problems though.

Excuse me **Whack-a-Mole **, but either you misread my post or you contradicted yoursef with a later post.

As you said (and I said) time slows down the closer you get to the speed of light. You dont have to reach the speed of light to travel forward in time. You already know about time dilation. I dont know what the exact formula is but you can dilate 1 hour of time (inside your one way time machine) into 1 year of time (outside the time machine) and not have to reach the speed of light.

Heres something to whack your brain with. The theory of relativity forbids you from traveling at the speed of light. Nothing there that says you cant travel faster than the speed of light which are what tachyons are theoretically able to do.

If time travel were possible, would that mean that there is no free will? That is if you go back in time, then that time exists. Since you also exist, then that means that your own time must exist somewhere in the continuum. If that is true, then it is logical to surmise that the future also exists. If it already exists, then how can it be changed?

There is the classic paradox about going back in time and you kill your grandfather before he ever met your grandmother. The fact that you exist means that no matter what you do or how hard you try, there is no way you could kill your actual grandfather. Which is to say that you could not have changed the future, thus there is no free will. Anything you do in the past, you will have already done by the time you’re born and travel backwards in time. If you’ve already done it, you were “supposed” to do it. No free will.

What if you go forward in time and you want to make yourself rich by winning the lottery? If you are not rich in the future, then you must not have won the lottery in spite of your efforts to gain the knowledge of the winning numbers. No free will.

But what if you did get the numbers and you did win the lottery? What happened to the pre-existing future? Did it cease to exist? How can it cease to exist if it already exists in the continuum? At the moment you put your plan into action, would you have started a new “branch” to the timeline? That is, the future you once saw is no longer accessible to you and you must continue on the new one.

I’m thinking that if time travel were possible, then either there is no free will because all times exist at the same time in the continuum; or free will exists, but you must abandon your own timeline and “shift” to another plane.

So what do you think? One continuum and no free will? Or free will and infinite continuums?

Ok here is a scenario about time travel.

If you were to travel back 20 years and tell yourself not to flick off your boss or something than it wouldnt work. Heres why:

If you did travel back in time and you told yourself not to do something (and assuming that you listened) you would not have done whatever it is that you did. Plus you would have remembered yourself going back in time and talking to you and telling you not to flick off your boss, so it never would have taken place and then why would you do it?

Final analysis:

It never would have happened in the first place so your present self would see no need to tell your past self not to do something because you never did it (in the past, your future self already told you).

Also i personally dont believe time exists.

And im still asking myself SentientMeat’s question…

Hope that helps, i know its a little confusing…

:stuck_out_tongue:

** Johnny L.A.** that is exactly what I was eluding to when I brought up the religious idea of our lives being pre-ordained. Whack-a-Mole introduced this concept of parallel universes, which would explain man779977 paradox. If you went back in time and changed something you would only spawn one of these parallel universes. Now if this is the case you cannot change the universe you exist in and that explains why we have not seen those time travelers from the future, that would be experienced by the we in the parallel universe(s).

I didn’t contradict myself…I was just speculating hypothetically what would happen if you moved at the speed of light or faster although I realize it is not possible for us to actually do so.

To further bend your brain…if you travel faster than the speed of light you can NEVER slow down to below light speed. This is true for anything moving faster than light…even tachyons. Once you’re on one side of the light speed fence you stay there and cannot cross the light speed barrier going in either direction.

I wonder why ‘light-speed’ is the limit then? Not light-speed +/- 1 mps.