# Why is then Time Travel possible?

1. Jun 4, 2004

### HIGHLYTOXIC

Hi guys,

Relativity says (actually, proves) that nothing & absolutely nothing can be faster than light (assuming the body has mass)..

Now, its also said that we can reverse or slow down time, if we move with the speed of light or even approach it..

Its a proven fact that time slows down for an object that is approaching the speed of light, it stops when it reaches the value of c and starts reversing when we reach a velocity higher than c..

If we were to go in the past, we have to use the third part of the above statement..right? But then its just not possible, is it?

So, how can some scientists still say that Time Travel is possible? Is there any other way to move back in time?

Thanks..

Last edited: Jun 4, 2004
2. Jun 4, 2004

### asdfjkl

A few things.

It is possible to have particles that can only travel faster than light (they are called tachyons). I haven't heard of any experimental investigations into the existence of tachyons (but there probably are some constraints).

Usually when scientists talk about time travel it is by taking one end of a worm hole and moving it next to the other end. The movement causes the time read at one end to retard with respect to the other end. Therefore, if you were to step into one side you would come out of the other at a previous time. There are some problems with this that are usually brushed aside. 1) The acceleration of the worm hole (to get it from one place to another) is not taken into account (I think this negates the scenario but I'm not sure). 2) We don't know if worm holes exist. 3) If they do exist, we don't know if ordinary matter can pass through them.

3. Jun 4, 2004

### chroot

Staff Emeritus
Wormholes aside, relativity says we can only effective travel into the future, not the past.

Wormholes are mathematically valid, but the chances of such freaks of nature actually existing in the universe are, IMO, very small.

- Warren

4. Jun 4, 2004

### Parth Dave

Einstein didn't say nothing can travel faster than the speed of light. He simply said nothing can accelerate to faster than it (ie if you are slower, you cannot make yourself faster).

5. Jun 5, 2004

Staff Emeritus
And if you are faster (tachyonic) you can't make yourself go slower than light. In both cases there is an infinite singularity of energy at c. BTW, a tachyonic mass is pure imaginary, a multiple of $$\sqrt{-1)$$. That's how theorists recognize them, for example in bosonic string theory, you get a particle with $$mass^2 < 0$$.

6. Jun 5, 2004

### DW

You are talking about special relativity, not general relativity and special relativity is only applicable in the absence of gravitational sources or in local limits, and general relativity is also not a quantum applicable theory. In short, when you include gravitation more is possible than what special relativistic physics would otherwise allow. If you are interested in general relativistic investigation into time travel see section 3 of chapter 12 at
http://www.geocities.com/zcphysicsms/chap12.htm#BM12_3

7. Jun 15, 2004

### Silverious

I just finished reading a book by Nick Herbet called "Faster Than Light." In it he says that there are things in the universe that actually do travel faster than light. But most of these are either completely random, or completely monotonous. Meaning that we cannot encode them in order to communicate back in time. As for traveling back in time and reexperiencing the past, our best bet is to make a singularity. One example is Tipler's cylinder. If you were to rotate a large enough cylinder fast enough, it would eventually warp space so much that time would be reserved inside the singularity. Of course no material we have can witstand such centrifugal forces. But he suggests quasars could help us understand this more.

Good read go check it out.

8. Jun 17, 2004

Speed of Light in what

Just a quick point which most of you are sure to know, intended for those as new to the subject as me - yeah, can matter travel faster than the speed of light? Well, a charged particle can move faster than light in specific media. I believe an experiment by a Russian physicist demonstrated this in water. I know thats not the topic here, but its a minor correction I would've felt itchy for not pointing out.

Of what I have read on the topic, time travel is possible because the time not being thing ("Newton, forgive me..."). Since velocity affects time, you can time 'travel' at a different speed from others based on your frame of reference. But there's the second question; what I just said only affects traveling in the 'right' direction, the future.

Or maybe I'm completely wrong? I apologize beforehand if I am.

9. Jul 9, 2004

### VantagePoint72

Yes, that does only allow "time travel" to the future. In the original post it said:
Special relativity does NOT say that time reverses if you travel faster than the speed of light. If plug a value greater than c in for v in the Lorentz equation for time, the result is not a negative number that would denote time reversal, it's an imaginary number, simply illustrating that accelerating to faster than light is impossible.

You can "travel" to the future by travelling at a significant percentage of the speed of light, so that time passes more quickly for everyone on earth. This is what everyone learns in high school physics.

Travel to the past may or may not be possible. There are theoretical highly concentrated forms of energy called cosmic strings, thinner than an atomic nucleus but millions of kilometres long. As I said, their existence is theoretical. If they do exist, two cosmic strings circling each other could cause such a warp in spacetime than if you were to fly a circle around them, you could arrive back at your starting point before you left.

An important thing to remember is that it is commonly viewed that what we call the past and future are forever frozen in spacetime. Do to the prohibition of simultaneous events in all frames of references in SR, it's a requirement. The seperation of events is also magnified by distance. If travelled at about 15 km/h toward the earth at a distance of about 10 billion LY from the earth, your little trip would be viewed as simultaneous with something happening in around the year 2100 on earth from your perspective (though you could never know for 10 billion years.) The point is, the past and future are there, how much we can access them remains to be seen.

10. Jul 10, 2004

### omin

Time Travel

I am studying relativity for the first time. Time travel is something I'm struggling with. Here's how I understood time travel before I began hearing these strange relativity theories:

If a person wants to travel forward in time, they would have to do one of two things.

1. Apply a magnificient force to everything in the universe that would accelerate everything, simultaneously, in it's current direction, accept for the person wanting to time travel, and then apply an equal and opposite force that would perfectly deccelerate everything back to it's original velocity (relative to the time traveler) when the amount of time passed that the person wanting to time travel would want to travel.

2. Or, leave the universe alone and only perfectly slow the time traveler's matter and physiological processes down with another magnificiently accurate force acting upon every element of their being that would also be applied to accelerate the person's matter and processes up once the desired time of travel would have passed.

Is there anyway this easy but physically improbable understanding can help me understand the relativity time travel theory?

And, if 2 is somewhat accurate, it seems to contradict high speeds creating time travel, because in this analogy, when I sleep I travel through time, because I've slowed down while everything went by me.

Wake me up if you can, Omin.

11. Jul 11, 2004

### VantagePoint72

Before answering your question, we need to define "time travel". Do you consider it "warping" out of your time and into another time, or in any one getting to a time that would normally be beyond your lifespan? I can't say I have ever heard of method one, I doubt it has anything to do with relativity. Method two is only time travel depending on which definition you choose. If you consider time travel the second definition, then yes going to "sleep" would technically be considered this. Here's an exerpt from a previous post of mine explaining the theory of relativity's way of dealing with time travel:

According to Einstein's theory of Special Relativity, as you increase your speed relative to another observer, that observer will note time as passing more slowly for you than for them. There is an extremely good article at http://science.howstuffworks.com/relativity.htm that explains why this happens and does so in very easy to understand, non mathematical terms, if you would like to know more about it. Once you accept time dilation as it's called (which actually refers to what the moving observer sees), then you can see how it can be used to "travel" to the future. If you were to blast off in a space ship and accelerate to 0.9999999999999 the speed of light, for every 1 second that passes for you, about 2236068 seconds would pass for everyone back on earth (to anyone who knows how to use the Lorentz equations, yes I did calculate this, I left the math out for simplicity's sake). If you maintained this speed for 24 hours, over 6,000 years would pass back on earth. You could then return to earth having hardly aged but millenia after you left, effectively "travelling" to the future. Unfortunately, reaching such fantastic speeds is no easy task. Another relativistic effect is that as you increase your mass, more and more energy is required to accelerate. This number becomes infinite at the speed of light (the Universe imposes many contraints making accelerating to the speed of light impossible). Particles in atom smashers are accelerated to speeds comparative to the one I mentioned, but accelerated a space ship to that speed would be quite a feat indeed. Still, it's possible. Time travel to the past is a different matter. It's not simply a matter of travelling faster than light (not only is this impossible, but plugging such a value into Einstein's equation yield's an imaginary number, as opposed to the negative number that would suggest time reversal). There are possibilities though. There are objects called cosmic strings, their existence is purely theoretical but if they do exist they are highly concentrated strings of energy, remnants of the Big Bang, that are thinner than a nucleus of an atom but several million kilometres long. Two of these strings circling each other would warp spacetime (now we're dealing with Einstein's Theory of General Relativity) so significantly, that spaceship flying around them could theoretically arrive back at it's starting point before it even left! Again, the existence of these strings is purely mathematical. Still, it is possible. Special relativity says that events that are simultaneous in one reference frame aren't necessarily simultaneous in another (see the article I mentioned). That means one observer could see an event happen and it's in their past, while it has yet to happen for another observer, it's in their future. This suggests that what we call the past and future are forever frozen in spacetime and the present is just our fleeting impression of the Universe as it is as we hurdle through time. If that's the case, the past and future are always there, always part of the Universe, and it's only accessing them that's the question. Perhaps the methods I suggested are the ways of going about this.

It's important to note that the purpose of relativity (special and general) is not time travel, the "ability" to do so only comes from basic fact that time is relative. "Time travel" to the future is insanely more easy than travel to the fast. In fact, relativistic future time travel happens all the time, as particles that are travelling at extremely high speeds, come to rest in our frame of reference, thereby "jumping" to the future.

Another important thing to note is that you are ALWAYS travelling through time. Trees grow, particles decay, people die...in the sleep case, nothing related to relativity is happening, as relativity's method of time travel involves changing your velocity through time. In this example, your velocity through time doesn't change, you really only increase your lifespan. So, "time travel" is really a misnomer, since we're always time travelling.

The whole concept can be pretty complicated, but I hope that helps.

12. Jul 11, 2004

### omin

I define time as a specific distance an object travels in a linear motion across that mass. Time represents my experience of the usual movement of physical things in my world in general. And of course, if all things weren't moving, I wouldn't sense light, sound...the fives senses; therefore I wouldn't recognize time. Time is my sense of movement.

Travel in terms of time travel means an unusual amount of motion. If I could make the motion of a substance decay faster, I think this substance would then be moving unusually fast. Relative to me, I think I am time traveling into it's future.

I'll check out that web page.

Last edited: Jul 11, 2004
13. Jul 14, 2004

### C3l7

Just wondering.. even if you were faster then the speed of light "Time Travel" would be just looking at the light coming off of objects as it approaches you?..

14. Jul 14, 2004

### JD

It may be the case that you would "see" an object that was moving faster than light before it reached you.

15. Jul 14, 2004

### Richard Harris

in a sense, you will be run down by the train, Then you will see the train about to hit you.

much like the way a supersonic jet can fly past you before you hear it.

16. Jul 14, 2004

### JD

Given that the train is operating (in one sense) in an opposite way, how could you perceive it? What might the end result be? Would you be run over first and then be untouched afterwards?

Last edited: Jul 14, 2004
17. Jul 14, 2004

### Richard Harris

as Percieved by the Victim, you will experiance being run down, then (if still Able to) see the train moving backwards towards the location it started in.

It's being "(run down)-1" by a train that's moving backwards.

18. Jul 14, 2004

### Richard Harris

edit: It's the oposite of being...

you would be Perfectly fine when you percieve the Train beind you. but injured when the train is in front of you.

If an object is moving faster than the speed of light, then it's most recent Activitys will be portayed first. and it's Previous movements will be shown afterwards.

Last edited: Jul 14, 2004
19. Jul 14, 2004

### JD

Is there necessarily a size limit for FTL particles/objects?

20. Jul 14, 2004

### Richard Harris

no known limitation, i'll leave that to those with Highschool or greater qualifications, Maths is greek to me.