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1. although it only means a little to me....
2. so if tchyons are always travelling at the speed of light, they're continuously travelling back in time?!
3. (Original post by Robusty)
so if tchyons are always travelling at the speed of light, they're continuously travelling back in time?!
Tachyons are just as incapible of traveling AT the speed of light as any normal particle. Except with Tachyons, the 'slower' they go the more energtic they become, and the energy velocity graph becomes asymtotic at the speed of light. At super luminal velocities it is believed that Tachyons travel backwards in time yes. I do not understand the theory of cause and effect with respect to Tachyons.
Note Tachyons are predicted by theory in so much as magnetic monopoles exist in theory. The idea is that by working backwards from our theory we can deduce possible solutions to the equations, such as by having an "imaginary mass", thus the particle must be superluminal. But the idea of Tachyons may just throw up too many contraditions for the universe to even handle so maybe impossible.
4. in another words, Tachyons are just theories? they are not even real?
5. Nothing has ever been observed moving faster than light and a particle doing so would contradict notions like 'mass'. The square of a tachyon's mass is negative, so it's mass could be something like i kilograms. Perhaps you could just about imagine negative mass, but what is imaginary mass? How long is i metres? How long is i seconds? The quantities are totally 'unphysical'.

It's hard to say "That's totally impossible' with 100 certainty, but if tachyons were discovered, they'd require the defining of a totally new and almost 'etherial' (not to be confused with 'the aether') quantities since mass, time and distance would be very odd things to them.

When string theory was being developed in the early 80s, it was striken with the problem of predicting tachyons. It wasn't until a few bright sparks through up something to remove them that string theory really was taken with any kind of serious attitude. None of the string theories being worked on at the moment have tachyons in their physical theories. Don't know about stuff like quantum loop gravity.
6. thanks for your help on this question. but i understand that if you move faster than the speed of light you will reach a given piont before it so you may see things from the past, but thats not really the same as actually being in the past, and being able to influence it. Or am i missing something?
7. The problem is with trying to accelerate to the speed of light is that you cannot do it. If you always have existed moving faster than light the universe would be a very strange place compared to how we see it.

For instance, you cannot use such a method to go back in time and help William the Conquer at the Battle of Hastings. You or I cannot accelerate enough to move back in time. If you were always moving faster than light, you'd never be able to decelerate enough to have time move in 'the usual' direction.

Therefore you have to think up more crafty ways of going back in time which exploit gravity and space-time (ie general relativity) rather than just velocity (ie special relativity). Hence the giant massive spinning toilet roll, it exploits space-time and gravity to get the job done.
8. (Original post by AlphaNumeric)
The problem is with trying to accelerate to the speed of light is that you cannot do it. If you always have existed moving faster than light the universe would be a very strange place compared to how we see it.

For instance, you cannot use such a method to go back in time and help William the Conquer at the Battle of Hastings. You or I cannot accelerate enough to move back in time. If you were always moving faster than light, you'd never be able to decelerate enough to have time move in 'the usual' direction.

Therefore you have to think up more crafty ways of going back in time which exploit gravity and space-time (ie general relativity) rather than just velocity (ie special relativity). Hence the giant massive spinning toilet roll, it exploits space-time and gravity to get the job done.
The reason you can not move faster than the speed of light when you begin slower is simple. There is a quantity called the 4-momentum*. This vector is comprised of 4 scalar quantities, three of them momentum resolved in each direction and the fourth, energy. As one Lorrenz boosts a 4-momentum, faster and faster, we see the fourth quantity grow wildly, until we finally try to boost it to c, where the energy component becomes "over zero", forming a singularity point. But if we boost past this point (ignoring a universe where we need to go thru all the intemediate velocities we get another 4 momentum that kind of weirdly makes some sense, if not physical sense.)

[This is a vector qunatity in SR, who's square is invariant under lorenz boosts, like the 4-position.
9. So if you go really really fast, faster than the speed of light you go back in time? Does that mean that if you go really really slower than the speed of light you go forward in time?
10. (Original post by Troyes)
So if you go really really fast, faster than the speed of light you go back in time? Does that mean that if you go really really slower than the speed of light you go forward in time?
As I have just explained you can not go at the speed of light. Therefore you can't go faster than the speed of light. What I saw was if there was a way to going faster than the speed of light without first going at the speed of light...etc etc...
11. (Original post by Troyes)
So if you go really really fast, faster than the speed of light you go back in time? Does that mean that if you go really really slower than the speed of light you go forward in time?
You are going forward in time now, you always go forward in time. If you mean can you go forward in time faster than when you are stationary then you would just have to move faster to take advantage of time dilation. When you stop you will be farther forward in time than you expect to be.

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