# At the speed of light

1. Sep 16, 2006

### Skhandelwal

I read in physics that it is impossible to get at the speed of light b/c it requires infinite force and I HEARD that if you do you can go to the future and if you cross it, you go to the past. Now I am having problem understanding the HEARD part. I mean what makes you reverse or accelerate entropy?

btw, they were talking about some objects that nevertheless do travel faster than light. I think one of them was photons and then there were some that havn't been proven to exist. Do any of you guys know what were they?

2. Sep 16, 2006

Staff Emeritus
First of all speed in relativity is relative, duh. So all statements about going some speed or other need to be qualified with stating the observer ("inertial frame") which the speed is relative to. Observers seeing another body at a speed relative to them will experience that body's length, time, and energy as altered by a formula that depends on the relative speed. Lengths are shorter, times are dilated, and kinetic energy increases, relative to the observer's own values. Note that many differently moving observers are possible, each one with its own relative speed experience of the body and its own formula for adjustment.

As the relative speed approaches the speed of light, lengths and time durations shrink toward zero, relative to any observer, while kinetic energy increases without limit. So no, a body which has mass cannot get to the speed of light if it has mass. On the other hand, a body which is massless, like the photon must travel at exactly the speed of light.[/quote]

You have mixed up a discussion of massless particles, including the photon, which must travel at the speed of light relative to every observer; that is AT the speed of light, not over it!

The math of relativity allows for particles that always travel faster than light. They have the same length, time and energy problems slowing down to c that our particles do in accelerating up to it. So they can't cross the c-limit from above and we can't cross it from below. These particles are entirely hypothetical; no-one has ever detected one, and many people have tried. These FTL particles (there could be many types of them) are generically called tachyons, and our kind of particles are similarly called bradyons. These names are from the Greek words for "fast" and "slow" respectively. Some physical theories, notably string field theory, predict tachyons, and there is theoretical work on what properties they would have. One thing that relativity requires is that tachyons have mass a multiple of $$\sqrt{-1}$$.

So there are generically three kinds of particles in pure theory:
1. Bradyons, our kind, have masses > 0 and must travel slower than light relative to all observers.

2. Massless particles, have mass = 0, and travel exactly at the speed of light.

3. Tachyons have masses that are multiplied by $$\sqrt{-1}$$ and must travel at speeds greater than light.

3. Sep 16, 2006

### Skhandelwal

Does inertial frame really exists? Since the whole galaxy is traveling and who knows, may be the universe is too!

4. Sep 16, 2006

Staff Emeritus
If you can't detect any acceleration or forces, you can assume your frame is inertial fo all practical purposes. In practice, even on the rotating earth, laboratories and their instruments are considered inertial with respect to fast moving subatomic particles.

5. Sep 16, 2006

### Skhandelwal

So you are saying that even if it isn't, it doesn't really make a difference. Right? Btw, how did they came up w/ tachyons? I mean I am sure it wasn't a random guess.

6. Sep 17, 2006

### michael879

the equation you can see evidence of them from is $$m_r = \gamma*m_0$$ and $$\gamma=1/\sqrt{1-v^2/c^2}$$ if v < c then $$m_r$$ increases with speed. if v = c then $$m_r$$ is infinite and if v > c then $$m_r$$ is imaginary. Also, $$\delta t = 1/\gamma$$ for a reference frame moving at v relative to you. If something goes at c, its time difference compared to yours goes to 0 which means they are basically stopped in time relative to you. If it goes over c it has imaginary time relative to you but I believe there is a way to manipulate a situation in which something goes faster than c in order to send information to the past.

o yea also, $$E = m_r*c^2$$ shows the energy of something with a velocity (not sure if its the right equation it may be $$E = 1/2*m_r*v^2$$) so as v->c, E -> infinity (or -infinity if its from the >c direction).

Last edited: Sep 17, 2006
7. Sep 17, 2006

### Skhandelwal

What about that time traveling thing?

8. Sep 17, 2006

### DaveC426913

When I turn on the engines of my rocket, am I now in an ertial frame?

9. Sep 17, 2006

Staff Emeritus

As long as your engines were the only (measurable) source of acceleration, yes. Usually in these thought experiments that is assumed to be the case. When you are coasting through space at a constant speed relative to other observers you are inertial, and your physical experiments should work just as if you were at rest.

10. Sep 17, 2006

### DaveC426913

Is an object's tendency to be pushed around easily known as its ertia?

"Now most of the these landmines are inert, however some of them are ... ert."
- Sgt Hulka, Stripes (1981)

Last edited: Sep 17, 2006
11. Sep 17, 2006

### michael879

if you have something going faster than light there is a way to send information to the past. It has to do with the time difference being imaginary, not really sure how to do it tho.

12. Sep 17, 2006

### Skhandelwal

Is it like that time keeps getting slower the faster we travel and at the speed of light, it stops so after, we imagine that it will will traveling in the past?

But then how do we go into the future?

13. Sep 17, 2006

### michael879

not exactly. If you have something going at faster than c relative to you, it's clocks will go some imaginary time for every second the clocks go in your frame. Its hard to say exactly what that means. However if you multiply two imagary numbers you get a negative number. A negative time represents the frame going backwards in time relative to you. Like I said, Im not even positive if its possible but Ive read multiple times that if you can find something going FTL you can send information to the past. I gave you the equation that shows that time slows down relative to you as something speeds up and then becomes imaginary.

to go into the future you just do nothing lol. You cant really prevent yourself from going into the future without going >= c. To go into the future faster than everyone else, you just move relative to them.

14. Sep 17, 2006

### Skhandelwal

What do you mean by saying, moving relative to them inorder to go to future faster?
Btw, what is making us move to the future? I heard something about entropy but didn't exactly get it.

15. Sep 17, 2006

### JesseM

It has to do with the fact that different reference frames in relativity disagree about "simultaneity", which means that if you have two events that happen at the same time in one frame, they happen at different times in another frame. But it works out so that if you send a signal at the speed of light or slower, then if the event the signal being sent happens before the event of it being received in one frame, then every frame will agree the event of being sent happens before being recieved. However, if you could send a signal faster than light, it would always work out so that some inertial frames would see the signal being recieved before it was sent! Not only that, but since relativity says the laws of physics must work the same in every frame, that means if it is possible in some frames to send a signal such that it is recieved before it is sent, then that means it must be true in every frame. Because of this, if FTL signals were possible and the laws governing these signals worked the same way in every frame, then if I was moving away from you at slower-than-light speeds, you could send me a signal FTL in your frame and backwards in time in my frame, then I could send a reply FTL in my frame and backwards in time in your frame, and you would actually recieve my reply before you sent the original signal.

16. Sep 17, 2006

### michael879

well if you move at some high velocity relative to earth and then come back at that same velocity, earth will have aged more than you. (due to general relativity, not special). One theory explaining time (that I like to use to visualize it) is that the universe is a hypersphere with our 3d universe on its surface. The radius of this hypersphere is time so that the farther away from the center you are, the farther in time youve traveled. The big bang is what makes the hypersphere expand.

17. Sep 17, 2006

### JesseM

This can actually be understood using either special relativity or general relativity--see this FAQ on the twin paradox for more. SR does only deal with inertial reference frames, but you can calculate how much time will elapse on an accelerating clock by integrating the rate of its ticking throughout the journey as seen in a given inertial frame.

18. Sep 18, 2006

### Skhandelwal

Ok, so if I invent a time machine, and I go to the past, would I meet myself, or would time reverse?

19. Sep 18, 2006

### michael879

thats a good question. Does anyone really know what would happen if you traveled into the past? Its theoretically possible..
I know in movies youd actually see everything like it was back in the past and youd be able to change the future but time isnt like that in reality is it? Like if the universe were to contract, would everything replay itself except in reverse? or would it still seem like time were going forward?

20. Sep 18, 2006

### Skhandelwal

the thing is, if you mess up w/ time, the universe corrupts(grandfather paradox) so I dont think that would happen but even if we do back in time, just reversing the time, it would create this thing where you come to future, you go back to time, you come to future again, then go back to time, till eternity. One thing I am unsure about is that would you put the whole world in cycle or just yourself? Another thing that can happen is just seeing the past but how you described it to me, I don't think that would happen by this method, what do you think?