# Homework Help: Call me crazy but, I think there might a speed faster than C

1. Dec 14, 2005

### suckstobeyou

if you have one train inside another train both traveling close to C in the same direction what would be the speed of the second train inside of the first one for an observer standing on the ground?

lets extend this to a billion embedded trains what's the speed of the inner-most train to an obserevr on the ground?

Last edited: Dec 14, 2005
2. Dec 14, 2005

### Danger

I don't know the math, but you're not going to get that inside train to light speed. You'll still run into the 'infinite energy required' problem, regardless of whether it's strictly under its own power or is in part parasitical upon the outer train.

3. Dec 14, 2005

### Staff: Mentor

The speed would be close to but less than C.

The speed would be given by the relativistic addition of velocities formula:
$$U' = \frac{U + V}{1 + \frac{U V}{c^2}}$$

V is the speed of the first train with respect to the ground, U is the speed of the second train with respect to the first, U' is the speed of the second train with respect to the ground. You can see that the relative speed of the second train will never exceed the speed of light.

4. Dec 14, 2005

### Danger

Thanks for the formula, Doc. I never figured there'd be one simple enough for me to work out by myself.

5. Dec 14, 2005

### suckstobeyou

that sux:grumpy:

6. Dec 14, 2005

### Pengwuino

*slaps danger*

What kinda physisist are you :tongue2:

7. Dec 14, 2005

### Danger

I'm not a physicist (although I know how to spell it :tongue: ), or an engineer, or anything else of the sort. As I've stated in other areas, I never finished high-school and have a grade 9 math education.

8. Dec 14, 2005

### dicerandom

Actually I think it's quite wonderful, although I think I understand how you feel about it. The ideas behind Special Relativity are extremely disconcerting at first, you have to adjust your entire perspective of space and time in order to come to terms with the ideas. However, once you have done so the theory is extremely simple and beautiful, it's really quite amazing.

9. Dec 14, 2005

### suckstobeyou

10. Dec 15, 2005

### Danger

Not me... for the above-mentioned reason.

11. Dec 17, 2005

### x2468

hey guys. new here.

I remember reading that a group of scientists managed to slow down light? also, if at the point a start becomes a black whole, the light it was giving off should now begin to be pulled into the blackhole by gravity correct? thus the light must slow down and change direction. or in any case, a blackhole must be slowing light down at some point? thus, if light can be slowed down then it is not constant and there is little reason to beleive it can't be sped up as well? that ideas been on my mind for a while. been lookin for some answers.

12. Dec 17, 2005

### Morbid Steve

But you've got that incorrect. This black whole has mass, and therefore something to 'stop' the light. When you shine light at a brick wall does it keep going? Nope. It's merely stopping and being refracted into -blackness- (black is simply the absence of color).

13. Dec 18, 2005

### aalmighty

umm, I don't think mass is something which 'stops' light, a sheet of glass had mass the last time I checked.