# On a crash at light speed

1. Jul 26, 2004

### Clausius2

Some day I was driving my spaceship that can go as faster as light velocity. (in year 2175 all of us believe Einstein was wrong). Suddenly, a dumb driver appeared in the galactic highway with a old fashioned spacecraft, and when I reach him I stood behind him wanting to express my angry. To my surprise, I sounded a lot of times the horn, but nobody of the cars in front of me heard no sound. Then, I thought in the Mach cone and this sorts of things and using those unpractical tools of the physics, I invoked the special relativity postulated and switched on the frontal lights like flickering. Hey, I discovered that going at light speed my lights worked! (After all Einstein was not so wrong).

To point this in another way more serious, sound interaction seems to be a perturbation over some type of steady mesh (not participating of the spacecraft movement). But light rays seems to be a little bit different, isn't it?. Although I think they might be similar. The question that anyone of such stupid drivers can hear me if I sound my horn if I'm going supersonically, but can see me if I switch on the lights, seems to be a little bit puzzling!.

2. Jul 26, 2004

### Locrian

To say the least. Sound is composed of varying pressure in a medium, whereas Light is a concept we use to describe the way some forces occur. Since it involves causation, your headlight questions become fundamental questions about how our universe works. Those types of thought problems are constantly considered in relativistic mechanics.

3. Jul 27, 2004

for that question, you can use Special Relativity and calculations. If you look at velocity addition at speeds close to the speed of light, you will see your result. If I am not mistaken, relativistic velovity addition is:
u' = v + u / (1 + vu/c^2)
if you sub in your speeds (speed of space ship = 1c)
(speed of emitted light = 1c)
u' = 1c + 1c / (1 + (1c)(1c)/c^2)
u' = 2c / (1 + (1c^2)/c^2)
u' = 2c / (1 + 1)
u' = 2c / 2
u' = 1c
which means the light emitted by your headlights will propagate ahead of you at the speed of light. It is hard to picture this but that's special relativity.

4. Jul 27, 2004

### FZ+

No you can't, since:

If you are travelling at c, and is massive, no current law can tell you what is going on, because the current laws say rather bluntly: No massive object can travel at c!

5. Jul 27, 2004

### omin

Does that mean the mass Earth is not moving at .8c in comparison to some stars? Or how about in comparison to light reflected away from earth?

6. Jul 28, 2004

What law is that? I have never read that or heard anything like that.

7. Jul 28, 2004

### KrazyIBKid

299,792,458 m/s - Speed of light (.8 = 239,833.966 km/s) Speed of light info

149,600,000 km - Earth's orbit away from sun. Orbit info

Therefore circumference of orbit = (pi(d)) 3.14*299,200,000 = ~939,488,000

therefore if the earth were traveling at 8/10 the speed of light the earth would revolve around the sun once every 3917 seconds or ~every 65 minutes... so no the earth is NOT traveling at 8/10 the speed of light.

Last edited: Jul 28, 2004
8. Jul 28, 2004

### Staff: Mentor

The earth is certainly not traveling with respect to the sun at 8/10 c. Did anyone say it was?

Relativity tells us (as FZ+ said) that no massive object can travel at speed c with respect to any inertial frame, and thus with respect to any other massive object.

Of course, I have no idea as to Clausius2's original point, as he has things moving at (or greater than?) light speed and sound traveling in space.

9. Jul 28, 2004

### KrazyIBKid

some stars includes our own, so i used that as a provable refernece, or am I just not understanding what he meant? And if c is the S.o.L. Why would it change from what you were measuring it with respect to? wheter you judged it from our sun, orhter stars, Mars, or even comets?

10. Jul 28, 2004

### KrazyIBKid

299,792,458 m/s - Speed of light (.8 = 239,833.966 km/s)

149,600,000 km - Earth's orbit away from sun.

Therefore circumference of orbit = (pi(d)) 3.14*299,200,000 = ~939,488,000

just for fun. the speed of the earth in m/s (in reference to the orbit around our sun)..

there are 31,536,000 seconds in a year (using a 365 day year)(and not including the 1/4 of a day each year) if the orbit of the Earth is app. 939,488,000 KM and the Earth makes that trip in exactly 1 year, the Earth travels at ~ 29.790 KM/s or 29,790 m/s which is app. 99/1000000 the speed of light or, .000099369

And yes this accounts for the elliptical shape of the Earths orbit.. not circular.. but remember thats using the other assumptions

Last edited: Jul 28, 2004
11. Jul 28, 2004

### LURCH

.8 c?, certainly. c?, never!

12. Jul 28, 2004

### KrazyIBKid

totally lost me on that response?

13. Jul 29, 2004

### LURCH

In response to the question whether earth is moving at .8c relative to "some stars", I am saying that it certainly is. It is a near certainty that there is some stars somewhere in universe traveling at that velocity relative to earth.

However, FZ+ did not say that the laws of relativity prohibit earth from traveling at .8c, but only that it cannot travel at c.

14. Jul 29, 2004

Hubbles law tells us that distant galaxies are travelling away from us at very high velocities; velocities proportional to their distance from us.

There is an observable edge to the Universe - past which galaxies appear to be travelling away from us at greater than the speed of light, so their light never reaches us. Of course, they aren't really travelling at greater than c, just the expansion of space means that distant 'stationary' galaxies are all travelling further apart as the space between them expands - as do dots on a balloon as you inflate the balloon.

Is this what Omin is referring to perhaps when he talks about stars travelling at very high speeds?????

Just a thought.....

15. Aug 7, 2004

### pervect

Staff Emeritus
Well, make something up then - because standard theory won't cover it.