# Speed of light in any man made vehicle

1. Apr 15, 2005

i do not know if the theory behind this will work, but seems plausable

now as far as i know, we have not attained the speed of light in any man made vehicle, however, 1/4 the speed of light seems a much more plausable situation. in this experiment, the set up looks somthing like this

the back of each one of those capsules is a flat edge that has a miror face and a small camera on it (if 1/4th the speed of light is not unbearable for humans then it is a small capsule for veiwing instead) now each of hese are spinning on an axis the opposite direction of eachother so that for a few brief miliseconds, the two back faces are facing eachother, they are each spining at 1/4th the speed of light, am i wrong to asume that say, if you weere on the capsule, s you veiwed the other face/mirror moving away from you, first of all, you have a velocity of -1/4th the speed of light, you are veiwing the other going at a velocity of 1/4th the spead of light, so basically, you see the other face mving away from you at 1/2 the spead of light, now with the mirrors on these cpsules, the whole process is reversed, the light is refracted off the thing going 1/4th velocity witch then finds you at another -1/4th, this adds andother 1/2 the spead of light to it, the total should in theory be the spead of light, it should be at least observable.

i realise there are many many flaws in this experiment, namely that im not sure that 1/4th spead of light has ever been attained, however, this is the spead of light , would actually veiwing the process be possible as veiwing needs light to work?

at least tell me if this idea is at all interesting.

Edit: apparently i cant get the graphic to load, no matter, it looks just like 2 centrifuges moving in opposite directions right next to eachother, the explination in the post should explain it somewhat

Last edited by a moderator: Apr 15, 2005
2. Apr 15, 2005

### Staff: Mentor

I have no idea what you are getting at with this apparatus. If the two capsules are each moving away from each other at 1/4 light speed (with respect to the earth), then their speed relative to each other is not 1/2c, but only about 0.47c. In any case, so what?

3. Apr 27, 2005

well, lets say c = 4mph, we have 2 cars, each's rear end is a big mirror with a mirror tainted window so that the passenger looking back can see out, but to anyone behind, it looks like a mirror, these 2 cars are faced back to back, they each start driving at 1mph away from eachother, the passenger inside the left moving car is moving at a velocity of -1mph right, his brother in the other car is also moving at 1mph and is therefore going at 1mph right, if there were no mirrors, the first brother (Bob) sees his brother's (Joe) car moving away from him at 2mph. however, with the mirrors, the image of his own car is reflected off of Joe's car going 1mph right and out of this Joe sees himself moving away at 4mph, in other words, c. if i had a vidio camera and mirors and such i could probably explain this better, but i dont, so i cant. the apperatus i first mentioned was simply the more plausable way, i thought of ataining 1/4c, however, in this case, a simple adding to that 1/4c would allow the camera, or in this case, Bob, to observe more than 4mph, more than c, more than the speed of light.

4. Apr 27, 2005

### Danger

There are an awful lot of problems with that whole scenario, but I'll just point out the simplest one. c is not relative. It's the one constant against which others can be measured. Your receding images would redshift themselves flat and you wouldn't see anything.

5. Apr 27, 2005

### Staff: Mentor

I think I finally see what you are saying. First, to avoid pointless diversion, forget having two cars driving away from each other. (If you insist on that scenario, you must add the velocities relativistically.) You can make your point with a mirror fixed with respect to the earth and a single car moving away from it at whatever (sub-light) speed you like. What I think you are saying is that the image one sees in the mirror appears to move at twice the speed that the car moves. (After all, if one is a distance D from a plane mirror, then one's image appears to be at a distance of 2D.) An interesting thought!

The first thing to realize is that the image in the mirror is not a "thing", so that even if it appears to move faster than c, so what?

To work out the details, realize that the apparent position of the image seen at any time depends on where the mirror was when the light reflected from it, not where the mirror is now. Also realize that with respect to the observers in the car, the light still moves at speed c.

My calculation says that the apparent speed of the image would be $2vc/(c+v)$ if the car moves away from the mirror. (I hope someone can double check that.) Thus the apparent speed of the receding image ranges from 2v (for low speeds) to a limit of c as the speed of the car approaches that of light.

On the other hand, if the car drives towards the mirror, the image would seem to approach the car at a speed of $2vc/(c-v)$!

6. Apr 27, 2005

### siddharth

You want an image to attain the speed of light? Then I think that this could work.

Imagine that you have your hand in front of a light source at a fairly close distance. Then 1000 light years away, there is a giant shell which has a screen. First switch on the light and wait. Eventually the shadow will fall on the giant screen. Now move your hand rapidly in front of the light source. If you move it fast enough, the shadow on the giant screen should travel faster than c.

But if you want to travel faster than light, why not consider this.

Correct me if i am wrong, but if you pass light through a material with sufficiently high refracitve index or through Bose-Einsten condensate, then you can slow the light down
so much that, humans can move faster than it.

7. Apr 27, 2005

### Mk

Last edited by a moderator: May 2, 2017
8. Apr 28, 2005

### ZapperZ

Staff Emeritus
Not just that, but to "slow" it down to 0 km/s! I think an ant can move faster than that.

Zz.

9. May 2, 2005

but, and i am probably very wrong, but if we can slow down light, how can we say taht speed of light is a constant?

i soppose there would be really no use for this expriment lol the car would allow it to see itself moving at speed of light, would probably do no good lol,

also as for the car, yes it would see twice it's own speed if one of the mirors was fixed, but, it isnt, there are 2 caqrs moving away from eachother at 1/4c anyway, taking this slowing down of light one step furthur, might it be possible to not only slow it down but to stop it? and tehn perhaps, to reverse it?

10. May 2, 2005

### ZapperZ

Staff Emeritus
This is a perfect example of making sure that one clearly understands the FULL extent on how things are defined in physics.

The speed of light is a constant as defined in VACUUM. The speed of light in a medium changes to lower value due to scattering, absortion-reemission, etc. Furthermore, one needs to clearly know HOW the speed of light is typically measured, especially in a dispersive medium. We define such speed via its GROUP VELOCITY.

When light is stopped in the Lena Hau's experiment, it means that both its energy and phase are "stored" in the medium, which in this case is a very cold gas. So the info about the absorbed light is stored coherently! This is very different than light being absorbed by a "black" paper, for example, because the absorbing material loses any coherent info and energy is absorbed completely.

When we start testing and "theorizing" various exotic properties of light, it is imperative that we are very clear on how exactly its properties are defined and measured. Without that, things will make for a very confusing picture.

Zz.

11. May 2, 2005

### Staff: Mentor

No, we can't slow light down. If you are thinking about refraction, refraction involves interaction with atoms, making it appear to slow down. But light goes from atom to atom at the speed of light.

12. May 3, 2005

### Staff: Mentor

Please reread my post where I describe how images are seen in a moving mirror. All that matters is the relative speed of the mirror and the car. Having two cars just introduces more room for confusion.