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# Rare physics question watch

1. hypothetically:

if one was travelling at the speed of light through space in a car and turned on the headlights what would be seen?
2. Nothing?

You won't be able to see the light as your not moving slower than it.

That would be one heavy car lol
3. is it really nothing though? Because obviously as you approach the speed of light, time outside slows down to allow you to see light travel at a normal speed. Also lengths contract so that the measured speed is valid. Thus when you reach the speed of light, lorentz contraction is infinite and time dilation is infinite - you are everywhere at once and for effectively 0 time. To even contemplate turning on the lights is absurd, because where would the light go? You are everywhere? It has nowhere to go!
4. (Original post by vinny2256)
hypothetically:

if one was travelling at the speed of light through space in a car and turned on the headlights what would be seen?
as you approached the speed of light, everything would still look the same to you as if you are sitting still in a car (disregarding the acceleration).

you wouldn't be able to hit the speed of light (physically, not practically) so the question, although hypothetical, doesn't have an answer.
5. Well, provided you are not CHANGING speed at that moment we are still in the Special Relativity domain - You and We are in different INERTIAL FRAMES - so Mr. Einstein has told us exactly what to expect.

Special Relativity says we have NO WAY of determining how fast we're moving. So in your car's frame of reference, nothing will be any different from right now. You will see your headlights come on, they will shine out ahead of you as normal, and be their normal colour, etc.

And if you make an experiment to see how fast the light from your headlights is travelling, it will STILL be the same as in our Earthly frame of reference, 3 times 10 to the eighth metres per second (I hope I got that right!) You in your car, and We will agree on that.

But viewed from OUR frame of reference things will look very different. If we are able to measure YOU, we will find that you are much THINNER in the direction you are travelling, and much heavier - your mass measured by us will approach infinity as you approach the speed of light relative to us.

The beams of light from your headlights? They will still be travelling at the universal light speed as measured by us. The distance they cover in a set time will be the same.

But as with the light emitted by galaxies far away from us, we will see your headlight beams a different colour. The light will shifted towards the red end of spectrum. This is analogous to the Doppler effect in sound that we experience when a police siren moves past us, although is in fact slightly different in details. In this case the light viewed from our frame keeps its same speed, but since relativity has changed its wavelength from our perspective, its FREQUENCY is different.

I guess I am assuming you are travelling away from us - so we will see a red-shift. If you were travelling TOWARDS us we would see a blue shift.

OK now I am in difficulties - I am thinking the relativist 'shortening' of the wavelength of the headlight beam is the same no matter which direction the car is travelling - so why don't we get a blue shift in all cases??
6. (Original post by vinny2256)
hypothetically:

if one was travelling at the speed of light through space in a car and turned on the headlights what would be seen?
To reach the speed of light you would have to be travelling through some medium. As no total vacuum exists, it is theoretically possible for something with zero rest mass (or near as damn it zero mass) to approach light speed.

If you were to fly in a airplane doing EXACTLY the speed of sound, what would you hear? I guess that would be equivalent to what you would "see" at light speed.
7. (Original post by vinny2256)
Well, provided you are not CHANGING speed at that moment we are still in the Special Relativity domain - You and We are in different INERTIAL FRAMES - so Mr. Einstein has told us exactly what to expect.

Special Relativity says we have NO WAY of determining how fast we're moving. So in your car's frame of reference, nothing will be any different from right now. You will see your headlights come on, they will shine out ahead of you as normal, and be their normal colour, etc.

And if you make an experiment to see how fast the light from your headlights is travelling, it will STILL be the same as in our Earthly frame of reference, 3 times 10 to the eighth metres per second (I hope I got that right!) You in your car, and We will agree on that.

But viewed from OUR frame of reference things will look very different. If we are able to measure YOU, we will find that you are much THINNER in the direction you are travelling, and much heavier - your mass measured by us will approach infinity as you approach the speed of light relative to us.

The beams of light from your headlights? They will still be travelling at the universal light speed as measured by us. The distance they cover in a set time will be the same.

But as with the light emitted by galaxies far away from us, we will see your headlight beams a different colour. The light will shifted towards the red end of spectrum. This is analogous to the Doppler effect in sound that we experience when a police siren moves past us, although is in fact slightly different in details. In this case the light viewed from our frame keeps its same speed, but since relativity has changed its wavelength from our perspective, its FREQUENCY is different.

I guess I am assuming you are travelling away from us - so we will see a red-shift. If you were travelling TOWARDS us we would see a blue shift.

OK now I am in difficulties - I am thinking the relativist 'shortening' of the wavelength of the headlight beam is the same no matter which direction the car is travelling - so why don't we get a blue shift in all cases??
But where does it travel to? lorentz contraction would mean the entire universe would become infinitly thin - so where would the light travel to? There is no in front of you, and there is no behind you!
8. If the question was the same except you wanted to know what it looked like when you were traveling at 0.999c we could tell you. As it is, it is not possible to say because the concept of travelling at the speed of light is not a physical one.
9. you would see the light from the headlights going out at the speed of light
10. Professor Jelly at Oxford said that it has been known for rare and random particles to travel even faster than the speed of light.
11. (Original post by Invisible)
Professor Jelly at Oxford said that it has been known for rare and random particles to travel even faster than the speed of light.
who's professor jelly?
12. (Original post by elpaw)
who's professor jelly?
Physics professor at Oxford, I had a 45 minute on-one tutorial with him. Even one of the people doing lectures to us passively mentioned it.
13. (Original post by Invisible)
Physics professor at Oxford, I had a 45 minute on-one tutorial with him. Even one of the people doing lectures to us passively mentioned it.
what was his *real* name?
14. (Original post by elpaw)
what was his *real* name?
That was his name, Professor Jelly. He's a professor at Lincoln college. Look it up if you don't believe me.
15. (Original post by Invisible)
That was his name, Professor Jelly. He's a professor at Lincoln college. Look it up if you don't believe me.
you mean Jelley
16. (Original post by elpaw)
you mean Jelley
OK, but it sounds exactly the same.
17. If you were everywhere at light speed wouldn't you 'see everything'? Besides if Lorentz contraction were true and the universe was infinitely thin you still wouldn't see nothing because you couldn't occupy the same amount of space as your body (as we percieve) would then occupy the entire universe so you wouldn't see it from one perspective.

Bah, that doesn't even make sense when I try and type it out.
18. (Original post by Willa)
is it really nothing though? Because obviously as you approach the speed of light, time outside slows down to allow you to see light travel at a normal speed. Also lengths contract so that the measured speed is valid. Thus when you reach the speed of light, lorentz contraction is infinite and time dilation is infinite - you are everywhere at once and for effectively 0 time. To even contemplate turning on the lights is absurd, because where would the light go? You are everywhere? It has nowhere to go!
Your time slows down respect to someone standing still with respect to you. But the person in the car just experiences everything as they normally would (the relativity principle), so they would just see everything happening normally
19. Doesn't the car obtain infinite mass, and thus cause a big crunch? OOPS, thats when it travels BEYOND the speed of light.

I was up in cambridge for an open day, sitting in on a lecture by some professor and she was discussing this. Apparently the light moves at the speed of light, regardless of the vehicles velocity. However, AT the speed of light, I do not believed you would see anything in front of you.
20. Things moving faster than the speed of light- erm quantum fluctuations?

I would say you can't travel at the speed of light. It is a bar near the ceiling that we cannot reach, however high we jump-restricted to photons and other things. Besides, travel at the speed of light relative to what exactly? Relative to the Earth? Relative to the Milky Way?

There are equations that show how two approaching trains, each travelling at 0.51c relative to the platform, will not record each other's velocities to exceed c.

I don't agree with this part:

"Special Relativity says we have NO WAY of determining how fast we're moving"

I think special relativity says, when asked how fast we are moving, says "compared to what?". What is this concept of speed? Am I moving? No, of course I'm not, to me. If I get on a train, I'm still not moving. The Earth is moving! Look at the Earth. The Earth is not moving! The sun is orbiting us! Why not be self-centered in thinking we are always stationary? We are not given any reason to think that we aren't moving, so why give in and say we do move when we get on a train?

Rather than having no way of determining how fast we're moving, or whether we are at "absolute rest" - not moving relative to something that is special (luminiferous aether) - we simply say we can only measure the difference between two speeds.

Like distances. What place in the universe is so lucky to be chosen at zero displacement? None. We can still measure the height of everest... that is the displacement of the peak relative to the base. We measure the difference, like we do with velocities.

That's a key I've found is helpful in understanding this. Be selfish. You are never moving. Things you see moving are moving, never yourself.

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