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# faster than the speed of light

joggerjayne | 14:42 Wed 02nd Dec 2009 | Science
So, to paraphrase the well known paradox ...

What happens if you are in a vehicle, travelling at the speed of light ...

... and you turn on the headlights?

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why would you turn on the lights ? if you're travelling at the speed of light it wouldn't be dark
presumably the light wouldn't go anywhere?
it's impossible to answer because nothing can travel at the speed of light.

if you were at a near light speed then the headlights would work as normal, light would still travel away from your reference pane at the speed of light.

Einstein covered it early last century with the special theory of relativity

1. The rules of physics have to work in all frames of references-moving or at rest, as long as the moving frames are not accelerating. Things have to work as usual.

2. The speed of light in a vacuum is a constant.
Question Author
My chelsea tractor is happy with the cruise control on at 110

(erm ... hypothetically speaking, of course)
you of course mean 110KPH don't your jayne :)
The light would not be able to move away from you therefore it would stay dark.

Bit of a pain if you come to a bend.
TTG, I'm just getting my head around the speed of light and you have to introduce bends !!
not true TTG

The speed of light is always consent to any observer no mater what reference frame they are in, so to a person in the car the light would travel away from the car at the speed of light (c) in relation to them.

Just to really confuse matters though, if a person standing still was to measure the speed of the light beams coming from the headlights it would also be traveling at c for them too, whereas you would expect it to be traveling a 2xc as it's kinda got a push start, but this doesn't happen.

Funny stuff light!
Question Author
KPH chuck?

Oh, purleeeeez !!
Just as an aside the constant speed of light does not come from relativity. It was discovered in a series of experiments not least by the Americans Michaelson & Morely.

Einstein used this fact to derive special relativity - E=mc² wasn't in the theory either - Einstein rushed it out as an addition to the main paper
To expand a little on the expectation that Chuck 'IT God & Lord of the Cyber Wave' Fickens mentions (that the light from the headlights would approach a stationary observer at 2c) you would certainly expect that to be the case. (Just as you would expect a rifle bullet fired at, say, 1,000mph, forwards from an aircraft travelling at 500mph to approach the observer at 1,500mph).

Simple Newtonian physics dictates that the combined velocity would be (u + v) (where u and v are the velocities of the two objects).

However, Einstein’s theories suggest that nothing (including light) may travel relative to something else at a speed greater than c. To accommodate this there is a formula for calculating the relative speeds:

(u+v) / (1+(uv/c2))

Substituting low values for u and v results in very little difference whichever formulae is used. In my case of the rifle bullet, the difference is just 0.0000000017mph.

However, two “Starships” (as in Star Trek) converging, with each travelling at 0.8c would converge not at 1.6c, but at a more reasonable 0.976c. Using the same formula and substituting 1.0c for the car and the light from its headlamps the result is 1.0c.

If you examine the formula you will find that as long as the individual values of u and v do not exceed c (and this is impossible under Einstein’s theories) then their relative velocities cannot exceed c.
And don't ask. I don't know where "Chuck 'IT God & Lord of the Cyber Wave' Fickens" came from. Perhaps he or she could elucidate.

Nothing's been said about traffic signals. If you stopped at one you would suddenly see your headlight beams!

LOL

I was one of the users that helped to test the swear filter on here and the Ed left in the entry in the swear filter that changes my full username. there are a handful of other users that get their names changed as well

If you type "the prof" without the space in it you get The Most Notable Professor

(thanks for expanding on what I said though,it's been a lot of years since I studied relativity)

(oh and I'm a he BTW:))
From another persective . . .
To travel at the speed of light the only possibility is for you to be light for which time does not elapse, From a photons point of view, provided it does not encounter and is not reabsorbed by matter, it would arrive at the end of all time and the end of the universe in an instant with no time whatsoever to even think about turning on anything.
Don't forget that all this time dilation stuff is only observed by someone in the OTHER time frame. You, yourself, travelling at near-light speed, don't see any effects at all on your own clocks. If you observe his clocks then you do of course
It's important to realise that 'No experiment can ever detect a uniform translation through space'. The speeds are all RELATIVE speeds. In a sense we can all claim we are travelling at any speed we like, as long as it's less than the speed of light.
Also what we are discussing here is called Special Relativity - as distinct from General Relativity. Special relativity only applies between time frames in uniform relative motion.
There are no accelerations and hence no forces and hence no gravity - for that you need General relativity, and that's a whole new ball-game.
Just to throw a curve ball in - special relativity does not explicitly prohibit something travelling faster than light.

The formulae show that mass increases at where the velocity is c you get a division by zero - faster you get a negative square root - a complex number.

Now that doesn't actually mean it has to be impossible it just tells us that something special happens

Of course the way we normally think of things you have to reach 60mph in order to travel at 100mph - it's common sense isn't it? - but as so often happens common sense isn't always right

In quantum mechanics energy levels can be jumped - maybe with enough energy to get close enough to c that speed could also be jumped - of course that sort of energy would only be possible shortly after the birth of the Universe - but still there could be partilcles travelling faster than the speed of light. People called them Tachyons and went looking for them.

Alas no body has ever found even the merest indications that they actually do exist and so Tachyons remain a nice theory broken on the wheel of an ugly fact.

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