Speed of light

[Champagne]

If you were travelling in a car at the speed of light and then turned on the headlights, what would happen?

[Tim123]

A car cannot travel at the speed of light, so the question is meaningless.

[jake-the-peg]

Firstly - the obvious answer - you cannot have any body with mass travelling at the speed of light - It's a bit like asking what would happen if I went to the North pole and kept going North.

But if you were travelling at just under say 99.99% the speed of light then the light from your headlights would seem to you as if it were travelling away from you at the speed of light.

It's a classic question and the answer can be at various levels of detail from a slightly misleading "it looks the same" to a complicated mathematical explanation of relativity.

Here's a rather informal one: http://www.straightdope.com/classics/a3_143.ht ml

but otherwise try "car speed of light headlights" in google and you won't be short of explanations at various levels

[ladyshy]

<Smiles Smugly >

Told you someone here would know Champers!

And thank God for that, was starting to grate.

[JudgeJ]

If we ignore the practical impossibility of the situation, your instinct would tell you that the light from your headlamps would be travelling (towards the brick wall that you are about to hit) at twice the speed of light. After all, if you were travelling in the same car at a more reasonable 50mph, and throw a ball forwards from it at 50mph, you would expect the ball - ignoring wind resistance - to travel towards the wall at (50+50) = 100mph.

This is the simple explanation arrived at using Newtonian physics and the combined speed is calculated by the formula (u + v) where u and v are the speeds of the two objects.

Einstein�s physics dictates that nothing (not even light) can travel relative to something else at a speed greater than c, the speed of light. The simple formula for combined speed has to be modified to accommodate this limitation and it becomes:

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

(Where c is the speed of light)

Substituting low values for u and v results in very little difference between this formula and the simple formula. Two trains approaching each other at 100mph each close at 200mph under Newton, and 199.999999999996mph (or thereabouts) under Einstein and the two are close enough for most purposes. However, two objects approaching each other each travelling at 0.75c would exhibit an impossible (according to Einstein) closing speed of 1.5c under Newton, but only 0.96c under Einstein. The formula ensures that relative speeds cannot exceed c.

So the answer to your question is that, provided tou accept Einstein's theories of relativity, the speed of the light from your headlamps would approach anything in its path at 1.0c (the speed of light). This is the result of the Einsteinian formula if you substitute 1.0c for both u and v.

[jake-the-peg]

Just to pick a tiny hole.

It's a common misconception that relativity dictates that nothing can travel faster than light as JudgeJ states.

If fact relativity takes the observation that the speed of light is an absolute limit and draws it's assumptions from this observation.

Strictly speaking relativity also does not forbit something travelling faster than the speed of light either. It's just that the energy to travel at the speed of light is infinite. If you could get to velocities greater than the speed of light without actually ever having to travel at that speed the equations involve a negative square root which does not necessarily mean that it's impossible.

Having said that we have no reason to believe that objects like tacyons which do this actually exist.

[JudgeJ]

I bow to your superior knowledge and understanding, jake!

I was just trying to explain things in (my) simple layman's terms!!!

[Champagne]

Okay Jake, you lost me in your last post!!

I was with you up until the 3rd paragraph. :o\

[shomer42]

It depends whether you have a valid MOT? What colour is the car?

[Champagne]

hahahaha! It's a pink ford capri. :oD

[mibn2cweus]

Assuming you are in the car traveling at said speed (relative to what?) when you turn the headlights on, then (You the car the headlights etc.) have no motion relative to each other (all are in the same reference frame). Therefore the light would travel at the same apparent velocity as it always did/does from you point of view, relatively speaking!

[JudgeJ]

Yes. My post explained things from the point of view of an outside observer standing at the side of the road watching all this!

[Its John!]

Apparently, as you get close to the speed of light, some of your energy turns to mass, so you can never get enough energy to travel at that speed... its theory tho i think.

What if your driving against the earth's rotation at the speed of the earth's rotattion? are you travelling at a sepcific velocity? or are you jus stayin in one position, the earth's rotation counteracts your speed, like running on a running machine.

Relative to the earth, you are moving, but relative to the space above you, you havnt moved.

[oglecorp]

if you're almost at the speed of light, the light from the headlamps would travel away from you at the speed of light. however if you travelled AT the speed of light you would become infinitely heavy. because of this you would have infinite gravity and everything which exists would be pulled towards you with an infinite force. therefore, the instant you reach the speed of light, the entire universe, no matter how far away it is, will be sucked into you by your infinite gravity and nothing at all would exist.

[sidlaws]

The beam of light would leave the car at 186,000 miles per second. The second postulate of the Special Theory of Relativity states that " the velocity of light is always constant relative to an observer."