Headlights

[Judge]

If I am travelling at 100mph and I turn on my headlights, does the beam go away from me at the speed of light, and if so does that mean it is the speed of light plus 100mph?

[gwatkin]

No. The speed of light is relative to it's source. If you are moving at 100mph, the source of light is also moving at 100mph. The light leaves the source at the speed of light.

[ndarroch]

The previous answer is correct if you are considering the light relative the source. However relative to a person standing on the road in front of the car then I suppose the light would in fact be travelling towards them at speed of light +100 mph. I guess too that if the car was reversing at 100 mph away from a person standing in front of it, then relative to that person the light would be travelling at the speed of light minus 100mph. However, since light travels at ~186,000 miles per second (~669,600,000mph) then plus or minus 100mph (~0.0000015%) is unlikely to make a significant difference.

[platinum]

The speed of light is fixed for any observer. If in the same car you were to throw a ball out of the window towards the direction of travel it would appear to travel away from you at say 20mph. Someone by the side of the road would see it travel at 120mph. However with light this is different, both the driver and the person by the side of the road see light travel at the same finite speed (186,000 miles per second).

[Tra_j_d]

Also, speed is an instantaneous measurement. At the point in time the speed is measured, the positions the speed is being measured at and from are fixed (albeit for a very short length of time). Therefore the speed of light is constant where ever it is measured from.

[ANSWERBANK69]

Everyone's right, but no-one's addressed the real issue yet. The speed of light (in a vacuum, as everyone forgets to mention) is near as makes no difference 186,000 miles per second. Coming towards you, going away from you makes no difference. That's just the way it works. It's constant, hence the interesting part is not how fast it goes, but what happens when it gets there. If you're heading towards the mythical stationary observer, there will be more than than the expected number of light waves/photons arriving at their eye every second. This is perceived as being a shorter wavelength, hence more blue than the actual light emitted. If you're travelling away from the observer then your light would be red-shifted. It's called the Doppler effect, but as one of the respondents noted, 0.0000015% is not noticable in any event, even if it were monochromatic light, like a laser rather then the polychromatic spectrum given off by the incandescent piece of wire in a bulb. The doppler effect applies to sound too - it's what makes police sirens appear to change pitch as they go past you at speed.