Celestia Forums

Say you're traveling at 55 mph in your car with your headlights off and you turn them on. Does the light from your headlights travel at the speed of light + 55 mph?

No, that's what relativity is all about. The "speed of light", c, is the speed limit for the physical universe. Just as 0 degrees Kelvin a.k.a Absolute Zero is the "coldest" anything can get, i.e. all motion stops.

Relative to you, the light from your headlights shoots off at c. To a bystander's perspective, the distance you travel and the time it takes change, but the light's speed stays a happy constant, c.

Same thing is true if you traveled at 90% of c and turned on a flashlight. I am sure you have heard that time slows down for you (relatively, compared to the people you left on Earth) as you approach c. To you, in your ship, perceiving time at it's normal rate, light would shine across the cabin and hit the other wall at c. To an observer outside, your time in the ship is perceived to be slower and your distances shorter so c remains constant.

Everyone learns that distance = rate * time in school. If the rate is c, constant, d=c*t, or c=d/t. Since c cannot change, d/t must.

In your example, even at 55 mph, distances shorten and time slows down for the person in the vehicle, from the perspective of the person watching him pass. Therefore it allows c to stay c. Mass grows too, but that's another story...

All correct for now.. just one addition:
Indeed, particles can be faster that the speed of light in its surrounding medium, but NEVER faster than the speed of light in vaccum. For example high energetic cosmic particles can traverse water with a speed faster then the speed of light in water.. causing cherenkov radiation (similar to the supersonic 'bang' for acoustics).

None of this violates the theory of special relativity.. and in fact, this effect is used by particle physicists on a daily basis.

Cheers,
Achim

shouldn't this be in the P&A forum?