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The Question

(Submitted January 02, 1997)

I looked all over but, this is the only place I found that may answer two questions of mine.

  1. If I were traveling at the speed of light (out in space), and turned on a flashlight (facing forward), would it illuminate my console or will the light "stall"? Also, could I illuminate a target outside, if I turned on my headlights?
  2. I believe this is the same question, just in a different setting. If I shot a bullet that travels at 600 ft/sec, and my car is speeding at the 600 ft/sec, would the bullet fly out of the barrel? Can the bullet actually accelerate to 1200 ft/sec?
Not only would I greatly appreciate the answers, I would really like to know a good place to learn this type of physics (like books, WEB, etc..)


The Answer

These are very good questions, and thinking about questions like these motivated Einstein to develop his Theory of Relativity. It will be easier to answer the second question first. In that case, the bullet would accelerate to 1200 ft/s relative to the ground. The reason is that the force of the gunpowder explosion increases the velocity of the bullet by 600 ft/s relative to the gun. If the gun happens to be traveling at 600 ft/s relative to the ground, then the final speed of the bullet relative to the ground will be 1200 ft/s. Keep in mind that essentially all speeds are relative to something else. For example, the Earth is rotating and moving around the Sun, so you are moving through space even when you are "at rest" relative to the ground.

I said "essentially" because there is one exception... the speed of light is always the same (specifically, 300000 km/s) to all observers , regardless of the speed of the observer or the light emitter (in this case, the flashlight). This is not very intuitive, as I hope the bullet explanation was. So even if you are traveling at 150000 km/s, a beam of light would still pass you going 300000 km/s or approach you going 300000 km/s. What happens is that as you travel faster and approach the speed of light, distances shorten and time slows down so that light still travels at 300000 km/s relative to you. This is not just a theory... these effects have been observed in experiments. According to Einstein's equations, it is impossible for anything with mass to reach the speed of light. So the answer to the first question is that you couldn't be traveling at the speed of light, but even if you were traveling at close to the speed of light, you would still be able to illuminate your console and shine a light on a target outside. The speed of a bullet is negligible compared to the speed of light, so in that case we don't have to worry about these effects.

A good general-level book on these topics is Einstein's Universe by Nigel Calder.

Andy Ptak
for the Ask an Astrophysicist team

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