# Q & A: Saying hello really fast

Q:
If one could travel at the speed of light, and you pass another person standing still, could that person hear you say "hello"?
- jim (age 15)
elma wa usa
A:
The good news is that people cannot travel at the speed of light. We can travel at lower speeds, and by adding energy, can travel at higher speeds. No amount of energy however is enough to accelerate a person to the speed of light -- the speed will always be a little less. Individual particles have been accelerated up to high speeds in particle physics laboratories. Beams of electrons, for example, have been accelerated to 99.99999999879% of the speed of light so far in the laboratory and it takes lots of energy to do that.

If a person is traveling very fast however, he may still say "hello", provided that there is some air around him traveling along with him for him to exhale in the process of saying that (sound needs air or some other medium to travel in). But the air must also be present for the listener to hear the "hello", so I'll worry about the case in which the air is stationary. At lower speeds (less than the speed of sound), the "hello" will be doppler shifted to higher frequencies if the traveler is approaching the listener, or to lower frequencies if the traveler is going away. Above the speed of sound, the traveler will create a shock wave of air molecules around him and the listener will hear a sonic boom. The traveler may not be able to say "hello" because of the difficulty exhaling air into the shock wave (or the low-pressure vortex-filled wake behind him if he's facing backwards). At higher speeds still, the air will rub on the traveler and heat him up. If the speed is high enough, the traveler will burn up in a short time, as a meteor does in the sky.

Removing the air and doing this in space with the traveler in a spaceship with a flashlight, he can blink "hello" in Morse Code with his flashlight. A stationary observer may still see these pulses of light, but they will be doppler shifted to higher frequencies when the traveler is approaching and to lower frequencies when receding. At speeds very very close to the speed of light, the "hello" may be stretched out for weeks in low-frequency light, and for the high-frequency case, may be compacted into a very short burst of X-rays. But they can still be detected.

Tom

(published on 10/22/2007)