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Q & A: dot going faster than c

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Most recent answer: 10/22/2007
Q:
You answerd one of my questons about "spinning faster then C" you said that the monemtium on the outside would be infinite and it would break But what if I had an extremly long beam of light and waved it back and forth if it was long enough the end would be going faster then the speed of light. If light doesent have mass it couldnt possibly break. Thanks
- James
U.S
A:
You're absolutely right that if you were in a huge dome and spun a laser around quickly the dot that the laser makes on the dome would move faster than light. The dot itself isn't really a thing- it doesn't carry information from place to place. If you want to change the dot that's going to get somewhere, the way to do it is not to do something to the dot now but rather to put something in the beam heading out to the spot you're interested in. And any change you make in the beam only travels out there at c.

This very example, as it happens, is one I often use in a course. We have a nice little laser that makes a dinosaur-shaped spot for illustration.

Mike W.

If you somehow were able to look at where all the laser beam's photons are at a single instant in the example above, they would form a spiral pattern. It's a lot like water coming out of a rotating lawn sprinkler -- all the water goes away from the middle with a certain, fixed speed, but in a direction that depends on the initial direction a particular drop of water was launched at.

Tom

(published on 10/22/2007)

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