A Moment of Science

You Can’t Outrun a Ray Gun

Sometimes on science fiction shows you see somebody fire a laser beam only to miss because the other person ducked in time. Can you see what's wrong with this?

Three toy ray guns

Photo: Andy Field (Hubmedia) (flickr)

If "ray guns" like these actually existed, no human would ever be able to dodge the laser

Occasionally on science fiction shows you see somebody pull out a laser beam and fire it, only to miss because the other person ducked in time.

Let’s just assume that some day there will be such devices as laser guns. Still, there’s a basic problem with this scenario that we can understand even in our own century. Can you see what it is?

If you are very fast, you might already have anticipated the problem; but no matter how fast you are, you can’t ever duck out of the way of a laser beam. Since light travels 186,000 miles per second, it’s coming at you considerably faster than any real person might react.

An alien with fantastic reflexes could at least be imagined who might be able to move away, or the beam could be coming from a great distance. The problem is that even an alien wouldn’t know TO react.

Let’s think of this in a simplified way. Two people face each other, one with a ray gun in her hand. When she pulls the trigger the other one ducks, but how does he know to begin ducking? Even assuming his brain commands his muscles with incredible swiftness, he won’t START ducking until he sees that trigger pulled.

Now you see the problem: you CAN’T see the trigger pulled until the light coming from that trigger finger reaches your eyes. That’s exactly what it means to “see” something. If the ray is travelling at light speed toward you, seeing that you are being fired upon, is the same as being hit.

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