So, let's assume that the "laser gun / phaser / beam weapon" from all sorts of Sci Fi were an actual thing. You know – a weapon that uses a laser to kill somebody.
I'm trying to imagine the physics of such a device, and two issues make me think that the real world physics of the thing make it an impossibility, even if you could get a laser to be that small in reality.
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The beam should keep travelling too far to be safe, right?
So, if you fire a normal gun, and miss your target, the bullet continues to travel for a bit. While there is disagreement as to how far, there is still a pretty good reason why you build a firing range against a hill – a stray bullet can still kill at considerable distance.
With light, however, I don't see anything that would stop the beam. If gravity were to affect said beam (and one question I have is, would it?), it would seem that the speed of light, unlike a Star Wars special effect, would mean that the 'bullet' should be able to travel around the entire earth more than 7 times in less than a second. While a bullet is clearly going to hit the ground due to gravity within a space of less than a mile, I wonder, how long would it take a laser beam to dissipate?
Since light has a speed that exceeds escape velocity, I can't see that gravity actually pulls the beam to the ground. My question is, does the beam continue in a straight line, eventually leaving the atmosphere, and going off into space, or would the curvature of the earth keep the beam orbitting the earth for some time?
And, how far would such a beam go? At a certain distance, I would assume that orbitting spacecraft would eventually be hit by the beam, or if it closely followed the earth, it seems like the beam would eventually cause a swath of destruction as far as it went.
So, how long would it take for the beam to be dissipated in the air just as a matter of course?
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The pulse should be way too long to be tactical, right?
I remember Admiral Grace Hopper showing an 11" bit of wire, and saying that is how far light would travel in a nanosecond. Her objective, of course, was to show how fast a nanosecond really is. Applying that to a laser, however, I'm imagining that no computer could actually be fast enough to actually start and stop a laser beam in a nanosecond.
Applying that to a laser gun – and let's assume that it was affected by gravity – means that a normal person triggering a pulse could actually start and stop in 1/8th of a second. Again, at the speed of light, that would mean the pulse should theortically be a long as the circumference of the entire earth at that point, making it tactically impossible to guide in any fashion.
I'm curious what a rational limit would be for a microprocessor would be in terms of time, and then how big of a bolt that would produce.
So, given just these two considerations – length of the bolt in relation to turn on/off times and distance that it would travel, how would a real-world "laser gun" actually work in practice?
Best Answer
The nearest I know of to a real ray gun is the Lockheed ADAM anti-missile laser (see also this popular article or Google for much more on the subject). The Lockheed laser is hardly portable, but who knows what a few centuries of power pack design might achieve and it shows that it can be done.
I don't understand your issue with the length of the laser pulse. Suppose you were aiming a torch at someone: as they move you just turn the torch to follow. Make that torch a gigawatt laser and there's your ray gun.