Okay, I know that there’s no blasting noise to laser technology as seen in Star Wars and other science fiction. However, I have only the simplest and most layman methods to explain it. Can anyone out there help me out and give me a good, scientific-terminology-filled explanation as to why lasers don’t make sound? It is important as I plan to use it in a short story that - when completed - I plan to submit to various publications, and I’d rather have some Tom Clancy-like explanation instead of my “light doesn’t make sound” version. I mean, it’s too short. And… not cool. So… any takers?
Psychopachik Vampire(Ugh, what a lame name. I regret it already.)
Lasers do make sounds - powerful lasers can heat the air in their path, causing an audible ‘pop’ and parts of the laser machinery itself can often make a loud humming sound.
Apart from that, how can you expand on the bare facts? sound actually consists of vibrating molecules (usually molecules of the gases that make up our atmosphere), light consists of photons. Light doesn’t make any sound because there is no reason why it should.
Well, light doesn’t make sound. That’s about all there is to it. Well, nearly. The power supplies that drive them could make sound, if they are poorly designed. Or if they used switched-mode supplies to charge caps, like flash units often do. If a laser were pulsed in the audio-frequency range, and had sufficient power to strongly heat the air the beam passed through, it could conceivably make a sound that way, as well–but that would take one hell of a powerful laser, since air absorbs very little visible light that passes through it. So, your scientific explanation is that lasers do not make sound because there is insufficient thermal expansion of the air the beam passes through to be detected by the human ear.
Hmmm – I seem to recall that powerful photo strobes make a popping noise, possibly due to air expansion or maybe just arcing electricity, and they are significantly less powerful than a laser weapon would be. An energy source dense enough to power a hand laser might involve a chemical explosion. So a laser may not be that silent. Not to mention that when the beam hits something, you’ll probably get a small explosion either of steam or molten whatever.
I have seen UV lasers go off on TV. They make lots of noise. A huge pop, caused by super heated air causing a vacuum in the beam’s path.
Low power lasers, such as your standard pointer can’t heat the air enough to make the pop, and lasers powerful enough to burn you don’t make a pop either. They don’t have enough energy in the beam to heat the air enough.
Welcome to the SDMB, Psychopachik Vampire!
simple answer:
flashlights don’t make sounds, even if you get all the light to line up, still won’t make sound
Welcome, indeed. We need more citizens such as Psychopachik Vampire We haven’t enough of them. Kinda sounds like a form letter doesn’t it? Haaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaah.
In Heinlein’s “The Moon is a Harsh Mistress”, a sentient computer tries out this riddle: “Why is a goldfish like a laser?” The answer was “Because neither one can whistle.” The nearby human points out that one could probably rig a laser beam to do just that.
So, Psychopachik Vampire, put that in your pipe and smoke it tonight…
Also, please note that sci-fi stories usually don’t use lasers. Star wars may call them “turbolasers” but they obviously are not. Star trek has Phasers, and so forth. And not one of them has anything that has the properties of a laser. DUne did have them, but the Frank Herbert was slightly-above an idiot and used them in the dumbest possible way.
The reason, of course, is that you can’t see a laser, so its kind of dull to show a big action scene.
[nitpick] Of course you can see them. First of all, the term laser refers to the device generating the beam, so they are always visible. Yes, I know you meant the laser beam, but that isn’t what you said. When this beam is emitted in the atmosphere, inevitably some of the light is scattered by dust and other airborne particulates and is therefore visible. I know you didn’t mean that either. What you meant to say is that in the near-vacuum of space, a laser beam is invisible. [/nitpick]
In space, no-one can see you lase. 
Larry Niven is a notable exception - the “flashlight laser” is mentioned in Ringworld. Works as an illumination source if defocused, and as a weapon if focused into a tight beam. He even dealt with the problems of using lasers as weapons - it had no effect on shiny armor, and little effect on things which were the same color as the laser beam. (Or white things, presumably.) It was described as a continuous beam rather than pulses or bursts.
Earthlight by Arthur C. Clarke had an interesting battle scene where light beams were used as weapons. The beams were invisible until the target started to vaporize. Unfortunately lasers weren’t invented when the story was written (or at least not known to the author) so it’s described simply as an intense light and subject to the inverse square law - it only worked as a close range weapon. To my knowledge this is the only fight/battle scene ever written by Clarke.
I worked for a while with a pulsed Nd:YAG laser and while it’s true that the laser beam itself made no sound, if you focussed it too tightly the air ionised quite splendidly around the focus - and that made a great sound
Russell
Sci-fi movies just wouldn’t be the same if the laser guns didn’t make sounds. Imagine watching StarWars without the added sound effect of the laser gun. “Die rebel skum!” (light flash, light flash) “Ahhhh!”
Movie wouldn’t be the same, though some arguably better, without the Foley artist who adds the sounds we expect to hear. Fistfight punches and breaking bones don’t sound like snapping celery stalks but we’ve been conditioned like so many salivating dogs to anticipate that sound. Weapons are a particular area where the added sounds are often completely disconnected from reality. Every long gun sounds like a pump action shotgun being racked even in situations when it should make no sound at all. Particularly irksome is the sound of a hammer on a handgun being cocked on weaopons that are already cocked or worse on weapons that have no hammer to begin with. Since sci-fi has a tenuous connection to reality at best why not add the most entertaining sounds?
In space, no one can year you get rubber in all four gears.
Ok, I did a rough calculation of how much energy a laser pulse would need to ionize air (and thus presumably make some noise).
Ionization energy of nitrogen = 1402.3 kJ/mol.
Nitrogen density = 22.414 mol/L * 2 atoms/mol = 44.828 mol/L.
Required energy density = 62.862 J/m^3.
Attenuation = 0.02 dB/km = 4.6e-6 1/m (assuming optical frequencies).
Laser diameter = 10 mm => area = 1e-4 m^2.
Therefore, energy required per pulse = 62.862 J/m^3 / 4.6e-6 1/m * 1e-4 m^2 = 1400 J.
I can’t calculate the pulse length (or the power requirement for a continuous beam) for the laser without knowing how quickly the energy is disapated. The pulse length must be significantly shorter than the disapation time constant.
Gack! Now I’ll turn on my reality-check circuits and do the units correctly.
1402.3 kJ/mol (no change).
44.828 mol/L (no change).
6.2862e10 J/m^3 (ouch, big change).
4.6e-6 1/m (ok).
1e-4 m^2 (ok).
1.4e12 J (much better).
That is a lot of energy for one pulse!
What’s always bugged me is when the hero cop enters a buidling where the bad guy ran into, draws his gun, then several minutes later, when he hears a faint sound or his Spidey sense tingles, he finally cocks the weapon. Gah!
Any real cop who did that would be called “target”.
Hey, I’m suprised that Spider-Man’s web gland didn’t make a “chunk-chunk” sounds like a Remington 870 shotgun.