Stopping a light wave?

[Icewalker]

Science!

So, I was working on something, but made a quite foolish mistake I now realize. I'm wondering if there's a good fix.

If two waves, of light, intersect with one another 100% destructively at a point, they cancel each other out, at that point. They then continue onward past it, yes? My mistake was assuming that they actually stopped at that point of interference. Is there a good way to get a wave to stop by using something like another wave?

[Amiel]

A "wave" of anti-matter?

[Icewalker]

Probably works, although rather difficult to do, and rather explodey. :smallbiggrin:

[Keld Denar]

An opaque object? Preferably one that absorbs the light rather than reflecting it. Light converted to heat = no longer light!

[Icewalker]

Yeah, that would work, but ideally this is something canceled by another projected wave or the like. ie, two source points on one object sending signals canceling each other out at some point some distance away from the object. Problem is they keep going after that point if you just use destructive interference.

To put it more clearly: Preferably you don't need an object at the point of cancellation.

[Manga Shoggoth]

Science!

So, I was working on something, but made a quite foolish mistake I now realize. I'm wondering if there's a good fix.

If two waves, of light, intersect with one another 100% destructively at a point, they cancel each other out, at that point. They then continue onward past it, yes? My mistake was assuming that they actually stopped at that point of interference. Is there a good way to get a wave to stop by using something like another wave?

Probably the best you can do is set up a standing wave, where the maxima and minima appear to be stationary at specific points.

(There used to be a section of motorway in the UK where the road was exactly between two broadcast towers where there is a standing wave set up at the frequency of one of the radio stations, and the radio fades in and out as you drive down the road.)

[factotum]

If you were to generate a wave that was exactly the same amplitude and opposite phase to the original one then they'd cancel each other out (see the principle of superposition)--I imagine that'd be quite tricky to do for a light wave, though!

[Sholos]

An opaque object? Preferably one that absorbs the light rather than reflecting it. Light converted to heat = no longer light!

Well, no longer visible (by humans) light, anyways.

[Flickerdart]

Are you planning to shoot down lasers with other lasers? Because that would be way cool.

[thubby]

If you were to generate a wave that was exactly the same amplitude and opposite phase to the original one then they'd cancel each other out (see the principle of superposition)--I imagine that'd be quite tricky to do for a light wave, though!

the tricky part is dealing with the change the wave undergoes as it moves along.

[Seffbasilisk]

As others have alluded to.

You'd need one of the same amplitude, and synch it with a neutralizing frequency. Basically match the valleys of your wave to the peaks of the one you want gone, and vice versa. Even then it'd be tricky.

Lights a particle too. :smallwink:

[mucat]

As others have alluded to.

You'd need one of the same amplitude, and synch it with a neutralizing frequency. Basically match the valleys of your wave to the peaks of the one you want gone, and vice versa. Even then it'd be tricky.

And even then, to answer IceWalker's original question, the wave would continue to travel on past the point of destructive interference. For example, the nodes in a standing wave on a guitar string are points of perfect destructive interference, but they don't keep the wave from continuing to propagate up and down the string.

If you were looking for a way to actually "erase" the energy of the wave, it can't be done. (Or rather, if it can be done, it would be a revolutionary discovery to rival Newton's, and we could pretty much say the discoverer gets every Nobel prize, ever.) Any time you've got a point of destructive interference between two waves, you will also have points of constructive interference, and the combined energy of the waves will still all be there.

Of course, an opaque object can absorb a light wave, turning its electromagnetic energy into some other form (such as thermal)...but that's not the same as simply canceling the wave into oblivion.

[Icewalker]

Are you planning to shoot down lasers with other lasers? Because that would be way cool.

To a large extent, yes. Although may not realize just how cool... :smallamused:

Sounds like it's not particularly physically possible, sadly. Guess I won't be making a lightsaber.

[Keld Denar]

The main issue here, I'd guess, would be the plane orientation of the light. Matching frequencies of visible light (color) isn't that hard, nor is amplitude (intensity), although matching phase at 180 degrees offset might be kinda tough. If, however, the planes don't match up, you're gonna have issues getting destructive resonance. If you could polarize the light to a degree where all wave motion is exactly vertial, you might be able to pull it off.

So yea, just figure out how to polarize light into a single infinitely thin plane with no deviation and no margin of error, and then figure out how to get the waves exactly 180 degrees out of phase, and you should have NO problem setting up destructive resonance.

Its kinda like fusion. Its a VERY simple concept. Getting the heat, pressure, and a way to contain that heat and pressure seems to be the primary reason we aren't using it to power our flying cars and jet packs.

[mucat]

So yea, just figure out how to polarize light into a single infinitely thin plane with no deviation and no margin of error, and then figure out how to get the waves exactly 180 degrees out of phase, and you should have NO problem setting up destructive resonance.

Sure. But even if you could pull that off, you would guarantee that no energy was deposited on that plane...the energy would continue to propagate forward past that plane along the original beam line, though.

So the "shooting down lasers with lasers" bit still wouldn't really work...

[Knaight]

Sure. But even if you could pull that off, you would guarantee that no energy was deposited on that plane...the energy would continue to propagate forward past that plane along the original beam line, though.

So the "shooting down lasers with lasers" bit still wouldn't really work...

Sure. Which means you need 100% overlap on the plane, which is doable, if you are being shot at. Though that goes well beyond fusion and into much bigger problems, starting with having no way to know you are being shot at until you are hit.

[Tirian]

Sounds like it's not particularly physically possible, sadly.

Not unless your interfering wave was generated at the exact same point as the original broadcast and at the exact same time. Which, of course, it wasn't.

[Knaight]

Pretty much, though it can be stopped over an area.

[Jimorian]

I haven't been able to successfully Google a link, but I recall that there was an experiment that was able to "freeze" some photons so that their velocity was zero (or near enough to it). I can't recall what happened when they let them go, though.

[Jokasti]

Is there a good way to get a wave to stop by using something like another wave?

Are you MAD, sir?
You cannot cross the waves.

[Manga Shoggoth]

Well, I suppose if you could get two lasers, a half-silvered mirror and a full-silvered mirror you could do something like the following:


Beam 1 Beam 2
| |
| |
| |
\---------------------\===================

Full mirror Half-silvered mirror
This relies on the fact that light from a laser is in phase and coherent. It also relies on my memories of laser theory, which I last studied about 20 years ago.

The distance between the mirrors can be adjusted until the two beams are exactly out of phase with each other.

You would have to be able to adjust the intensity of the two beams to allow for losses passing through the half-silvered mirror so that the intensities of the two beams after the mirror system are equal.

This won't "stop" the beam, but it should make the area of destructive inteference longer.

[Urpriest]

This (http://www.scientificamerican.com/article.cfm?id=what-visions-in-the-dark) may be relevant, and is I believe what Jimor was alluding to.

A beam of antimatter as suggested earlier wouldn't work. Light is its own antiparticle.

[Jimorian]

Yes! That was it exactly. There are some amazing experiments going on that at some point in the past would have been thought pure science fiction.

It's questions like the OP's that often lead to them, or something equally as cool. :smallsmile:

[Icewalker]

Yup! That is damned awesome, and I am very curious about that quantum computational stuff with light as a storage system it's talking about. Very interesting. Sadly, it does not accomplish my goal here, and therefore is not useful for making a lightsaber.

[factotum]

A lightsabre is clearly not a laser anyway, if that's what you're thinking. You can see it from the side, for a start (a true laser would be invisible apart from its interaction with airborne dust particles from anywhere other than straight on), and furthermore, lightsabre blades actually clash with each other and hold like normal sword blades do--laser beams don't do that!

[Icewalker]

No, but if this worked, you would still have a handle, from which a laser is sprouting to a distance of your choosing, such as standard sword length, then coming to an end. Get a sufficiently powerful laser and it would operate in a very similar fashion (save for the clashing)

[Xyk]

Yup! That is damned awesome, and I am very curious about that quantum computational stuff with light as a storage system it's talking about. Very interesting. Sadly, it does not accomplish my goal here, and therefore is not useful for making a lightsaber.

Read your comics. (http://darthsanddroids.net/episodes/0009.html)

[Icewalker]

Note: don't quote me on any of these, I'm no expert on plasma physics...

-I don't think it would be possible to magnetically contain plasma in a shape like extending from a handle (certainly not that could be moved quickly).
-I don't think that you could cut things with plasma like that as a lightsaber does?

(Added bonuses:
-A magnetic containment field would not reflect what it contains, it would just contain more of it, probably.
-Plasma fired in the form of a blaster shot I believe is also impossible, or at least somewhat ridiculous.)

... :smallbiggrin: That's such a great webcomic though.

I believe you need lasers that come to a stop at a given point, to make something operating similar in fashion to a lightsaber.

[thubby]

I believe you need lasers that come to a stop at a given point, to make something operating similar in fashion to a lightsaber.

i always assumed there was a physical rod inside the field with some kind of lens at the end. it makes the whole concept of them extending make sense.

[RebelRogue]

Yup! That is damned awesome, and I am very curious about that quantum computational stuff with light as a storage system it's talking about. Very interesting. Sadly, it does not accomplish my goal here, and therefore is not useful for making a lightsaber.
Having seen Lene talk about this in person, yes it's pretty damn awesome, but the need for a Bose-Einstein Condensate makes the practical exploitation of any of this rather complicated.