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Squashed
2007-Jan-30, 06:37 PM
In another thread a link to this site was given: Quantum Mechanics (http://www.ncsu.edu/felder-public/kenny/papers/quantum.html); in which I read the following (under the subheading: When Light Waves Meet):

"Depending on where you held them—specifically, depending on their distances from the wall—they might create a bright spot, or they might create nothing at all!"

I have known about this phenomenon before but this question occurred to me: Is the light completely canceled?

When an electron and a positron annihilate the two "entities" are converted to other entities but what happens to the light in the above circumstance?

If we are pointing the two flashlights at a wall is force (now invisible) still being applied to the wall?

The flashlights recoil (very, very slightly) by firing the photons but shouldn't the wall still experience the equal but opposite force that the flashlights experience?

Or has the light just completely cancelled out of existence?

korjik
2007-Jan-30, 07:04 PM
since you cannot exactly cancel out the full extent of both waves in three dimensions, it will mean that the light went somewhere else. If you have a dark fringe, you should have a light fringe somewhere.

crosscountry
2007-Jan-30, 08:10 PM
basically there is 0 probability of a photon reaching certain spots. There is no light there.

I just did a lab last week that worked with diffraction and such. I've done it many times before, but for some reason these silly professors here require it yet another time.

Kaptain K
2007-Jan-30, 10:38 PM
Just like "anti-reflection" coating on lenses, which work by cancellation, so that light is not reflected. The light that is not reflected goes on through the lens.

Squashed
2007-Jan-31, 01:51 PM
Thanks for the replies.

According to the description from the article the light cancels at a certain distance from the two slits (or flashlights) but then I wonder does this cancelled effect occur at regular distance intervals away from the source or is the cancellation zone a one time event/area?

If the cancellation occurs at multiple distances: visible 0.0-1.0 meters then invisible 1.0-2.0 meters then visible 2.0-3.0 meters then invisible 3.0-4.0 meters; then doesn't that imply that even though the light is cancelled it is still there in some other form?

If the cancellation only occurs at one specific distance at which and beyond which it is forever invisible then that would imply that the light truly is destroyed or cancelled out of existence.

I think the thing I am really wondering is: if we take a real powerful laser and then work it out so that the laser light is cancelled then does this "invisible" laser still apply a force to the object that is within the cancelled distance interval?

Or since electrons act in a similar wave-like manner and since electrons can deliver more momentum force than photons then could we develop an invisible force-field using said cancellation techniques?

Sock puppet
2007-Jan-31, 02:18 PM
Thanks for the replies.

According to the description from the article the light cancels at a certain distance from the two slits (or flashlights) but then I wonder does this cancelled effect occur at regular distance intervals away from the source or is the cancellation zone a one time event/area?

If the cancellation occurs at multiple distances: visible 0.0-1.0 meters then invisible 1.0-2.0 meters then visible 2.0-3.0 meters then invisible 3.0-4.0 meters; then doesn't that imply that even though the light is cancelled it is still there in some other form?

If the cancellation only occurs at one specific distance at which and beyond which it is forever invisible then that would imply that the light truly is destroyed or cancelled out of existence.

I think the thing I am really wondering is: if we take a real powerful laser and then work it out so that the laser light is cancelled then does this "invisible" laser still apply a force to the object that is within the cancelled distance interval?

Or since electrons act in a similar wave-like manner and since electrons can deliver more momentum force than photons then could we develop an invisible force-field using said cancellation techniques?

It might help to think of it like this:

The photons have an electric field, and a magnetic field, right? These fields have both direction and magnitude('strength'). And the only way they can interact with the matter we are familiar with, indeed, the only way we can detect them, is by these fields interacting with something else. Maybe it'll be knocking an electron off an atom (the electric field exerts a force on the electron).

But then, if we have two photons going through the same space, with their electric and magnetic fields equal in magnitude and opposite in direction, they will cancel. Any charged particle experiencing a force from the first photon will experience one in the opposite direction from the second photon, giving no net force. that means no work done on it, no interaction.

So the photons are still 'there' so far as photons may be considered to have a specific location, they just can't transfer their energy to anything.

tusenfem
2007-Jan-31, 02:24 PM
If you would just read up on interference, you would find out everything you need.

If you do the two slit experiment, than take the wavelength of the light, and calculate the distance from the slits to some place on the screen onto which you project, and if the difference in distance is 0.5 lambda than, in the wave interpretation of light, the two waves cancel, if it is 1 lambda they enhance eachother, so on the screen you get stripes with no light and "double the light" and everything in between. This is called an interference pattern. Naturally the location of the dark stripes depends on how far you put the screen from the slits. In all, the total power on the screen must integrate to the total power that is let through by the slits.

Only at one location on the screen may you see cancellation - noncancellation etc. if you move the screen further away from the slits, but at another location you will see something different (e.g. the opposite). You cannot look at one little spot on the screen by itself, it is a combined phenomenon, so you have to look at the whole screen.

Now, doing it the particle way, it gets more complicated, and experiments have shown that even if you fire off single photons through the slits, in the end you still end up with the interference pattern, as in the wave view. But that goes too far here.

Squashed
2007-Jan-31, 05:01 PM
Thanks for the replies.

I finally did read the whole article that my O.P references and now I am thinking that the double-slit patterns are more "probability" patterns rather than "interference" patterns.

Probability is gonna have me thinking for quite a while!!

tusenfem
2007-Feb-01, 08:20 AM
In this case probability patterns and interferece patterns are the same. This is just because you can see light as a wave or as a particle. Now if you would do this same experiment with water waves (which is basically done in every physics class when discussing waves) you will see that waves make an interference pattern.