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Author Topic: What would a very fast lighthouse do?  (Read 5298 times)

Offline Geezer

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What would a very fast lighthouse do?
« on: 27/04/2011 06:10:44 »
If I rotate a light source, possibly a "laser", so that its light is projected on to a surface that is a constant distance from the axis of rotation of the light source, will the projected image appear quantized if the distance and rotational speed are great enough?


 

Offline graham.d

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What would a very fast lighthouse do?
« Reply #1 on: 27/04/2011 08:55:32 »
A very interesting question. The answer must be "yes". You can imagine the following... If the receiving region is far enough away, the effect would be to diminish the light intensity. Eventually, any one point in the receiving region would only be receiving single photons infrequently in the same way that a closer region would be if the light intensity of the transmitter were reduced in power. It is easy to see that this means that the receiving area would be detecting single quantised photons.

In fact really any detector is always doing so by receiving quantised photons.
 

Offline imatfaal

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What would a very fast lighthouse do?
« Reply #2 on: 27/04/2011 10:31:36 »
In addition to Graham's answer - isn't angular momentum quantised as well?  On changing the rotation would this mean that only certain rpms were possible?  This is normally impossible to detect on a classical scale because the presence of planck's constant in the equations means that any quantisation is so small as to be undetectable in lumpen reality - but would this set-up act as a huge magnification device that would allow us to perceive the underlying quantum world
 

Offline JP

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What would a very fast lighthouse do?
« Reply #3 on: 27/04/2011 15:22:27 »
Are you asking about detecting individual photons?  For most light sources, you can detect single photons only when the light source is dim enough that your camera captures only a single photon per "picture" it takes.  Cameras also have an integration time, so when taking a picture they add up all the light reaching them over a certain very short time interval.  The result is that you want a light source that's dim enough so that over the camera's integration time only one photon is captured.

Rotating the light source would only help if it rotated significantly out of your field of view in a time period below the camera's integration time.  In that case, it will effectively reduce the integration time of your camera, decreasing the number of photons per snapshot.  Moving far away from the light source will also generally help, since, due to diffraction, the light gets more spread out as you move away from it, so the intensity at your camera will be less.
 

Offline Geezer

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What would a very fast lighthouse do?
« Reply #4 on: 27/04/2011 19:53:05 »
Thanks everyone!

I was wondering if the projected light would show up as a series of dots on the screen. Admittedly, they would probably be too faint to actually see, but perhaps a fancy detector array or even photographic emulsion would work.

Alternatively, it might be possible to synchronize the rotational speed in such a way that an image would build up in time.

If this actually works, I was thinking it might be an interesting way of demonstrating that even a powerful light source is composed of quantized light. But there are probably much better ways of doing that already.

Does anyone have a rough idea of what the "sweep speed" would have to be in order for this to work? I suppose that would depend on the photon emission rate at the source.
 

Offline JP

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What would a very fast lighthouse do?
« Reply #5 on: 27/04/2011 20:15:54 »
Does anyone have a rough idea of what the "sweep speed" would have to be in order for this to work? I suppose that would depend on the photon emission rate at the source.

Yep.  You'd have to compute it for a given camera integration time, power and sweep rate.  :)

I think this might be an overcomplicated setup, though.  You basically just want to take a powerful source and attenuate it enough that you get roughly one photon per integration period.    You can just put a highly absorbing material in the way, or use highly reflective mirrors to remove most of the light.
 

Offline Geezer

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What would a very fast lighthouse do?
« Reply #6 on: 27/04/2011 20:58:16 »

I think this might be an overcomplicated setup, though.


Typical academic NIH attitude  ;D
 

Offline lightarrow

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What would a very fast lighthouse do?
« Reply #7 on: 27/04/2011 21:48:58 »
If this actually works, I was thinking it might be an interesting way of demonstrating that even a powerful light source is composed of quantized light. But there are probably much better ways of doing that already.
Yes. Register the light from a very faint star. You need some time to collect enough photons to form a visible image.
 

Offline Geezer

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What would a very fast lighthouse do?
« Reply #8 on: 27/04/2011 21:52:36 »
I suppose I could use a rotating mirror with a stationary light source. If I could vary the rotational speed of the mirror, would expect to see some sort of interference patterns on the screen at certain speeds?
 

Offline JP

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What would a very fast lighthouse do?
« Reply #9 on: 27/04/2011 23:17:03 »
Not really.  There would be no reason for light emitted on the first rotation to interfere with the later light emitted from the second rotation.
 

Offline Geezer

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What would a very fast lighthouse do?
« Reply #10 on: 27/04/2011 23:26:42 »
Bugger er, well, maybe interference is the wrong term.

Would it be possible to emit photons at a constant rate so that, during each revolution, the photons hit the screen at the same spot? The time for one revolution would have to be some multiple of the emission period of course.
 

Offline Bill S

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What would a very fast lighthouse do?
« Reply #11 on: 28/04/2011 00:16:49 »
Quote from: Geezer
Admittedly, they would probably be too faint to actually see, but perhaps a fancy detector array or even photographic emulsion would work.

Or, you could see individual photons if you happened to be a frog, according to David Deutsch.  "The Fabric of Reality".
 

Offline JP

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What would a very fast lighthouse do?
« Reply #12 on: 28/04/2011 00:45:43 »
Bugger er, well, maybe interference is the wrong term.

Would it be possible to emit photons at a constant rate so that, during each revolution, the photons hit the screen at the same spot? The time for one revolution would have to be some multiple of the emission period of course.

Well, you can kind of think of the intensity on the screen as a guide to where you'll find photons when the intensity gets turned down very low.  Unless all your intensity is bunched at a point this won't happen.  If all your intensity is bunched at a point, you won't be gaining anything by rotating the light source, since you're intentionally not spreading the intensity beyond that point. 

If you did want to put all your intensity at a spot while rotating the light source, you could use a shutter or a pulsed laser.
 

Offline Geezer

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What would a very fast lighthouse do?
« Reply #13 on: 28/04/2011 08:53:42 »
Bugger er, well, maybe interference is the wrong term.

Would it be possible to emit photons at a constant rate so that, during each revolution, the photons hit the screen at the same spot? The time for one revolution would have to be some multiple of the emission period of course.

Well, you can kind of think of the intensity on the screen as a guide to where you'll find photons when the intensity gets turned down very low.  Unless all your intensity is bunched at a point this won't happen.  If all your intensity is bunched at a point, you won't be gaining anything by rotating the light source, since you're intentionally not spreading the intensity beyond that point. 

If you did want to put all your intensity at a spot while rotating the light source, you could use a shutter or a pulsed laser.

But I don't want to turn down the intensity  :D That's the whole point of the experiment!

What I want to do is demonstrate that a beam of light is quantized without altering its intensity by any means other than projecting it like a lighthouse beam. I was thinking if I could perhaps synchronize the rate of rotation with the emission rate, we would build up a series of visible dots on the screen, but I have a nasty feeling the photon emission rate is too indeterminate to make that work.
 

Offline JP

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What would a very fast lighthouse do?
« Reply #14 on: 28/04/2011 13:25:49 »
Yeah.  To control the emission rate, you're going to have to control the intensity, which is what you don't want to do.
 

Offline lightarrow

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What would a very fast lighthouse do?
« Reply #15 on: 28/04/2011 15:19:04 »

But I don't want to turn down the intensity  :D That's the whole point of the experiment!

What I want to do is demonstrate that a beam of light is quantized without altering its intensity by any means other than projecting it like a lighthouse beam. I was thinking if I could perhaps synchronize the rate of rotation with the emission rate, we would build up a series of visible dots on the screen, but I have a nasty feeling the photon emission rate is too indeterminate to make that work.
Photons are not emitted naturally in a definite sequence (like one every0.0000001 seconds), but casually. You should intentionally prepare a source emitting photons at a constant pace and every photon should be independent, that is its coherence lenght shorter than the spatial distance between two subsequent photons.
Anyway I'm not sure at all you could see the individual photons hitting the screen in precise points in this way, however it could be an idea.
« Last Edit: 28/04/2011 15:21:47 by lightarrow »
 

Offline Geezer

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What would a very fast lighthouse do?
« Reply #16 on: 28/04/2011 16:55:53 »

Photons are not emitted naturally in a definite sequence (like one every0.0000001 seconds), but casually.


That's what I was afraid of. Sounds as if they are like busses. You wait at the bus stop for half an hour, then five of them show up all at the same time.
 

Offline Geezer

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What would a very fast lighthouse do?
« Reply #17 on: 28/04/2011 18:01:50 »

Yeah.  To control the emission rate, you're going to have to control the intensity, which is what you don't want to do.


Dang! Another perfectly good experiment messed up by minor practical limitations.

Back to the drawing board.
 

Offline lightarrow

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What would a very fast lighthouse do?
« Reply #18 on: 28/04/2011 19:14:33 »
That's what I was afraid of. Sounds as if they are like busses. You wait at the bus stop for half an hour, then five of them show up all at the same time.
Instead I often experienced this when I left university: just before I could arrive at the bus stop, 2 busses in a line had just left it and I missed them for a few seconds... I had then to wait twice the normal time.
I began to think the fate was against me  :(
(Maybe it's even for this reason that I didn't get the degree?  ???)
« Last Edit: 28/04/2011 19:19:58 by lightarrow »
 

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What would a very fast lighthouse do?
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