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Swept-beam light experiment

What does a swept beam look like from the far end?
  [vote for,

This is an idea for an experiment that could be performed.

When talking about things moving faster than light, sometimes someone brings up the thought experiment of sweeping the beam of a across a distant object. the point of illumination can easily be moving faster than the speed of light. And of course that isn't very interesting in terms of faster than light travel because with that setup, no information moves faster than light.

But what does the beam of light look like from the far end when that happens?

If you think of light as photons, then if the beam swept across an array of telescopes looking toward the laser, some might catch a photon, but others would receive no photons because the path length is long in comparison to the rate that photons are being emitted form the laser.

On the other hand, if you consider light to be an electromagnetic wave which is continuous, at any one telescope it seems like you'd get a very stretched out version of part of a wave.

It seems like some kind of experiment to explore this might teach us something more about the nature of light. Maybe someone has already done this and I'd be interested to read about it.

I'll make a prediction that if you set up a standard double- slit experiment at a distance such that a laser on a spinning mount sweeps across the two slits one after the other, that if the point of illumination is moving at non-relativistic speeds, you'll see the dot behind one slit then behind the other. As the laser is spun fast enough (or the device is move far enough away) that the point of illumination approaches or exceeds C (probably with some factor related to the wavelength), will you start to see interference patterns, or because the wave arrives at one slit sooner than the other slit, do they not interfere?

scad mientist, Dec 07 2016

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       I don't know that the sweep can go faster than c. It is dependent upon the rate of the arrival of the photons at each point along the sweep.   

       I'm quite certain that this thought experiment has been explored before.
RayfordSteele, Dec 07 2016

       At a wild guess, it'd be like getting someone to ride a bike towards you, with a strong bike lamp, you'd get c + some velocity and so a bit of a doppler shift.
not_morrison_rm, Dec 07 2016

       This is a cool think, [scad].
MaxwellBuchanan, Dec 08 2016

       Pulsars are actually set up to broadcast digital TV; it's just an incredibly low bit-rate, but then again the civilisation that set it up is very ancient and very long lived. The last few hundred thousand years have been a commercial break, but programmes should re-start any millennium now.
MaxwellBuchanan, Dec 08 2016

       Photons and waves are only models - ways of thinking about light - and neither model captures all of the properties of light. Given that a dual- slit experiment shows interference patterns even when the light source emits only one photon at a time I think the experimental set-up you describe will definitely show an interference pattern.

The point of 'sweep' of the light beam will certainly move faster than light, but this is OK. Also, the experimental set-up you describe is absolutely identical in every way to one in which you have a stationary light-source and a rapidly rotating universe (principle of relativity).
hippo, Dec 08 2016

       This may be a silly question but...   

       Suppose I have a light beam, and the beam itself is being swept from left to right at a significant fraction of lightspeed.   

       Suppose I have a pipe, in line with the source, so that at some moment, the light beam is shining through the tube.   

       Given the sideways movement of the beam (and hence of all the photons), will the beam still shine through the pipe, or will the photons be travelling "sideways" and therefore hit the walls of the pipe?
MaxwellBuchanan, Dec 08 2016

       If you put a machine-gun (with zero barrel-length) at the centre of a merry-go-round and fire it outward whilst it's spinning, the bullets will fly out in straight-lines whose origins are at the centre of the merry-go-round. In other words, as soon as the bullet leaves the spinny-thing, it is no-longer under the effect of the centipedal (yes, I know) force of the spinny thing. Same with a torch. Does any angular momentum carry over as you move the machine gun out towards the edge of the merry-go-round, or as you increase the length of the barrel? My guess is not.   

       Then, swap a laser for the machine gun. What's changed? I think a photon emission is something that happens instantaneously, so I don't think any angular momentum gets picked up. Like a zero-length machine-gun barrel.
zen_tom, Dec 08 2016

       //I think a photon emission is something that happens instantaneously, so I don't think any angular momentum gets picked up. Like a zero-length machine-gun barrel.//   

       Now that is one of those useful analogies that makes things easier to understand. Thanks [zt]
MaxwellBuchanan, Dec 09 2016

       Yes, each photon travels out radially from the centre with no sideways momentum; the path of the whole light beam though looks diagonal. Remember (principle of relativity) it should not be possible to look at this set-up and determine whether it is the light source at the centre which is rotating or whether it is the entire universe which is rotating around a stationary light source.
hippo, Dec 09 2016


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