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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
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?
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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.
||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.
||This is a cool think, [scad].
||//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.//
||Well you do see a diffraction pattern, which is kind of the point. I think you got that backwards.
||I hate to think what pulsars are doing at a distance.
||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.
||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).
||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?
||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.
||//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]
||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.