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Author Topic: What would I see in a spinning mirror?  (Read 7845 times)

Whittle, David

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What would I see in a spinning mirror?
« on: 05/02/2009 22:30:03 »
Whittle, David  asked the Naked Scientists:
   Love your podcast, I am a regular listener from down under.

I have a question...

Standing on the platform as a train passes, I notice that I can see my roughly stationery reflection in the procession of train windows passing at 60 kmh.

That got me thinking that if I got a spinning mirror, that I would be able to see my stationary image in it, no matter how fast the mirror spun.

But is that true? What would happen if the mirror spun so fast that the area I was looking at passed at close to or at the speed of light? Would the light bounce off just the same or would something funny happen to the image?

I hope you can help!

Best Wishes,

David Whittle
Melbourne, Australia

What do you think?


 

Offline Vern

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What would I see in a spinning mirror?
« Reply #1 on: 06/02/2009 03:16:31 »
I guess you have guessed it correctly; you would see a normal image. We can't spin a mirror to close to light speed to test any hypothesis. The little dance that Feynman describes of light reflecting from a surface might encounter some distortions at near light speed, but we can't know what that might be.
 

Offline rogerscottq

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What would I see in a spinning mirror?
« Reply #2 on: 06/02/2009 06:40:30 »
Thanks for your response Vern.  And it leads me to a memory about a
unique movie-camera.  I don't recall its original purpose.  It was
basically a tube with a rod inside that had an angled reflective
surface that received a light/image-signal.  I believe the rod rotated
and simultaneously moved down the length of this tube. Yet it might have
been the tube that was spinning and moving . . . I don't recall.

The claim was that this device was capable of capturing millions of frames
per second in purely mechanical ways.  Evidently the 'tube' was lined with
some emulsion.

Have you heard of this?

In another vein, I have been told of a charge-injection type video
equipment as opposed to a charge-coupling device, that was used to
capture the 'wave-front' of a laser beam.

What I'd like to know is if, after David's trend of query, if any
evidence exists that shows an abberation of image at all.  I'd presume
that with such high revolutions, even in a short time, there'd be some
temperature changes and purely mechanical alterations of proportion.

With current techniques, one would assume such could be mathematically
compensated.  Thus, I wonder, who has actually tested the substance of his
question in actual experiment?

 

Offline yor_on

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What would I see in a spinning mirror?
« Reply #3 on: 06/02/2009 09:02:45 »
With spinning I assume that you mean the mirror rotating on its axis?
If you look at light like small balls they first will be reflected from your face into the mirror.
As the mirror spins some of the light will be reflected back to you, and some will be deflected otherwise.

I would expect that with a certain spin you will get an effect where your image becomes very visible again.
The same type of effect that makes us see 'moving pictures'.
That is a relation between your brains capability to handle visual information and the mirrors spin.

Also I have this feeling that the center of that mirror will be what reflects 'best' back towards you, and also what will give you the least 'deformed' image.
The mirror, if spinning near light, will create a framedragging effect that I would expect to 'keep' your light for ever. In fact you would have a 'black hole'.
 

Offline rogerscottq

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What would I see in a spinning mirror?
« Reply #4 on: 06/02/2009 10:03:53 »
What yor_on said makes me mindful of the term 'stroboscopic' which
implies some missing information, hence a 'time-lapse' compilation.

What is implied here is a perfect straight-line reflection . . . . even
if the abundance of signal is only partially conveyed.  What is sent off
in any diverging way is simply assumed as 'missing data'.

With any moving reflector, the question cannot be simply sequestered.

Any reflector implies a collection of atomic oscillators.  Where any
such exist, gravity also exists.  And, dispite relativity, an implication
can also exist that atomic structure is based on motion in some 'medium'.

The assumption that 'space' is empty and yet asking for a mathematical
stricture for a differential in a void near or remote from a gravitational
center leads to un-avoidable contradictions.

It is no help to speak of 'fields' or bending of fields or attempting to
introduce 'time' as if it were a dimension or a fourth 'extention'.

From the difficulties and internal contradictions of a particular brand of
relativity have grown a veritable quandry of new and apologetic theories.

Evidently, obsessive attention to these theories has diverted attention from
potentially illuminating experiments . . . either experiments actually
 historically performed or capable of being
performed . . . yet not.

In the case of the former, popular theory has actually created a blinded
eye or lack of popular attention.  In terms of the latter, money has
been typically diverted away from them. So, 'knowledge' today answers
 per theory and not truly from experiment. Contradictory experiment or
empirical data is rejected that doesn't agree with theory. Theory blocks
data.

Then you get into pyschology.


 

Offline Vern

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What would I see in a spinning mirror?
« Reply #5 on: 06/02/2009 11:20:33 »
Quote from: rogerscottq
Thanks for your response Vern.  And it leads me to a memory about a
unique movie-camera.  I don't recall its original purpose.  It was
basically a tube with a rod inside that had an angled reflective
surface that received a light/image-signal.  I believe the rod rotated
and simultaneously moved down the length of this tube. Yet it might have
been the tube that was spinning and moving . . . I don't recall.
I have seen designs for cameras similar to this whose purpose was to take extreme slow motion pictures; for example a slow motion image of a rifle bullet tearing through a playing card.

I don't know of any experiments with spinning mirrors; the reflecting light might be spin polarized in sync with the spin of the mirror; we can see spin polarized light with no problem. So, the reflection would seem normal for most spin speeds. We can only speculate about the outcome of spin rotation speeds close to the speed of light.

I am assuming the spin is in the plane of the mirror surface so that the surface faces the viewer all the time. If the spin were perpendicular to the viewer presenting front, back, front, back of the mirror to the viewer I suspect the viewer would only see a blur; no image.
 

lyner

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What would I see in a spinning mirror?
« Reply #6 on: 06/02/2009 11:35:51 »
It's a bit late in the thread but I have to ask. When you say "spinning" - what sort of spinning do you mean? What axis is it spinning around? A diagram might help.
 

Offline Chemistry4me

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What would I see in a spinning mirror?
« Reply #7 on: 06/02/2009 11:43:59 »
From what he said with the train, I presume that it would be like this:



-------

Whoops sorry, there's only meant to be one mirror/square
« Last Edit: 06/02/2009 11:46:25 by Chemistry4me »
 

lyner

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What would I see in a spinning mirror?
« Reply #8 on: 06/02/2009 12:16:43 »
Thinking classically, when the light (any e/m wave) is reflected at a conducting surface (shiny metal) the reflection is due to the conduction electrons being moved by the fields and then re radiating the energy. There is a slight phase lag in this process so the lighe from your moving mirror will come out from a slightly different position. This will displace the image a tiny bit.
For high speeds, the surface electrons will 'see' a transverse doppler shifted version and re radiate that. There will be an additional effect at re radiation. So I guess you would see a red shifted image, displaced in the direction of motion.
 

Offline Vern

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What would I see in a spinning mirror?
« Reply #9 on: 06/02/2009 12:17:02 »
Yes; Chemistry4me; that was my assumption.
 

Offline Vern

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What would I see in a spinning mirror?
« Reply #10 on: 06/02/2009 12:19:28 »
Quote
For high speeds, the surface electrons will 'see' a transverse doppler shifted version and re radiate that. There will be an additional effect at re radiation. So I guess you would see a red shifted image, displaced in the direction of motion.
I'm having trouble visualizing how that displacement would appear to the observer :)
 

Offline LeeE

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What would I see in a spinning mirror?
« Reply #11 on: 06/02/2009 13:18:47 »
I don't think that spinning the mirror about an axis that runs through both the observer and the face of the mirror is the correct model when compared with a passing train as it doesn't account for the changing angle of incidence between the observer and the plane of the mirror.  I think the correct model to use would be a mirror that spun on a vertical axis and from this it would seem that one side of the reflected image would be blue-shifted and the other red-shifted.

Rapidly spinning mirrors, used in conjunction with synchronised spinning shutters, was/is an easy way to achieve very high shutter speeds.  A conventional shutter, that opens and closes, is limited to the speed that it can operate by the mass of the shutter blades/curtains and furthermore, because of the moving masses, introduces movement into the camera.  A spinning mirror and shutter though, once it's up to speed, presents constant and much lower forces.  The speed that the combination spinning mirror and shutter disc can achieve is really limited by the size of the two components.
 

lyner

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What would I see in a spinning mirror?
« Reply #12 on: 06/02/2009 15:25:41 »
Quote
For high speeds, the surface electrons will 'see' a transverse doppler shifted version and re radiate that. There will be an additional effect at re radiation. So I guess you would see a red shifted image, displaced in the direction of motion.
I'm having trouble visualizing how that displacement would appear to the observer :)

I thought in the direction of the motion of each part of the surface of the mirror.
As it is rotating, the speed would vary with radial distance so I suppose you'd expect a 'twist' in the image. a la  Photoshop filter
« Last Edit: 06/02/2009 19:44:52 by sophiecentaur »
 

lyner

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What would I see in a spinning mirror?
« Reply #13 on: 06/02/2009 15:29:33 »
LeeE
I think they're assuming a different axis from yours.
What you're describing is used all over the place in one form or another - shutters etc.  -The reflected rays just sweep out like a lighthouse beam - giving a virtual speed of greater than c for distant objects.
 

Offline syhprum

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What would I see in a spinning mirror?
« Reply #14 on: 06/02/2009 15:30:49 »
Assuming the mirror is spinning about its horizontal axis you would see a lateral displacement of the image depending on your distance from the mirror and its rotational speed.
wasn't this what Michelson and Morley did to determine the speed of light ?.
 

lyner

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What would I see in a spinning mirror?
« Reply #15 on: 06/02/2009 19:43:47 »
I think you're referring to Michleson's method, using a rotating polygonal mirror. In that method, the rotation was about an axis IN the plane of the mirror. What we're discussing here (afaics) is rotation about an axis normal to the plane of the mirror. That's what the above picture shows, I think.
 

Offline syhprum

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What would I see in a spinning mirror?
« Reply #16 on: 06/02/2009 20:06:18 »
In that case there will be some rotation of the reflection as the Photons arriving at the surface of the mirror will spend a little time adsorbed by the atoms in the reflecting surface before being re-immited but the rate of rotation would have to be very high to produce any noticeable effect.
 

Offline LeeE

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What would I see in a spinning mirror?
« Reply #17 on: 06/02/2009 20:28:26 »
LeeE
I think they're assuming a different axis from yours.

Yup, that's why I commented in the first place.  A mirror spinning in the way suggested i.e. with the axis running through both the face of the mirror and the observer isn't the correct model for windows on a train passing a stationary observer because it doesn't account for the changing incidence - but I'm just repeating myself now...

Quote
What you're describing is used all over the place in one form or another - shutters etc.  -The reflected rays just sweep out like a lighthouse beam - giving a virtual speed of greater than c for distant objects.

Yup.  A couple of the comments seemed to show some uncertainty about why they were used in high-speed photography though, so I tried to explain.
 

Offline yor_on

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What would I see in a spinning mirror?
« Reply #18 on: 06/02/2009 21:58:24 »
What I thought about was a observer stationary in front of a mirror mounted on a vertical axle, with the mirror spinning around that same axle.
But one can understand it more ways than one.
« Last Edit: 06/02/2009 22:01:34 by yor_on »
 

lyner

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What would I see in a spinning mirror?
« Reply #19 on: 07/02/2009 00:37:07 »
It shows how useful it is to establish the actual question before we all hare off and answer our own!
I wonder what David Whittle meant.
 

Offline LeeE

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What would I see in a spinning mirror?
« Reply #20 on: 07/02/2009 17:05:23 »
Lol  ;D
 

Offline yor_on

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What would I see in a spinning mirror?
« Reply #21 on: 07/02/2009 17:33:37 »
I'm sure he found the answer somewhere:)
 

Offline David Whittle

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What would I see in a spinning mirror?
« Reply #22 on: 18/02/2009 06:28:16 »
Thanks all for your thoughts on this. I think I saw in there some of the things that I expected and others that I hadn't thought of.

Vern mentioned distortion
Yor_on mentioned framedragging (I was expecting this one)
Vern and Sophiecentaur mentioned red shift
rogerscottq mentioned - I don't know what.. I'm confused!

I regret not being more explicit about the details of the experiment and I've been slow getting back onto this (technical problems...) but reading all your comments, I think I can now be more specific.  I'm definitely striking now with a cold iron but have had some fun detailing an actual experiment. Many of you guessed at a flat mirror spinning on an axis that is perpendicular to its surface. That's not the only interesting experiment I can think of but it is the one in my head when I wrote it down. And it has nothing to do with trains anymore...

So for anyone still interested, here is an explicit description:

The Experiment
• The mirror is circular and it spins on an axis that intersects the centre of the mirror (red X in Exhibit 1) and is at a right angle to its surface. It is 2m in diameter.

• Adam has positioned himself in front of the mirror so that his image appears to him as shown in the attached image, with the top of his head just visible at the edge. With the mirror stationary, the image appears to Adam to be undistorted, bright and stable.

• The mirror begins to spin at a few thousand rpm (clockwise) but Adam doesn’t notice because the mirror is perfect – his image remains undistorted, bright and stable.

• The rate of spin increases until the outer edge of the mirror is moving at 99.9% the speed of light. If I’ve done my calculations correctly, that means that the mirror needs to spin at about 2.86 billion rpm.

The question is: Does Adam still see an undistorted, stationary and bright image of himself?

Assumptions
• Adam and the mirror are in outer space.
• Adam is wearing a space suit.
• The mirror’s reflective surface is made of silver.  It doesn’t have glass in front of the silver surface, but it won’t tarnish in the vacuum of space.
• The mirror’s substrate and frame are made of infinitely strong material and will not distort, break, or let go of the silver surface, no matter how fast it spins.
• There is infinite energy available to run an infinitely powerful motor to spin the mirror.
 

lyner

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What would I see in a spinning mirror?
« Reply #23 on: 18/02/2009 11:19:39 »
I think that, at the very least, there would be a 'twisting' of the image in the direction of rotation because of a phase lag during the reflection process.  The nearer to the edge of the mirror, the greater would be the effect. This isn't a relativistic notion - it's just the consequence of the time taken for em to interact with a conducting surface and to be reflected.
For an optical image (frequency of 10^15Hz ish) the displacement will be very small, of course. For a reflected radio wave, the shift of image could be much greater.
 

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What would I see in a spinning mirror?
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