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Rotating a laser beam faster than light?

  1. Aug 20, 2008 #1
    This is probably just nuts but lets say you switch on a laser, point it to the sky and leave it on for billions of years until the beam has traveled billions of light years. Then you rotate the laser 360 degrees in lets say 1 RPM. Does the laser spot that is now billions of light years away from the axis of rotation complete the rotation in 1 minute? If so then the laser spot that is now billions of light years from the axis of rotation would have to travel much faster than light right?
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  3. Aug 20, 2008 #2


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    A mathematical point can travel faster than light, relativity says you can't send information faster than light.
    You can sweep a laser accross the moon faster than light but it doesn't transfer information from one side of the moon to the other (it's known as the lighthouse paradox).
    You can even (in theory) make the point where a pair of scissor blades meet go faster than light, if you close them fast enough.
    Last edited: Aug 20, 2008
  4. Aug 20, 2008 #3
    I'm curious about this lighthouse paradox, but I can't really find anything on it. Do you know any sources for further reading?
  5. Aug 20, 2008 #4


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    It's just what the OP described.
    A lighthouse beam sweeps around in say 1second.
    So at a distance of 1mile the beam covers 6.3miles in 1s (ie. 2pi*r)
    Obviously as r increases the circumference increases but it still only takes 1 second.
    So at some distance (30,00miles) it will go >186,000 miles around the circumference in 1 second so faster than light.

    But the beam moving from point to point doesn't carry any information so no violation of relativity.
  6. Aug 20, 2008 #5
    Kind of a stupid question but can someone define what exactly is meant by information in this context.
  7. Aug 20, 2008 #6


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    Right, but the spot on the screen is not a single entity. It is a succession of different spots formed by different bundles of light as they hit the screen. Each bundle travels from the laser to the screen at speed c. Nothing actually travels from one point on the screen to the next.
  8. Aug 20, 2008 #7


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    It's very simple, anything you didn't know before!
    So turningthe lighthouse on/off sends information, on=team A won the superbowl, off=team B. So the light from the light house can't reach you faster than light because you would know who won. But a sweeping beam can't take any information from one side of the moon to the other because there is nothing anyone can do at the left point to change the beam that will arrive at the right point.

    But suppose you had a very long solid metal arm that you swung around in an arc, someone could stick a post-it note on the end as it went past them - which the next other person could read.
    So this proves that you can't just put a long metal rod on the lighthouse and move it fast enough for the end to go faster than light.
  9. Aug 20, 2008 #8
    People are confusing a moving laser dot with something tangible. The dot is made up of photons shooting outwards from the light source. The dot at starting point is not made up of the same photons as the dot at the end, and the idea that the dot at the start and the dot at the end are one in the same is a product of our imagination. My cats get confused over this very same thing.
  10. Aug 21, 2008 #9
    What if we replaced the laser with a metal disc with a radii of a billion light years and rotated it at 1 RPM. Would the outer edge of the disk be rotating faster than light?
  11. Aug 21, 2008 #10


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    This is at one with the old question about taking a rigid rod with length several light years and the "pushing" one end a centimeter. Doesn't the other end move a centimeter at the same time? Couldn't you use that to send a message several light years in an instant?

    The answer is that what that tells us is that there is no such thing as a "rigid" object, even theoretically, in relativity. The motion of the end of the rod will move as a wave, at a speed determined by the elasticity of the rod but less than the speed of light, up the rod.

    Similarly, your disk will distort so that the outer edge is not moving at the same speed as the center. You can't rotate a huge sphere at 1 rpm.
  12. Sep 28, 2008 #11
    I guess that the underlying but un-articulated point in most of the questions is this: do the photons, exiting from the rotating source of light, have any tangential velocity, in addition to the outward radial speed of light. Ie, are they travelling radially as the traditional view has it, or in some 'Catherine wheel', bent spoke trajectories? I read that some recent experiments with the lighthouse nuclear emissions (from the rotating samples) DO register angular shift between irradiation and emission.
  13. Sep 28, 2008 #12


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  14. Nov 15, 2008 #13
    In fact the light beam spot wont move faster than light, the light beam will curve as you move it, so if you pointed it at moon as suggested earlier, the spot cant move faster than light even if the angular movement of laser on earth does point toward a different place on the moon faster than light could move to that spot.

    So if the spot is an enormous distance in space on a surface of massive object, say a light year away, then the laser is moved just a few degrees, an observer on that planet would not see the laser spot move for a year, the beam of light would actually move in a curve shape with the spot lagging behind. If it did move instantly at that distance then information could be transfered from that observer to another one a light year from him by rapidly moving laser spot from one to the other in a digital sequence etc.

    Another example would be point at moon again, turn it on, but move away from moon in less than a second, if spot moves with laser at any distance like a straight rod then the laser spot would not hit moon, but that would`t be the case
  15. Nov 15, 2008 #14
    The light beam does not actually "curve", but I do know what you are suggesting and you are correct.

    Why some insist that a laser from earth can sweep the surface of the moon faster than light is beyond me. It just doesn't happen, can't happen and never will.
  16. Nov 15, 2008 #15


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    Not entirely sure what robbie is trying to say.
    The important point is that the point of contact of the laser with the moon can move faster than 'c' but the point of contact is just a mathematical notion - there is nothing actually there. No photon ismoving faster than 'c'.

    A simpler example might be to consider two cars driving to the east and west coast from the centre of the USA. They both drive the same distance at the same speed and arrive within a millisecond of each other. So the 'point of arrival' has traveled accros the
    USA (3000mi) in 1ms - much faster than light.
  17. Nov 15, 2008 #16
    Well, I certainly agree with that.
    Last edited by a moderator: Nov 15, 2008
  18. Nov 15, 2008 #17
    While I understand part of your point, your explanation is flawed. Using the example of the target being one light year away, if the laser starts to rotate yes it will take one year to notice this on the receiving end. At that point though the spot will start to appear to move faster than the speed of light at the receiving end.

    However it's already pointed out that a light spot is just an interaction. Light hits atoms and is reflected. I have used this example before, and I think it gives a good picture of what is happening. Imagine a line of light bulbs, and a switching system that turns them on then off in quick succession. We can rig this line of bulbs so that the time the first bulb blinks to the time the last bulb blinks is greater than the speed of light across the line of bulbs. The light would appear to travel faster that light speed (left to right as it were), but in reality there is no movement of the bulbs.

    Substitute the bulbs for the atoms reflecting the light, and the switching system for the laser pointer.

    I'm guessing by curved light you are referring to the type of situation where a rotating water sprinkler makes a curved stream of water. That's a fair comparison but the stream will still hit the receiver and cause interactions with atoms that propagate faster than the speed of light given the situation described here.
  19. Nov 15, 2008 #18
    Wow, I can't believe how hard it is for me to understand some things, even though I'm not entirely brain-dead. Maybe in time and through proper education/discourse I will begin to grasp these concepts.
  20. Nov 15, 2008 #19
    Interesting idea Halls.
    Something like this should actually be testable shouldn't it?
    You don't need a rod several light years in length.
    You simply need a rod of any length; the longer the better for measurement sake.
    Measure its length.
    Put two sensors at that length apart.
    Connect the two sensors via the same amount of wire to a central recorder which records time of contact. As the contacts are stationary with respect to each other this is no problem.
    Then use the rod with contact pad to press a message against the first sensor which, because of the rod, is translated to the second sensor via a second contact.
    All corresponding contacts are then sent to the central recorder.
    Then simply compare the duration between the first contacts received and the second contacts received and divide this by the length of the rod to get the speed of transmission.

    Has such an experiment ever been done?
  21. Nov 15, 2008 #20


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    Yes it's called measuring the speed of sound in a material.
    It's done everytime you ultrasound test a weld or metal pipe.
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