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Superluminal events which do not transmit information or energy

  1. Oct 25, 2005 #1
    What are the examples of superluminal events which do not transmit information or energy, and hence not violating causality but travels faster than light?
    My book here gives an example of a shadow of a bug flying across a projector. Any others?
  2. jcsd
  3. Oct 25, 2005 #2


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    Some other examples are the spot of a laser moving across the moon faster than light (see here), or the contact point between the blades of a giant pair of scissors moving faster than light, and thus cutting a piece of paper faster than light (see here).
  4. Oct 25, 2005 #3
    Surely with the laser on the moon there will be a lag as the laser pointer is moved here on earth, and the time it takes for more EM wave packets to leave the pointer and reach the moon? Therefore the light will look curved from a "top-down" view, if you like, until the first photon from the pointer in its new position reaches the moon. And surely moving the giant scissors will cause the scissors to just get deformed.

    For example, say we have a rigid sphere. When it hits the floor the bottom and top of the sphere instantaneously change velocity; but of course the bottom receives its upwards push from the floor; then through EM interactions the push is transmitted to the top of the ball at a speed less than or equal to c, and so the ball gets slightly deformed; therefore no such thing as a rigid body.

    Don't know why I added the last bit; it seemed relevant at the time.
  5. Oct 25, 2005 #4


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    The laser example isn't about the beam, it's about the spot on the moon. But you are right - the beam would appear curved, kinda like when you spray water from a hose and move it back and forth.
  6. Oct 25, 2005 #5


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    Sure, there'd be a lag between the moment you began to rotate the angle of the laser and the moment the spot began to move across the face of the moon, but the point is that once the spot begins to move, it can move faster than light if you rotated the angle of the laser fast enough. But no actual photon in the laser is moving faster than light here, and the "spot" is not a single physical entity, it's just a series of positions where different photons from the laser are hitting the face of the moon.
    That depends whether the angle between the blades is changing--you could just have two separate blades with a constant angle between them that move vertically relative to each other at constant velocity, for example. But yes, if you try to rotate a blade from a fixed pivot the blade will get distorted.
  7. Jun 29, 2011 #6
    Re: Superluminal

    Do they correct them though? I'm just starting out with physics so bear with me!

    As I understand it, redshift is permitted to be in excess of c when dealing with cosmological distance/time/velocity because it doesn't cause a violation of special relativity, given that redshift is moving into infinity from our 'static' point of view, as opposed to measuring a quantified distance, between the Sun and Proxima Centauri for instance?

    And while the vector sum you give in your example of two objects approaching each other at near c would be in excess of c.. that's only an abstract value composed from combined two empirical values?
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