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  Can information propagate with superluminal speed in this experiment?

+ 0 like - 1 dislike

No? Consider the following simple (thought) experiment. A strong laser pulse issued from a location on Earth sends encoded information bits to a point A near the surface of the Moon where the info arrives after 3.8 seconds. Then the laser pulse is rotated by an angle Theta = 90° (= 1.57 rad) during 1 second so that the info at the point of the laser sweeps over the lunar surface with speed v. The info is received and decoded by a station B at a distance d from the point A, so d = Theta x (distance Earth-Moon) = 1.57 x 380000 = 596600 km. During 1 second the info has travelled at a speed v = d/1s = about 2c near the moon. Note that in this experiment the info has a carrier which is the endpoint of the laser. - Is there a flaw in this reasoning?

asked Dec 27, 2022 in Chat by rhkail (0 points) [ no revision ]
recategorized Jan 2 by Dilaton

A laser beam is not a rod, in particular not a stiff rod which can be rotated as a single object. A laser pulse emitted in direction of point A will not sweep over the lunar surface when you turn the laser.

Is this because for any small change deltaTheta of the rotation angle of the laser on Earth the laser will deviate in direction?

It is because if you send a laser pulse (encoding some information, if you wish) towards a point A, then this laser pulse will continue moving towards point A, even after you have turned the laser. Only pulses emitted after you have turned the laser will travel towards a different point. A pulse emitted by the laser does not remain "connected" to the laser, it is not turned with the laser.

Thanks, I agree. And, according to SRT, even a "stiff rod" would not do the job, since according to the principle of action by contact there are no absolutely rigid bodies.


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