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bloodhound

Point of light travelling faster than light

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Imagine a laser beam shining at the moon. if you move the beam. with enough angular velocity. due to the fact the the points speed at distance r is spped = ang. vel times r. wont that point of light be moving faster than light on the mmooons surface. And if that is true, would we see a discrete set of points like firing a machine gun while moving fast enough.

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When you talk about discreet points, are you talking about individual photons? Because those would be difficult to see. But yeah, sure this seems true.

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was the set of points continuous in the first place? was taking light as a wave. it should be. but what happends once that point where the light strikes the moons surface starts moving faster and faster

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There is no such thing as rigidity in special relativity. As you move the point across the sky the photon beam will bend slightly as each successive photon will be ejected on a different trajectory. The same thing happens with solid objects. If you have a really long pole, for instance, and you try to use it to send messages faster than light by pushing at one end the pressure wave, placed on particles in the atomic lattice, is restriced to the speed of sound in the material. This is a similar concept to what I think your trying to get at here.

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Imagine a laser beam shining at the moon. if you move the beam. with enough angular velocity. due to the fact the the points speed at distance r is spped = ang. vel times r. wont that point of light be moving faster than light on the mmooons surface. And if that is true, would we see a discrete set of points like firing a machine gun while moving fast enough.

 

Yest, the point will move faster than c. No, it doesn't necessarily mean we see a discrete set of points - it depends on the photon flux. You could see this effect on a much smaller scale if the photons were emitted slowly (rate-wise) enough.

 

[pre-emptive]Oh, and this doesn't violate relativity, either.[/pre-emptive]

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I get what you mean but the light isn't realy moving sideways. Its just like shooting a machine gun. You fire bullets in rapid sucssesion while changing the angle of the gun. Each bullet reaches the moon at the point that its trajectory took it but it does not move side ways. You may have a good particle/ wave experiment though. If you moved the beam fast enough you would get seperate photon impact sites with particles.

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Yup. Not in violation much like firing a single photon in opposite directions causes their motion relative to one another to equal 2c.

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Yup. Not in violation much like firing a single photon in opposite directions causes their motion relative to one another to equal 2c.

 

Not strictly true as the two photons do not have relative speeds as they do not have refrence frmaes. Taking the limit of the compostion of velcoties law you actually get an answer of c.

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you would get lots of points on the moon (depending at what rate the photons are emmited. I think it would work much better if you used a laser that emitted one photon at a time (if this were possible) then assuming you moved the laser fast enough and looked at the moon you would see little points. but they would be scattered across the moon and not in a single line.

 

this is because the photons impact point is decided by a probability wave (two slit experiment) however I am not very familiar with the physics of this

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