# here's a thought

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i'm confused, how does doppler shifting work if light continues at c regardless of the velocity of the thing emitting it?

Hi Rocket Man,

I hope this helps.

Doppler shift is due to a change in the wavelength of light that occurs when an object is moving toward or away from an observer.

It might help to consider an electromagnetic wave as being(emitted)extruded from an object onto a conveyer belt of constant velocity c, to be later retrieved by an observer. The length of the observed extrusion is dependant on the relative velocities of the emitting object during the time of emission and the observer during the time of observation.

• If emitting object, and observer are relatively stationary, then the observed extruded wave length will be the same for both emitting object and observer.

• If the emitting object was moving away from the observer, the wave extruded onto the conveyer would be longer, however the extruded wave would still travel at the same velocity c, to the observer.

• If the emitting object was moving towards the observer, the wave extruded onto the conveyer would be shorter, however the extruded wave would still travel at the same velocity c, to the observer.

Also, i believe that the doppler effect, is not a relativistic effect. In addition to the doppler effect, the wave length is also subject to time dilation, which is a relativistic effect.
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righto, so the effect with the lightbulb would see the photons travelling away from the lightbulb (relative to the lightbulb) at 2c, and the observer would have photons travelling past at c but with 0Hz frequency so no energy.

very interesting.

the photons out the front would be shifted to twice ther frequency then?

in that case, wouldnt the momentum difference between the forward and backward photons cause a net backward force? (however negligible)

just a thought, supposing someone managed to make a 90 terawatt lazer, (1gram mass energy per second), would that have the propulsive force of 1gram per second travelling at c (following newtonian dynamics)?

one way of thinking of it is 1 gram moving away from a source, and momentum laws, the center of mass must keep in constant uniform motion.

if that's the case, gamma rays from antimatter/matter combustion would be an ideal propellant.

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sorry, the photons out the front are travelling at the same speed as the emmitter, so each osscilation is placed on top of the last. that would mean 0nm frequency and therefore infinite energy per photon (not so absurd when you think about the kinetic energy of the emmitter).

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lightbulb would see the photons travelling away from the lightbulb (relative to the lightbulb) at 2c

There's no backwards force, what are you talking about?

You're 90TW laser suggestion does not work.

gamma rays from antimatter/matter combustion would be an ideal propellant
They certainly would not. Not only do many pass straight through a material the explosion to produce enough photons would destroy the object.

the photons out the front are travelling at the same speed as the emmitter
In what reference frame?

each osscilation is placed on top of the last. that would mean 0nm frequency and therefore infinite energy per photon
No it wouldn't. You're thinking of continuous waves not inidivdual photons. There will be a defined non-zero frequency and the idea of an infinite amount of energy really is absurd.
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There's no backwards force' date=' what are you talking about?

You're 90TW laser suggestion does not work.

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which in the case of high energy gamma rays is quite substantial.

as for 90TW, i was talking hypothetically.

but otherwise you are entirely correct, which confuses me. how can a single photon occupy different positions after a certain time of travel when looking from different inertial frames? i thought photons were individual particles.

does this have a simple answer or is it simply a quirk of SR?

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• 2 weeks later...

Actually, theoretically, you would still see the light shining. the thought experiment is similar to the one where an observer travels alongside a light beam and does not see it frozen in motion. scientists think this observer might be witness to constantly moving light. The fact however is that the bulb is moving away, and so I suspect you wouldn't see a thing.

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