Create Photonic Force with Flashlight?

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Discussion Overview

The discussion revolves around the concept of generating a photonic force using a flashlight, particularly through the manipulation of photon frequency and its implications on momentum and acceleration. Participants explore theoretical aspects, potential applications, and the underlying physics principles involved.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that changing the frequency of light emitted from a flashlight could create an acceleration due to the relationship between momentum and frequency.
  • Another participant counters that changing the color of the light merely emits photons of different colors without affecting overall momentum.
  • A question is raised about whether a frequency shift of photons could lead to acceleration, and how such a shift could be achieved, such as through the Doppler effect.
  • One participant asserts that classical theory indicates no acceleration for photons in a vacuum, emphasizing that changes in momentum are due to moving charges rather than the photons themselves.
  • Another participant argues that photons, despite having no mass, possess momentum and do not need to accelerate to exert force.
  • A participant mentions that a flashlight already produces a small amount of thrust, likening it to a "photon drive," and states that the frequency of photons does not affect the momentum of the exhaust.
  • It is noted that the energy of a photon is dependent on its frequency, leading back to the relationship between momentum and frequency.
  • One participant elaborates on the conservation of momentum in electromagnetic fields and the distinction between photons and charged particles in this context.
  • A later reply clarifies that once photons are emitted, their frequency cannot be altered by the flashlight, which has already imparted momentum at the moment of emission.

Areas of Agreement / Disagreement

Participants express differing views on whether changing the frequency of emitted photons can create an acceleration or force. There is no consensus on the implications of frequency changes on momentum, and the discussion remains unresolved regarding the mechanics of photonic force generation.

Contextual Notes

Some participants reference classical physics and electromagnetic theory, while others introduce concepts from quantum electrodynamics (QED). The discussion includes assumptions about the behavior of photons in different contexts, such as in vacuum versus dispersive media.

Phymath
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while I am most likely completely wrong I've got an idea...
while we all know force is...
F = dp/dt

and the momentum of a photon is p = hf/c
then wouldn't you create an acceleration if u had a flashlight that changed frequence(color) extremely quickly, yes i know the momentum would be so small that you wouldn't experience it but shouldn't it still happen

dp/dt = h/c df/dt

a recoil from the frequencey change or a pulsing light would also work, from f = 0 to f = some color frequencey.

ma = h/c df/dt
[tex]a = \frac{h}{cm} \ \frac{df}{dt} \ \frac{1}{m}[/tex]
 
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You seem to have missed the point that as you change colour quickly, all you are doing is emmitting photons of different colours at different times. How does this create a change in momentum?
 
yes i see what u mean, but if the photon did have a frequency shift would there be an acceleration? (next question: how would u make a photons frequency shift? dopper effect? would that change the orginal problem?)
 
Don's you just hate it when someo one shines a flashlight on you and it nocks you back several feet?
 
Don's you just hate it when someone shines a flashlight on you and it nocks you back several feet?
 
Classical theory is quite suffficient to indicate no acceleration for the photon in vacuum. From Poynting's Thrm it's clear that any change in momentum, field + particles, is due to moving, accelerating charges, which, of course, experience forces. The field changes due to the motion of the charges -- emitting and absorbing photons

I suspect that it might be possible to "accelerate" a wave packet in a dispersive medium. Interesting problem.

Photons in vacuum do not accelerate. There's nothing to push or pull,
Regards,
Reilly Atkinson
 
while i don't really what ur trying to say about changes in momentum, other than i think your trying to say that momentum only changes with particles with mass...but anyways the point i was making is that the photons didn't need to "accelerate" to create a force, photons have not been discovered to have mass currently, but still they have momentum...
 
I don't quite get why you want to change the photons frequency. A flashlight will already produce a tiny amount of thrust. It's a very low power example of a "photon drive", a rocket where the exhaust is photons. The frequency of the photons in a photon drive does not matter - the momentum in the exhaust P is the energy of the exhaust E divided by the speed of light

p = E/c

irregardless of the frequency of the photons.
 
But the energy of a photon depends on its frequency by
E=hf,
so we are finally back at the starting relation p=hf/c.
 
  • #10
Phymath said:
...and the momentum of a photon is p = hf/c
then wouldn't you create an acceleration if u had a flashlight that changed frequence(color) extremely quickly, yes i know the momentum would be so small that you wouldn't experience it but shouldn't it still happen

Keeping the same intensity (= number of photons), a change in frequency should correspond to a change in momentum, simply because :

p = hf/c

where p is the momentum of the photon, and -p that of the emitter. If you have a blue laser and a red laser of equal intensity, the blue laser should provide more recoil than the red. Finely tuning the frequency from red to blue from a single laser however is more problematic.
 
  • #11
Momentum and Poynting's Thrm

Phymath said:
while i don't really what ur trying to say about changes in momentum, other than i think your trying to say that momentum only changes with particles with mass...but anyways the point i was making is that the photons didn't need to "accelerate" to create a force, photons have not been discovered to have mass currently, but still they have momentum...
Yes, photons have momentum because the E&M field has momentum. That this is true is one of the more profound results of classical physics. Further, a key result is that the momentum of charged particles added to the field momentum is conserved. There are some subtle points in the arguments, some of which deal with whether or not there is an ether. Many E&M and Relativity texts cover this material in great detail. Check it out.

Also, the structure of QED diagrams illustrate the profound difference between photons and charged particles. Charge lines go on forever -- loops are forever; photon lines end all over the place -- creation and destruction and all that.


Regards,
Reilly Atkinson
 
  • #12
Phymath said:
yes i see what u mean, but if the photon did have a frequency shift would there be an acceleration? (next question: how would u make a photons frequency shift? dopper effect? would that change the orginal problem?)

If you change the frequency of photons that are already emitted, the flashlight will not know it, it couldn't care less. The flashlight's momentum was given to it at the instant of emission. Whatever happens to photons afterwards is irrelevant to this problem.
 

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