Can high energy radiation create momentum?

  • #1
Suppose there's an atom in deep space, beyond the reach of gravity of any close-by galaxies. When this atom is bombarded with gamma radiation, and an electron is ejected, wouldn't this ejection also exert reverse 'thrust' on the atom itself, in accordance to Newton's 3rd law? In other words, would gamma or high energy x-ray radiation be capable of moving an atom if no other forces are exerted? And if not, why not?
 

Answers and Replies

  • #2
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p=h/lambda so All electromagnetic radiation has associated with it some momentum.
 
  • #3
p=h/lambda so All electromagnetic radiation has associated with it some momentum.

Does that mean that, if no other (sufficient) forces are applied, matter could be repulsed by light?
 
  • #4
sophiecentaur
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Each photon has momentum and this will transfer to an atom as it is absorbed. But if an electron is ejected at high speed, the nucleus will also head off in the opposite direction so that total momentum is conserved. The total momentum, after the event will be that of the original photon although the individual momentum of the electron may be higher than the individual momentum of the photon.

The phenomenon of light pressure is well known and is part of the reason why Comets' tails are always pointing away from the Sun and not, as one might think, trailing out behind the comet.
 
  • #5
A.T.
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The phenomenon of light pressure is well known and is part of the reason why Comets' tails are always pointing away from the Sun and not, as one might think, trailing out behind the comet.

Cool film showing stuff pushed by light:

220px-Schwassman-Wachmann3-B-HST.gif


Material coming off Component B of 73P/Schwassmann–Wachmann which broke up starting in 1995, as seen by the HST. This animation covers a span of three days.

http://en.wikipedia.org/wiki/Comet
 
  • #6
Very cool, but wouldn't that imply that photons have mass? If mass is 0 then momentum must be 0? Or am I missing something?
 
  • #7
sophiecentaur
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No. Photons have Momentum (h/λ) but not mass. Counter-intuitive, maybe, but true.
 
  • #8
Weird, but ok. How does that work then? Curiosity gland is pumping :D
 
  • #9
sophiecentaur
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Aamof, it works in Classical Electromasgnetism, too. If you do the sums for a classical electromagnetic wave hitting a metal reflector, it is not too hard to show that there is a force. The value of this force turns out to be the same as if you do the same with QM. That gobsmacked me even more, actually. It's there in my old Panofski and Phillips Electomag text book from the 60s.
 
  • #10
Well, I asked :P Anyway, putting it all together, if a planet were to somehow be ejected beyond the reach of gravity of its own galaxy, would the spectrum of light coming from the galaxy repulse the planet away?
 
  • #11
sophiecentaur
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There is nowhere "beyond the reach of gravity" of anything. Both the gravitational force and the intensity of the light will diminish at the same rate (the inverse square law) so your suggested effect couldn't occur. The forces would be in the same proportion as they are right near the Sun or a Galaxy.
 
  • #12
Drakkith
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Well, I asked :P Anyway, putting it all together, if a planet were to somehow be ejected beyond the reach of gravity of its own galaxy, would the spectrum of light coming from the galaxy repulse the planet away?

Small objects and particles near a star are repulsed away from it due to light pressure. This is also the reason a solar sail works. The less massive an object is the greater the effect of radiation pressure will be. This is why the earth isn't pushed away from the sun, it is far too massive!
 

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