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Does electromagnetic radiation actually carry kinetic energy?

  1. Mar 11, 2015 #1
    Hi all,

    Does electromagnetic radiation actually carry kinetic energy?

    Looking around the web gives different answers. I have seen explanations like radiation pressure experiments as evidence, but this has been argued being effects from energy absorbed by the recipient.

    In case it does not carry kinetic energy, the universe dynamics are challenged.

    In my understanding, based on the law of energy conservation, the universe rest mass energy, the rest mass kinetic energy and electromagnetic radiation should equate over the universe different epochs.

    In early epochs with electromagnetic radiation dominating over rest mass energy, the expansion rate/ kinetic energy must have been much higher in order for the energies to equate? At the moment just after singularity, the expansion rate must have been close to C? As rest mass start dominate expansion speed decreases? Energy equation is balanced by the speed of expansion/kinetic energy? G?

    What we actually see looking out in universe, back in time, to a point in time were most rest mass in fact already was created is a quite steady expansion rate (arguably some acceleration in “recent” times)
     
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  3. Mar 11, 2015 #2

    mfb

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    It depends on your definition of "carry kinetic energy". It carries energy and momentum.

    You can have momentum transfer with the recipient losing (or keeping the same) energy, too.

    There is no global energy conservation in general relativity. The expansion of space has nothing to do with motion or kinetic energy, and you cannot measure it as speed.
     
  4. Mar 13, 2015 #3
    Well, I’m a laymen in math…..

    If imagine an external view of an isolated system of decaying isotope, the energy is conserved and Newtonian physics should be correct?

    • The kinetic energy of escaping alpha particle and repelling nucleus, plus the escaping gamma energy would equal the loss of rest mass in the decayed nucleus, according to E-mc2

    • Mother Nature obviously balance kinetic energy, rest mass and electromagnetic emission
    If defining an arbitrary volume in universe as an independent system and with an outside observation (and disregard any energy exchange at the boundaries), the energy must be conserved in the expanding room over time. But simply roll-out the kinetic energy of the moving rest masses from an external observation point, I have hard to understand

    Also, if an infinite universe is assumed, I have hard to get that G should have an attracting force, as all galaxies have equal pull from surrounding galaxies in all dimensions in the expanding universe (like thinking “what is the gravity in the center of mother Earth”) Its expanding to hold the energy equilibrium?

    Sorry to bather you with perhaps silly thoughts….
     
  5. Mar 13, 2015 #4

    mfb

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    Newtonian physics is "correct" if you can neglect relativistic effects.

    There does not have to be a gamma emission.

    Which "expanding room" do you mean?

    You need general relativity for a proper description of that.
    There is no "energy equilibrium", and no global energy conservation in general relativity.
     
  6. Mar 13, 2015 #5
    Electromagnetic energy is purely kinetic energy, as there is no rest mass.
     
  7. Mar 15, 2015 #6
    There does not have to be a gamma emission. Which "expanding room" do you mean?

    Reference https://www.physicsforums.com/threa...lly-carry-kinetic-energy.802609/#post-5040710

    Electromagnetic energy is purely kinetic energy, as there is no rest mass.

    Reference https://www.physicsforums.com/threa...lly-carry-kinetic-energy.802609/#post-5040710

    Sure, there are diffrent kind of decay.


    It would be interesting to actually be able observe and measure a of a single atom decay.

    - An emission of an alpha particle (a percentage of rest mass in relation to the emitting nucleus) would result in a recoil and an increased kinetic energy of the emitting nucleus?

    - A directional strong gamma emission would not I guess??? (if gamma emission is perfectly isotropic, there will be no recoil obviously)

    There can be difference in the actual physics and wave / particle duality math’s should perhaps be applied carefully.

    With “expanding” room I mean a expanding finite bubble in the universe; with the expansion rate we currently measure.
     
  8. Mar 15, 2015 #7

    mfb

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    That has been done.
    The recoil is the increased kinetic energy.
    The photon goes in a single specific direction - the direction is random but the emission cannot be isotropic for a single atom.
    That's why we have quantum mechanics, to explain things like the Mößbauer effect.

    This has nothing to do with the expansion of space.
     
  9. Mar 15, 2015 #8
    A photon carries energy.
    I don't see the point in qualifying its energy as "kinetic".
    As far as I know, the term "kinetic" is only used to distinguish between different types of energy when dealing with massive particles.
     
  10. Mar 15, 2015 #9
    This was the route I was thinking as well since the most basic and famous energy formula at least to me is E=mc^2 so my interpretation as a "layperson in math" is that when there is energy there also has to be mass?

    This makes sense coming from my understanding of kinesiology, anatomy and biomechanics. I mostly lurk on here and read more than post but I thought EM radiation had mass until one of the first respondents of this thread said that it didn't, thus appearing to be at odds with einstein's equation.
     
  11. Mar 15, 2015 #10

    mfb

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    This formula applies to objects at rest only. Photons have energy but no mass and they cannot be at rest.
     
  12. Mar 15, 2015 #11
    How do you define "at rest only"? Wouldn't this be problematic since the theory itself defines c as the "speed of light" so when I hear that I think of how fast the mass is moving in relation to light.
     
  13. Mar 15, 2015 #12

    mfb

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    A system that has a center of mass that is not moving relative to you.
    This does not exist for photons, so the equation is meaningless for photons on their own.
     
  14. Mar 15, 2015 #13

    Drakkith

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    The full version of the formula is: E2=M2C4+P2C2
    P is the momentum of the object. Photons have zero mass, which is M, but they have non-zero momentum, so the equation turns into E2=P2C2, which then simplifies to E=PC.
     
  15. Mar 18, 2015 #14
  16. Mar 18, 2015 #15
    A photon moves. Kinetic derives from Greek and means associated with motion.
    A photon does not have rest energy nor rotational energy nor potential energy.
    Kinetic energy if ever there was any.
     
    Last edited: Mar 18, 2015
  17. Mar 18, 2015 #16
    You cannot seriously suggest that this idea is new ;-).
    Read more about recoil and emission at http://en.wikipedia.org/wiki/Mössbauer_effect
     
  18. Mar 18, 2015 #17
    Electromagnetic standing waves can be considered to be at rest.
    An otherwise massless resonance cavity with infinite quality containing a standing light wave with (purely kinetic) energy E behaves as an object with rest mass E/c^2.
     
    Last edited: Mar 18, 2015
  19. Mar 18, 2015 #18
    A most reasonable definition of kinetic energy is total energy minus rest energy. The entire energy of a light is kinetic energy.
     
  20. Mar 18, 2015 #19
    You cannot seriously suggest that this idea is new ;-).
    Read more about recoil and emission at http://en.wikipedia.org/wiki/Mössbauer_effect

    Well, is it really the same?

    Assume a isolated system of one atom in a energy transition

    1. In a strong alpha emission, the emitting nucleus will recoil in the opposite direction at a velocity proportional on restmass of resp. particle (weight)?

    2. In the example of gammal emission, the emitting nucleus (if not fixed in matrix) will "recoil" by vibrating (increased temperatur, resonance widening (doppler)), but it would hardly gain any velocity in the opposite direction of gamma photon?
     
  21. Mar 18, 2015 #20

    mfb

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    Yes it is.
    It will gain velocity. The effects a crystal can have on moving atoms afterwards are independent of the decay process.
     
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