High speed and low speed photons

4,510

Solution 1

The animation is unfortunately misleading. The speed of light is constant and all photons, of any energy, travel at the same speed. Higher energy photons have smaller wavelengths (or, equivalently, higher frequency) but not a different speed.

Unfortunately, this is difficult to illustrate clearly. The reason the illustration shows the higher energy photons as faster is because of the naturalness of equating speed with kinetic energy of an object. It "makes sense" to most people that a more energetic particle would move faster, even if this isn't an accurate description of the phenomenon.

Solution 2

No. the energy of a photon is not linked with his speed. all photons move at the same speed $300.000 km/s$, that corresponds to the speed of light. The different energies of pictures are associated with frequency or wavelength of this. the greater the wavelength, lower energy and vice versa. this is: $$ E = h.\nu $$

Depending on their wavelength, electromagnetic radiation is given different names, ranging from energy gamma rays (with a wavelength of about picometers) to radio waves (wavelengths of the order of kilometers) through the visible spectrum (the wavelength is in the range of tenths of micrometer).

Solution 3

In the answer by user48649 an example is given of an observation that can be interpreted as two different velocities.

A telescope viewing a supernova from over 16 billion light years away recently clocked the low energy photon arriving 5-7 seconds later than it's high energy equivalent.

This publication shows that actually this statement is partly true, only the distance and certainty of the conclusion is wrong. Dr Robert Wagner, of the Max-Planck-Institut für Physik, München, found that there was up to a 5 second gap between low energy and high energy photons coming from an active galactic nucleus (Markarian 501) SEVEN billion light years away. This is not to say that this is definitive proof, as Dr Wagner notes "We cannot exclude, however, the possibility that the delay we find, which is significant beyond the 95% C.L., is due to some energy-dependent effect at the source."

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Updated on August 01, 2022

Comments

  • Jonathan.
    Jonathan. over 1 year

    Looking at the discovery of the neutron, and I came across this page: http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron3_1.htm

    The animation on the left, talks about low energy photons and high energy photons. And it implies the more energy a photon has the faster it moves. I don't understand the whole light is photons and waves at the same time thing, but I thought the speed of light was constant, that Gamma rays travel at the same speed as visible light, infrared, microwaves, etc.

    So how is that some photons can move faster than others (with more energy)?

    • Vineet Menon
      Vineet Menon about 12 years
      photons always move at same speed $c$. By low and high energy the author probably meant the inherent energy manifested by the frequency of that photon.
  • Ted Bunn
    Ted Bunn about 12 years
    I can understand the temptation to use speed as a stand-in for kinetic energy in the animation. Worse, in my opinion, is the the text below it, which says that the photons "needed to move extremely fast".
  • Jonathan.
    Jonathan. about 12 years
    So kinetic energy is not (necessarily?) related to speed? (what exactly is it then?)
  • David Z
    David Z about 12 years
    @Jonathan: it'd make a good question to ask what the KE of a photon is. But see if this helps you first.
  • Ted Bunn
    Ted Bunn about 12 years
    By the way, although this answer has the physics right, I think the wording is confusing at one point: the phrase "because of the naturalness of equating kinetic energy with total energy of an object." For the photons in this experiment, equating kinetic energy with total energy is correct, so this isn't the problem. Rather, the problem is implying that higher kinetic energy means higher speed.
  • Sandro Vitenti
    Sandro Vitenti about 11 years
    This supernova would come from outside of our observable universe and on top of that from before the big bang? Sorry but this and the rest are just nonsense.
  • anna v
    anna v over 9 years
    this was formatted as a comment to another answer that has been down voted. I editted so that it will not be deleted as out of format. You can roll back the edit and wait for the moderators of course.
  • anna v
    anna v over 9 years
    My observation is that GeV photons do not come from run of the mill reactions . This must be a huge cosmic accelerator there, and to imagine the source in time is a great leap for my imagination . Maybe it pulsed, and higher energy synchrotron is followed by lower energy synchrotron radiation
  • dmckee --- ex-moderator kitten
    dmckee --- ex-moderator kitten over 9 years
    @SandroVitenti Oddly enough the size of the visible universe (expressed in light-years) is larger than the age of the universe (expressed in years). This is because space has been growing as the light traveled.
  • rob
    rob over 9 years
    As an example, at this accelerator the electron beam makes several passes around the racetrack to gain energy from the same accelerating cavities more than once. This means that the beam pipe simultaneously contains electron bunches with energy 1–12 GeV. But they don't overtake each other, because even 1 GeV is strongly relativistic for an electron and so all the bunches travel at the same speed, $c$.