Take the 2-minute tour ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free, no registration required.

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)?

share|improve this question
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. –  Vineet Menon Sep 7 '11 at 5:31
add comment

3 Answers

up vote 3 down vote accepted

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.

share|improve this answer
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". –  Ted Bunn Sep 6 '11 at 17:28
So kinetic energy is not (necessarily?) related to speed? (what exactly is it then?) –  Jonathan. Sep 6 '11 at 17:34
@Jonathan: it'd make a good question to ask what the KE of a photon is. But see if this helps you first. –  David Z Sep 6 '11 at 17:39
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. –  Ted Bunn Sep 6 '11 at 18:19
@Ted: True, thanks. –  AdamRedwine Sep 6 '11 at 20:09
add comment

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).

share|improve this answer
add comment

Actually, the speed of light is not constant. 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 was used as proof as to the fabric of space time not being smooth, but likened to a frothy foam that can affect the way photons travel. The scientist at the centre of this research was quick to note that his findings would actually undermine a long held principle of physics... that the speed of light is anything but constant!!!

share|improve this answer
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. –  Sandro Vitenti Sep 21 '12 at 22:49
add comment

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.