im not a student, teacher, researcher in fact I've never even taken a physics class. But I Love to argue!, i spent a day proving (in my mind anyway) that, 1. a photon does have mass. 2. its impossible to say a photon's mass is x, because its variable, and can not be at rest.

so how is the speed of light is possible in nature since photons can only travel at the speed of light in a vacuum, a vacuum has no gravity, gravity it a result of particles. in space there is particles, gasses, dust, antimatter, gravity, and on and on. in nature there is no perfect vacuum. is my thinking flawed? someone said the space between atoms, but what about quantum foam. and a vacuum is the absence of matter, if you have matter in a jar thats not full, is the epmty space a vacuum? so eithet space IS a PERFECT vacuum, or its NOT a PERFECT vacuum. thoughts?

response to a comment.............

In the center of momentum frame, the colliding antiparticles have no net momentum, whereas a single photon always has momentum (since it is determined, as we have seen, only by the photon's frequency or wavelength—which cannot be zero)

A so-called massless particle (such as a photon, or a theoretical graviton) moves at the speed of light in every frame of reference. In this case there is no transformation that will bring the particle to rest. The total energy of such particles becomes smaller and smaller in frames which move faster and faster in the same direction. As such, they have no rest mass, because they can never be measured in a frame where they are at rest. This property of having no rest mass is what causes these particles to be termed "massless." However, even massless particles have a relativistic mass, which varies with their observed energy in various frames of reference,

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    $\begingroup$ Umm... physics doesn't work like philosophy. $\endgroup$ – Jimmy360 May 17 '15 at 7:42
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    $\begingroup$ You can ponder all you want, but that won't change experimentally verified fact. $\endgroup$ – Jimmy360 May 17 '15 at 7:42
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    $\begingroup$ How did you come about the "fact" that "a photon does have mass"? $\endgroup$ – Jimmy360 May 17 '15 at 7:43
  • $\begingroup$ jimmy360, i responded to your comment in my post $\endgroup$ – Jason Lowe May 17 '15 at 7:50
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    $\begingroup$ @Jimmy360 Haha, I hadn't really noticed the date. But it is encouraging to know that we are humanly capable of changing opinions on such topics. :P $\endgroup$ – Dvij Mankad Jul 14 at 22:02

You are basically correct. In the presence of any charged particles a light wave interacts with the particles to form an entangled system. Once this has happened we no longer have a photon and (for example) an electron, but instead we have a single system described by a single wavefunction that includes both particles. For weak interaction the system is still roughly describable as two particles, and if we do this we find the photon travels slower than light and behaves as if it has a mass. For more see my answer to What is the status of massless photons traveling through a medium? (actually this is awfully close to a duplicate of your question).

However, if we are going to be precise then a photon does still has zero mass and travels at the speed of light. That's because the entangled system described above is no longer a photon.

In any case, the question of the speed of light in a vacuum is a somewhat pedantic one. The density of baryonic matter in intergalactic space is about one hydrogen atom in every two cubic metres. This density is so low that no conceivable experiment is ever going to measure its effect on the speed of light.


In nature, there is a perfect (classical) vacuum (ignoring gravity, which I will get to). Think about atoms. In space, there are atoms, but if you "zoom in" there are gaps. In these gaps, couldn't light travel impeded? And about the gravity argument, gravity changes the frequency of light but not its velocity.

  • $\begingroup$ yes, i agree, im my argument with my friend i said the speed of light for travel would be impossible as 1micron, or 1 mile might be clear. $\endgroup$ – Jason Lowe May 17 '15 at 7:52
  • $\begingroup$ im thinking about major distance though $\endgroup$ – Jason Lowe May 17 '15 at 7:53
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    $\begingroup$ @JasonLowe Well sure, the speed of light probably cannot be maintained for the length of the observable universe. $\endgroup$ – Jimmy360 May 17 '15 at 7:58
  • $\begingroup$ That doesn't affect or contradict modern physics though. $\endgroup$ – Jimmy360 May 17 '15 at 7:59
  • $\begingroup$ sorry i got a phone call, but i thought about this, and did a bit of "research"., you said between atoms light could travel at light speed, and i agreed. but now im not so sure. $\endgroup$ – Jason Lowe May 17 '15 at 9:40

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