# Thread: What is the non-rest mass of the photon?

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## What is the non-rest mass of the photon?

The rest mass of the photon is zero. Right?

But a photon is never at rest.

So, in a perfect example where photons always go at the speed of light in vacuum (c), what would their mass be, given that E=mc2?
Thanks

2. I think energy=hf

so non-rest mass must be hf/(c^2) where h is planck's constant, f is the frequency of the photon, and c is the speed of light.

but I could be wrong.

10-36kg?

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Yeah, I forgot about the frequency. Thanks for that!

Let's say, for comparison, an infrared photon and an ultraviolet photon. Exact frequencies aren't necessary, just an average will do.

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Mass is the resistance of an object to acceleration by
a force. That is, apply a known force to an object, and
measure the object's acceleration in response to that
force.

The problem is, you can't apply a force to a photon.
You can't accelerate a photon. It always moves at the
same speed. Because it has no mass.

-- Jeff, in Minneapolis

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Photons can be accelerated, Jeff. As when they change optical medium from a non-vacuum to a complete vacuum. They will accelerate to the speed of light.

6. anyway the mass equivalent is around 10-36kg, I think.

7. Originally Posted by Xibalba
Photons can be accelerated, Jeff. As when they change optical medium from a non-vacuum to a complete vacuum. They will accelerate to the speed of light.
That's not true. Light travels more slowly in a medium because photons hit atoms and are absorbed, and that atom emits a new photon. But it doesn't happen instantaneously. The photons themselves always travel at the speed of light. It's the effect of the photons (light) that can be slowed down or accelerated.

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Originally Posted by primummobile
That's not true. Light travels more slowly in a medium because photons hit atoms and are absorbed, and that atom emits a new photon. But it doesn't happen instantaneously. The photons themselves always travel at the speed of light. It's the effect of the photons (light) that can be slowed down or accelerated.
Thanks for the correction.

However, I found a cool Wikipedia page that gives the relative mass of the photons at different frequencies : http://en.wikipedia.org/wiki/Electromagnetic_spectrum

9. Originally Posted by Xibalba
The rest mass of the photon is zero. Right?

But a photon is never at rest. ...
A neutrino has a non-zero rest mass, but it's mass is so small that it always seems to travel at c for just about any situation in which you'd measure the speed (SN1987A might be a place where we observed a difference). Photons might have a zero rest mass, but if their mass is several orders of magnitude smaller than the equivalent energy in very long wavelength radio waves, we'd have no way to observe the mass, and a photon could still never be at rest.

I say this not to say you are wrong, but because you asked if your statement was right, and the answer really is that we don't know for sure.

10. If you could come up with a container with perfect reflectivity, then if it contained photons, I would presume, that that container would weigh more because of those photons.

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Oh. I think maybe you want to know how much mass
would have the same total energy as a given photon.
That's straightforward.

As Frog March says, E=hf. The peak emission of an
ordinary tungsten lightbulb is just a little way into the
infrared, with a wavelength of about 1 micrometer, so
a frequency of about 3x10^14 Hz (300 THz).

h = 6.6262x10^-34 Js
hf = 1.986x10^-19 J

The Lyman alpha line of hydrogen is in ultraviolet,
with a wavelength of 121.6 nm, so its frequency is
2.465 x 10^15 Hz (2.465 PHz).

hf = 1.634 x 10^-18 J

My math aversion is making me nervous, so I'll let
you square the speed of light (299792458 m/s) and
divide those energy figures by it:

m = E / c^2

-- Jeff, in Minneapolis

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Sheesh. I started working on my second post immediately
after finishing my first post. It took an hour and two minutes.

-- Jeff, in Minneapolis

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Originally Posted by Frog march
If you could come up with a container with perfect
reflectivity, then if it contained photons, I would
presume, that that container would weigh more
because of those photons.
explained that, huh?

Yes, a box of light weighs more than an empty box,
although the mass equivalent of light is so miniscule
that the mass difference could never be measured
in reality.

-- Jeff, in Minneapolis

14. Originally Posted by Frog march
I think energy=hf

so non-rest mass must be hf/(c^2) where h is planck's constant, f is the frequency of the photon, and c is the speed of light.

but I could be wrong.

10-36kg?
Are you saying that non-rest mass of photon is 10-36kg?

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He is saying that the mass equivalent to the energy carried by a low frequency photon is 10e-36 kg.

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