# Is light infinitly heavy?

Asked by jacdesign (5) June 3rd, 2007
If light is made of photons, photons exist in the universe so they have mass, photons are also energy, and E=mc^2 then something going to infinite energy like a photon should have infinite mass (multiplied by a constant) right? I am sure there is fault in my logic because I walk outside and I am not blown back inside by the sun.
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Photons actually have zero mass.
brownlemur (4086)
at what point does the no-mass thing acquire mass?
nomtastic (974)
I think the fault in your logic is the "infinite energy" part. A photon should have some discrete amount of energy. There are some fancy equations for the amount of energy in a photon, but none of them equal infinity.
ben (8713)
photons do not have zero mass. they weigh 1.67262158 × 10^-27 kilograms. however, this weight is a proton's resting mass. a photon in motion (as they all are becuase light is a wave as well as a particle) does not have true mass, otherwise light would fall down because of gravity which would be really weird and confusing.
ezraglenn (3499)
ok, then if a photon has a very small mass then shouldn't it have very little energy? If so is it shear volume (quantity) that makes light a powerful energy source?

what do you mean "powerful energy source"? light is not really a very powerful energy source at all. a photon within the range of visible light carries 4x10 -19 joules of power... that means around 500 quadrillion photons will add up to one joule, or about 1 quarter of a calorie. not a lot of power. you'd need 37 septillion photons to boil a cup of water.

i sense that you are veering towards a question about the viability of solar power... the reason solar power is so appealing is that, as you stated, we get a gigantic number of photons from the sun, at a very consistent rate, and capturing those photons can give us energy without taking something else away (well, if you captured enough photons from the sun, you could cool the planet because those photons would not strike the earth and heat it up... but that's going to have to be a *very* large solar panel...)

samkusnetz (1210)

@ezraglenn: Photons do have mass, and therefore photons are affected by gravity. How do you think black holes 'work'? d-:

Here's an excellent (long) article on the subject: http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html

bpeoples (2551)

There seems to be some confusion here. First of all, John Baez’s site listed by bpeoples is very good. However, I think this can be answered relatively quickly. When we talk about “mass”, what we often mean is rest energy. That is, if you have a particle in a frame where it is not moving, its energy is given by E=mc^2, where c is the speed of light, and m is the mass. The more complete formula for energy is given by

E^2=m^2c^4+p^2c^2

where p is momentum. The fact that the photon is massless is equivalent to the statement that its energy is just given by E=pc, which is also sometimes written as E=h\nu where h is planck’s constant and \nu is the frequency of the photon.

A photon can never be at rest—it is always moving at the speed of light. This does not mean that photons are without energy. Indeed, as alluded to by elliottcable, the reason that black holes are “black” is that light cannot escape from them. This is due to the fact that, gravity is a force which couples to energy and not just mass.

In response to nomtastic, a particle which is massless can acquire a mass through the Higgs mechanism (also sometimes the Anderson-Higgs mechanism), which should have a good entry on wikipedia. However, this is not something that we expect to happen to the photon—the requisite scalar fields which are electromagnetically charged don’t seem to exist.

chris (409)

In response to ezraglenn, the mass you gave is for a proton (which forms the nucleus of hydrogen). A photon is very different from a proton. As chris explained the photon really is massless. Many photons can be in the exact same state at the same time (this is what makes laser light so intense) while a only one proton can be in a particular place at a particular time (just like electrons).

jterning (4)

Actually, light is without limitations, therefore there is nothing that can hinder it.

That is why it is called “LIGHT” in the first place because it possesses the ability to fill all and still accommodate all without being a burden unto none..

The opposite is also true, darkness is that which possesses infinite mass or very, very, heavy and is a burden unto everything, cannot accommodate nothing neither can it fill anything.

kess (3917)

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