# I Volume of light

#### Janez

For light it is said it has no volume and also it it waves of electric and magnetic field. But for electric and magnetic field you need space? So can they both be at the same time?

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#### Dale

Mentor
For light it is said it has no volume
Where specifically is that said? please provide an exact reference

#### Janez

I read on the internet, I cant renember where exatly. So is this wrong? Does light have volume?

#### Dale

Mentor
I don’t know. I would have to see the quote. I could think of ways it could be right or wrong depending on the details of the actual statement. Your vague recollection is simply not enough to go by.

#### Janez

Could we measure volume of light?

#### Dale

Mentor
Could we measure volume of light?
It depends what you mean by that. What sort of experiment are you thinking of? Or what quantity would you accept as being the volume of light.

I could make a 1 L box and shine a flashlight in it. Would you call that a 1 L volume of light? If not, what would you call a 1 L volume of light.

#### DaveC426913

Gold Member
There are two types of particles in the universe: fermions and bosons.

Fermions - such as protons and electrons - obey the Pauli Exclusion Principle - among other things, this means two can't be in the same place at the same time. They are the normal "volume-filling" particles that matter is made of.

Light is made of photons, which are bosons.
Bosons do not obey the Pauli Exclusion Principle - so that means you can have as many photons in a volume as you like.

ie. the fact that electromagnetic waves may have a measurable amplitude does not mean they "take up room" in the sense that other things can't occupy the same space.

You can just keep adding photons to that 1L box as long as you have photons to add. It never gets full.

#### olgerm

Gold Member
For light it is said it has no volume and also it it waves of electric and magnetic field. But for electric and magnetic field you need space? So can they both be at the same time?
Light means waves in electromagnetic field. You need to define spatial boundaries of a wave, to determine its volume.
Similarily it would be hard to determine volume of sound(soundwaves) or surfacearea of waterwaves.

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#### davenn

Gold Member
Light means waves in electromagnetic field.
no it doesn't. .... "light" refers primarily the visible, to us, part of the EM spectrum plus/minus a bit ... ie ... IR through visible to Ultraviolet

You need to define spatial boundaries of a wave, to determine its volume.
pretty difficult to do that for an EM wave when it's spreading out pretty much to infinity

Similarily it would be hard to determine volume of sound(soundwaves) or surfacearea of waterwaves.
a reason for running those words together ?

try sound waves, surface area, water waves

Dave

#### Droidriven

Could we measure volume of light?
Light is quantized as photons but this quantification is a representation of an amount of energy, not a representation of an amount of matter or mass, photons are considered "mass-less", meaning, they are not made of a substantial material that occupies space. They are not made of an amount of physical material. If they were, that material could be measured and given a mass, weight and volume.

#### weirdoguy

They are not made of an amount of physical material. If they were, that material could be measured and given a mass
System of at least two photons have non-zero mass so your reasoning is flawed.

but this quantification is a representation of an amount of energy, not a representation of an amount of matter or mass, photons are considered "mass-less", meaning, they are not made of a substantial material that occupies space.
1. Define "substantial material".
2. Energy is a property of particles/systems, not a particle (or whatever) itself. Photons also have momentum, so why do people seem to focus so much on the energy part and neglect other things? And it's always in a spirit like if massive particles didn't have energy...

#### Janez

Would it be posible to meausre(theoreticaly) magnetic filed and electric field for example if a single photon wolud be travling trought space?

#### Droidriven

System of at least two photons have non-zero mass so your reasoning is flawed.

1. Define "substantial material".
2. Energy is a property of particles/systems, not a particle (or whatever) itself. Photons also have momentum, so why do people seem to focus so much on the energy part and neglect other things? And it's always in a spirit like if massive particles didn't have energy...
So, two or more particles with zero mass have a combined total of greater than zero mass? How is this an exception to the mathematical principle of 0+0=0? A photon is a particle of energy instead of a particle of matter such as a proton or electron, isn't it?

Correct me if I'm wrong, but, doesn't some of the mass of particles come from their momentum.

1) of substance, of matter

2) isn't momentum also energy?

#### Dale

Mentor
So, two or more particles with zero mass have a combined total of greater than zero mass?
Yes. Mass is the norm of the four-momentum. So the norm of a sum of four-vectors is greater than the sum of the norms of the original four-vectors. This is the four-vector equivalent of the triangle inequality

How is this an exception to the mathematical principle of 0+0=0?
It is a different mathematical object. Mass is the norm of a four-vector, so it doesn’t add like a real number.

isn't momentum also energy?
No.

#### weirdoguy

1. Mass of a system is equal to the norm of four-momentum of the system, so mass of two massless particles can be non-zero. In relativity mass is not an additive quantity.
2. There is no such thing as "particle of energy", just like there is no "particle of velocity". Energy is a property of a system, not a system itself. Photons are quanta of electromagnetic field, they have energy, but that is not the only property of photons.
3. Momentum is definitely not the same thing as energy.

#### Stephenk53

Couldn’t you say that the light from let’s say a flashlight traveled a distance of d in say t time, and spread out into a cone shape taking up a volume of v space travelled in t time. Also in this case I assume that the flashlight stayed on and thus the light “takes up” that volume since if you stand anywhere in that volume of space you would see the light.

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#### DaveC426913

Gold Member
Couldn’t you say that the light from let’s say a flashlight traveled a distance of d in say t time, and spread out into a cone shape taking up a volume of v space travelled in t time. Also in this case I assume that the flashlight stayed on and thus the light “takes up” that volume since if you stand anywhere in that volume of space you would see the light.
You could say that, sure...

It could be deemed a little misleading, for a number of reasons.

#### weirdoguy

Wave packets, and energy is one of the things that characterise it. Besides energy light has also momentum, but no one says that light is "pure momentum". Electromagnetic filed can also have angular momentum.

#### Dale

Mentor
After cleaning up several speculative posts this thread is closed

"Volume of light"

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