# Yet more questions about light

If light is an electromagnetic wave in the electromagnetic field, why does it not respond to magnetism?
Is magnetism a form of light?
No.
And why do not virtual photons create virtual light?
They are virtual light.

How can photons have momentum when they don't have mass?

"Momentum can be defined as "mass in motion." All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion. The amount of momentum that an object has is dependent upon two variables: how much stuff is moving and how fast the stuff is moving. Momentum depends upon the variables mass and velocity. In terms of an equation, the momentum of an object is equal to the mass of the object times the velocity of the object."--- https://www.physicsclassroom.com/Class/momentum/u4l1a.cfm

How can photons have momentum when they don't have mass?

"Momentum can be defined as "mass in motion." All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion. The amount of momentum that an object has is dependent upon two variables: how much stuff is moving and how fast the stuff is moving. Momentum depends upon the variables mass and velocity. In terms of an equation, the momentum of an object is equal to the mass of the object times the velocity of the object."--- https://www.physicsclassroom.com/Class/momentum/u4l1a.cfm
See the equation at the top of James's post 75. (p = momentum.)

You are quoting a teaching aid for schoolchildren, which is explaining Newtonian mechanics. Since the advent of quantum mechanics and relativity we now know Newtonian mechanics is only an approximation to how nature is found to work, which breaks down under some circumstances. This is one of them.

You don't teach QM and relativity until the 6th Form, generally.

See the equation at the top of James's post 75. (p = momentum.)

You are quoting a teaching aid for schoolchildren, which is explaining Newtonian mechanics. Since the advent of quantum mechanics and relativity we now know Newtonian mechanics is only an approximation to how nature is found to work, which breaks down under some circumstances. This is one of them.

You don't teach QM and relativity until the 6th Form, generally.

So if this physics website got the definition of momentum wrong as you say, what is the true definition of momentum?

So if this physics website got the definition of momentum wrong as you say, what is the true definition of momentum?
Good question. Offhand I think the best simple definition I can give, which both photons and massive particles obey, is the ability to impart an impulse. Impulse is force x the time for which it is applied, so F x t, or Ft and this is equal to the change in momentum experienced by the body receiving the impulse. So Ft = Δp.

Radiation pressure is a known phenomenon arising from this. Even massless photons exert a pressure on a surface on which they fall: https://en.wikipedia.org/wiki/Radiation_pressure

However I'm not a physicist so stand willing to be corrected by one, if he or she comes along.

Good question. Offhand I think the best simple definition I can give, which both photons and massive particles obey, is the ability to impart an impulse. Impulse is force x the time for which it is applied, so F x t, or Ft and this is equal to the change in momentum experienced by the body receiving the impulse. So Ft = Δp.

Radiation pressure is a known phenomenon arising from this. Even massless photons exert a pressure on a surface on which they fall: https://en.wikipedia.org/wiki/Radiation_pressure

However I'm not a physicist so stand willing to be corrected by one, if he or she comes along.
I'm sure Write 4 U will be along shortly.

How can photons have momentum when they don't have mass?
I answered that question back in post #75, the first time you asked.

Are you going to do me the courtesy of reading my replies to you? If not, I might not bother replying to your questions in future. At least, not to try to help you. And are you going to thank me for answering the other questions you asked? Didn't your mother teach you basic manners?
"Momentum can be defined as "mass in motion." All objects have mass; so if an object is moving, then it has momentum - it has its mass in motion. ...
Physics textbooks don't define momentum as "mass in motion". Momentum is defined in physics by a mathematical definition.

It is true that for objects with mass, which aren't moving at a significant fraction of the speed of light, momentum can be defined by $\vec{p}=m\vec{v}$, where $m$ the mass of the object and $\vec{v}$ is its velocity.

However, it turns out that this definition is only an approximation, which needs to be modified to make the concept of momentum useful for objects that are travelling at reasonable fractions of the speed of light, and for things that travel at the speed of light.

Photons are things that travel at the speed of light and $p=mv$ does not apply to them.

Note that momentum is, fundamentally, a mathematical construct. It is defined in the way(s) that it is because it is useful for various types of calculations in physics. In particular, it is useful because it is a conserved quantity in systems on which no external forces act. The definition $p=mv$ works well enough for massive objects travelling at everyday speeds, to conserve momentum in systems on which no external forces act. However, with that definition, momentum is not conserved in relativistic situations in which one or more of the constituents of a system are moving very fast. So, we modify the definition to make sure that momentum conservation works in those systems, as well. Using the more complicated, relativistic definition, we find that the momentum, as defined, approximates to $p=mv$ for massive objects moving at low speeds.

The equation I gave earlier, $E^2=(pc)^2+(mc^2)^2$ applies to all objects at all speeds, provided that we use the relativistic definitions of the momentum $p$ and the total mechanical energy $E$, and take $m$ to be the rest mass of the object (which happens to be zero for photons).
So if this physics website got the definition of momentum wrong as you say, what is the true definition of momentum?
I already told you what it is for photons. It's $p=E/c=hf/c=h/\lambda$, where $E$ is the photon energy, $h$ is Planck's constant, $f$ is the photon frequency and $\lambda$ is the photon wavelength and $c$ is the speed of light. For particles with non-zero rest mass, it is $p=\gamma mv$, where $\gamma=\frac{1}{\sqrt{1-(v/c)^2}}$ is the Lorentz factor.

Last edited:
Don't hide behind obscure equations. Just tell me in plain English what momentum is.

Albert Einstein — 'If you can't explain it to a six year old, you don't understand it yourself.'

Don't hide behind obscure equations. Just tell me in plain English what momentum is.

Albert Einstein — 'If you can't explain it to a six year old, you don't understand it yourself.'
There is no evidence Einstein ever said anything of the kind, and good reason to think he would never have done.

Don't hide behind obscure equations. Just tell me in plain English what momentum is.
Tell me what you did not understand in my previous explanation and I'll try to dumb it down for you.

What's the problem? Can't understand math?

It's obvious you don't have a plain english explanation for what momentum is. Neither does Wikipedia. Sounds like another case of science making up something just to make the math work out. Oh well..

It's obvious you don't have a plain english explanation for what momentum is. Neither does Wikipedia.
Here's wikipedia:
wikipedia said:
In Newtonian mechanics, momentum (... more specifically linear momentum or translational momentum) is the product of the mass and velocity of an object.
That looks like a plain English explanation of what momentum is, to me.

Are you struggling with what the mathematical word "product" means?

Let me know if you need more help. I can dumb it down some more for you, if necessary. There is a limit, however.
Sounds like another case of science making up something just to make the math work out. Oh well..
On that point, I have written a helpful explanation here:

It seems to me like you don't have much of an idea about how science is done. Oh well.

That looks like a plain English explanation of what momentum is, to me.

I already quoted the Newtonian definition of momentum. I was told that has become inadequate since Einstein. Then came the obscure equations. I asked for the plain English version of that, and you regurgitate the Newtonian definition and add some pissy comments. Do me a favor and just admit you don't know what the hell it is. Moving on..

I already quoted the Newtonian definition of momentum. I was told that has become inadequate since Einstein.
Correct. I told you, explicitly, how the Newtonian definition has to be modified, too. You didn't thank me for helping you. On the contrary, you have been quite rude. Why?
Then came the obscure equations.
They are not obscure. They can be found in many introductory university-level texts. This is undergraduate stuff.
I asked for the plain English version of that, and you regurgitate the Newtonian definition and add some pissy comments.
Well, I already gave you a more complicated explanation. It's all right there in post #88, above. But you're still struggling to cope with even the simplest explanation. I invited to you to ask questions. I offered to help you. You have not yet taken me up on my offer. Nor have you explained what you're struggling to understand. I'm not sure what more you want from me. The problem appears to be on your end.
Do me a favor and just admit you don't know what the hell it is.
I already told you what the hell it is. It's right there in post #88, above. It's not my fault that you can't get yourself up to speed, even with me holding your hand.
Moving on..
Giving up, are you? This seems to be a pattern with you, when things get even a little bit beyond your immediate grasp. Perhaps you should try a little harder.

I already quoted the Newtonian definition of momentum. I was told that has become inadequate since Einstein. Then came the obscure equations. I asked for the plain English version of that, and you regurgitate the Newtonian definition and add some pissy comments. Do me a favor and just admit you don't know what the hell it is. Moving on..

He just demonstrated that it is the classical formula modified by the Lorenz factor to account for the relativistic changes in motion.

Last edited:
He just demonstrated that it is the classic formula modified by the Lorenz factor to account for the relativistic changes in motion.
MR is in troll mode at the moment. I gave an explanation in words in post 85, which he actually gave a "like" to, so all this subsequent shit must be just trying to wind James up.

I note he's started another thread, warming to his new troll theme, to pursue the same silly claim that "everything is physics is made up to make the math[sic] work out".

Last edited:
Can light be destroyed?

Can light be destroyed?
Of course. When you turn the light off in a windowless room it goes dark. The light is absorbed by the surfaces and is light no more.

Guess what I ran across.

### How Heavy Can a Particle of Light Be? Scientists Just Figured It Out​

We have a new upper limit for the mass of light.

According to measurements of pulsing stars scattered throughout the Milky Way and mystery radio signals from other galaxies, a particle of light – called a photon – can be no heavier than 9.52 × 10-46 kilograms.

It's a tiny limit, but finding that light has any mass at all would significantly impact how we interpret the Universe around us, and our understanding of physics.
Photons, typically, are described as massless particles. These discrete quantities of energy zip through space-time at a constant speed, unable to accelerate or slow down in a vacuum. This constant velocity implies masslessness, and there isn't evidence to the contrary.

However, we don't know for absolute certainty that photons are massless.

A non-zero mass would have profound implications. It would contradict Einstein's special relativity, and Maxwell's electromagnetic theory, probably leading to new physics, and possibly answering some giant questions about the Universe (although raising many more in the process).
If a photon did have mass, it would need to be extremely small to not have major effects on the way the Universe appeared, which means that we just don't have the tools to measure it directly.
But we can take indirect measurements that will give us an upper limit for this hypothetical mass, and this is exactly what a group of astronomers did.

Would it confirm Bohmian Mechanics?

The de Broglie–Bohm theory, also known as the pilot wave theory, Bohmian mechanics, Bohm's interpretation, and the causal interpretation, is an interpretation of quantum mechanics. It postulates that in addition to the wavefunction, an actual configuration of particles exists, even when unobserved.

Last edited: