Yet more questions about light
- Thread starter Magical Realist
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Magical Realist
Valued Senior Member
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
"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
exchemist
Valued Senior Member
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.
Magical Realist
Valued Senior Member
So if this physics website got the definition of momentum wrong as you say, what is the true definition of momentum?
exchemist
Valued Senior Member
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.
Seattle
Valued Senior Member
I'm sure Write 4 U will be along shortly.
exchemist
Valued Senior Member
Thank God he stays out of physics.
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?
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).
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.
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Magical Realist
Valued Senior Member
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.'
Albert Einstein — 'If you can't explain it to a six year old, you don't understand it yourself.'
exchemist
Valued Senior Member
There is no evidence Einstein ever said anything of the kind, and good reason to think he would never have done.
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?
Magical Realist
Valued Senior Member
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..
Here's wikipedia:
That looks like a plain English explanation of what momentum is, to me.wikipedia said:
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.
On that point, I have written a helpful explanation here:
https://www.sciforums.com/threads/d...up-to-make-the-math-work.166382/#post-3728113
It seems to me like you don't have much of an idea about how science is done. Oh well.
Magical Realist
Valued Senior Member
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..
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?
They are not obscure. They can be found in many introductory university-level texts. This is undergraduate stuff.
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.
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.
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.
Seattle
Valued Senior Member
He just demonstrated that it is the classical formula modified by the Lorenz factor to account for the relativistic changes in motion.
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exchemist
Valued Senior Member
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".
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Magical Realist
Valued Senior Member
Can light be destroyed?
exchemist
Valued Senior Member
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.