# Does momentum "get transferred" during collisions?

#### Pmb

PHF Hall of Fame
Thank you. I read your link and I can see that energy can be called a property of matter and treated in a bookkeeping way.
That never came from what I wrote. The most important think on that page is the quote from Richard Feynman from The Feynman Lectures on Physics, Vol I by Feynman, Leighton, and Sands, Addison Wesley, (1963)(1989). Section 4-1 What is energy? page 4-1
It is important to realize that in physics today, we have no knowledge of what energy is. We do not have a picture that energy comes in little blobs of a definite amount. It is not that way. However, there are formulas for calculating some numerical quantity, and we add it all together it gives “28” - always the same number. It is an abstract thing in that it does not tell us the mechanism or the reasons for the various formulas.
Now I have 3 other related questions:
1) Can force be called a "property" of matter? If not then what would you call it?
2)Can torque be called a "property" of matter? If not then what would you call it?
3) Is there such a thing as matter?
The answer to 1 and 2 are no. Matter is a poorly defined quantity in physics. That's why the best of them avoid it as much as I do. Its no better than the term "stuff." Is there such thing as stuff?

See: https://en.oxforddictionaries.com/definition/matter
Physical substance in general, as distinct from mind and spirit; (in physics) that which occupies space and possesses rest mass, especially as distinct from energy.
Some physicists, such as myself, think that's a poor definition.

#### studiot

I want to get the words right.
OK so you do not want to have a discussion with me.

I will leave you to enjoy the tender mercies of PMB.

#### Woody

What are Mass & Energy?

Physics is perhaps surprisingly bad at providing answers to these sort of questions.
Similarly poorly defined concepts include Distance and Time!

One tends to get into circular arguments:
Energy is the quantity that satisfies these equations; these equations define energy...

Physicists might argue that "what" these quantities "are" is a meta-physical question.
Physics concentrates only on what these quantities "do".

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#### Farsight

Mass, or should I say rest mass, is resistance to change in motion. It's easier to stop a skateboard than a car, because the latter has more mass. It's also a measure of a body's energy-content, as Einstein said in his 1905 paper Does the inertia of a body depend upon its energy content? As to why, see a paper by Martin van der Mark and Gert (not the Nobel) ‘t Hooft called Light is Heavy. It talks about a balance scale, and explains why a box containing hot gas is heavier than a box containing cold gas. Imagine something like a gedanken spring steel box in space full of gedanken spring steel bullets ricocheting around inside. When you push the box, you have to push against the bullets bouncing off the side you’re pushing against. When I tap my magic wand and make those bullets move faster, you find it harder to get the box moving. So the box has more mass. Van der Mark and ‘t Hooft go on to replace the gas with light, whereupon the same principle applies. When you push the box, you have to push against the photons bouncing off the side you’re pushing against. When I tap my magic wand and increase the photon frequency, you find it harder to get the box moving. So the box has more mass.

Note that the mass of a body is a measure of its energy-content. Energy is a real thing. Matter is made of it. As for what energy is, that's more difficult to explain, because energy is fundamental. I can tell you that your sideboard is made of wood, and I can tell you that wood is made of carbon atoms plus other atoms, and that a carbon atom is made up of electrons plus other particles. Then I can tell you that we can make electrons and positrons out of photons in pair production. I can also tell you what a photon is. It's an E=hf transverse wave "soliton" in space that propagates at c. In addition I can tell you that Einstein considered it to be energy. However there's also neutrinos, and black holes which are comprised of energy, and they aren't photons. So in the end I'm reduced to saying energy is spatial stress-energy-momentum.

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#### Pmb

PHF Hall of Fame
Energy is not a real thing. You can't pick, it up. It can't be detected. You can't see, feel or hear it. Energy is merely a booking concept.

#### Farsight

It isn't just some bookkeeping thing. Read Einstein's E=mc² paper. Note this line: "The mass of a body is a measure of its energy-content". Also think of an ocean wave. When you take some of the energy out of that wave, the wave is diminished. When you take all the energy out of that wave, the wave isn't there any more. It's the same for a photon. And note that we can make matter out of photons in gamma-gamma pair production.

I don't know where you've got this bookkeeping idea from, but it doesn't square with Einstein, or with the hard scientific evidence. It's wrong. Sorry.

#### studiot

Farsight, your material is a farcry from the OP, who seems to be just learning elementary classical Physics.

Ok so there is no such thing as mass, energy or momentum..they are just "properties" of matter.
Can force be called a "property" of matter?
Can torque be called a "property" of matter?
Is matter a "thing"?

Thanks, I want to get the words right.

In particular photons are not 'matter'.

#### Farsight

Yes, I know photons are not matter. But when you trap a massless photon in a gedanken mirror-box, it's harder to move as a result. It's mass is increased. Then when you open the box and the photon flies out, the mirror-box is a radiating body that loses mass. It's all pretty simple stuff, see https://arxiv.org/abs/1508.06478.

#### Pmb

PHF Hall of Fame
Farsight, your material is a farcry from the OP, who seems to be just learning elementary classical Physics.

In particular photons are not 'matter'.
That's subject to opinion. Some authors (Feynman, Guth, etc.) sometimes use relativistic mass and as such a photon (or a directed beam of radiation) has mass and as such its matter.

Hans C. Ohanian refers to the EM field as the fourth state of matter.

#### Farsight

Hans C Ohanian also wrote What is Spin? which I think is a great paper.