Mass

Can something without mass bounce off something with mass?

 

I'm not sure what you mean. The only known massless things are sub-atomic particles that are constantly moving at the speed of light. They do interact with massive (i.e. having mass) particles but bouncing is not something any sub-atomic particles do.

 

This is legitimate curiosity, MikeE.

If sub-atoimic particles do not bounce, then what do they do?

A photon is a massless particle, no? yes?

So if a photon strikes an Electron, then what happens the moment it comes into contact with the Electron?

A photon may be massless, but doesn't it also have a charge measured by its wavelength?

Thanks for the reply,

Darrell

 

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A classical particle has momentum p=mv so to have momentum this equation suggests that particle must have mass. However there are a very small number 2 known and 3 more possible that have 0 mass. The most well known is the photon which 'is' light. Light does have momentum that is calculated using the DeBroglie relation (p = hk) that should be an h-bar but I have no idea how to get one of those in this forum. neither h or k are dependant on mass, this relation essentially says that a wave can have a momentum as well as a particle. So I suppose the simple answer to your question is no . You can see these effects everytime you look in a mirror, the light refelcting off of a mirror is essentailly an elastic collision between a photon and a particle (sort of).

 

A beam of light will reflect from a mirrored surface. However, you can not extend this to get an idea of the behaviour of an individual photon.

Sub-atomic particle interactions are very weird. The main reason for this is the Heisenberg priciple. The uncertainty in the position of a particle times the uncertainty in its momentum is always postive. This means that neither uncertainty is zero. This is not a statement about the cleverness of the experiment, but a statement about the fundimental unknowablity of the universe.

The study of the motion of massless particles (in the small scale) is called Quantum Mechanics. To realy understand it you need a lot of math. English or any other language is inadequate to describe what happens. I would recomend reading one of Issac Asimov's fact books to get a picture of how weird Q.M. is.

 

Bouncing maybe a feature of very large things but the idea of an elastic collision applies at all levels. Not all types of momentum are totally unknowable, for example the angular momentum along one orthogonal axis can be known to arbitary accuracy, just as can energy in an eigenstate. Though you are correct that there is a position-momentum uncertainty.
Also just because the particle leaving isnt the same that arrived doesn't change the fact that momentum must be conserved. I appreciate that my mirror analogy was simplstic (and possibly not a particularly good one) but the light imparts a momentum to the mirror, which must be conserved.

 

If it has energy, it must have mass, right? What about billions and billions of them? They could accumulate mass (out of space debris or etc) or accumulate INTO mass, like a big Sun. So it's gotta have some. I thought everything had mass, except depletions. Like GMOs' (Genetically modified organisms) mass has been tampered with, there's possible depletions there and should be tested for cancer causing agents. Cancer has become rather a loose term, I think, and it shouldn't have to be terminal to be listed as cancerous, (some cancers live awhile and then die out), in order to take legal action to get them off the market. To make up the depletion, should we shoot them with photons? God, then they would glow!

 

We do blast cancer with photons its called radiotherapy.

 

Wrong.

Photons and neutrinos both are massless, yet have energy. When we say massless, we mean having zero rest mass. Relativity says that the mass of an object changes with the velocity as m=m0 / sqrt{ 1 - (v^2/c^2) }
Where m is the relativistic mass, m0 is the rest mass, v the velocity and c is the speed of light.

If v=c then m=m0 / sqrt ( 1-1) which is division by zero. Thus anything moving at the speed of light must have a zero rest mass.

 

and what about impulse? if we talk about bouncing...

 

As the impulse is the integral of a force between two times. Or put another way a measure of change in linear momentum. I suppose that the equation could be applied to a quantum mechanical system but I've never seen it done so, I suspect the result would not correspond to any existing observable. I guess as a concept its a very classical idea. So I suspect youcould find a way of applying the equations but im not sure they'd mean anything.
The second definition of impulse is a force applied over negligable time, this really just there to the students dont have to worry about initial acceleration conditions. Although it is a reasonable approximation to a few things such as a hammer hitting a surface.

 

Please explain ...

 

I really am sorry to have to edit this to escalate it to the top of the forum, but I really need to understand this ... how can something without mass "bounce" off something with mass ???

It seems that massless objects, if object can be applied to something massless, have the inability to come into contact with objects of mass ... that's like saying no-thing can come into contact with some-thing which seems to be a contridiction in terms.

So I ask again ...

Please explain ...

 

'Bounce' is consequence of the physical principle of conservation of momentum. In any collision momentum must be conserved, this law applies to all areas of physics even down to studying the interaction of two fundamental particles. So for something to bounce without mass all you need to do is show that something without mass can possess momentum. It is indeed possible and mathematically is wraped up in the DeBroglie relationship. p = h/l (p is momentum, h is the plank constant and l is the wavelength). This is a very important equation and one of that starting points of quantum mechanics. But it also shows that a wave has a momentum even if it doesn't have a mass. Finding Schrodingers Cat adn Finding Schrodingers kittens (books are John Gribbin) are a very good non-mathematical intro to quantum mech, it gives an idea of the importance of waves.

Its a hard thing to concieve because in our everyday life we're so used to picturing this law as snooker balls and car crashes, but its MOMENTUM not mass that causes 'bouncing', if you can appreciate that you don't need mass to have momentum then I guess your most of the way there.

 

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nice pic but i got slated for using the mirror analogy the other day.

 

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Actually many physists now beleive that photons have mass. If black holes have such strong gravity that they pull in light than light must have mass. The energy given off by the break down of matter entering a black hole can escape, but is it due to lack of mass or to energy strong enough to pull free? They are still debating this subject and there is some good research on both sides.

 

Mmmm im not that photons have mass. If they did then they couldn't travel at the speed of light. Relativity clearly states that for a body with mass to be accelerated to the speed of light in a vacuum will require infinite energy. Clearly this is not the case as a lightbulb in space will emit light travelling at the speed of light in a vacuum off a 12V battery which is some way short of infinite energy.