Massless particle sends electron flying?

[Brainguy]

massless particle sends electron flying?

Hi, I'm an 11 year old boy and I was listening to a lecture on my ipod by proffesor Steven Pollock, who I have been litening to for a while and I would be really surprised if he's giving me false information. More likely I just misunderstood him. But he was getting ready to explain the strong nuclear force which holds a nucleus together, and explained that electrons know each other are there because the emit photons which smack into the other electron and send it hurtling backwards. there is also a reaction where the electron that emitted the photon flys back in recoil. I paused the lecture and thought about this for a second. Then I remembered something, photons have no mass! F=ma, so a particle with no mass, even if it does travel at the speed of light, it couldn't possibly exert any kind of force on an electron. My first reaction to this was excitement that I was able to piece together a lot of what I have learned: Newtons laws of motion, the properties of light while govorned by a particle state, and F=ma. Tell me if I understood him correctly, because My particle zoo app tells me that a gluon is the force carrier of strong nuclear force, not a photon, and I am completely sure he never mentioned gluons in any of his lectures I have listened to, and they have a specific order so I couldn't have missed that one.
-Brainguy

 

[Doc Al]

Then I remembered something, photons have no mass!

They have no mass, but they have momentum.

F=ma, so a particle with no mass, even if it does travel at the speed of light, it couldn't possibly exert any kind of force on an electron.

The laws of physics are quite different at the particle level--you cannot apply Newton's laws of motion at that scale, you must use relativity and quantum mechanics.

(Photons are mediators of the electromagnetic force between electrons.)

 

[Parlyne]

F=ma, so a particle with no mass, even if it does travel at the speed of light, it couldn't possibly exert any kind of force on an electron.

There are two problems with this statement. First, as Doc Al stated, $\vec{F}=m\vec{a}$ doesn't correctly apply when relativity is involved. But, it looks like you also have some confusion about the meaning of the terms in Newton's Second Law.

In words, you should understand $\vec{F}=m\vec{a}$ to mean that the sum of all forces (or "net force") acting on some object is equal to that object's mass multiplied by its acceleration. In other words, this tells you how the forces on an object affect its motion. It doesn't tell you anything about the force that that object can exert on another object.

 

[Brainguy]

oh, but why does Einsteins relativity prove me wrong? I thought the essence of relativity was that physics applies everywhere, even at the smallest particles. Even at the largest galaxies. thanks to both of you. F=ma does not tell you what things will do when they hit other things, that's momentum. thanks alot, now I can re-listen to the lecture and take everything in.

 

[Stcloud]

If photons had no mass, how could we tell they existed? They would pass through all matter, leaving no trace?

 

[DaveC426913]

If photons had no mass, how could we tell they existed? They would pass through all matter, leaving no trace?

They interact via the EM force, usually with electrons.

 

[Stcloud]

''They interact via the EM force, usually with electrons.'

How can something that has no mass (immaterial - non-physical - does not 'exist') 'interact' with anything at all? How many angels can you fit in the space between electrons?Well, only half joking!

 

[DaveC426913]

''They interact via the EM force, usually with electrons.'

How can something that has no mass (immaterial - non-physical - does not 'exist') 'interact' with anything at all? How many angels can you fit in the space between electrons?Well, only half joking!

A light beam is immaterial, and yet it interacts quite nicely with a plant and your skin.


The irony of your question is that it's backwards. Energy is more fundamental than matter. The better question is: how does anything with mass interact with anything else? The answer is via energy. All mass interacts with other mass via one of the four fundamental forces.

 

[cubzar]

momentum of a photon=h/$\lambda$.
A photon cannot have rest mass. momentum=mv/sqrt(1-v²/c². The denominator is 0 for light, so the momentum would be infinite if photons had rest mass.

 

[BruceW]

Yeah, photons don't have rest mass, but do have momentum, which is why they can be detected.
Also, the photons which mediate the electromagnetic force are virtual photons, that cannot be detected (most people view them as simply a mathematical construct).

 

[Jack23454]

My first thought would be that if photons are unable to interact with mass, how then did I get a sunburn?

 

[nickthrop101]

the photons don't exactly push back the photons, but the photon will contain energy which will be absorbed by the electron, in effect giving it its energy, this energy will make the electron vibrate more and possibly even bouncing it up to a higher energy level, if the photon contains enough energy then it may even delocalise the elctron creating an ion and a free electron :D