Basic Photon Question

[JeffreyE]

Please be gentle to a newbie question, and to my limited understanding of the subject.

 

E=Mc2 as I understand it means that there is an equivalency between Mass and Energy, that essentially, mass is energy, and so the more energy of one kind or another there is, the more mass there is.

 

In that case, since we know there are several kinds of energy in photons, how can they have no mass?

 

Thanks :)

 

All My Best,

Jeffrey

[Erasmus00]

E=Mc2 as I understand it means that there is an equivalency between Mass and Energy, that essentially, mass is energy, and so the more energy of one kind or another there is, the more mass there is.

 

The key thing to realize is that when a physicist says "mass" , what he is really referring to is known as "rest mass." Perhaps the formula should more properly read

 

[math] E= m_0 c^2 [/math]

 

which tells us that an object at rest has energy just because it has mass. This mass is the mass of an object that we would measure if we did experiments on a stationary object. A moving object would actually appear to have more mass.

 

For moving objects we can modify our formula

 

[math] E = \frac{ m_0 c^2}{\sqrt{1-\frac{v^2}{c^2}} [/math]

 

This works really well unless we set v=c, and then it has problems because of division by 0. In fact, But, what we do know from this is that IF an object of 0 mass can exist, it must move at v=c (or else we know for sure it has 0 energy).

 

Now then, how can we establish the energy of a moving photon? Well, we use a slightly different formula we can also establish from relativity.

 

[math]E^2-p^2 = m_0 c^2 [/math]

 

From this we see that if rest mass is 0, a particle has E=p, where p is the momentum of the particle.

 

Hope this helps.

-Will

[JeffreyE]

Maybe that's where I'm going wrong. To me, relativity has always meant that all mass, even rest mass, is just lots and lots of energy. Things may appear solid, but the implication in my mind was that there is no difference between the two. Is this wrong?

[sanctus]

In a way you can say the rest mass is always constant, while the mass is the quantity which tends to infinity when approaching light speed (hence mass is what physically makes it impossible to reach for anything massive-ie. rest

mass different from zero- the speed of light). Was this the question?

 

And by the way welcome back!

[JeffreyE]

Hi, Santcus--

 

Yes, I think that's right. Basically, I'm having a hard time understanding the concept of why things can't travel faster than c.

 

So from what I understand, although photons have a rest mass of 0, their relative (is that term still used?) mass can still approach infinity the same way all matter does.

 

Can I then say that the reason nothing can travel faster than c is because its mass would be infinite? If so, why can photons travel at c without becoming infinitely massive as well?

 

Thanks for the welcome back... It's nice to be here

 

All My Best,

Jeffrey

[sanctus]

You can say that nothing can travel faster than cc as when approaching c the mass apporaches infinity and hence the force needed to accelerate of 1 m/s tends to infinity as well.

No, the photons always travel at c that's where they are at rest, hence their rest-mass is the one they have travelling at c and hence is zero.

[JeffreyE]

Ok, then I'm really confused. They're at rest traveling at c?

 

All My Best,

Jeffrey

[Erasmus00]

Ok, then I'm really confused. They're at rest traveling at c?

 

All My Best,

Jeffrey

 

I wouldn't try and think about it that way. I think a better way to think about it is as I stated above.

 

Relativity tells us any particle with a rest mass CANNOT travel at a speed c.

 

Relativity also tells us any particle WITHOUT a rest mass cannot travel at a speed other than c.

 

In otherwards, there is a huge difference between having a small mass, and having none at all. Even if you have small mass, you can exist at a large variety of relative speeds. If you have none at all you will always be moving at c.

-Will

[JeffreyE]

Does that mean photons are... the universe "waving"; not traveling in the sense we normally understand it?

 

Sorry, I'm trying to visualize what's "actually" happening. This is basically the place I'm stuck at understanding relativity.

[Erasmus00]

Sorry, I'm trying to visualize what's "actually" happening. This is basically the place I'm stuck at understanding relativity.

 

Here is the problem: visualization is perhaps the most difficult thing for any quantum theory. Some would argue its completely impossible.

 

May I suggest picking up Feynman's book on QED for a readable (but occasionally difficult) account of how photons work.

-Will

[JeffreyE]

Before I do that, from reading my posts, do you think it might be too difficult for me...?

 

Also, that question needs a whole book...?

 

All My Best,

Jeffrey

[Erasmus00]

Before I do that, from reading my posts, do you think it might be too difficult for me...?

 

It depends on how much you care to learn. It is a tad difficult, but it is essentially a transcript of lectures aimed at a popular audience. Plus, you can find used copies for about $5. Search for Feynman and QED: The Strange Theory of Light and Matter.

 

 

Also, that question needs a whole book...?

 

To really and truly understand, most deep questions require many books. To really understand what a photon "is" you need to first understand how classical E/M fields work and their wave properties. You then need to understand how quantum theories work, and what we mean by "quantizing" an electromagnetic field. This is centuries of work from some of the brightest minds on Earth. Feynman's book does a good job of laying some groundwork on a non-technical level, and discusses quantum electrodynamics, which is the best theory of nature that we have.

-Will

[JeffreyE]

k. I'll give it a try...

 

Thanks

 

All My Best,

Jeffrey

[Farsight]

E=mc² as I understand it means that there is an equivalency between Mass and Energy, that essentially, mass is energy, and so the more energy of one kind or another there is, the more mass there is. In that case, since we know there are several kinds of energy in photons, how can they have no mass?

 

This answer is not accepted as "mainstream", so Craig might take issue, but IMHO it's an interesting subject well worthy of debate:

 

A photon is moving and whilst it has no mass, it does have energy/momentum. If it collides with you as per Compton Scattering you'd feel a bump and say it was the result of the photon momentum.

 

 

But relativity tells us motion is not absolute, so we can consider that it was you moving rather than the photon. Then when you collide with it, you would describe that bump as the result of inertia rather than momentum. The photon would feel like it was something solid, with mass.

 

OK you can't quite do this because you always measure c at 300,000km/s, so you can't stop a photon, can you? Actually you can. Via pair production:

 

Pair production - Wikipedia, the free encyclopedia

 

 

A gamma photon of at least 2 * 511KeV is converted into an electron and a positron of 511KeV apiece. We "create" mass out of energy/momentum. Yes, there's a little wastage, but most of it is re-presented as inertia. If we simplify matters by discarding the positron and considering the electron to be at rest we can arguably say:

 

E = hc/λ -> mc² and therefore m = h/λc.

 

This is saying a photon does have mass. It sounds wrong. However we already know that both mass and energy cause gravity. A 511KeV gamma photon causes the same amount of gravity as the electron. It has the same gravitational mass. However the photon has no inertial mass whilst the electron has a non-zero inertial mass. Which means their gravitational mass is the same whilst their inertial mass is different. What's going on?

 

What did pair production actually do? We converted travelling kinetic energy aka "relativistic mass" into non-travelling energy aka "rest mass". So rest mass is effectively a measure of how much energy is not travelling. You can see this if you consider a photon bouncing back and forth in a mirrored box. The photon has no "mass". But the box has more mass because of the photon in it.

 

See RELATIVITY+ for details. But note that it's speculative and unvalidated, so don't take it as gospel truth.

[JeffreyE]

I'm beginning to think that maybe I've asked a question there isn't really a clear answer to as yet.

 

All My Best,

Jeffrey