# Radiation energy of a moving particle

1. Jan 4, 2004

### tenzin

If an electron gains mass as it moves towards the speed of light where does this mass-energy come from? Does this not violate conservation of energy?

2. Jan 4, 2004

### HallsofIvy

Exactly! With relativity there is no "conservation of energy", there is a "conservation of mass-energy" because mass and energy are different aspects of the same thing.

3. Jan 4, 2004

### tenzin

Ok then...

Where does the gain in mass come from.

4. Jan 4, 2004

### lethe

Re: Ok then...

you have to do work to accelerate the object. all the energy stored in its motion which we call kinetic energy, comes from the work you did to accelerate it.

actually, the situation is the same as in the nonrelativistic case, only, in the nonrelativistic case, we don t call kinetic energy mass (sensible people don t call it mass in the relativistic case either, but that s a different story)

5. Jan 4, 2004

### tenzin

Interesting

Given the equivalence of mass and energy why is it that massive objects can not go the speed of light but massless objects can go the speed of light. It seems to have a relation with mass as a material manifestation of energy.

6. Jan 4, 2004

### lethe

Re: Interesting

i am going to strongly discourage you from using relativistic mass. it confuses the issues more than it helps.

when i say mass, i mean the lorentz invariant length of the momentum of the particle. energy and mass are not equivalent, using these definitions.

anyway, massive objects cannot go the speed of light because it is dynamically ruled out: it would require an infinite force to accelerate to that speed, or a constant force for an infinite amount of time. in either case, an infinite amount of energy would be required.

7. Jan 4, 2004

### tenzin

Re: Re: Interesting

So then E = mc^2 is not correct according to you.

You seem to have a good grasp of the mathematics and on this basis can rule out a massive object going the speed of light. There are many intelligent people who do not really think though. You can regurgitate what you learned but you still have not answered my question. It is a why question.

Why is it that massive objects can not travel the speed of light but massless objects can.

8. Jan 4, 2004

### lethe

Re: Re: Re: Interesting

correct. this equation is not true in general, according to this definition. it does happen to be true in the rest frame of a particle though.

well i already told you why massive objects cannot travel at the speed of light.

as for massless particles, it is not so much why they can go the speed of light (which is the only natural speed that the universe has other than 0), but rather why they cannot go slower than light. they cannot go slower than light because this is kinematically forbidden: if a massless particle is going slower than light in some frame, then there is a frame in which it is at rest. in this frame, we see that there is no particle at all.

9. Jan 4, 2004

### tenzin

Re: Re: Re: Re: Interesting

So in the rest frame of a photon the it is true?

No all you told me is what the equations allow and do not allow. Secondly, it is a question as to why massless particles can go the speed of light. Either way you answer you are only regurgitating results from formulas. You need to interpret what the equations are saying.

Why is it that no one has even tackled the question as to why light is measured the same speed by all intertial reference frames? Without answering this question there is no way to progress with anything resembling real understanding.

10. Jan 5, 2004

### UltanByrne

You should and on that!(and thank lethe for his help in the mean time )

Ultan!

11. Jan 5, 2004

### tenzin

Lethe had nothing to do with my thinking. I have been considering this for over 10 years.

12. Jan 5, 2004

### lethe

Re: Re: Re: Re: Re: Interesting

photons do not have a rest frame. they are massless, remember? this was the whole point of my above post. did you read it?

well, listen man, this is how physics works:

2. derive the mathematical consequences of those assumptions.

you asked about why some physical phenomenon happens, and i explained to you how it follows from the principles of physics.

to recap: massive particles cannot travel at the speed of light because the basic laws of physics do not allow it.

what more do you want? if you want to see a mathematical derivation, i can surely provide it, but now i am thinking it would be a waste of my time and of yours.

if you want to know why the basic laws of physics are what they are, then i have nothing more to say. i have neither the inclination, nor the expertise, to give history lessons.

i don t know if this is supposed to be insulting, but i do find it so.

this question has been tackled, and the answer is understood. if you are implying that i lack understanding, then i guess i have nothing more to say to you that wouldn t be a waste of your time.

13. Jan 5, 2004

### tenzin

to recap: massive particles cannot travel at the speed of light because the basic laws of physics do not allow it.

How do you know the so called basic laws of physics are always correct?

what more do you want? if you want to see a mathematical derivation, i can surely provide it, but now i am thinking it would be a waste of my time and of yours.

Don't worry your level of math does not impress me. I know more than I have let on. I learned QED renormalization in about 15 minutes of reading a book I just picked up off the shelf.

if you want to know why the basic laws of physics are what they are, then i have nothing more to say. i have neither the inclination, nor the expertise, to give history lessons.

So you don't know.

14. Jan 5, 2004

### lethe

well, i guess i just don t have as high a math level as you. i feel so humiliated, you know it is my only source of self worth, to know that i can impress people on message boards with my math knowledge.

but truth be told, you are right, the proof that massive objects cannot go the speed of light is mathematically well beyond my level. renormalization too.

arrogant prick. your medal for learning renormalization in 15 minutes is forthcoming.

i ll tell you what: if you want to ask questions about physics, go ahead and do that. i don t think i will be answering any more of your questions, as i think you are an ass-hole, but perhaps someone else will.

Last edited: Jan 5, 2004
15. Jan 5, 2004

### tenzin

It didn't take 15 mintues because of me. It took 15 mintues to learn because it is easy. Any person who inderstands the probablity of coin flips can understand QED. The stange thing is how easy these things are once the shroud of imaginged difficulty is lifted. The so called intellectuals purposely make things harder than they are in order to promote themselves as smart.

Why start now.

16. Jan 5, 2004

### lethe

oh yes, the international conspiracy of physicists trying to make themselves look smart and exclude the common man.

you hit the nail right on the head.

17. Jan 5, 2004

### tenzin

I don't know where I implied conspiracy.

18. Jan 5, 2004

### BigRedDot

If I were as gifted and smart as you truly are tenzin, I think I'd find a more useful or at least profitable out let for my gifts than trolling internet messages boards. That would just be me, though. I mean, whatever floats your tub toys.

19. Jan 5, 2004

### tenzin

A. I am not trolling.

B. I don't know where you get the idea that I am gifted and smart.

C. who cares what you think.

20. Jan 5, 2004

### BigRedDot

A matter of opinion, surely. It looks to me like you posted your question, with your answer already in mind ("I know more than I have let on.") just so that you could pounce on a well-meaning respondent. I call that a troll.
In fact I don't; I was being facetious.
A few people come to mind.