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Why doesn't mass of light equal infinity 
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#1
Jan2613, 12:12 AM

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I'm trying to wrap my head around what happens as mass is converted to energy. In a nuclear reaction, it is my understanding that mass is converted to energy. It is also my understanding that as matter approaches the speed of light it's mass approaches infinity. If that is so, why is the mass of light itself zero? (In my mind, matter that has been converted to energy is going the speed of light). I apologize if this is a really elementary question, but could you set me straight on where the error in my thinking is?
I suppose what I am trying to figure out is what physical process takes place at the instant mass is converted to energy? 


#2
Jan2613, 12:22 AM

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#3
Jan2613, 12:29 AM

P: 4

So the answer is... Nobody knows where the mass goes and suddenly the matter is going the speed of light without having its mass approach infinity? So what's all the talk about needing an infinite amount of energy to accelerate mass to the speed of light?



#4
Jan2613, 12:33 AM

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Why doesn't mass of light equal infinity
One is "rest mass" or "invariant mass" (the latter is a better name for reasons which will appear in a moment). Ordinary objects, and matter particles like electrons, have nonzero invariant mass; light has zero invariant mass. (The case of light illustrates why "rest mass" is not a good name: light always moves at the speed of light, so it's never at rest, but it still has a welldefined invariant mass of zero.) The other is "relativistic mass", which is really just total energy. This is the "mass" that increases without bound as an ordinary object, or a matter particle like an electron, approaches the speed of light. The term "relativistic mass" is out of favor now among physicists because, first, it causes confusion, and second, we don't need it because we already have a perfectly good word, "energy", for the same concept. (Note that light itself also has energy, but the concept of "relativistic mass" is never applied to light; we just talk about its energy.) 


#5
Jan2613, 12:39 AM

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#6
Jan2613, 12:59 AM

P: 4

Thank you both for your insightful comments. I do remember now the +pc, I had forgotten. Also, thanks for straightening me out on the definitions of mass.



#7
Jan2613, 01:19 AM

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#8
Jan2613, 07:31 AM

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#9
Jan2613, 08:07 AM

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http://www.physicsforums.com/forumdisplay.php?f=210 I'm posting this not just for your information, but also something we have to do regularly to make sure other new members are aware of such a source. Zz. 


#10
Jan2613, 08:31 AM

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#11
Jan2613, 09:58 AM

P: 4

Dalespam, so if mass has energy and energy has mass, is light not energy? It has no mass. But perhaps I'm confusing the two different concepts of mass again.
I'm trying to reconcile everyone's replies now. So I think what you are saying is relativistic mass has energy and energy has relativistic mass. Or, in other words, the way peterdonis explained it, since relativistic mass is just total energy, we are saying that total energy has energy and energy has total energy. :) 


#12
Jan2613, 10:52 AM

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[itex]m=P=P_{e^}+P_{e^+}=(1,0,0,0)+(1,0,0,0)=(2,0,0,0)=2[/itex] After the anhillation the mass of the system is: [itex]m=P=P_{\gamma_1}+P_{\gamma_2}=(1,1,0,0)+(1,1,0,0)=(2,0,0,0)=2[/itex] Note that the mass of a system of particles is different from the sum of the masses of the individual particles. The former is conserved, the latter is not. The latter is what you are refering to, but it is not the same as the mass of the system. 


#13
Jan2613, 10:57 AM

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The relationship between (invariant) mass and energy is: [itex]c^2 m^2=E^2/c^2p^2[/itex] Momentum is in there also. So light has as much energy as momentum (in units where c=1) and therefore has no (invariant) mass. For something at rest, p=0 and you get the familiar E=mc˛. 


#14
Jan2613, 11:34 AM

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#15
Jan2613, 11:35 AM

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