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Author Topic: Why does E=MC squared?  (Read 2746 times)

Offline allan marsh

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Why does E=MC squared?
« on: 08/07/2014 21:42:16 »
Curious?  If photon has no mass does that mean it  it can't convert to any energy?
« Last Edit: 10/07/2014 09:43:33 by Georgia »


 

Offline chiralSPO

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Re: E=MC squared!
« Reply #1 on: 08/07/2014 22:23:10 »
it already is (has) energy
 

Offline PmbPhy

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Re: E=MC squared!
« Reply #2 on: 08/07/2014 22:37:59 »
Quote from: allan marsh
Curious?  If photon has no mass does that mean it can't convert to any energy?
No. That is a common misconception. First of all E = mc^2 does not mean that mass can be converted into energy. That too is a common misconception. It means that the form of mass and the form of energy can change. But in all cases the total mass and the total energy remains the same. For example; when an electron and a positron annihilate they disappear and two photons are produced traveling in equal and opposite directions. The energy in the beginning was the sum of the kinetic energy and the sum of the rest energies of both particles. When the particles disappear then all the energy is in the form of photons. You can think of the energy of photons as being entirely kinetic energy. So the form of the matter went from electron/positron to photons. The form of the energy went from kinetic + rest energy to pure kinetic energy. So it was the form that changes. The conversion that physicists speak of is that the form of the mass and energy is converted from one form to another.

As far as what “mass” means; I assume that you’re familiar with the fact that many terms in the English language can at times have two different meanings, correct? The same thing happens in physics. For example; when you study quantum mechanics you have to keep in mind that when you’re talking about momentum you’re not talking about, mechanical momentum, you’re talking about canonical momentum. Therefore if someone asks you How is mechanical momentum defined? you can’t just answer then with one or the other. You either have to ask for the context or given them both meanings. The same is true in relativity with mass. The term mass has two meanings. It can refer either to relativistic mass (aka inertial mass) or proper mass (aka rest mass). The inertial mass of a photon is the m in p = mv. So if you know the momentum and since v = c for a photon we have m = p/c. Since for a photon E = pc -> p = E/c we have m = p/c = (E/c)/c = E/c^2. That’s the simplest way to obtain that relationship for a photon that I know of. For particles which have finite proper mass see

http://home.comcast.net/~peter.m.brown/sr/inertial_mass.htm

http://home.comcast.net/~peter.m.brown/sr/einsteins_box.htm

To understand the mass-energy equivalence relationship E = mc^2 please see

http://home.comcast.net/~peter.m.brown/sr/mass_energy_equiv.htm

I hope that was helpful? :)
 

Offline PmbPhy

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Re: E=MC squared!
« Reply #3 on: 08/07/2014 22:39:56 »
it already is (has) energy
That statement is correct in the form It already has energy. It's incorrect to say that a photon is energy.
 

Offline allan marsh

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Re: E=MC squared!
« Reply #4 on: 09/07/2014 14:56:54 »
Thanks dear members ... Now understand more
 

Offline chiralSPO

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Re: Why does E=MC squared?
« Reply #5 on: 12/07/2014 22:58:18 »
So, if the photon counts as matter, does that mean that matter is created and destroyed when photons are emitted and absorbed, say by electronic transitions in a molecule?
 

Offline Ethos_

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Re: Why does E=MC squared?
« Reply #6 on: 13/07/2014 02:39:37 »
So, if the photon counts as matter, does that mean that matter is created and destroyed when photons are emitted and absorbed, say by electronic transitions in a molecule?
No............The photon is not destroyed, it only changes it's character by becoming an electron.
 

Offline PmbPhy

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Re: Why does E=MC squared?
« Reply #7 on: 13/07/2014 07:54:00 »
Quote from: Ethos_
No............The photon is not destroyed, it only changes it's character by becoming an electron.
That's quite wrong. First of all, no single photon can change into a single electron. And as soon as any particle changes it does mean that it's destroyed so your conclusion is wrong on that front. It's impossible to say that any particle is another particle but merely with a different character. At least not in mainstream physics.
 

Offline Ethos_

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Re: Why does E=MC squared?
« Reply #8 on: 13/07/2014 17:23:17 »
Quote from: Ethos_
No............The photon is not destroyed, it only changes it's character by becoming an electron.
That's quite wrong. First of all, no single photon can change into a single electron. And as soon as any particle changes it does mean that it's destroyed so your conclusion is wrong on that front. It's impossible to say that any particle is another particle but merely with a different character. At least not in mainstream physics.
OK;....... I will concede to your point based upon your chosen words. Let me say it another way. The total mass/energy of the photon will not be destroyed, only changed into another form of mass/energy.

Conservation of mass and energy.
 

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Re: Why does E=MC squared?
« Reply #8 on: 13/07/2014 17:23:17 »

 

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