# atomic mass + photon

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question.When an atom absorbs a photon is there an increase in the atoms mass?

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Let E be the energy of a particle, m its mass, and p the magnitude of it's momentum. E>=0, m>=0, p>=0. Furthermore, let's E, m, and p be given in suitable units such that E*E = m*m + p*p. The energy of the atom after the absorption equals that of the atom before the absorption plus that of the absorbed photon (which is called "conservation of energy"). If the momentum of the atom was zero before the absorption, then after the absorption its momentum equals that of the photon (due to "conservation of momentum").

That should suffice to calculate the mass of the atom after the absorption. Have fun!

Edited by timo
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OK.thanks just one other point,does all the energy get absorbed by the atom internally.Or is the atoms velocity or direction of travel effected?In other words if the photon is massless does the atom react as if it had been hit.

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That should be obvious, not only in a qualitative way but as an actual relation, from the calculation I proposed above (hint: Conservation of momentum). Oh, and the mass of a photon is zero, in case that was unclear.

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ok.I'll take a stab at it and say,that if the photon is massless it has zero momentum,and therefore does not effect the atoms velocity or direction of travel,only its internal structure.yes/no?

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The worst thing that can happen is that you realize what went wrong.

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ok.I'll take a stab at it and say,that if the photon is massless it has zero momentum,and therefore does not effect the atoms velocity or direction of travel,only its internal structure.yes/no?

http://en.wikipedia.org/wiki/Photon

Photon has momentum.

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Thanks,for that link,so the photon is massless but adds to the mass of atom,that extra mass must be thought of as travelling in the direction of the photon,so its momentum is the mass it adds to the atom x c.

f x h/c^2

h/c^2 = 7.37249493 X 10^-15 kg/s

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OK.thanks just one other point,does all the energy get absorbed by the atom internally.Or is the atoms velocity or direction of travel effected?In other words if the photon is massless does the atom react as if it had been hit.

Energy can distort the fabric of space, and it does increase the mass of an atom temporarily.

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The mass increase has been measured in the case of a nuclear excitation to a long-lived excited state

http://blogs.scienceforums.net/swansont/archives/278

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sorry typo there it should be:-

h/c^2 = 7.37249493 x 10^-51 kg/s

so if a photon has a frequency of 10^14 does an atom gain 7.37249493 x 10^-37 kg/s.

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I have a problem in understanding energy and frequency.

If a photon adds to the mass of an atom,in what time frame does it do so,10^-14 seconds,which gives me h/c^2 or 1 second which gives me h/c^2 x 10^14.My question,how do I think about mass per time?Unless my mass is constantly changing.

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There is no mass per time to think about. Photon frequency is related to energy via E = hf, and that corresponds to the increase in mass. h/c^2 doesn't relate to anything.

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ok.thanks.

so i have to compare all actions(at atomic scale) in the same time frame,1 second.

I could only compare actions of 10^-14 seconds if I could analyse a single cycle of a photon.

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• 2 weeks later...

So if I say that a photon with a frequency of 10^14 travels as a wave,with matter and anti-matter popping in & out of existence in 10^14 different places per second,but once absorbed by an atom then matter & anti-matter is then popping in and out of existence 10^14 times per second in the same place,thereby adding mass to the atom.Would I be thinking correct?

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question.When an atom absorbs a photon is there an increase in the atoms mass?

It depends on what do you mean by mass. If by mass you mean, for example, the m that appears in (1/2 mv2), this mass is constant. What varies is the energy of the atom not its mass m. This is the same for an electron. The mass of an electron me does not vary when the electron is accelerated by absorbing a photon.

Some people uses an old concept of mass (sometimes named relativistic mass) M, which varies with energy as M = E/c2. This M varies when energy E varies.

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question.When an atom absorbs a photon is it the electron that absorbs the photon?

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question.When an atom absorbs a photon is it the electron that absorbs the photon?

It's a bound system, so the atom absorbs the photon. Since the nucleus has almost all of the mass, the effects are noticeable on the electron. However, the atom, i.e. including the nucleus, does have to recoil to conserve momentum. A free electron can't absorb a photon, it can only scatter them; there is no way to conserve energy and momentum with a photon absorption.

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• 2 weeks later...

ok thanks.

next question if the atom keeps absorbing photons until an electron is emitted and the atom becomes an ion,it then remains an ion until when?When hit but another electron which knocks out a photon?

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ok thanks.

next question if the atom keeps absorbing photons until an electron is emitted and the atom becomes an ion,it then remains an ion until when?When hit but another electron which knocks out a photon?

Basically yes. The ion will capture another electron, which emits a photon in order to become bound.

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The worst thing that can happen is that you realize what went wrong.

Just before you die.

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Fatalities rates from the summation of two vectors are negligible.

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Is a photon a 2 dimensional wave function that spins on an axis through 3 dimensions/time?

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