How come massless photon has energy

In summary: Thanks for asking! A photon is an energy-carrying particle. In general, it has enough energy to keep traveling through the cosmological constant, but it can't carry energy if it has mass.
  • #1
Rico L
37
0
I don't much about it.. but according to Einstein's equation that E=MC squared which suggests that the mass is also a factor, therefore in theory that photon suppose not to have any energy but however... photon travels through the space without stoping..just like infinity.. does photon ever runs out energy ? is it because photon are massless and it never runs out energy?? and how is it massless if it has energy? what is the difference between EM wave and photon??


thanks ppl.. I am confused..
 
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  • #2
Hi,
photon has no mass. But it has relativistic mass.
There is no difference between EM and photon.
few examples of EM: X-rays, gamma ray, radio wave, light.
 
  • #3
thanks.. but can you be more precise? what kind of relativistic mass ?
 
  • #4
The concept of mass deserves a more detailed explanation.

If you look at a massive body with rest mass[tex]m[/tex], its total energy [tex]E[/tex] is (I use [tex]c=1[/tex])

[tex]E(v) = \frac{m}{\sqrt{1-v^2}}[/tex]

Of course this formula does not apply to massless particles.

But there is a more general formula whoich shows that the rest mass is an invariant concept. For all particles the following relation between energy and momentum holds

[tex]E^2 - p^2 = m^2[/tex]

Looking at the same particle in different frames both energy and momentum will change, but the rest mass always has the same value. This formula even makes sense for massles particles like the photon:

[tex]E^2 - p^2 = 0[/tex]

So the photon can have energy and momentum.
 
  • #5
Rico L said:
but according to Einstein's equation that E=MC squared which suggests that the mass is also a factor, therefore in theory that photon suppose not to have any energy but however..

The famous relation E=Mc^2 holds when the particle is rest.
More general equation including the case of moving particle is
E^2 = P^2 c^2 + M^2 c^4
where P is momentum. You can see P=0 leads E = Mc^2
Relation for massless M=0 particle is E=|P|c that hold for photon.

Relativistic mass is historical concept. You do not need it in modern physics.

Regards.
 
  • #6
sweet springs said:
Relativistic mass is historical concept. You do not need it in modern physics.

In principle this is correct, but you will often find explanations containing the equation

[tex]E = mc^2[/tex]

where it is not specified what [tex]m[/tex] really means. There are two interpretations, namely

a) based on the rest mass [tex]m[/tex] or sometimes [tex]m_0[/tex] which is zero for the photon; then the formula means "rest energy"

b) based on the relativistic mass [tex]m[/tex] or sometimes [tex]m(v)[/tex] (which is not defined for the photon) as it always moves with c and you would get 0/0 in this formula; then the formula means (velocity dependent) total energy for which I introduced [tex]E(v)[/tex] above

It is correct that one does not need the concept b) of relativistic = velocity dependent mass, but you have to be aware of the fact that the meaning of the formula [tex]E = mc^2[/tex]
is based on this concept implicitly quite often - w/o mentioning it.
 
  • #7
Hi, tom
I think I understand you. But
Rico questioned "what happens in E=mc^2 when m=0?".
The formula E=mc^2 does not work well here even m is not m0.
Regards.
 
  • #8
This is what I said before: the E=mc² equation does not make sense for photons.
 
  • #9
tom.stoer said:
The concept of mass deserves a more detailed explanation.

If you look at a massive body with rest mass[tex]m[/tex], its total energy [tex]E[/tex] is (I use [tex]c=1[/tex])

[tex]E(v) = \frac{m}{\sqrt{1-v^2}}[/tex]

Of course this formula does not apply to massless particles.

But there is a more general formula whoich shows that the rest mass is an invariant concept. For all particles the following relation between energy and momentum holds

[tex]E^2 - p^2 = m^2[/tex]

Looking at the same particle in different frames both energy and momentum will change, but the rest mass always has the same value. This formula even makes sense for massles particles like the photon:

[tex]E^2 - p^2 = 0[/tex]

So the photon can have energy and momentum.



this make sense.. cheers ppl and tom
 
  • #10
Sorry to jump in, but does this imply that a photon's energy must equal the magnitude of its momentum?
 
  • #11
mg0stisha said:
Sorry to jump in, but does this imply that a photon's energy must equal the magnitude of its momentum?

yes it does
 
  • #12
Hi. Rico

tom.stoer said:
This is what I said before: the E=mc² equation does not make sense for photons.

You are right. Why do not you say more that the E=mc² equation does not make sense for any moving particles? We have no reason to make effort saving the famous E=mc^2 by introducing artificial m(v) instead of scalar m.

Regards.
 
  • #13
The Photon

Yeah, that brings up a good point on how does a photon continue to travel the cosmological constant, but how can it carry energy if it is mass less. A photon is an energy carrying particle. Stating that it has carry energy. Can somebody help with some reasoning?
 
  • #14
First, making your words huge doesn't help and in fact may hurt your cause. Please, don't do that.

Second, a photon carries energy because it has momentum, and thus produces a value in the equation [tex]E^2=(pc)^2+m^2 c^4[/tex], even though m=0 for the photon.
 
  • #15
sweet springs said:
We have no reason to make effort saving the famous E=mc^2 by introducing artificial m(v) instead of scalar m.
If we start with the "axiomatic" approach for RT you are right; but in practice many RT lessons use the "historical" approach. Therefore one has to understand how to "find" RT "bottom-up". I think both approaches are not contradictory but complementary. This is comparable to the "old" quantum theory of Niels Bohr e al. Today we know that its "wrong", nevertheless it makes sense to study it.

He must so to speak throw away the ladder, after he has climbed up on it (Wittgenstein)
 
  • #17
Rico L said:
I don't much about it.. but according to Einstein's equation that E=MC squared which suggests that the mass is also a factor, therefore in theory that photon suppose not to have any energy but however... photon travels through the space without stoping..just like infinity.. does photon ever runs out energy ? is it because photon are massless and it never runs out energy?? and how is it massless if it has energy? what is the difference between EM wave and photon??


thanks ppl.. I am confused..

Hi Rico

Here is how I understand it, someone please correct me if I am wrong:

Photons are created from the destruction of mass, simular to how fire is created from the burning of wood. The photons were created from the mass at some point, just as the fire was created from the wood. The photons can have energy without mass, like fire can have energy without wood.

E=MC2 simply states that if you act upon some object to make it create electromagnetic radiation (like turning on an incandescent light bulb), the objects mass (filimant of light bulb) will be reduced by the total amount of electromagnetic radiation energy given off, divided by the speed of light squared. Or, if you act upon an object to transform its entire mass into electromagnetic radiation, the total amount of radiation produced will equal the mass multiplied by the speed of light squared.

Photons are created from the interactions of electric and magnetic fields (EM waves), so they are totally different. I'm not sure, but I think a photon can be thought of as each peak (or cycle) of the electromagnetic wave??
 
  • #18
what is the difference between EM wave and photon


The photon is the quantum of the electromagnetic interaction and the basic "unit" of light and all other forms of electromagnetic radiation and is also the force carrier for the electromagnetic force...Electromagnetic (wave) radiation has particle-like properties as discrete packets of energy, or quanta, called photons

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

See also in the Wikie article: wave model (described by Maxwell's equations) and particle model (described by quantum mechanics_ ...
 

1. How can a photon have energy if it has no mass?

Although photons have no rest mass, they do have energy due to their motion and frequency. According to Einstein's famous equation E=mc^2, energy can be converted into mass and vice versa. Photons have energy in the form of electromagnetic radiation, and since they are constantly moving at the speed of light, this energy is equivalent to their mass.

2. What is the relationship between a photon's energy and its frequency?

The energy of a photon is directly proportional to its frequency. This means that the higher the frequency of a photon, the more energy it has. This relationship is described by the equation E=h*f, where E is energy, h is Planck's constant, and f is frequency.

3. Can a photon's energy be measured?

Yes, a photon's energy can be measured using a variety of methods such as spectroscopy or photoelectric effect experiments. The energy of a photon is typically measured in electron volts (eV) or joules (J).

4. How does a photon's energy relate to its wavelength?

The energy of a photon is inversely proportional to its wavelength. This means that longer wavelength photons have less energy, while shorter wavelength photons have more energy. This relationship is described by the equation E=hc/λ, where c is the speed of light and λ is the wavelength.

5. Can a photon's energy be changed or transferred?

Yes, a photon's energy can be changed or transferred through various interactions such as absorption, emission, and scattering. When a photon is absorbed by an atom or molecule, its energy is transferred to the absorbing particle. Similarly, when a photon is emitted, it transfers its energy to the surrounding environment. The energy of a photon can also be scattered, causing it to change direction and potentially transfer its energy to another particle.

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