# Does electromagnetic radiation actually carry kinetic energy?

Hi all,

Does electromagnetic radiation actually carry kinetic energy?

Looking around the web gives different answers. I have seen explanations like radiation pressure experiments as evidence, but this has been argued being effects from energy absorbed by the recipient.

In case it does not carry kinetic energy, the universe dynamics are challenged.

In my understanding, based on the law of energy conservation, the universe rest mass energy, the rest mass kinetic energy and electromagnetic radiation should equate over the universe different epochs.

In early epochs with electromagnetic radiation dominating over rest mass energy, the expansion rate/ kinetic energy must have been much higher in order for the energies to equate? At the moment just after singularity, the expansion rate must have been close to C? As rest mass start dominate expansion speed decreases? Energy equation is balanced by the speed of expansion/kinetic energy? G?

What we actually see looking out in universe, back in time, to a point in time were most rest mass in fact already was created is a quite steady expansion rate (arguably some acceleration in “recent” times)

## Answers and Replies

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mfb
Mentor
It depends on your definition of "carry kinetic energy". It carries energy and momentum.

I have seen explanations like radiation pressure experiments as evidence, but this has been argued being effects from energy absorbed by the recipient.
You can have momentum transfer with the recipient losing (or keeping the same) energy, too.

There is no global energy conservation in general relativity. The expansion of space has nothing to do with motion or kinetic energy, and you cannot measure it as speed.

Well, I’m a laymen in math…..

If imagine an external view of an isolated system of decaying isotope, the energy is conserved and Newtonian physics should be correct?

• The kinetic energy of escaping alpha particle and repelling nucleus, plus the escaping gamma energy would equal the loss of rest mass in the decayed nucleus, according to E-mc2

• Mother Nature obviously balance kinetic energy, rest mass and electromagnetic emission
If defining an arbitrary volume in universe as an independent system and with an outside observation (and disregard any energy exchange at the boundaries), the energy must be conserved in the expanding room over time. But simply roll-out the kinetic energy of the moving rest masses from an external observation point, I have hard to understand

Also, if an infinite universe is assumed, I have hard to get that G should have an attracting force, as all galaxies have equal pull from surrounding galaxies in all dimensions in the expanding universe (like thinking “what is the gravity in the center of mother Earth”) Its expanding to hold the energy equilibrium?

Sorry to bather you with perhaps silly thoughts….

mfb
Mentor
Newtonian physics is "correct" if you can neglect relativistic effects.

The kinetic energy of escaping alpha particle and repelling nucleus, plus the escaping gamma energy would equal the loss of rest mass in the decayed nucleus, according to E-mc2
There does not have to be a gamma emission.

Which "expanding room" do you mean?

Also, if an infinite universe is assumed, I have hard to get that G should have an attracting force, as all galaxies have equal pull from surrounding galaxies in all dimensions in the expanding universe
You need general relativity for a proper description of that.
Its expanding to hold the energy equilibrium?
There is no "energy equilibrium", and no global energy conservation in general relativity.

Electromagnetic energy is purely kinetic energy, as there is no rest mass.

There does not have to be a gamma emission. Which "expanding room" do you mean?

Reference https://www.physicsforums.com/threa...lly-carry-kinetic-energy.802609/#post-5040710

Electromagnetic energy is purely kinetic energy, as there is no rest mass.

Reference https://www.physicsforums.com/threa...lly-carry-kinetic-energy.802609/#post-5040710

Sure, there are diffrent kind of decay.

It would be interesting to actually be able observe and measure a of a single atom decay.

- An emission of an alpha particle (a percentage of rest mass in relation to the emitting nucleus) would result in a recoil and an increased kinetic energy of the emitting nucleus?

- A directional strong gamma emission would not I guess??? (if gamma emission is perfectly isotropic, there will be no recoil obviously)

There can be difference in the actual physics and wave / particle duality math’s should perhaps be applied carefully.

With “expanding” room I mean a expanding finite bubble in the universe; with the expansion rate we currently measure.

mfb
Mentor
It would be interesting to actually be able observe and measure a of a single atom decay.
That has been done.
- An emission of an alpha particle (a percentage of rest mass in relation to the emitting nucleus) would result in a recoil and an increased kinetic energy of the emitting nucleus?
The recoil is the increased kinetic energy.
- A directional strong gamma emission would not I guess??? (if gamma emission is perfectly isotropic, there will be no recoil obviously)
The photon goes in a single specific direction - the direction is random but the emission cannot be isotropic for a single atom.
There can be difference in the actual physics and wave / particle duality math’s should perhaps be applied carefully.
That's why we have quantum mechanics, to explain things like the Mößbauer effect.

This has nothing to do with the expansion of space.

A photon carries energy.
I don't see the point in qualifying its energy as "kinetic".
As far as I know, the term "kinetic" is only used to distinguish between different types of energy when dealing with massive particles.

Entanglement and Symmetry777
Electromagnetic energy is purely kinetic energy, as there is no rest mass.
This was the route I was thinking as well since the most basic and famous energy formula at least to me is E=mc^2 so my interpretation as a "layperson in math" is that when there is energy there also has to be mass?

A photon carries energy.
I don't see the point in qualifying its energy as "kinetic".
As far as I know, the term "kinetic" is only used to distinguish between different types of energy when dealing with massive particles.
This makes sense coming from my understanding of kinesiology, anatomy and biomechanics. I mostly lurk on here and read more than post but I thought EM radiation had mass until one of the first respondents of this thread said that it didn't, thus appearing to be at odds with einstein's equation.

Symmetry777
mfb
Mentor
This was the route I was thinking as well since the most basic and famous energy formula at least to me is E=mc^2 so my interpretation as a "layperson in math" is that when there is energy there also has to be mass?
This formula applies to objects at rest only. Photons have energy but no mass and they cannot be at rest.

This formula applies to objects at rest only. Photons have energy but no mass and they cannot be at rest.
How do you define "at rest only"? Wouldn't this be problematic since the theory itself defines c as the "speed of light" so when I hear that I think of how fast the mass is moving in relation to light.

mfb
Mentor
How do you define "at rest only"?
A system that has a center of mass that is not moving relative to you.
This does not exist for photons, so the equation is meaningless for photons on their own.

Drakkith
Staff Emeritus
This was the route I was thinking as well since the most basic and famous energy formula at least to me is E=mc^2 so my interpretation as a "layperson in math" is that when there is energy there also has to be mass?
The full version of the formula is: E2=M2C4+P2C2
P is the momentum of the object. Photons have zero mass, which is M, but they have non-zero momentum, so the equation turns into E2=P2C2, which then simplifies to E=PC.

Shinaolord and audire
A photon carries energy.
I don't see the point in qualifying its energy as "kinetic".
As far as I know, the term "kinetic" is only used to distinguish between different types of energy when dealing with massive particles.
A photon moves. Kinetic derives from Greek and means associated with motion.
A photon does not have rest energy nor rotational energy nor potential energy.
Kinetic energy if ever there was any.

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mfb said "The photon goes in a single specific direction - the direction is random but the emission cannot be isotropic for a single atom".

Reference https://www.physicsforums.com/threa...lly-carry-kinetic-energy.802609/#post-5040710

Have any recoil been tested/measured on the nucleus from a gamma photon emission?

If not, this open up for some new thinking?
You cannot seriously suggest that this idea is new ;-).
Read more about recoil and emission at http://en.wikipedia.org/wiki/Mössbauer_effect

This formula applies to objects at rest only. Photons have energy but no mass and they cannot be at rest.
Electromagnetic standing waves can be considered to be at rest.
An otherwise massless resonance cavity with infinite quality containing a standing light wave with (purely kinetic) energy E behaves as an object with rest mass E/c^2.

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Khashishi
A most reasonable definition of kinetic energy is total energy minus rest energy. The entire energy of a light is kinetic energy.

You cannot seriously suggest that this idea is new ;-).
Read more about recoil and emission at http://en.wikipedia.org/wiki/Mössbauer_effect

Well, is it really the same?

Assume a isolated system of one atom in a energy transition

1. In a strong alpha emission, the emitting nucleus will recoil in the opposite direction at a velocity proportional on restmass of resp. particle (weight)?

2. In the example of gammal emission, the emitting nucleus (if not fixed in matrix) will "recoil" by vibrating (increased temperatur, resonance widening (doppler)), but it would hardly gain any velocity in the opposite direction of gamma photon?

mfb
Mentor
Well, is it really the same?
Yes it is.
2. In the example of gammal emission, the emitting nucleus (if not fixed in matrix) will "recoil" by vibrating (increased temperatur, resonance widening (doppler)), but it would hardly gain any velocity in the opposite direction of gamma photon?
It will gain velocity. The effects a crystal can have on moving atoms afterwards are independent of the decay process.

A photon moves. Kinetic derives from Greek and means associated with motion.
A photon does not have rest energy nor rotational energy nor potential energy.
Kinetic energy if ever there was any.
Sure but it's redundant to say it. It's the only kind of energy a photon carries, and by definition, photons have motion.

Sure but it's redundant to say it. It's the only kind of energy a photon carries, and by definition, photons have motion.
Now that you agree, I am having second thoughts ;-). Light is an oscillation. and oscillations in general require a restoring force and therefore potential energy. Also a simple ball and spring model reduces to the massless wave equation in the continuum limit. So there's a case for potential energy.