- #1

- 5

- 0

Does light contain kinetic energy as it moves, or does it require mass.

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter Ingrid Eldevj
- Start date

- #1

- 5

- 0

Does light contain kinetic energy as it moves, or does it require mass.

- #2

mfb

Mentor

- 35,809

- 12,537

It has kinetic energy, equal to its total energy because it has no mass.

- #3

- 4

- 2

Yes, light has kinetic energy and the photon is a massless particle so the light has no mass.

- #4

- 1,531

- 554

It has kinetic energy, equal to its total energy because it has no mass.

This is interesting.. If I make a Fourier series or integral to represent an electromagnetic wave traveling in vacuum, the individual plane wave components act similarly to harmonic oscillators, don't they? Then the individual modes should have Hamiltonian functions that consist of terms that are equivalent to the kinetic and potential energies in ##H = \frac{p^2}{2m}+\frac{1}{2}kx^2##... Does the total amount of "kinetic energy", as defined by this equivalence, have some actual name and physical significance?

- #5

mfb

Mentor

- 35,809

- 12,537

The concepts are not completely unrelated, but I'm not sure if we can call that "similarly".the individual plane wave components act similarly to harmonic oscillators, don't they

The amount of kinetic energy of a beam of light is just its energy. It doesn't need an additional special name because "energy" is a good name already.

- #6

- 1,531

- 554

- #7

- 2,167

- 500

It has kinetic energy, equal to its total energy because it has no mass.

It has kinetic energy, equal to its total energy if it has no mass. Light can have mass and that's not limited to exotic objects like a geon. Almost all light you see in everyday life (e.g. sunlight) has mass. Even single photons can have mass (e.g. a single photon bessel beam).

- #8

Drakkith

Staff Emeritus

Science Advisor

- 21,485

- 5,388

It has kinetic energy, equal to its total energy if it has no mass. Light can have mass and that's not limited to exotic objects like a geon. Almost all light you see in everyday life (e.g. sunlight) has mass. Even single photons can have mass (e.g. a single photon bessel beam).

Mind elaborating on this?

- #9

- 2,167

- 500

Mind elaborating on this?

I'm afraid my English is not good enough for this phrase.

- #10

Drakkith

Staff Emeritus

Science Advisor

- 21,485

- 5,388

I'm afraid my English is not good enough for this phrase.

Would you mind elaborating about your previous post? Would you explain it in more detail?

- #11

- 2,167

- 500

[itex]\frac{{p_1 \cdot p_2 }}{{\left| {p_1 } \right| \cdot \left| {p_2 } \right|}} = \cos \left( \alpha \right)[/itex]

If every wave has the energy E/2 then we also have

[itex]\left| {p_1 } \right| = \left| {p_2 } \right| = \frac{E}{{2 \cdot c}}[/itex]

That results in the total momentum

[itex]p^2 = \left( {p_1 + p_2 } \right)^2 = \left[ {1 + \cos \left( \alpha \right)} \right] \cdot \frac{{E^2 }}{{2 \cdot c^2 }}[/itex]

and therefore in the mass

[itex]m = \sqrt {\frac{{E^2 }}{{c^4 }} - \frac{{p^2 }}{{c^2 }}} = \frac{E}{{c^2 }} \cdot \sin \left( {\frac{\alpha }{2}} \right)[/itex]

of the resulting light wave which travels with the speed

[itex]\left| v \right| = \frac{{\left| p \right| \cdot c^2 }}{E} = c \cdot \cos \left( {\frac{\alpha }{2}} \right)[/itex]

along the bisecting line between the original waves. Of course normal light and Bessel beams are much more complex but the principle is the same. Plane parallel light waves (e.g. lasers) are exceptional cases in everyday life.

Share: