Comment by zepearl | Hacker News Reader

Comment by zepearl | Hacker News Reader

Sorry, here comes probably the usual question asked whenever somebody talks about "light":

if I point a flashlight at an object, does the target object move (in theory, if no other external force exists) because of the light hitting it?

I could expand the question asking: when the light hits, does it lose energy (because some of its energy is transferred to the object that is hit? (e.g. target object heats up and/or moves and/or don't-know-what-else-could-happen) => if yes, then light would disappear if it would bounce around enough times (e.g. if I let light enter a box and then I close it and never open it again, is the light absorbed after a while by the box?)

Personally, I am totally confused. I think that the last time I did some investigations, the simplest statement I found said "light has momentum but no mass" (is this correct, in general?).

That "light has momentum but no mass"-statement confused me even more as I thought that gravity has influence only on things that have mass (what I keep thinking about is the example of the light being bent when it passes nearby a black hole).

Additionally, about "momentum", its definition needs mass, at least as per Wikipedia:

> momentum (pl. momenta) is the product of the mass and velocity of an object. ( https://en.wikipedia.org/wiki/Momentum )

Therefore if light does not have a mass then it shouldn't be bent by gravity nor have any momentum?

Thx :)

  if I point a flashlight at an object, does the target object move (in theory, if no other external force exists) because of the light hitting it?

Yes, the target object gains velocity due to conservation of momentum. The photon goes from momentum p to -p, so the target object needs to gain 2p of momentum to balance it out. For photons, this amount of momentum is very small.

  when the light hits, does it lose energy (because some of its energy is transferred to the object that is hit?

For perfect reflection, no. (What you're referring to is light absorption, which is a different effect that raises the temperature of the target object and red-shifts the incoming photon.)

  Personally, I am totally confused. I think that the last time I did some investigations, the simplest statement I found said "light has momentum but no mass" (is this correct, in general?).

Pretty much. Photons are usually described by their momentum and energy. For our peace of mind, we can derive an effective "mass" using p=mv, where p is the electron's momentum and v is its velocity; however, the notion is not very useful since it doesn't correspond nicely to our everyday experience of mass (whereas electron momentum does correspond nicely to our everyday experience of momentum).

> For perfect reflection, no.

That's not true, it wouldn't conserve energy. The kinetic energy of the object that's hit changes, so the light must lose or gain energy. I think for a reflection this must be because of a red- or blueshift.

> What you're referring to is light absorption, which is a different effect that raises the temperature of the target object and red-shifts the incoming photon.

Absorption usually means a whole photon is absorbed, not redshifted. A lower energy photon can sometimes be emitted after absorption (however the direction and phase relation with the previous photon are lost)

> That's not true, it wouldn't conserve energy. The kinetic energy of the object that's hit changes, so the light must lose or gain energy.

You mean that as the "path" of the photon is changed by hitting the target object, the vector of the target object must change accordingly? (if yes then how "can" the new vector of the target object computed, if the source object/photon has no mass (I would basically miss a variable for the equation)? IF it does have no mass...)

Yes, using its momentum and energy.

Here you can read the relations between a photon's momentum and things like wavelength. Without any mass :)

https://en.wikipedia.org/wiki/Photon_momentum#Physical_prope...

(You need to scroll down a bit for the formulas)

These results come from special relativity(and a bit of quantum mechanics), which you really need if things don't have mass.

Thx a lot - next questions follow... :)

(I tried, but the wikipedia chapter is too complicated for me => trying to simplify here)

> ...it's momentum and energy.

Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right?

[puah - for some reason I cannot reply to the my post's - typothrowaway's - reply...]

> (I tried, but the wikipedia chapter is too complicated for me => trying to simplify here)

I can imagine. That stuff on the wiki is not even in every physics bachelor. It's definitely not for a general audience, but I thought it might help a little :)

> Therefore, if the photon does not have a mass but is still able to have an effect against the target body it impacts with, then the photon must lose energy, right?

Yes, or gain it if it slows down the object, if the object is traveling towards the light.

Or the internal energy of the object is changed. This happens when a photon is absorbed.

Thx!

So here are my next questions :) (I know nothing about physics etc... - I'm just a simple IT-employee)

> "Yes, the target object gains velocity due to conservation of momentum. The photon goes from momentum p to -p, so the target object needs to gain 2p of momentum to balance it out."

1) If the target object gains velocity, then the photon slows down? Meaning: a) are there then slow and fast photons and b) is the speed-of-light constant of ~300KM/s in vacuum valid only for not-sped-down-photons?

2) Then if I would put in orbit a bunch of mirrors and make them all reflect the sun's light to point to a fixed object, that fixed object would start moving because of the photons hitting it? (forgetting any other factor that does not have something to do with photons)

> For perfect reflection, no. What you're referring to is light absorption, which is a different effect that raises the temperature of the target object and red-shifts the incoming photon.

So, ignoring "perfect reflection", some photons will be reflected, some will be absorbed, which will raise the temp of the target object?

> Pretty much. Photons are usually described by their momentum and energy.

So, a remix of the original question, haha: if we say that a photon has no mass, but at the same time we say that momentum needs mass, then if we say that a photon has momentum we're screwed, no?

1)

A photon's momentum is related to its wavelength, not it's speed. They all have the same speed in a vacuum.

2)

Sure.

> So, a remix of the original question, haha: if we say that a photon has no mass, but at the same time we say that momentum needs mass, then if we say that a photon has momentum we're screwed, no?

We would be screwed, but momentum doesn't need mass.

1) So, the wavelength is basically its "energy"?

2) Therefore "momentum" needs only "energy" and velocity instead of "mass" and velocity?

Sort of, yes.

It's hard to put reasons and interpretations behind all this, because in the end we just use formulas and numbers and check them in experiments. It's usually simpler that way.

Be aware that a longer wavelength means a lower energy. Frequency is proportional to energy, which is inversely proportional to wavelength.

Can we kill photon by increasing its wavelength too much? Like put him between two mirrors so they would eat its energy multiple times (without absorbing)?

No, as far as I know, there is no limit to a photon's wavelength, so it will just keep on stretching.

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