# Does light have mass...?

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Does light have mass - a question relating to this question requires this bit of information.

Does it also mean that if light can be said to have mass then does all energy have some mass (How ever small)?

Thanks for the help, another question to come if the outcome is as I think it is

Cheers,

Ryan Jones

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From what I've heard on the subject, photons have a mass, however VERY small. I cannot quote the information, I could be wrong, but I could swear I heard something on the subject.

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Thanks Ricker

In that case, how can these travel at light speed?

Anything with mass accelerated to light speed gains infinate mass and thus cannot travel light speed - am I wrong? This has been bugging me for a while, if I have made a mistake then pleas epoint it out

Cheers,

Ryan Jones

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photons are massless but they have momentum. you are porbably thinking of E=mc^2 but this does not apply to the photon unless it transforms into a matter-antimatter pair

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Photons don't have mass, although, apparently, they can be assigned a relativistic mass from their energy (E=mc^2 and all that).

Ok, thanks both

Cheers,

Ryan Jones

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From what I've heard on the subject, photons have a mass, however VERY small. I cannot quote the information, I could be wrong, but I could swear I heard something on the subject.

I have heard that satelites are hacked and that one can see this in an increased eye-blinking of the people on television. I could even provide the link.

No, honestly: Photons do have zero mass unless one equates mass with energy. This is sometimes done but not the common understanding of the term "mass" because this new mass ("relativistic mass") is not a property of the particle class anymore, but a property of each individual particle.

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Does light have mass - a question relating to this question requires this bit of information.

Photons have no rest mass, but they do have relativistic mass.

Does it also mean that if light can be said to have mass then does all energy have some mass (How ever small)?

All energy has relativistic mass. E=mc^2...

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Does light have mass

The answer depends entirely on which convention for mass you choose to adopt. Just about every particle physicist and relativist out there rejects the idea of "relativistic mass" and instead adopts the convention that mass is the (Lorentz invariant) norm of the 4-momentum, in natural units. In this convention light has no mass, period. And since that is the convention used by the great majority of physicists, I would advise adopting that one.

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The answer depends entirely on which convention for mass you choose to adopt. Just about every particle physicist and relativist out there rejects the idea of "relativistic mass" and instead adopts the convention that mass is the (Lorentz invariant) norm of the 4-momentum, in natural units. In this convention light has no mass, period. And since that is the convention used by the great majority of physicists, I would advise adopting that one.

Cheers,

Ryan Jones

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If you plug m=0 into the famous e=mc^2 equation you will notice how you get 0 energy, well we know that photons do have energy!

So there's an extension to the formula which goes like this;

$e^2 = (pc)^2 + (mc^2 )^2$

when m=o we get, basically,

$e = pc$

where p is momentum (e is still energy and c is still speed of light).

When we say "mass" what we normally mean is when an object stops moving and is at rest we measure its mass, so we have its "rest mass".

As an object accelerates or gains energy of some kind it gains another kind of mass, we know this because e=mc^2 so when e increases so must m (because c^2 is constant). This is relativistic mass (I'm 99% certain thats the right name), it is different from rest mass.

A photon has no rest mass (aka mass) because it can never be truely at rest, so we cannot measure its rest mass, it doesn't exist.

2 little extras:

(1) You can measure a photon's energy using the equation

$e = hf$

Where

e = energy

h = Plank's constant

f = frequency

(2) There is a theorised model in which mass doesn't exist, only energy exists and in different forms. It's not the commonly accepted/known model, but when I heard about it I just thought it was an interesting concept, very much related to this thread.

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Doesn't that de Broglie wave thing hint that light has mass?

$\lambda = \frac{h}{mc}$

Where the lambda is wavelength,

h is the Planck constant

M is the mass

C is the speed of light

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The deBroglie relation is:

$\lambda=\frac{h}{p}$,

not

$\lambda=\frac{h}{mc}$.

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and from knowing that

$\lambda=\frac{h}{p}$

and that

$e = pc$

thus

$p = \frac{e}{c}$

then

$\lambda=\frac{h}{e / c}$

so

$\lambda = h * \frac{c}{e}$ or $\lambda = h\frac{c}{e}$

which simplifies to

$\lambda=\frac{hc}{e}$

which rearranges as

$e = \frac{hc}{\lambda}$

That's kinda irrelevant, I was just doing it outta personal interest to see what we ended up with!

btw, how are you meant to do a multiplied sign in LaTeX? x doesnt work and * gives a star, are you meant to just leave it like xy to represent x multiplied by y?

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You can use \cdot: $\lambda = h \cdot \frac{c}{e}$.

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what does it mean when they (narrators on documentaries) say X amount of pounds (weight) in photons hit this planet everyday?

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what does it mean when they (narrators on documentaries) say X amount of pounds (weight) in photons hit this planet everyday?

Maybe they are refering about a relative mass...?

Cheers,

Ryan Jones

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what does it mean when they (narrators on documentaries) say X amount of pounds (weight) in photons hit this planet everyday?

You´d better ask them, not us. I could bet they won´t know that themselves. A possible explanation would be something like: The pressure exerted by the light that hits earth everyday is the same as if a mass of xxx pounds was constantly pushing against it. But then, I´ve never heard someone saying this so I don´t even know the context.

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well it went on to say that even when you turn on a flashlight there is a "kick" like recoil, only its too tiny to notice.

then they powered up this HUGE mega power laser and fired it at this little piece of black material suspended in a vacuum chamber and the material Moved when the light hit. dont know if thats of any help at all?

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Recoil is transfer of momentum. Photons do have momentum and they are also able to "transfer" it to other particles. You could say, the momentum transfer is as if a mass of vvv kg hit with a speed of lll m/s.

Did you marry lately or is there another reason why your avatar changed?

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Hmmm... I dont get it? it has no Mass (I accept that 100%) it has Speed (I accept that also), so what actulay "Hits" and shoves it forwards to move?

whats the "interaction" if thats the right word?

it feels like Im accepting "Facts" without any basis, so therefore it must be a limitation on my part as a Knowledge Gap.

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well it went on to say that even when you turn on a flashlight there is a "kick" like recoil' date=' only its too tiny to notice.

then they powered up this HUGE mega power laser and fired it at this little piece of black material suspended in a vacuum chamber and the material Moved when the light hit. dont know if thats of any help at all?[/quote']

Isn't it possible that when the laser hit it (the black material) that some of the material vaporized and that is what cuased the movement?

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I dont know? abblation was not mentioned, I remember wondering that myself

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Hmmm... I dont get it? it has no Mass (I accept that 100%) it has Speed (I accept that also)' date=' so what actulay "Hits" and shoves it forwards to move?

whats the "interaction" if thats the right word?

it feels like Im accepting "Facts" without any basis, so therefore it must be a limitation on my part as a Knowledge Gap.[/quote']

It's the fact that photons have momentum (loadsa equations have been said already for the momentum of a particle with no mass).

There are loads of "conservation of ____ " laws in physics, one of these is the conservation of momentum.

When a photon hits you with momentum x it transfers that momentum to you. This forces you in the direction the photon was going. Understand that even when there are millions of photons hitting you this force is still unnoticeable, but that doesn't mean it's not there.

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all thats fine and good, but HOW does it "transfer" this momentum if it has no mass?

its a bit like a ghost hitting a punch bag, it wont move unless it interacts or has Mass behind it.

THAT is the part I`m trying to get at/understand.

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