Does light have mass?

magick · 26-08-2007 10:29pm #1

well does it ?

tolosenc · 26-08-2007 10:35pm

How much exposure to Physics do you have?

Crucifix · 26-08-2007 10:40pm

Particles of light (photons) have zero rest mass. However, they can have an effective mass.

PS: I think.

Fremen · 26-08-2007 10:41pm

No, anything that moves at the speed of light has zero mass. Light can have momentum though. The physics behind that is a bit tricky.

johnnyrotten · 26-08-2007 10:43pm

biko · 26-08-2007 11:07pm

Einstein found that mass can be transformed into energy, and energy into mass. Gravity sees no difference between mass and energy, so something can feel gravity because it has mass or because it has energy, or both.

If something goes faster, then it has more energy, which corresponds to more mass, so you could say that the mass of something has two parts: one that is due to the extra energy that the thing has (such as energy of speed [kinetic energy] or heat − there are still more of them), and one that remains if you remove all of the extra energy (which means that the thing doesn't move at all and is as cold as it can be). That last part of the mass is called the rest mass: the mass that the thing would have if it were completely at rest. The rest mass does not depend on the state of the thing (on its speed or temperature, for example). For example, the rest mass of all electrons is the same, so you can put that in a table in a book. The "mass-with-all-energy-taken-into-account" is different for each electron, and can be different from one moment to the next, so you cannot put that one in a table in a book. If physicists or astronomers talk about mass, then they usually mean the rest mass.

It turns out that matter has (non-zero) rest mass, but light does not. If you take all extra energy out of light, then nothing at all remains. When you ask a physicist or astronomer if light has mass, then they can answer "no" if they mean rest mass, or "yes" if they take the extra energy into account.

Light and matter are the same in some respects (for example, both of them notice gravity), but are different in other respects. For example, light can easily travel through a piece of glass without bothering the glass, but matter cannot do that. So, it is still useful (even for scientists) to distinguish between light and matter.

Things such as light that have no rest mass cannot be put on a scale to be weighed, so they have no weight (but they do have mass, if you take the energy into account). Mass and weight are not the same. You don't feel weight when you're floating freely in space, but you then still have mass, otherwise the force of gravity would not keep pulling you down and/or keep changing the direction of your movement.

Terry · 26-08-2007 11:17pm

Catholic light has mass.

Kali · 26-08-2007 11:19pm

Terry wrote:

Catholic light has mass.

Best post ever.

Plug · 26-08-2007 11:21pm

magick wrote:

well does it ?

Candels at a church, ever think of that bumbuster?

galwayrush · 26-08-2007 11:23pm

Terry wrote:

Catholic light has mass.

Explained, next question please.:D

m83 · 26-08-2007 11:49pm

Terry wrote:

Catholic light has mass.

you did teh funneh!

2 stroke · 26-08-2007 11:59pm

Something I always wondered about. Is light always moving unless it is absorbed by something? i.e. can light stand still?

Sonnenblumen · 27-08-2007 12:04am

magick wrote:

well does it ?

Light= energy = mass x force.

Sonnenblumen · 27-08-2007 12:08am

biko wrote:

Einstein found that mass can be transformed into energy, and energy into mass. Gravity sees no difference between mass and energy, so something can feel gravity because it has mass or because it has energy, or both.

If something goes faster, then it has more energy, which corresponds to more mass, so you could say that the mass of something has two parts: one that is due to the extra energy that the thing has (such as energy of speed [kinetic energy] or heat − there are still more of them), and one that remains if you remove all of the extra energy (which means that the thing doesn't move at all and is as cold as it can be). That last part of the mass is called the rest mass: the mass that the thing would have if it were completely at rest. The rest mass does not depend on the state of the thing (on its speed or temperature, for example). For example, the rest mass of all electrons is the same, so you can put that in a table in a book. The "mass-with-all-energy-taken-into-account" is different for each electron, and can be different from one moment to the next, so you cannot put that one in a table in a book. If physicists or astronomers talk about mass, then they usually mean the rest mass.

It turns out that matter has (non-zero) rest mass, but light does not. If you take all extra energy out of light, then nothing at all remains. When you ask a physicist or astronomer if light has mass, then they can answer "no" if they mean rest mass, or "yes" if they take the extra energy into account.

Light and matter are the same in some respects (for example, both of them notice gravity), but are different in other respects. For example, light can easily travel through a piece of glass without bothering the glass, but matter cannot do that. So, it is still useful (even for scientists) to distinguish between light and matter.

Things such as light that have no rest mass cannot be put on a scale to be weighed, so they have no weight (but they do have mass, if you take the energy into account). Mass and weight are not the same. You don't feel weight when you're floating freely in space, but you then still have mass, otherwise the force of gravity would not keep pulling you down and/or keep changing the direction of your movement.

I think you're confusing mass with weight, the latter is a force the former isn't and therefore are measured differently. BTW coated glass eg mirror reflects light and there can also be light defraction with glass.

Tar.Aldarion · 27-08-2007 5:00pm

'If light is trapped in a box with perfect mirrors so the photons are continually reflected back and forth in both directions symmetrically in the box, then the total momentum is zero in the box's frame of reference but the energy is not. Therefore the light adds a small contribution to the mass of the box. This could be measured--in principle at least--either by the greater force required to accelerate the box, or by an increase in its gravitational pull. You might say that the light in the box has mass, but it would be more correct to say that the light contributes to the total mass of the box of light. You should not use this to justify the statement that light has mass in general.'

Anyway, light has no mass, it is made of massless particles called photons. It has a "relativistic mass".

If something goes faster, then it has more energy, which corresponds to more mass

No, mass stays the same, you are talking about relativistic mass.

Sleepy · 27-08-2007 5:20pm

I need a drink after reading this!

L31mr0d · 27-08-2007 5:47pm

isn't it impossible for light to have mass? Something do to with the fact anything with mass can't travel at the speed of light. I vaguely remember that the energy needed to move anything at that speed increases exponentially or something. So you can get something of mass very close to the speed of light, but not equal it or go faster than it.

Correct me if i'm wrong here. I need to know if all those years of star trek where lying to me

Maximilian · 27-08-2007 6:55pm

I'd love to help but I can't read the weighing scales with the bloody lights off.

Reku · 27-08-2007 7:26pm

Anything with mass experiences an exponentially increasing mass the closer it gets to the speed of light, which makes it more difficult to increase its velocity further.

However light is considered to have a mass due to the fact that it has energy and E=MCC (since I can't do a squared symbol here)
E of a photon = hv, where v = the frequency of the photon.
=> hv = MCC

C = vLamda, where Lamda = wavelength

=>h/Lamda = MC

=>h/vLamdaLamda = C

Solar sails are believed to work off the principle of light having a momentum which it can impart to them.

The fact that gravity can affect light is also evidence that light has an effective, non-zero, mass. Since Gravitational force requires both bodies to have a non-zero mass.

Gf = G(Mass1)(Mass2)/(the square of the distance between Mass1 and Mass2)

This is why theoretical physics is sometimes seen as about as made up as statistics, look up schrodingers cat for more theoretical tomfoolery... yes, the cat is both alive and dead it's only our act of looking at the cat that makes it alive OR dead...
Theoretical physics = way too much theorising, assuming and imagining, too little real evidence and even Stephen Hawking had doubts in some of his own work (when making a wager he bet against himself just so if he was wrong in his theories at least he'd win the bet).

Capt'n Midnight · 27-08-2007 7:28pm

If an atom/molecule captures a photon it becomes heavier.

WindSock · 27-08-2007 7:44pm

light is light, not heavy

I myself was wondering recently if photons have mass. So now I know. Back to the drawering board

dudara · 27-08-2007 7:51pm

2 stroke wrote:

Something I always wondered about. Is light always moving unless it is absorbed by something? i.e. can light stand still?

Light generally slows down when it enters a refractive medium

v = c / n

where v is the speed of light in a medium of refractive index n and c is the speed of light in vacuo.

In 2000 experiments appreciably slowed down light pulses, in fact to speeds as low as 1.6 km/hr by passing the light through a Bose-Einstein condensate. These developments are seen as having important implications for computer networking

MooseJam · 27-08-2007 7:58pm

if you made a photon much much bigger what would it look like ?, is it solid ? why does light travel at the speed of light ? does it accelerate to this speed or is it always moving at this speed, if it is always moving at that speed doesn't it break some law or other as infinite energy would be required for infinite acceleration, if it isn't always moving at that speed what force causes it's acceleration ?

aequinoctium · 27-08-2007 7:58pm

no but it does have momentum

Tar.Aldarion · 27-08-2007 8:52pm

farohar wrote:

Anything with mass experiences an exponentially increasing mass the closer it gets to the speed of light, which makes it more difficult to increase its velocity further.

This is relativistic mass.

However light is considered to have a mass due to the fact that it has energy and E=MCC (since I can't do a squared symbol here)

Nuh uh. Can't explain, late!

Capt'n Midnight · 27-08-2007 10:16pm

MooseJam wrote:

if you made a photon much much bigger what would it look like ?, is it solid ? why does light travel at the speed of light ? does it accelerate to this speed or is it always moving at this speed, if it is always moving at that speed doesn't it break some law or other as infinite energy would be required for infinite acceleration, if it isn't always moving at that speed what force causes it's acceleration ?

AFAIK it's always moving at the speed of light of the material it's in. So no acceleration and no force needed for acceleration.

Don't forget that photons have that old wave/particle thing going. When you want to look at the particle it behaves like a wave smeared over the whole universe but most of it being where you think it should be.

Manic Moran · 27-08-2007 11:05pm

OK, so how are solar sails supposed to work? The impact of the photons is supposed to cause movement of the object with the sail. First blush implies that this is a transfer of kinetic energy, no?

NTM

Capt'n Midnight · 27-08-2007 11:29pm

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

For example, the radiation of the Sun at the Earth has an energy flux density of 1370 W/m2, so the radiation pressure is 4.6 µPa (absorbed)

Steez · 27-08-2007 11:45pm

I am light

Schlemm · 27-08-2007 11:52pm

So does a lightbulb actually loose mass by radiating light if E=mc2?

Steez · 27-08-2007 11:56pm

Excuse me if i'm wrong here, but by schlemm's theory, it seems that if you left a light bulb on long enough it'd disappear.
No in other words. I don't think. If that was true though it'd be funky, never have to change a lightbulb again. Just have to replace it. No more burnt fingertips!

Schlemm · 28-08-2007 12:01am

Steez wrote:

Excuse me if i'm wrong here, but by schlemm's theory, it seems that if you left a light bulb on long enough it'd disappear.
No in other words. I don't think. If that was true though it'd be funky, never have to change a lightbulb again. Just have to replace it. No more burnt fingertips!

If that were true, what would happen to the sun?!

Anto McC · 28-08-2007 12:06am

The smart question is does mass have light?

Oh sorry i did i say smart question, i meant nonsensical question.

Steez · 28-08-2007 12:07am

Schlemm wrote:

If that were true, what would happen to the sun?!

:eek:

Schlemm · 28-08-2007 12:12am

Anto McC wrote:

The smart question is does mass have light?

Oh sorry i did i say smart question, i meant nonsensical question.

It does if you burn something I suppose? If leaving cert chemistry taught me anything, if you put something in a flame and burn it and look through this special lens thingy, it gives out light....and you can identify certain substances by burning them and seeing what light they give off...yes?

Steez · 28-08-2007 12:35am

Schlemm wrote:

if you put something in a flame and burn it and look through this special lens thingy, it gives out light....

Really?:p

Schlemm · 28-08-2007 12:38am

Steez wrote:

Really?:p

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

Capt'n Midnight · 28-08-2007 1:24am

Anto McC wrote:

The smart question is does mass have light?

Oh sorry i did i say smart question, i meant nonsensical question.

all matter above absolute zero radiates photons.

if you mean visible light then the mass would have to be above a temperature of 430 degrees C

Also black holes and protons decay into radiation too over very long amounts of time. Free neutons have a half life of a few minutes so they would disappear once the protons decay.

2 stroke · 28-08-2007 2:26am

Steez wrote:

Excuse me if i'm wrong here, but by schlemm's theory, it seems that if you left a light bulb on long enough it'd disappear.
No in other words. I don't think. If that was true though it'd be funky, never have to change a lightbulb again. Just have to replace it. No more burnt fingertips!

The light from a bulb comes from the electricty absorbed not from the bulb.

Naikon · 28-08-2007 2:58am

Would I be correct to say that light bulbs are horribly inefficient devices because most energy is lost as heat?
Big ass L.E.D's ftw!!!:)

Crucifix · 28-08-2007 8:19am

Capt'n Midnight wrote:

all matter above absolute zero radiates photons.

if you mean visible light then the mass would have to be above a temperature of 430 degrees C

Also black holes and protons decay into radiation too over very long amounts of time. Free neutons have a half life of a few minutes so they would disappear once the protons decay.

Have they observed protons decaying yet?

Reku · 28-08-2007 8:29am

Tar.Aldarion wrote:

Nuh uh. Can't explain, late!

:eek: It's the entire basis behind DeBroglie's theorem that won him a Nobel prize, no offence but who are we to believe, a forum poster of unknown credentials or someone who the physics community felt came up with something so significant that they gave him the most prestigious prize there is in the field?
All waves can be treated as particles with an associated momentum (and therefore a mass) and all particles can be treated as waves with an associated wavelength.

Ishmael · 28-08-2007 10:26am

farohar wrote:

:eek: It's the entire basis behind DeBroglie's theorem that won him a Nobel prize, no offence but who are we to believe, a forum poster of unknown credentials or someone who the physics community felt came up with something so significant that they gave him the most prestigious prize there is in the field?

The one that can ban us from this forum

Reku · 28-08-2007 12:46pm

Ishmael wrote:

The one that can ban us from this forum

Well in my entire time on these forums I have only once seen a moderator (and it wasn't Tar) behave slightly immaturely so I doubt Tar would stoop to such.
Am curious to hear his/her argument against DeBroglie's work though at least find out where the "nuh uh" stance is coming from.

Tar.Aldarion · 28-08-2007 12:58pm

farohar wrote:

:eek: It's the entire basis behind DeBroglie's theorem that won him a Nobel prize, no offence but who are we to believe, a forum poster of unknown credentials or someone who the physics community felt came up with something so significant that they gave him the most prestigious prize there is in the field?
All waves can be treated as particles with an associated momentum (and therefore a mass) and all particles can be treated as waves with an associated wavelength.

'Sometimes people like to say that the photon does have mass because a photon has energy E = hf where h is Planck's constant and f is the frequency of the photon. Energy, they say, is equivalent to mass according to Einstein's famous formula E = mc2. They also say that a photon has momentum, and momentum p is related to mass m by p = mv. What they are talking about is "relativistic mass", an old concept that can cause confusion. Relativistic mass is a measure of the energy E of a particle, which changes with velocity. By convention, relativistic mass is not usually called the mass of a particle in contemporary physics so, at least semantically, it is wrong to say the photon has mass in this way. But you can say that the photon has relativistic mass if you really want to. In modern terminology the mass of an object is its invariant mass, which is zero for a photon.
Sometimes people say "mass" when they mean "relativistic mass", mr but at other times they say "mass" when they mean "invariant mass", m0. These two meanings are not the same. The invariant mass of a particle is independent of its speed v, whereas relativistic mass increases with speed and tends to infinity as the speed approaches that of light, c. They can be defined as follows:
mr = E/c2
m0 = sqrt(E2/c4 - p2/c2)

where E is energy, p is momentum and c is the speed of light in a vacuum. The speed-dependent relation between the two is
mr = m0 /sqrt(1 - v2/c2)

Of the two, the definition of invariant mass is much preferred over the definition of relativistic mass. These days, when physicists talk about mass in their research, they always mean invariant mass. The symbol m for invariant mass is used without the subscript 0. Although the idea of relativistic mass is not wrong, it often leads to confusion, and is less useful in advanced applications such as quantum field theory and general relativity. Using the word "mass" unqualified to mean relativistic mass is wrong because the word on its own will usually be taken to mean invariant mass. For example, when physicists quote a value for "the mass of the electron" they mean its invariant mass.
At zero speed, the relativistic mass is equal to the invariant mass.'

Who are you going to believe, some French Physicist or a moderator on boards.ie? ;-)
Also, they were joking with you about me banning you.

L31mr0d · 28-08-2007 1:15pm

Tar.Aldarion wrote:

...
At zero speed, the relativistic mass is equal to the invariant mass.'

ok it all makes sense to me except for this bit... if it is possible to slow light down, and to eventually stop it, then you are saying it would have an ACTUAL mass (invariant mass), but when its moving it does not?

how is this possible that is somehow manages to gain a mass greater than 0 by merely stopping?

Tar.Aldarion · 28-08-2007 1:25pm

The rest mass of a photon is zero(although a photon can never be at rest).
It doesn't get a mass greater than 0 by 'stopping'.
Light is always invariantly massless. At zero speed both the relativistic mass and invariant mass are zero.
So, light is not gaining mass by stopping, it has zero mass at zero speed. It can however gain a relativistic mass when it is moving.

off to the cinema, any more questions will be answered on the morrow.

Reku · 28-08-2007 1:39pm

Well thought out Tar (and I know they were joking, as I said I've only ever once seen a Boards.ie mod behave that childishly), but since relativity tells us that mass increases by a factor related to your current speed relative to the speed of light even if this results in the mass we see for photons they must have an initial, at zero momentum, mass in order for this multiplied mass to be a non-zero value.
Also since mass is a measure of the difficulty in changing a thing's velocity, if photons had no mass then this would either mean that:
any force regardless of how small could change the velocity of a photon, rendering the "constant" (since light can be pulled into a black hole it is a reasonable assumption that light heading towards it can in fact travel faster than C) C non-existant
or that nothing could infact effect the velocity of a photon, which would mean that light would travel at the same speed in all materials regardless and more to the point could never stop or be absorbed by electrons and molecules.
Anyway, have to get back to work, will be back later, nice to have a genuinely thought inspiring thread in AH.

zuutroy · 28-08-2007 1:46pm

Good Lord there's some bad science in this thread.

Thomas_S_Hunterson · 28-08-2007 1:52pm

Naikon wrote:

Would I be correct to say that light bulbs are horribly inefficient devices because most energy is lost as heat?
Big ass L.E.D's ftw!!!:)

They're about 5% efficient, so yea

kevmy · 28-08-2007 2:53pm

Sure if they find the Higgs Boson then mass won't even have mass as we know it.

Does light have mass?

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