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tommac
2008-Apr-23, 08:33 PM
How does one prove that a photon of light has 0 mass?

hhEb09'1
2008-Apr-23, 08:37 PM
How does one prove that a photon of light has 0 mass?What are the allowed assumptions in the proof? Special relativity?

alainprice
2008-Apr-23, 08:38 PM
It has 0 rest mass because it doesn't rest.

While moving, which is the only thing it does, it does have momentum based on the wavelength of light.

I'm sure someone else can go into more detail if needed.

George
2008-Apr-23, 09:01 PM
It has 0 rest mass because it doesn't rest. :) It's hard to hog-tie the restless feet of a photon. ;)

tommac
2008-Apr-23, 09:02 PM
What are the allowed assumptions in the proof? Special relativity?

whatever ... I understand that this is taken as fact but I just want to know what the evidence is.

tommac
2008-Apr-23, 09:04 PM
OK so it has 0 rest mass but it does have momentum.

Now follow up question ( should I create a new thread or just ask here ). Kind of ATM ... OK will question it in ATM



It has 0 rest mass because it doesn't rest.

While moving, which is the only thing it does, it does have momentum based on the wavelength of light.

I'm sure someone else can go into more detail if needed.

RalofTyr
2008-Apr-24, 06:06 AM
If light always has momentum, then what happens when it enters a black hole?

tommac
2008-Apr-24, 01:23 PM
If light always has momentum, then what happens when it enters a black hole?


relative to? I think the light still has momentum but just doesnt escape the receeding "fabric of space-time". From what I understand this is different slightly than when a galaxy receeds past the line of the visible universe.

When entering a black hole light becomes blue shifted and actually gains momentum. But then past the event horizon it disappears.

Now I guess a good question here. As light recesses on the edge of the visible universe. its wavelegnth goes to infinity and its energy goes to 0 ( period goes to 0 ?? ) whatever it does but something goes to 0.

For blue shifting stuff. What happens to the light just as it passes the event horizon? Is there a max blueshift value? Does light just as it approaches a black hole do anything strange?

mugaliens
2008-Apr-25, 04:45 PM
That's easy. The lorentz transformation factor (http://en.wikipedia.org/wiki/Lorentz_transformation) stipulates that as something approaches the speed of light, it's mass approaches infinity.

If just one photon had any mass at all, at c it would weigh more than the entire universe.

Photons do have momentum (http://en.wikipedia.org/wiki/Photon#Physical_properties), however, and it's directly proportional to its frequency (alternatively it's inversely proportional to it's wavelength).

tommac
2008-Apr-25, 05:53 PM
If just one photon had any mass at all, at c it would weigh more than the entire universe.

What if the universe was infinite and contained an infinite number of these mass-photons?
This is being asked with regards to the math of your statement. Please ignore any consequences in the physical impossibility.

Amber Robot
2008-Apr-25, 05:56 PM
What if the universe was infinite and contained an infinite number of these mass-photons?

In an infinite universe, pretty much everything will appear in infinite numbers. What is then important is density, which can be finite, even in an infinite universe.

Ken G
2008-Apr-25, 06:04 PM
How does one prove that a photon of light has 0 mass?
As hhEb09'1 alluded to, one does not prove that. One constructs a model for a photon, and finds that the model works great, within the axioms of relativity, which also work great. Are you asking "what is the observational upper limit established for the photon mass in any model that would work as well as the zero-rest-mass model"? That's a way you could phrase it in a scientifically meaningful way-- unfortunately I have no idea what the answer is other than "very small indeed".

tommac
2008-Apr-25, 06:57 PM
In an infinite universe, pretty much everything will appear in infinite numbers. What is then important is density, which can be finite, even in an infinite universe.

exactly ... so a photon of light even if it was of infinite mass would not be greater or even equal to the mass of the universe.

my infinity is stronger than your infinity.

tommac
2008-Apr-25, 06:59 PM
One of these days I will learn to be eloquent.

Thanks for the clarification.



As hhEb09'1 alluded to, one does not prove that. One constructs a model for a photon, and finds that the model works great, within the axioms of relativity, which also work great. Are you asking "what is the observational upper limit established for the photon mass in any model that would work as well as the zero-rest-mass model"? That's a way you could phrase it in a scientifically meaningful way-- unfortunately I have no idea what the answer is other than "very small indeed".

Tim Thompson
2008-Apr-25, 11:50 PM
The rest mass of a photon must be zero, if special relativity is correct. Since special relativity is well established theory, it is normally assumed that the rest mass of a photon is in fact zero. Laboratory experiments cannot prove that the rest mass is in fact zero, but they can establish upper limits on the rest mass. Luo, et al., 2003 (http://adsabs.harvard.edu/abs/2003PhRvL..90h1801L) established an upper value of the rest mass for a photon at 1.210-51 grams in a torsion balance experiment. That's pretty close to zero. Astronomical observations can also set limits on photon rest mass. If the photon has a rest mass, then light of different wavelengths should travel at different speeds. So establishing observational limits on the wavelength dependence of the speed of light can indirectly establish limits on the photon rest mass. Schaefer, 1999 (http://adsabs.harvard.edu/abs/1999PhRvL..82.4964S) established an upper limit of 4.210-44 grams; not as stringent as the laboratory limit, but still pretty close to zero. And since these are all upper limits, they are consistent with a true zero rest mass.

tommac
2008-Apr-26, 03:20 AM
what does rest mass of a photon mean?


The rest mass of a photon must be zero, if special relativity is correct. Since special relativity is well established theory, it is normally assumed that the rest mass of a photon is in fact zero. Laboratory experiments cannot prove that the rest mass is in fact zero, but they can establish upper limits on the rest mass. Luo, et al., 2003 (http://adsabs.harvard.edu/abs/2003PhRvL..90h1801L) established an upper value of the rest mass for a photon at 1.210-51 grams in a torsion balance experiment. That's pretty close to zero. Astronomical observations can also set limits on photon rest mass. If the photon has a rest mass, then light of different wavelengths should travel at different speeds. So establishing observational limits on the wavelength dependence of the speed of light can indirectly establish limits on the photon rest mass. Schaefer, 1999 (http://adsabs.harvard.edu/abs/1999PhRvL..82.4964S) established an upper limit of 4.210-44 grams; not as stringent as the laboratory limit, but still pretty close to zero. And since these are all upper limits, they are consistent with a true zero rest mass.

alainprice
2008-Apr-26, 07:41 AM
Rest mass is what you measure on a scale, or better yet, a balance.

edit: It's like saying it is the mass you would feel if you held it in your hand. Unfortunately, you can't hold a photon; you can only be hit by one.

mugaliens
2008-Apr-26, 12:57 PM
Astronomical observations can also set limits on photon rest mass. If the photon has a rest mass, then light of different wavelengths should travel at different speeds. So establishing observational limits on the wavelength dependence of the speed of light can indirectly establish limits on the photon rest mass. Schaefer, 1999 (http://adsabs.harvard.edu/abs/1999PhRvL..82.4964S) established an upper limit of 4.210-44 grams; not as stringent as the laboratory limit, but still pretty close to zero. And since these are all upper limits, they are consistent with a true zero rest mass.

We could always put a nuke on the next deep-space probe, install a 50-year timer, then observe the resulting flash, compare it to known models, and check to see if any frequencies arrived sooner than others...

Nah. The greenies would never go for it. Er, unless we didn't tell them...

From what I understand about upper limits, it's simply due to the statistics of the errors inherent in the measuring equipment, ergo, "It measures zero, but with a 95% confidence interval, the measurement could be +/- 4.2x10-44 grams off." Naturally, since it can't have negative mass, the limit is simply 4.2x10-44 grams.

tommac
2008-Apr-27, 02:22 AM
i Just had a strange thought ... I think I need to head to ATM ... but to give you a preview ...

What if light moved at infinite speeds .... but the speed we detect as a constant is based on the local warping of space time.

Vanamonde
2008-Apr-27, 08:22 AM
One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass.

mugaliens
2008-Apr-27, 12:03 PM
i Just had a strange thought ... I think I need to head to ATM ... but to give you a preview ...

What if light moved at infinite speeds .... but the speed we detect as a constant is based on the local warping of space time.

Er... <brakes screeching, wildly>

Observational experiments with differing forms of electromagnetic radiation (of which visible light is a small subset) have universally concluded that no electromagnetic radiation (including light) moves at "infinate speeds."

So please hold you ATM post and save others some grief trying refute yet another ATM theory with no basis in fact (and a heap of evidence to the contrary), and take the time to research your ideas before wasting our time refuting them.

Thanks.

tommac
2008-Apr-28, 03:05 PM
Er... <brakes screeching, wildly>

Observational experiments with differing forms of electromagnetic radiation (of which visible light is a small subset) have universally concluded that no electromagnetic radiation (including light) moves at "infinate speeds."

So please hold you ATM post and save others some grief trying refute yet another ATM theory with no basis in fact (and a heap of evidence to the contrary), and take the time to research your ideas before wasting our time refuting them.

Thanks.

Yes I know that relative to us the speed is not infinite, but what I was going to post to ATM is that what we see is only the curvature of space-time as a photon of light moves through it. Space-time would get more distored in a gravitational field and less distorted in a perfect vacuum. The percieved speed of light would then be somehow derived from the distortion of space-time. To feed this theory would be youngs double slit experiments. Where light seems to interfere with itself. The state of light would desire to go to an infinite wavelength and 0 energy.

I will hold the question ... and not post to ATM until I have done more research on the matter.

alainprice
2008-Apr-28, 04:01 PM
Explain how such a model can allow light to behave as a particle in one instance and then a wave in another and you will be making ground. Feel free to do so in ATM.

Neverfly
2008-Apr-28, 04:13 PM
Yes I know that relative to us the speed is not infinite, but what I was going to post to ATM is that what we see is only the curvature of space-time as a photon of light moves through it. Space-time would get more distored in a gravitational field and less distorted in a perfect vacuum. The percieved speed of light would then be somehow derived from the distortion of space-time. To feed this theory would be youngs double slit experiments. Where light seems to interfere with itself. The state of light would desire to go to an infinite wavelength and 0 energy.

I will hold the question ... and not post to ATM until I have done more research on the matter.

:lol:
:D

tommac
2008-Apr-28, 04:35 PM
Explain how such a model can allow light to behave as a particle in one instance and then a wave in another and you will be making ground. Feel free to do so in ATM.

The wave is the bending of space ... The particle ( still need to think if it is a really a particle or a quasi particle ) is its natural state.

dcl
2008-Apr-28, 07:47 PM
Vanamonde, you said, "One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass."
It's not as simple as that. Under special relativity, there is an equivalence between matter and energy as expressed by Because of E = mc exp 2, even energy has mass. However, there is a simple answer to the question of whether the photon has mass. It depends on how fast the particle is moving. The special theory of relativity tells tells us how the mass of a particle varies with its speed. In particular, it tells us that anything moving at light speed and having finite mass necessarily has zero rest mass. A photon moves at light speed and has finite mass. It follows that its rest mass must be zero.

tommac
2008-Apr-28, 08:25 PM
doesnt e=mc^2 mean that matter is made of energy?
I am not sure if energy needs to be in a state of matter though. But who knows ... I am mostly just ATM



Vanamonde, you said, "One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass."
It's not as simple as that. Under special relativity, there is an equivalence between matter and energy as expressed by Because of E = mc exp 2, even energy has mass. However, there is a simple answer to the question of whether the photon has mass. It depends on how fast the particle is moving. The special theory of relativity tells tells us how the mass of a particle varies with its speed. In particular, it tells us that anything moving at light speed and having finite mass necessarily has zero rest mass. A photon moves at light speed and has finite mass. It follows that its rest mass must be zero.

alainprice
2008-Apr-28, 08:28 PM
It means that matter 'could' be made of energy. They are equivalent, but not necessarily the same.

Different arrangements of matter even have difference masses. For example, a helium nucleus is lighter than 2 protons and 2 neutrons.

mugaliens
2008-Apr-29, 05:48 PM
Space-time would get more distored in a gravitational field and less distorted in a perfect vacuum.

But space-time isn't distorted, except on a very local level. It's largely flat.

Only near matter (the mean density of which, throughout the observable universe, black holes and all, is a whopping 6 hydrogen atoms per cubic meter), is space-time curved to any appreciable degree.


The percieved speed of light would then be somehow derived from the distortion of space-time. To feed this theory would be youngs double slit experiments. Where light seems to interfere with itself.

All Young proved is that light has wave-like properties such that depending upon the photon's wave-like state when it went through the slit, it might have been refracted this way, or that way, or a little bit that way, or not at all.

mugaliens
2008-Apr-29, 06:00 PM
Vanamonde, you said, "One point I would like to add, is a photon is a unit of energy. Mass is a characteristic of matter. So, by definition, a photon must have no rest mass."
It's not as simple as that. Under special relativity, there is an equivalence between matter and energy as expressed by Because of E = mc exp 2, even energy has mass.

Exactly! Which is why it's said that a photon's rest mass is zero. But photons are hardly at rest, and at speed (c), they do impart a momentum that's equavalent to their energy/mass equivalence.


However, there is a simple answer to the question of whether the photon has mass. It depends on how fast the particle is moving.

Actually, I believe it depends on it's wavelength! All photons have the same "amplitude" (I dilike using that term, as the wave theory only stretches so far). Photons with shorter wavelengths have more energy, thus more momentum, thus more energy/mass equivalence.

Still no actual mass, however, as no mass can move at c.


The special theory of relativity tells tells us how the mass of a particle varies with its speed. In particular, it tells us that anything moving at light speed and having finite mass necessarily has zero rest mass. A photon moves at light speed and has finite mass. It follows that its rest mass must be zero.

Yes, rest mass zero. Yet no mass can move at c. The equations governing this are clear that it's apparent mass (still exerts a gravitational attraction) is proportional to it's E=Mc^2 (energy-mass) equivalence. The shorter the wavelength, the more energetic the photon, and thus the more mass-equivalence it has, therefore the more gravitational attraction that photon exerts, which, amazingly enough, is all tied together via Planck's equations, as well as Einstein's and Lorentz's!

Historians will probably record that they were secretly collaborating with one another behind closed doors...

mugaliens
2008-Apr-29, 06:10 PM
doesnt e=mc^2 mean that matter is made of energy?

Not exactly... Matter consists of various types of quarks (http://en.wikipedia.org/wiki/Quark) (another link to follow) in various states of energy bondage.

Pay particular attention to, and follow all the links in the Flavor (http://en.wikipedia.org/wiki/Quark#Flavor)section.

Complicating this is the difference between bosons (http://en.wikipedia.org/wiki/W_and_Z_bosons)and mesons (http://en.wikipedia.org/wiki/Meson) and neutrinos (http://en.wikipedia.org/wiki/Neutrinos).

Welcome to the world of particle physics! I've been dabbling in this soup since 1981, and I don't know anywhere near 1% of what the real frontiersmen in the field know.

mugaliens
2008-Apr-29, 07:10 PM
The wave is the bending of space ... The particle ( still need to think if it is a really a particle or a quasi particle ) is its natural state.

While light (http://en.wikipedia.org/wiki/Light), even just one photon, has energy, and thus through E=Mc^2, a mass equivalence which matches the momentum light, as a function of it's frequency (and thus it's Energy state) has been measured to impart to absorbing bodies, as well as off reflecting bodies (and this has indeed been measured in the laboratory - it's described by these equations (http://en.wikipedia.org/wiki/Light#Quantum_theory)), other than the very slight gravity imparted to a single photon, light doesn't "bend space."

Rather, it's comprised of both an oscillating electric field (think of static electricity) and, at right angles to that electric field, an oscillating magnetic field (think "magnet"). That's why they call it "electromagnetic radiation." The rate of oscillation for each of the fields is identical, and is equal to it's frequency (crest to crest). It's velocity through vacuum, c, is simply the relationship between it's wavelength (meters per wave) divided by it's frequency (waves per second). The result, c, is given in meters per second.

Since c is a constant, if you increase the frequency, you must corresponding decrease the wavelength.

When it comes to energy, E=Mc^2.

For light, it's energy is proportional to its frequency. Thus, E=h*f, where h is Planck's constant (figured out by Max Planck years ago), and f is it's frequency. Thus, the greater the frequency, the greater the energy. That's why infrared only warms you while UV rays can burn your skin, X-rays (if you get enough of them) can scramble your DNA, and gamma rays can kill you.

Meanwhile, you're awash in a veritble souple of very low frequency electromagnetic radiation (power transmission lines, the motor in your refridgerator, the wiring in your house, etc.), and it has no affect on you whatsoever (those who claim it does are probably using insecticide and/or weedkiller without either gloves or masks...).

Getting back to light/EM radiation...

So, if we know the wavelength of a photon, we can calculate it's frequency (f). With the frequency (f) we can calculate the photon's Energy. With the photon's Energy, we can calculate it's effective mass (it's still massless, as no mass can travel at the speed of light - however, it has an effective mass, or mass-like effect due to it's energy).

Thus, light has a momentum, as given by p=E/c (p stands for momentum).

It also exerts, via it's effective mass, a gravitational attraction.

The only way light "bends space" is via this gravitational attraction, and that bending, per photon, is so very small as to be utterly beyond detection by any instrument we can devise.

Even with a billion-billion (1*10^photons, or 1,000,000,000,000,000,000 photons) we still wouldn't be able to detect it's gravitational attraction! Such is gravity.

Gravity is the weakest force in the universe! (somone correct me if I'm wrong...)

It takes incredible volumes of mass to produce even the slightest detectable trace of it. Compare that with radio waves (also, in the electro-magnetic arena). A chip that weighs less than a postage stamp can easily produce radiation in the E-M spectrum which is readily detectable.

Case in point. Have you ever heard of the chips you can install in your car that are interrogated as you pass the entrance to a toll booth, and automatically tell the toll booth (as you're passing through at 60 mph) you're so-and-so and the next month you get a bill (that's almost always automatically paid via your credit card))?

Sound familiar?

As you pass through the toll booth, EM radiation given off by the toll booth (usually in the form of microwaves) is received by what's known as a "rectenna" (receiving antenna). That received energy actually powers the device! No batteries required. And that energy boots up a very simple computer whose sole job is to say, "I, Mugaliens, am passing through." Actually, it merely sends a simple header followed by a unique ID consisting of 1's and 0's. But it does so many times, constantly repeating that series of 1's and 0's until it's out of range and dies back into nothingness. The reason it does this is to ensure that the toll booth got the right signal. On the better systems, there are authentication/authorization cycles, such as "I'm Mugaliens, attempting to pass through. My unique ID is 1001001001111100001001001. Ok by you?" The response is either, "yep, go for it" or "WOAH!" in which case your microwave-powered device might start beeping at you because you didn't pay your bill!

Postage-stamp devices that we can detect at a distance.

Gravity of such a postage-stamp device? We'd never detect it in a million years, unless it smacked our interstellar windshield as we blew through it at 32,000 feet per second!

Back to the lesson...

Light. In a broader sense, EM radiation. Particle schmarticle. Wavicle schmavical.

Both and neither!

Light exhibits both some characteristics of a particle, as well as some characteristics of being a wave.

Physicists the world over have argued that photons are either one, the other, both, or neither. It's my take that they're both both, AND neither. Ocean waves differ quite significantly from E-M waves. Recently advances in plasmonics (http://en.wikipedia.org/wiki/Plasmonics)have given us a few surprises as to just how different this wave theory really is. Light waves are far more akin to sound waves than they are to ocean waves (which is like comparing apples and yak herds).

Well, enough for this post. Not going to solve everything here. I look forward to the responses!

a1call
2008-Apr-29, 07:47 PM
I don't know what the following add up to, but perhaps someone can see a link.

Some facts:

*- A photon has momentum
*- Static electric and magnetic fields possess momentum (http://en.wikipedia.org/wiki/Momentum#Momentum_in_electromagnetism)
*- I assume that a static magnetic field is not associated with photons (please feel free to correct)
*- varying the magnetic field suddenly creates electromagnetic radiation which is regarded/observed as photons

Amber Robot
2008-Apr-29, 08:08 PM
Not exactly... Matter consists of various types of quarks (http://en.wikipedia.org/wiki/Quark) (another link to follow) in various states of energy bondage.

Some matter does.

alainprice
2008-Apr-29, 08:44 PM
To add to Amber's post:
correct, some matter is composed of quarks. Other matter is not. Even then, I wonder if a quark is an indestructible particle, however, I seriously doubt it.

In response to a1call:
Einstein saw himself that there were problems when talking about electric fields versus magnetic fields in special relativity. Then answer lies in the fact that different things happen in different reference frames. An electric field can become a magnetic field under the right conditions.

mugaliens
2008-Apr-30, 05:29 PM
I don't know what the following add up to, but perhaps someone can see a link.

Some facts:

*- A photon has momentum
*- Static electric and magnetic fields possess momentum (http://en.wikipedia.org/wiki/Momentum#Momentum_in_electromagnetism)
*- I assume that a static magnetic field is not associated with photons (please feel free to correct)
*- varying the magnetic field suddenly creates electromagnetic radiation which is regarded/observed as photons

If you move a magnet, that's varying a magnetic field. No photons are released.

An antenna works by alternately charging and discharging a wire, which causes a current flow (which in turn generates a magnetic field) as well as charging the antenna with more, then less electronics (which generates an electric field).

Photons are both electric and magnetic, hence the term, electromagnetic radiation. Merely changing a magnetic field alone will not generation EM radiation (photons).

mugaliens
2008-Apr-30, 05:33 PM
Some matter does.

Ahhh, true - I forgot about those pesky leptons!

From Wikipedia: Matter consists "of elementary fermions. These are the leptons, including the electron, and the quarks, including the up and down quarks of which protons and neutrons are made."

And the energies binding these particles together also contribute to their mass.

alainprice
2008-Apr-30, 05:38 PM
And the energies binding these particles together also contribute to their mass.

Without even thinking about it, this seems wrong at first glance.

If you fight the binding energy, the original(higher) mass is restored. But with the binding energy in play, the total mass is lower than the sum of the parts.