# Thread: Does light have a mass?

1. I have not studied relativity - just so you know where I'm coming from - but to me it seems hat accepting the theory is not saying "this is the way it is", but more like "this seems to fit the evidence we have so far". Because to me it is not intuitive (and I know people would like to toss in that term "couter-intuitive").

As I understand light, photons are not particles but are, as some say "packets" or measures of electromagnetic radiation/signal. It seems to me people sometimes use the term "particle" when discussing light only as a means of picturing activity. To me, light is a wave propagating, affecting particles and being affected by particles. I have not yet seen anything that shows me these interactions are anything but mechanical/Newtonian. What I mean there is that if I look at some light striking a particle, it imparts energy, maybe an electron jumps up a little them drops back down, then photons are emitted as that excess energy. I'm not very good at describing it, but it does seem very ordinary to me. The only really worrying bit in this view I have is the question: "If light is not particles, is this propagating wave then an action of some ether or plenum." And I know exactly what people think of that idea...

And as I see space and the propagation of light, it looks to me as though space is not curved, but that the path of light is bent by massive presence. After all, the edge of this desk upon which my computer sits is straight. It's not curved. Yes, I am aware that this probably involves all sorts of maths I have not encountered yet. But straight is straight.

Could not the interaction of light and gravity be something simple like the (speed of light) = (c/(strength of gravity))? So that the closer it is toward a gravity source, the slower it is? I have no idea, I"m just guessing.

But I do appreciate these attempts to explain it all, so thanks.

If you take a classic gold-leaf electroscope, charge it then shine strong UV light at it the charge is lost. Why?

The charge is due to individual point like particles, electrons. Light is a wave motion. How can they interact? Only if for some reason, light is behaving like a particle.

Another mind bender. Diffraction occurs when the spacing between the grates is about the wavelength of the incident wave upon it. So, what happens if you fire particles at a diffraction grating? You would expect them to either go through in a straight line or impact the edges of the grate and not go through.

If you do this experiment you observe a diffraction pattern. As if the particles are behaving like waves.

Hmmm? Weird.

Another one. To follow Einsteins thought patterns.

Einstein realised that you can not tell the difference between acceleration and gravity, the principle of equivalence. So what happens if light goes through a gravitational field. By this principle it must accelerate, but the speed of light is invariant.

You can solve this paradox by realising that an acceleration can involve a change in direction while still maintaining the same speed. But light always travels in straight lines.

Light always travels the shortest path allowable which if space is euclidean its a straight line. But in a gravitational field is light folows a curved path. Which in turn means that space itself is non-euclidean in the presence of gravity. I.E. Space itself is curved.

What he later showed is that Gravity is entirely due to this curvature. The more mass, the more curvature and the more gravity.

3. How's the view about light and mass in Super String theory, Chaos theory, M theory, (and else)?

(note: I don't know nothing about these theories)

4. Thed,

You just can't accept the cold, hard truth. Photons are proof that Einsteins theory is flawed. They interact with other matter just like any other objects with mass: They interact with gravity, and they have inertia and momentum.

Your attempting to avoid the fact that photons have inertia and momentum, just like all mass, by introducing the ridiculous concept that momentum can exist without mass. In other words, Einstein's theory does not match the real world, so you are trying to change the real world so that it would match his theory.

Unfortunately, all the responses I get from you sound like your reading them from a textbook. Even though I did not go to college, I'm familiar with almost all of the examples you have given which, supposedly, prove Einsteins theory to be correct. Most, if not all, of these examples have also been proven with just the use of classical physics.

Finally, from your responses, I can tell that your still wet behind the ears. I doubt that you ever had an original thought dealing with physics. If I'm wrong, and you actually did once have an original thought, please share it with me instead of always quoting high school physics textbooks.

Tom

5. One thing I do know about all this is that there are many theories and many people who would love to punch each others' noses in about them, and we will probably not know for sure either way until someone makes some gadget that uses light in ways which can settle the dispute. For example, I'm pretty sure many people will never believe in the possibility of faster than light communications until (if) someone invents a device which introduces that capability to common use.

6. ## Straight is straight.

To amplify on Thed’s point about Euclidean space vs. curved space…

On a flat, i.e. Euclidean, plane, the angles of a triangle sum to 180 degrees. Imagine that the triangle now lies on a non-Euclidean surface such as a sphere. On the sphere the angles sum to greater than 180 degrees. Thus by measuring the angles of a triangle one can detect and measure the local curvature of space.

One can view the universe from the perspective of curved space in which case light doesn’t “bend” but does follow the curve of space around a star. Or one can view “curved space” as being embedded in a three-dimensional “flat” Euclidean space. Viewed in that flat space light does “bend”, as it does not follow a Euclidean geodesic.

Adam posted: “But straight is straight.”

The edge of your computer desk would remain “straight”, i.e. follow a geodesic curve, and yet could follow curved space so that the ends of your “straight” desk met in a circle. (Would require a very strong gravity field and a very long desk.) A “straight” ruler light years in length would follow the same path that light follows.

7. ## Tom

Thanks for the laugh, I needed that.

So your familiar with the examples are you? So why don't you understand why it supports Einstein?

You, and I mean you, are allowed to question conventional wisdom and have original thoughts when you first show an understanding of the basics.

It's an unfortunate fact that the Universe does not do what we want it to, as you seem to think.

8. Originally posted by Joeblow93132
Thed,

You just can't accept the cold, hard truth. Photons are proof that Einsteins theory is flawed. They interact with other matter just like any other objects with mass: They interact with gravity, and they have inertia and momentum.
Tell you what, give me one shred of evidence that that statement is correct and I'll believe you. Where's the experimental evidence that photons have Inertia.

I'll give you that they have momentum and intreact with Gravity. But the conclusion is flawed. It's like saying that because electrons have quantum mechanical properties that have been well tested, it disproves quantum mecahnics.

9. Thed,

How is inertia measured?? Isn't inertia the tendacy of an object to continue moving at a constant speed and in a straight line??

If this is the case, then the inertia of an object can be measured by the force that the object exerts on another object that is attemping to stop it or to speed it up(or change it's direction).

In laymans terms, if I throw a rock at your head, the force of the rock against your head is proof of the inertia of the rock. If the rock didn't have inertia, it would come to a complete stop without injuring you.

Using the same analogy, when light hits an object, the light itself creates a force on the object that it hits. This is proof that photons have inertia. Not to mention the fact that atoms recoil when they emit photons, just like a gun when it fires a bullet.

If photons didn't have inertia, atoms wouldn't recoil as photons are emmited, nor would photons exert a force on objects that they hit. If this was the case, I would have to agree with you that photons do not have mass. But unfortunatly, as you can see, photons possess all the qualifications of mass(except the fact that they travel at light speed).

If you have a different definition of inertia or believe my reasoning is faulty, please share.

Tom

10. All matter is comprised of photons. Matter cannot reach the speed of light, however photons travel at the speed of light. The speed of light is a finite speed and is the maximum speed limit in the universe. IF the speed of light were not the maximum and photons were able to travel at infinite speeds, inertia would not exist. Therefore, inertia is the result of the finite velocity of photons.

11. May I add a quote which might be usefull in this debate?

The question then is: how are such particles accelerated? In some way, energy has to be put into them; and in practice this is done electronically, typically by electromagnets, as in a cyclotron. This is the only controlled way to make the things really move.

So we have a situation in which (say) a charged electron is made to accelerate by applying a charge, which is supposed to repel or attract it, depending on whether it's negative or positive. When such experiments were carried out, and relying on estimates of the mass of an electron derived from Millikan's oil drop experiment, it was found that, as more energy was put in, the electron's speed increased, but not as much as would be expected. So it must be getting heavier! And moreover the limit was the speed of light!

Sadly, there appears to be a defect in reasoning here, pointed out by Phil Holland. [Though I don't know if this argument is original with him—RW.] The point is that electromagnetic radiation itself has a velocity, namely the speed of light in the medium it's travelling in. Since energy can be transferred to an electron, presumably, only when a wave of energy catches up with it, obviously it's impossible for the electron to ever reach the speed of the wave influencing it.

If you can't see this immediately, consider these everyday models of the situation, which I've tried to make as varied as possible to get the point across.

Imagine a wave-making machine in a swimming-pool, and a floating toy boat which is pushed along by the waves. However big the waves are, the boat won't go faster than these waves.
Or consider a boy throwing stones, every second, at the same speed, at a floating piece of wood; however heavy the stones, the piece of wood will never travel faster than the stones. (Or at any rate once it travels faster than the stones, the stones can't catch up with it). But nobody would imagine the piece of wood must be getting heavier as it picks up speed, because it moves less when it's hit.
Or imagine a children's roundabout, the sort turned by hand. If an adult regularly swings his arm to turn the roundabout, as it approaches the rate the arm's swinging at, will never speed up beyond the velocity of the arm.
It seems physicists, looking at electrons and measuring their speeds as they vary with energy, ignore this simple fact. They interpret the result as the particle getting heavier, with limiting speed that of light, without realising that the limit is imposed by their equipment. They assume in one part of their minds that electromagnetism travels at infinite speed.

12. c'est moi,

Your post is amazing!!! It's so simple yet I even failed to see it. Thanks!!!!

It shows how a phenomena can be perceived in a wrong way. Now I have to question my beliefs.

Tom

13. ## Tom

Your argument sounds good. Good enough that I went back to my copy of Mechanics, Volume I, second edition in the Berkeley Physics Course.

This is a classic case of arguing on cross purposes. It may do well to restate some of this as many will have forgotten it and some may learn.

First off, mass is either inertial mass or gravitational mass. Depending on how you measure. Their ratio is thought to be 1. But it leaves open the question that they can be different, relativity can and does do funny things with mass.

If E=M<sub> &gamma;</sub>c<sup>2</sup>. Then the inertial is mass M<sub> &gamma;</sub> = E/c<sup>2</sup>. Given that for a photon E=h &nu; then the effective inertial mass of a photon is,

M<sub> &gamma;</sub> = h &nu; / c<sup>2</sup>.

This is what causes radiation pressure and what you alluded to before.

But do they have gravitational mass. The answer is yes. It is found by imagining what happens if you dop a photon a height L and it gains kinetic energy with gravitational accelereration g. As it gains energy the frequency changes to &eta;' and

h &nu;' ~ h &nu; + h &nu; g L / c <sup>2</sup>

This has been experimentally proven by Pound and Rebka in 1960 and 1965.

So, as I originally said, you said and I argued against, a photon behaves as if it has mass.

But does this disprove Einstein? bearing in mind all the above came out of a text on Relativity. Short answer no.

A photon has no <i>rest mass</i>, which I was thinking of and did not say, is zero.

The problem is that your inertial mass increases with velocity but has a fixed value when you are at v=0. For a photon this is zero. It's a given that a photon never has v=0.

Another way of looking at this is that light is a wave. If you examine the Maxwell equations for EM radiation there is no mass term.

As I say, the Universe behaves in more complex ways than we think and understand.

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edited to change Eta to Nu, if I have my Greek right
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14. This is getting very confusing.

"A photon has no rest mass, which I was thinking of and did not say, is zero. "

in other words you are suddenly saying that a photon DOES have a mass since we know it is never at rest ...

15. ## C'est Moi

In short, yes, but with caveates.

Due to Einstein's insight that energy and mass are interchangeable a photon, having energy, can behave like it has mass. That is, it transfers momentum as Tom is saying.

Frankly, this gets into the very heart of basic, basic physics. What IS mass and energy. Why do particles behave like waves and vice versa. What is, really, a wave function. Deep questions that aren't answered by anyone yet.

As you, or some one said, Physics only models things. It does not explain why something is the way it is. The why's of the matter are beyond us at present.

16. "Frankly, this gets into the very heart of basic, basic physics. What IS mass and energy. Why do particles behave like waves and vice versa. What is, really, a wave function. Deep questions that aren't answered by anyone yet. "

I completely agree Thed
btw, may I just give a warm thanks to people like you Thed, and James R, Q, Crisp, Imahamster, for their great patience and good postings; but of course also to people like Adam who bring creativity or Joeblow who keeps everyone thinking and poundring around here ... don't kill me if I forgot someone!! (I'm sure i did)

but, here comes the big but, this is nonetheless important for the e=mc²! (look further)

"As you, or some one said, Physics only models things. It does not explain why something is the way it is. The why's of the matter are beyond us at present."

i said that one time indeed!

"Due to Einstein's insight that energy and mass are interchangeable a photon, having energy, can behave like it has mass. That is, it transfers momentum as Tom is saying."

few remarques:

1. matter is composed of particles, particles are "frozen" paquets of energie (in waveform/point)
2. matter is a different manifestation of energy, in other words, but basically it is the same "stuff"
3. if light can behave "like" it has mass, then we are playing with words here, i.e. in science you cannot say something does "like" this or that --> or it is or it isn't <-- if it has everything that can qualify it for having mass (note: we leave rest mass aside) then it must have mass. but then we get E = mc² as a problem. but maybe it's not a problem after all considering my previous quote.

so far some thoughts

17. "... don't kill me if I forgot someone!! (I'm sure i did)"

/kill
/stab
/stab

So... i see it this way: photons have no mass, but carry enegry... but since energy and mass are equvalent?

18. Originally posted by c'est moi

1. matter is composed of particles, particles are "frozen" paquets of energie (in waveform/point)
And that is the problem. A particle is a mass, energy, a point source and a wave function all at the same time. Physics chooses the correct model to use based on the problem. It has yet to address what they are.

2. matter is a different manifestation of energy, in other words, but basically it is the same "stuff"
In a way, yes. This gets very complicated very quickly. Energy is a distinct property that many things have. So is mass. Though the two are interchangeable it does well to understand they are different as well. A brick is a brick and electricity is elecricity. You never use equations of mass when talking about electricity and vice versa.

3. if light can behave "like" it has mass, then we are playing with words here, i.e. in science you cannot say something does "like" this or that --> or it is or it isn't <-- if it has everything that can qualify it for having mass (note: we leave rest mass aside) then it must have mass. but then we get E = mc² as a problem. but maybe it's not a problem after all considering my previous quote.

so far some thoughts
Absolutely. This is the problem.

It is why Physics uses a very distinct set of words to describe what you are dealing with, ala gravitational vs. intertial mass. In most cases mass will do but a in depth study requires you to start splitting hairs.

I'm as bad as most and will use terms sloppily.

19. Thed,

Our discussion of whether a photon does or doesn't have mass seems irrelevent compared to c'est moi's previous post.

If what c'est moi indicated is correct, then mass is ALWAYS constant, regardless of speed. This idea could be a MAJOR upset to the world of subatomic physics.

Tom

20. ## Tom

For me it's late. I've been up since 5am, it's now 9.36 pm and I've had a bottle of wine, the next one is open and breathing. I'll address the point when I get into work tomorrow at about 7am, after a gallon of coffee

Interesting point C'est Moi brings out but goes against my knowledge. Ho hum.

Let me sleep on it, zzzzz

"!£\$£\$T\$^%Y^%U^&U*&I

SIGNAL LOST

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