# Does light have a mass?

Discussion in 'Physics & Math' started by GRO, Apr 6, 2002.

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1. ### thedIT GopherRegistered Senior Member

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1,105
I'll throw a curve ball into this.

This whole matter

of light and mass gets into the heart of basic Physics.

We know energy and mass are related via Einstein. We know that mass has a wavelength and vice versa via DeBroglie and Schroendinger. QM goes a way towards relating these fundamendal quantities but only so far. So just what is energy, mass and wavelength?

The person to answer this gets the extreme smartass of the millenium award.

3. ### (Q)Encephaloid MartiniValued Senior Member

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20,855
Subtle is the Lord
The Science and the Life of Albert Einstein
ABRAHAM PAIS

Joeblow - Photons have no mass.

5. ### (Q)Encephaloid MartiniValued Senior Member

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20,855
Thed

So just what is energy, mass and wavelength?

The person to answer this gets the extreme smartass of the millenium award.

Now thats an award I'd be proud to put on the mantle.

I love a good challenge. Lets get started.

7. ### CrispGone 4everRegistered Senior Member

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1,339
Hi Tom,

"Photons DO have mass. Here are the proofs:"

Perhaps this is not what you want to hear, but those are not really proofs.

"1. Atoms recoil as they emitt photons, just like a gun."

This has nothing to do with mass but with conservation of momentum.

'2. Photons are influenced by gravitational fields(their paths bend when passing massive objects)."

Curvature of spacetime. I know you don't buy that one

. However.... [see below]

"3. Photons exert force on objects they impact(like the plates of the radiometer you were talking about)"

Again, conservation of momentum.

"The reason everyone on this thread is telling you that photons do not have mass is that , because if they did, Einstein would be wrong. And since many people on this board have inflateable Einstein dolls under their beds, they could never accept the posibility that Einstein was wrong."

I call mine "Einy" at night when ... oops... I'm going into too much detail here

.

"Under Einstein's relativity theory, a particles with a mass could never reach light speed, since at light speed, it's mass would become infinite(Einstein believed that as a particle traveled faster, it's mass would increase as well). Since a photon travels at light speed, the people on this board believe that it can't have mass."

Ok, let me first suprise you: Einstein WAS wrong. Without any doubt I am sure that sooner or later one of his theories will experimentally be shown to fail (especially in the world of quantum mechanics were we are discovering more and more strange things every day). However (and this is the continuation of above)... once again the goal of physics is -IMHO-misunderstood.

Physics is about the ability to predict what will happen when you perform a certain experiment. In principle, the underlying theory is irrelevant. NO theory can ever claim to be entirely correct; you simply cannot perform ALL possible experiments to verify it (there are an infinite amount of experiments to perform). That must come as a suprise from someone who is about to finish his studies in theoretical physics

.

What you can do with a theory is to interpret it as a simplified model of how nature works. This simple model is simple enough so our tiny minds can comprehend it and work with it. And the goal is to keep it as simple as possible. So then you have a few choices: either you start complicating things and try to grasp to your daily intuition ("it must have mass because it bends when passes the sun") or you can simplify the calculations and assume spacetime to be curved. You don't have to take my word for it, but I can assure you that assuming that spacetime is curved is more simple than assuming a flat Euclidian spacetime (you'd have to introduce a lot more unexplainable things to get your theory, your model compatible with experiments then). The theories that are "common" in the scientific community happen to be the theories that are both compatible with the experiments up to date, and that are the simplest in comparision to other theories that explain the same phenomena.

So you can throw overboard the curvature of spacetime if you'd like, and develop your own theory of why light bends in the vicinity of massive objects and that has probably been done. I simply can't think of an experimental way to distinguish between the two theories (assuming they don't fail at some point ofcourse). One theory is adopted by convention.

To conclude: what about the real workings of nature then ? At first I thought that physics would give the answer to that question, but I've grown a bit more cynic since then and I think that it can't. Physicists only speak in models, and sometimes we forget to mention that. When I claim that the photon has no restmass, I should add the sentence "According to the theory of special relativity", but I am a lazy guy

- and so are most physicists I think. However, up to a certain point, when experiment after experiment seems to speak in favour of a certain theory, you could start to argue that the model/theory you have made of nature is perhaps not that bad after all. Nature *could* perhaps work that way, but there is no way to tell it for sure. Ever.

That realization made me appreciate physics even more actually

Bye!

Crisp

8. ### Adam§Þ@ç€ MØnk€¥Registered Senior Member

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7,415
Einstein had some of his theories proven wrong, and he admitted he was wrong on several occasions. Such as the whole anti-gravity force, or Cosmic Fortce, or whatever he called it. He admitted it was wrong after two Californian university students explained redhsift to him. He also admitted he was terrible at maths. And I'm reasonably certain he said that some day people would disprove or improve his relativity ideas. That same consideration should be held for all theories. One day we'll most likely learn more, and know better.

9. ### ElmoRegistered Senior Member

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45
Joe:

Hey Joe:

Nobody just took Einstein's word for it, he had to prove it to the whole physics community, which he did. Relativity is now accepted, and experimentaly verified.
If you want to go along with Newton still, then please do. But he didn't finish the picture, his equations have a big hole. Or maybe you have your own theory for explaining everything.
I don't want to sound aggro, but it seems to me like you're one of those people who doesn't get enough attention so you try to make yourself an 'individual' and argue against everything just for the sake of it, correct me if I'm wrong.

10. ### Joeblow93132Guest

Elmo,

Welcome to Sciforums!!!

We can always use open-minded people like yourself.

Tom

11. ### ElmoRegistered Senior Member

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45
apology

Hi there, you caught me at a bad moment, for reaons I won't go into. I'm not usually as aggro, and usually more open minded, sorry.
The picture made me laugh though!

cheers.

12. ### Joeblow93132Guest

Crisp,

Mass is measured in ONLY two ways: Its gravitational effect and its inertia or momentum.

As you probably know, photons qualify as having mass under, not one, but BOTH cases.

Even if we assume that space is curved, it still does not explain the inertia of photons. Thed has indicated that even a massless object can have inertia, but if that is the case, then I can prove that bowling balls are massless as well(as I tried to sarcastically describe in an earlier post).

If inertia doesn't require mass, and gravitational interaction doesn't require mass, can you prove that anything has mass????

We can't bend the physical world so that it fits someones theory. Theories are based on the physical world, not the other way around.

Tom

Last edited by a moderator: Apr 9, 2002
13. ### ElmoRegistered Senior Member

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45
Light doesn't need mass for gravitational interaction. It bends past massive bodies, but not past other photons. We'd see all sorts of crazy shit in the telescopes if it did.

14. ### Joeblow93132Guest

Elmo,

I also found the picture funny, and sent it to a few of my friends.

"Light doesn't need mass for gravitational interaction. It bends past massive bodies, but not past other photons. We'd see all sorts of crazy shit in the telescopes if it did."

I don't think that it would be possible to bring so many photons close to each other so that they would react to their gravitational fields. After all, gravity is a very weak force and can only be observed in large objects.

If it were possible to create two parallel pulses of laser light that would be bright enough in order to measure the gravitational fields of the photons, the effects of the electric and magnetic fields of the photons would tarnish the results.

Tom

15. ### ElmoRegistered Senior Member

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45
I guess we'll never know.
I'll agree to differ with you for now. I'm only doing A-levels at the present and so can't offer anything more in-depth than what's already been said.
There's some interesting points here though.

16. ### c'est moiall is energy and entropyRegistered Senior Member

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583
I think Joeblow explained what he stands for in a good way for the first time
I'm impatient to know what the response will be ...

17. ### thedIT GopherRegistered Senior Member

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1,105
Joeblow

Light absolutely does not have mass. The example I posted was purely to describe how a massless particle can exert a pressure. Once you understand that momentum is a property independent of mass you'll have made an intellectual leap in understanding how wierd the Universe is. A common trick in Physics is to solve problems in momentum space, a plot of momentum and time only. We are not interested in mass in all cases.

To address your points. Light does not have gravitational properties. If it did a sufficiently stong source would attract something. Light is pure energy, a wave. But, and a big but, according to quantum mechanics waves can behave as particles, in this case a photon. Which is what I was trying to show by equating mass to wavelength. In the same way particles can behave as waves. In this case we model them according the energy of the particle. It's why in particle physics a particles mass is given in electron-volts, a measure of energy, and not in grams.

The reason light 'curves' in a gravitational field is that it follows the shortest path through spacetime. This path, in the prescence of a gravitational field, is not a straight line in 3D. Think of this, a solar mass star has a given gravitational field. A photon travelling near the stellar surface, through this field, deviates by some angle &theta; due to curvature. Now consider a solar mass black hole. The same photon travelling near the surface of the hole will follow a much more curved path. It deviates by an angle &phi; where &theta; << &phi; . The curvature depends not on the mass of the hole or star or photon only on the amount of curvature due to the gravitational <i>field</i>.

Momentum and inertia are not the same thing. Inertia is a resistance to a change in motion. In a way it is what mass is. Photons have momentum but no inertia.

18. ### Joeblow93132Guest

Thed,

First of all I want to state, as I mentioned many times before, I do not believe in curved space. The main reason I don't is because it can't account for the electric/magnetic interaction, which, in many ways, is identical to gravity. I believe that gravity will someday be explained as something SIMILIAR to the electric/magnetic interaction, not COMPLETELY different as Einstein proposed.

"Once you understand that momentum is a property independent of mass you'll have made an intellectual leap in understanding how wierd the Universe is. "

If I give you a rock to throw, can you change the momentum of the rock without changing it's speed??? Can you make a feather have the same momentum as your rock at the same speed?? If you can, then I'll agree with you that momentum is seperate than mass.

"To address your points. Light does not have gravitational properties. If it did a sufficiently stong source would attract something."

Do you realize how strong light would have to be in order for it's gravity to influence something??? I don't think it would even be possible to create light that bright.

"Inertia is a resistance to a change in motion. In a way it is what mass is. Photons have momentum but no inertia."

Inertia is also a resistance to change in speed. Photons do have inertia. The force experienced by objects hit by light is the result of photons resisting their change in speed.

How do you classify and measure mass? What can you do to illustrate that a rock has mass but a photon doesn't??

Tom

19. ### thedIT GopherRegistered Senior Member

Messages:
1,105
The evidence is against you I'm afraid. Countless experiments have shown spacetime to be curved. You can believe what you want, the Universe does not care.

Last I heard Einstein did not say EM was completely different to gravity. Relativity does not supplant Maxwells equations. It only addresses Newtons viewpoint.

Modern Physics accepts that all forces (EM and Gravity amongst others) are aspects of one Unified force.

Yes, by changing it's velocity. Can you see the difference.

If it is the same mass on the same velocity vector, no I can't. Can you see why this is? your example is at best specious, it assumes a feather has less mass than a feather. I'll point you to the expriment of Apollo 8 (mumble) where a feather and rock where dropped together, they hit the moon together.

The important thing is velocity here. If you can't understand that please duck out now.

You are the one claiming light has some mass based one several points as above. You have not put a limit on that mass so using the above example as a counter proof is also specious.

I'll say again, if light has mass a sufficiently strong source will attract a mass. Is the Sun not strong enough?

If light has mass it would have an anomalous effect above and beyond the mass of the Sun in this case.

No it is not.

Inertia is the resistance to a force, an acceleration. Speed is a purely scalar quantity. Acceleration includes change of direction, a vector quantity.

Your evidence in the face of countless experiments is?

It could also be due to a change in momenum a expleained. <b>F</b>=d<b>p</b>/dt after all. Note the use of <i>vectors</i> here.

Normal way, with a scale.

In a way, very simple. A photon travels at lightspeed, mass does not. It's not that simple really, I'll admit to that. A full description requires a full book and then some.

Dave Barlow

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Editd to make a force, F, a vector quantitiy.
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Last edited: Apr 9, 2002
20. ### Joeblow93132Guest

Thed,

I'm very dissapointed with your responses. I would have expected a more logical response to my questions. I hardly know where to start. Here I go:

"The evidence is against you I'm afraid. Countless experiments have shown spacetime to be curved. You can believe what you want, the Universe does not care."

Please explain these so-called experiments. I have never heard of any experiment that proves that space can be curved. How exactly was the curvature of space measured??

"Last I heard Einstein did not say EM was completely different to gravity."

According to Einstein, gravity is the result of curved space. Einstein never bothered to explain the electric/magnetic interaction because it would then prove that attractive forces ARE possible WITHOUT curved space. So if the attractive forces between negatively and positively charged objects are NOT the result of curved space, then why would gravity be???

""If I give you a rock to throw, can you change the momentum of the rock without changing it's speed???"
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Yes, by changing it's velocity. Can you see the difference. "

Momentum is the result of mass and velocity. You claimed that momentum is independent of mass. I asked you to change the momentum of a rock without changing it's speed. You avoided the question by saying you would change it's velocity. Nice Try.

"You are the one claiming light has some mass based one several points as above. You have not put a limit on that mass so using the above example as a counter proof is also specious."

I didn't think that I would have to explain that the mass of a photon would be very small. I thought that would be obvious.

"I'll say again, if light has mass a sufficiently strong source will attract a mass. Is the Sun not strong enough?"

The light from our sun hitting a football field on Earth has the force equivalent to the weight of one marble. As you can see, the suns light is not sufficiently strong.

""Inertia is also a resistance to change in speed."
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No it is not.

Inertia is the resistance to a force, an acceleration. Speed is a purely scalar quantity. Acceleration includes change of direction, a vector quantity."

"Change in speed" IS acceleration as well.

""The force experienced by objects hit by light is the result of photons resisting their change in speed."
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It could also be due to a change in momenum a expleained. F=dp/dt after all. Note the use of vectors here. "

There you go again saying that a photon has no mass, but has momentum. As I asked you before, change the momentum of a rock without changing it's velocity(or it's mass).

"How do you classify and measure mass?
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Normal way, with a scale."

Again you avoided the question!! A scale measures gravitational interaction. The same interaction that cause a photons path to curve.

""What can you do to illustrate that a rock has mass but a photon doesn't??

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In a way, very simple. A photon travels at lightspeed, mass does not. It's not that simple really, I'll admit to that. A full description requires a full book and then some.""

And how do you know that mass can't travel at light speed??? Let me guess, Einstein told you!!!!

Tom

21. ### James RJust this guy, you know?Staff Member

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37,766
Tom,

There are many experiments which show the curvature of spacetime: gravitational lensing observations, the precession of the perihelion of planets, black hole observations, time dilation of atomic clocks at different heights, etc.

<i>According to Einstein, gravity is the result of curved space. Einstein never bothered to explain the electric/magnetic interaction because it would then prove that attractive forces ARE possible WITHOUT curved space.</i>

Perhaps you should look at superstring theory. Guess what? It explains BOTH gravity and electromagnetism as curvature.

<i>Momentum is the result of mass and velocity. You claimed that momentum is independent of mass.</i>

Momentum depends on mass for massive objects. If something has mass m it has momentum m<b>v</b>. If it has no mass, it has momentum p = E/c.

<i>And how do you know that mass can't travel at light speed??? Let me guess, Einstein told you!!!!</i>

Experiment supports that conclusion. When we accelerate a proton from 99.7% the speed of light to 99.8% the speed of light in a particle accelerator (which is done every day), it takes less energy than to accelerate it from 99.8% to 99.9%. In fact, it is observed that the higher the speed, the more energy you need, and a massive particle NEVER gets to the speed of light, no matter how much energy you put into it.

That's not theory, that's what is observed experimentally.

22. ### thedIT GopherRegistered Senior Member

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1,105
Classic kookery!

Then you will be really disappointed with these

I can understand your confusion. People who have not fully researched and studied Relativity often do not fully understand it's implications. Maybe after you have sat a course in it you will understand and learn of the overwhelming evidence supporting it.

You want to me to explain the findings of hundreds upon hundreds of experiments supporting Relativity? Go read a book on it, plenty about. Start with "Gravitation" by Misner, Wheeler and Thorne.

To help you along your quest I'll give you a few pointers. You can of course dismiss these, I suspect you will, but what the heck.

During a Solar eclipse the position of a star near the suns limb was deflected from it's expected position by an amount predicted by Relativity. This was the defining proof that brought Relativity into the spotlight. Only possible if the mass of the Sun curved space near it.

Shine a laser down a gravitational well (from a large crane will do) onto the floor. It is redshifted in accordance with gravitational redshift.

The observed precession of the perihelion of mercury.

The tidal bulge of the Earth follows the gravitational potential of our gravitational field. Notice it's shape. Tides are a direct consequence of Relativity.

The retardation of the period of binary pulsars. Exactly as Relativity says.

The cosmic mirowave background. Predicted from Cosmology using Relativity as a base theory.

Redshift of Galaxies. Observed first, modelled in Relativity later.

Many, many objects observed that have properties of Black holes. First predicted as a result of Relativity.

Please explain all the above without Relativity. No hand waving arguments and do so in accordance to Occams Razor. That is, use less assumptions than GR. I.E. Only one assumption or Axiom.

Which part of "Maxwell explained Electromagentism first" are you unaware of and do not understand?

Einsteins work came out of a problem making EM work in a Newtonian universe.

Different causes maybe?

Using this specious argument can you also explain the weak and strong nuclear forces. They are attractive and repulsive as well and do not rely on curved space to a first approximation.

Indeed momentum is a result of the product of mass and velocity in a purely Newtonian universe. Relativity shows us that momentum can exist without mass. You would do well to understand this.

Explain Photodissasociation and Compton Scattering without this please?

Your ignorance of vectors is showing. You can not propose a modification to Relativity and basic Physics without first understanding the absoute basics needed to understand this.

The answer I gave is a perfectly accurate one. Your inability to understand it is at issue. Speed is the modulus of velocity I.E. s=|V|. Change velocity (which includes direction) and you change momentum.

You are making a claim against fully tested Physics without qualifying your statements. Are you know backpedalling against an internal inconsistency of your claims?

Yet gravitometers routinely measure smaller deviations than this and no anomalous affects are seen. How do you explain this.

Make your mind up. I said Inertia is resistance to a change in speed, you say it's not then agree with me. Inertia is a resistance to force, which can also be a a change in speed.

You see, you do understand the difference.

Funny how we suddenly decide to invoke vectors here and not before. This is a classic kook trick of selective argument and choosing which facts to argue with to support a weak position.

No, you asked me to change it's momentum without changing speed. There's a world of difference.

Given the new question I'll answer, I can't.

But this has nothing to do with massless objects having momentum. Which part of p=E/c do you not understand?

[/quote]

Now you are the one changing things.

You're argument is that space is not curved and photons have mass. Are you know agreeing with me?

No, I've spent considerable time in the past in Labs doing experiments and analysing data. I've yet to see any evidence that light has mass.

What is your proof that a photon has mass. I'm waiting.

23. ### thedIT GopherRegistered Senior Member

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Joeblow

As you asked for evidence supporting Relativity, have a read of these links I put together. They should assist you. Also, read these ones as well.

They are not a definitive guide nor do I claim to be authorative on this subject. It's far too big a topic to cover without dedicating your life to it. But I have researched it up to and beyond undergraduate level.