Light: Does It Have A Mass?

Well, the answer is no, but then how does it get "sucked" into a black hole if it has no mass?

Wikipedia might help:

Photon

My reckoning is that the immense gravity from the "black hole" warps space-time, bending the light.

Think of a trampoline. You put a marble on it, and it stays put. Then you put a bowling ball in the middle. The bowling ball's the black hole, and the marble as light rolls towards it. The trampoline material is in effect space time, and the bowling ball's weight's warping it.

The idea here has already been slightly touched upon. The problem with your thinking is that you're thinking of gravity as a kind of "string" or "tug" between one piece of mass somewhere in the universe, and another piece of mass elsewhere. In Newtonian physics, light should not be affected by gravity as it has no mass.

However we have observational evidence that disproves this (gravity lenses are a perfect example). However, General Relativity does explain it.

You see, it's not a tug, or a string, or a pull - but rather the warping of the FABRIC of space-time itself.

Take a piece of rubber, and flatten it out. Now, draw a straight line parrallel to one of the sides, and make sure it is closer to one side than the other. Now, push down on the middle of the rubber, without breaking through. Observe the line. Is it still straight?

You'll notice that besides the 3-dimensional change in its length, it will also change position 2-dimensionally, with the center of the line moving inwards towards the middle of the rubber. This is because the space that the line travels along is CURVED, or warped if you will.

Now, take this thought experiment and change two things.

First, we live in a 3-dimensional world, with 4-dimensional space-time.
Second, the line is a photon of light, and it will try to move 3-dimensionally forward as straight as possible.

However, when the light encounters the change in space-time, it will still be moving 3-dimensionally forwards, but outside observers will see its trajectory change and bend.

This is why light is affected by gravity. Because the space that it moves along is "bent".

In fact, this is also why you are affected by gravity. When you jump into the air, you don't fall down because something is pulling you down, you fall down because space-time curves in such a way as you will end up back on the ground.

Throw a baseball forward. It doesn't fall because something pulls it down, but rather the space in front of the ball is changing in constant with the time.

So, if we go back to Newton, his laws of motion still apply; "An object in motion will stay in motion unless acted upon by an outside force."

When you jump into the air, you stay in motion, and still have the energy to jump, but the space in which you are jumping through is changing because of the presence of matter.



Wierdified yet?

Originally posted by tek_604
Wikipedia might help:

Photon

If you Deny Ignorance...then don't Wiki

: In theory, Wikipedia is a beautiful thing - it has to be a beautiful thing if the Web is leading us to a higher consciousness. In reality, though, Wikipedia isn't very good at all. Certainly, it's useful - I regularly consult it to get a quick gloss on a subject. But at a factual level it's unreliable, and the writing is often appalling. I wouldn't depend on it as a source, and I certainly wouldn't recommend it to a student writing a research paper.

I know the jazz about the photon having no rest mass but the photon has been proved to have mass at c. Thus when a photon is obstructed it does transfer some energy to the object.
This I think has been proved by the Solar Sail experiment. Well it is basically a piece of extreemly thin material (dont know what exactly it is !) that deflects when exposed to a concentrated beam of photonic energy which can be say a laser. But according to the known theory Photons do have mass while they travel at the speed of light. They have a rest mass of 0. This is necessary in order to compensate for the increase in mass due to relativistic theory.

So yeah light does have mass !

Originally posted by SpittinCobra

Originally posted by tek_604
Wikipedia might help:

Photon

If you Deny Ignorance...then don't Wiki

: In theory, Wikipedia is a beautiful thing - it has to be a beautiful thing if the Web is leading us to a higher consciousness. In reality, though, Wikipedia isn't very good at all. Certainly, it's useful - I regularly consult it to get a quick gloss on a subject. But at a factual level it's unreliable, and the writing is often appalling. I wouldn't depend on it as a source, and I certainly wouldn't recommend it to a student writing a research paper.

According to the Science journal Nature Wikipedia is almost as accurate as Britannica... Britannica averaged 3 errors per article while Wikipedia average 4 errors per article. The catch is that they were Science based articles...

www.nature.com...

For topics like this Wiki is good enough. Now back to our regularly scheduled program.

I know the jazz about the photon having no rest mass but the photon has been proved to have mass at c. Thus when a photon is obstructed it does transfer some energy to the object.

According to who? I've asked the same question of physists and they laughed at my question and said no, Photons have no mass even at C. What happens is that they transfer a tiny bit of energy due to the imperfect reflective surface of the sail and when they bounce back they lose a little bit of energy(which gets transfered to the sail and imparts a slight push). They don't need mass to do that.

[edit on 21-1-2006 by sardion2000]

i believe it would be better understood by lookin at it in the 4th dimension world where light bends

Originally posted by sardion2000
According to who? I've asked the same question of physists and they laughed at my question and said no, Photons have no mass even at C.

Well your physicist should go look up planks equation and maybe even see General relativity and relativistic mass.
Light has energy hf at c and this energy is subject to some momentum. Now this momentum(p) is = mv. When light is accelerated to c with energy E the photon aquires a relativistic mass of hf/c^2.
Because, E=mc^2 and for a photon E=hf (h=planks constant, f= freq) and p=mv
p=hf/c or hf / wavelength then m=hf/c^2.
So the photon has a relativistic mass at c of hf/c^2 and this mass is accordance with the relativistic model .

Now comming to the gravitational attraction of photons by a blackhole, the answer to this is due to two reasons,
1) Light has energy and momentum which is coupled by mass, the energy-momentum vector of the photon acts along the curvature of space-time and a black hole is known to be a "sink"in space-time. The photon with its energy-momentum vector would be drawn to the objects gravitaionally becasue of the mass association where energy and momentum are related by m=E/c^2, E=hf, p=hf/c and thus by theory would be attracted in towards the space-time "sink".
2) as the photon's relativistic mass is present at c this makes light susceptible to the gravitational effects on its relativistic mass thereby drawing it in to the black hole.

So basically photons have two masses a mass of 0 at rest called the "rest mass" and a mass called the "relativistic mass " .

No, photons are massless particles. Don't even try to convince yourself otherwise.

Originally posted by T_Jesus
No, photons are massless particles. Don't even try to convince yourself otherwise.

I just love when people tell me what i can and can not do! Where should i start convincing you i am done with myself.

Energy = Mass if you never noticed so try work from there before getting agressive.

Stellar

Hrmm...

I have a 4-year degree in physics and mathematical sciences. I've seen equations used in this thread used out of context, and a Modern Physics course would most likely teach you what you need to know.

If you want to convince yourself, either read sardion2000's post, or take a course in Modern Physics =)

Originally posted by T_Jesus
If you want to convince yourself, either read sardion2000's post, or take a course in Modern Physics =)

That's what they all say. I stand to be corrected by i am not going to maky myself misinformed by reading University text books thanks. If you want to discuss the point go ahead and use as much detail as you like as i have the time and the energy to figure out what i do not allready know.

Stellar

It seems that your confusion of photons having mass at C stems from the solar sail arguement.

The important thing to remember is that the light is NOT "pushing" the sail.

What's happening is that the light hits the sail and is absorbed by the electrons in the atoms in the sail. Those electrons jump up to a higher orbit, and then jump back down as they release a photon in the opposite direction (that's right, the photon isn't bouncing off, it's being absorbed and released very quickly).

The released photon doesn't have quite the same amount of energy that it came in with. Some of the energy was lost to other energy types, mostly heat, but also some kinetic force.

Since there is no friction in space, this kinetic force will progressively build up, since each additional push will get it moving faster.

And thus does the solar sail take flight.


Now, does energy = mass?

Yes, and no. Energy can be changed to mass, and mass can be changed to energy, but on the atomic scale the two are quite different (quantum mechanics start getting fuzzy on this at REALLY tiny sizes, with wave-particle duality). However, outside of that, energy will not act like mass in any circumstance, nor mass like energy.

So, a photon is not a particle, just like an electron is not a particle. Photons tend to act like waves, but not always - and electrons tend to act like particles, but not always. The photon is simply the area occupied by the energy it contains. It has no mass unto its own.

Also, what's with that whole university textbooks being misinformation bit? I agree, I only know that I do not know, but where are we unless we try? So what if those textbooks will have to be completely redone in 30 years (even though they redo them every year for some stupid reason... oh wait, to make money...) - they're still the best we have to go on right now. Using the information that we think we know, we have to build upon that to find out the truth. And we must continue questioning what we think we know to ensure that all the conclusions that we based upon it are correct. That's why every few years you hear "Einstein's General Relativity verified!" - because people think of new ways to test the theory. And if one day that theory fails the test, then we either have to find a fault with the test, or there's a fault or something missing with/from the theory.

I've seen bad textbooks, but they're typically not used in classes. If they are, the professor will let it be known what's wrong.

I've liked most textbooks I've used, Thorton was great for Physical Mechanics, Griffiths was great for EM theory and quantum...the only textbook I thought was really horrible was Brown and Churchill Complex Variables and Applications.

Internet sources are okay to get a vague idea, but you can't really understand things without textbooks and working out problems.

All right, done with my rant.

Originally posted by Yarium
What's happening is that the light hits the sail and is absorbed by the electrons in the atoms in the sail. Those electrons jump up to a higher orbit, and then jump back down as they release a photon in the opposite direction (that's right, the photon isn't bouncing off, it's being absorbed and released very quickly).

In short the momentum of the photons is being transfered to the sail.
What you are talking about sounds a lot like the photo electric effect where the photons incident on the atoms tend to raise the outer most electrons to higher energy levels generating an electron "swamp" if you will.

So, a photon is not a particle, just like an electron is not a particle. Photons tend to act like waves, but not always - and electrons tend to act like particles, but not always.

Can you say that a photon exhibits no properties of a particle at all ?? Because the last time I was in collegue attending a physics class I can remember very distinctly something about the dual nature of light and inability to explain light as merely a wave just as the de broglie explaination of an electron as matter waves.

The photon is simply the area occupied by the energy it contains. It has no mass unto its own.

So you are saying that a photon is just energy ! No momentum associated with it at all ?? I mean the concept of "quanta" has become obsolete now.

Also you claim that in quantum physics their is some distrortion to the mass increment effects suggested by Relativity. But by the tests done to prove mass increment due to relativity was apparently done on a sub-atomic particle in an particle accelerator and brought close to c to find that the sub atomic mass did infact increase.

Okay you say that mass of a photon is 0 at rest ( I agree with this.) Then how does the photon which accelerates to c have a mass that is exactly similar to c at rest ?? [ In case you wonder how a photon accelerates imagins that a photon is generated in an extreemly dense medium and at the other end is let out into free space, the relative change in velocities ]
Also if the mass of the photon is 0 at c also then when it is forced to travel slower than its free space velocity, say enters an atmosphere and penetrates the ocean then how does it decrease its mass form 0 ? irrespective of however minute the mass decrement is ??

math.ucr.edu...

The short answer is "no", but it is a qualified "no" because there are odd ways of interpreting the question which could justify the answer "yes".

Light is composed of photons so we could ask if the photon has mass. The answer is then definitely "no": The photon is a massless particle. According to theory it has energy and momentum but no mass and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons it was known that light carries momentum and will exert a pressure on a surface. This is not evidence that it has mass since momentum can exist without mass. [ For details see the Physics FAQ article What is the mass of the photon?].

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 is related to mass p = mv. What they are talking about is "relativistic mass", an outdated concept which is best avoided [ See Relativity FAQ article Does mass change with velocity? ] 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 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.

Well guys luckily i dont suck these ideas out of my thumb so if you want to argue your free to do with people who actually have the degrees needed to be believed. :0

galileo.phys.virginia.edu...
plato.stanford.edu...
dbhs.wvusd.k12.ca.us...l
theory.uwinnipeg.ca...

As you probably know, there are other forms of energy than just the rest energy and the kinetic energy. There is heat energy, chemical energy, binding energies of atoms and nuclei, etc. etc. It turns out that all forms of energy are reflected in the total mass of the body. So although we have justified E = mc2 in terms of the kinetic energy, mass-energy equivalence is quite a bit more general.

www.upscale.utoronto.ca...

www.physlink.com...

www.newton.dep.anl.gov...

Stellar

Well guys luckily i dont suck these ideas out of my thumb so if you want to argue your free to do with people who actually have the degrees needed to be believed.

If you want people to actually take you seriously for a change stop talking down to them.

Whenever I see someone disagree with you, you go and say something like the above quote.

You said that University textbooks are misinformed remember that? No that wasn't it you said...

i am not going to maky myself misinformed by reading University text books thanks.

Now you say go argue with people who have degrees who had to be misinformed from those same textbooks in order to get said degree. WHICH IS IT!?!?! You're contradicting yourself.

[edit on 23-1-2006 by sardion2000]

[edit on 23-1-2006 by sardion2000]

I'll take this one on dude, some people need explaining.

The point he was trying to drill through your head is that people like you only tend to believe those with degrees, he's not going to waste his brain space learning a big fat book's worth of crap and clutter wrote by a bunch of people who admit they don't really understand how "it all" works so how do we know learning the teachings and techniques in it will give us the right thought processes we need to understand the universe, and finally, go ahead and argue with someone like that because you'd obviously find it much more fascinating although not necessarily elightening.

There are points in learning when breakthrough discoveries are made and we supposedly can't predict what the future will be like because we don't know how we'll build ourselves around these discoveries. Being stuck in one mindset won't help you "break on through to the other side."