Q, You just contradicted yourself. Mass is known by two things: its gravitational field or gravitational interaction AND, as you pointed out, it's momentum. Funny, because photons have both. They're paths curve when they approach massive objects, and they exert a force on objects they hit. How am I supposed to prove that photons have mass, if can't use the two most important properties of mass as the proof? Note: Photons slow down as they enter into denser matter like glass, and they accelerate back to c when they exit it. Tom
Nice link(If you assume the Theory of Relativity is correct) Oh oh! Now you've done it! Didn't Einstein once say that it will be one experiment that will prove relativity wrong. So far there have been countless experiments, millions if not billions of dollars spent and a parade of scientists throughout the past 80 some years that have tried to find that one experiment. All have failed. Relativity works. For the record, I did not contradict myself. Photons do not have mass, they follow straight lines and they do not accelerate. That said, it looks like we've reached a stalemate. I will only be repeating myself. Good luck on your theory. Please Register or Log in to view the hidden image!
Q, Maybe, over the last 80 years, people have so blindly accepted relativity that no one attempted to prove him wrong. And if they did, would you listen? I personally don't understand how so many people can believe a theory like relativity that's so illogical. Maybe people just use their imagination too much. Was it Einstein who said "Imagination is more important than knowledge"? I'm sure he meant imagination is more important than reality. Tom
Excuse me guys, I would like to take a moment to pull out my soapbox and make a little speech. Thanks in advance. Please Register or Log in to view the hidden image! We have in the Global Positioning System (GPS) an example of an actual engineering system in which the special and general theories of relativity are not merely of scientific interest, but rather are essential to its operation. The GPS serves as a kind of laboratory for the demonstration of relativistic effects on satellite clocks and as a model for the appropriate application of relativity algorithms in other systems. Typically, the effects of relativity become important for individual time measurements made to a precision of one microsecond or better. For any satellite system in which there are on-board clocks and on-board time-tagging of events, it is possible that relativistic effects may be even more dramatic than in the case of GPS, such as a satellite in a highly elliptical orbit. It is therefore important that these effects should be appropriately identified and taken into account.
Lets not forget about the countless experiments done in particle accelerators that extend the lifespan of a radioactive particle by double, triple and even more times it's original. How? They use Relativity. They accelerate the particle to about 99% the speed of light and its radioactive lifespan (from our viewpoint) becomes much longer than the same particle at rest. This conforms exactly to Relativity. There's no doubt about it, relativity works. You can believe otherwise, but then you would have to explain why it works so damn well for every experiment we can think of Please Register or Log in to view the hidden image! Photons do not have rest mass. If they did, it would be impossible for them to accelerate to their current speed. Their apparent mass increases as their kinetic energy increases. Suppose you found a photon at rest. It has no mass. Theoretically, any amount of energy could be applied to it to acheive an instantaneous velocity of c. Since there is no mass to overcome, any amount of energy will do. Once it reaches c it has apparent mass due to it's energy. For this reason it will still curve in space, because it does have mass as long as it's moving. btw, don't quote me on that thought experiment, I really don't know if it's right, just something I thought of. Please Register or Log in to view the hidden image!
JoeBlow, A good maxim to bear in mind is "Know thy enemy." You can't begin to try to refute a theory unless you first understand what it says. I therefore suggest you go away and read up on general relativity. Then you won't look so silly when you try to explain what that theory says and why it is wrong. To get you started, here are a few predictions of general relativity: * photons have no rest mass. * photons always travel at the same speed, according to all observers. They do not accelerate. * photons have energy and momentum. * the path of light can be bent by the gravity of a massive object due to the curvature of space and time.
Just thought I'd throw this in. About light slowing and accelerating when entering different media; the light only seems to slow down because it takes the entire waveform of the light pulse longer to get through the material. The individual particles are still travelling at c, but there is now more stuff "in the way" that they have to contend with. Thus, it seems as if the light slows down, when in fact the entire pulse of light is still travelling at c. Again, if I'm wrong here, someone correct me. I'm still learning Please Register or Log in to view the hidden image!
Photons striking the electrons of a transparent material will be absorbed and then re-emitted. The photons are passed along from one atom to the next until they are re-emitted out the other side of the material.
That's what I meant. And since there are more atoms inside a denser material (ie. more electrons) the photons will be absorbed many more times than if they were travelling through a vacuum. Thus, they appear to be travelling slower, while they actually aren't. Am I at least on the right track? Please Register or Log in to view the hidden image!
This is a most interesting thread. From the very start of wave mechanics (I have only had introductory classes so far), it seemed to me that these EM waves (which is what this is all about) were not the motion of particles at all. If you look at one of those demonstration diagrams of how particles act at various stages of a wave (exam questions: which way do the particles move at these points due to the wave? that sort of thing) it looks not like the particles are propagating through space, but as though they are being moved by motions of energy fields. Just as iron filings move to the will of magnetic fields. I personally think so far that (not counting atomic decay, alphas and betas and all) EM propagation or fields is much the same in its interaction with mass as plain old magnetic fields and iron filings. So in looking for mass of the photon, we will find particles which are affected by these things but are not actually the component matter of these things. Question: If all photons instantaneously propagate at light speed, why do photons not travel at light speed? Seems to me that even from the moment of creation, they travel at most at C/refraction index. Thus all those experiments involing slower light. Or is that idea of all photons propagating at C just a short-hand slang so people don't have to go into all those extra bits? Question: There's all this talk of photons as discrete particles. I was under the impression that "photon" was originally used as a way of conceptually working with a quantity of light or some other part of the spectrum, but was not actually a particles. More like a unit of measurement such as a litre or a metre. Indeed, my astronomy textbook defines a photon as a unit of measurement of electromagnetic energy rather than a particle. So it seems to me that when people refer to the mass of a photon, it makes about as much sense as talking about the mass of a metre. Can someone explain this further please?
If all photons instantaneously propagate at light speed, why do photons not travel at light speed? Seems to me that even from the moment of creation, they travel at most at C/refraction index. From what I understand, the speed of light is always constant. No matter what material it moves through, photons will always move at about 3x10^8 m/s. The reason it looks as if light slows down inside a refractive medium is because there are more electrons to absorb the photons and then re-emit them. This takes time, and although it happens very quickly, it can happen several million times to a single photon. This increases the time it takes for a photon to traverse the medium, which suggests that the photon is travelling slower than it would be in a vacuum (if the length of the journey is kept the same). As for your second question, I'll let James R. answer it as he probably knows more about it than I do Please Register or Log in to view the hidden image!
James R, Xelios, and Q, You are all wrong about the belief that photons get absorbed and re-emmited as they pass through dense material like glass. Glass is optically inert. The atoms in the glass do not absorb any photons. If a material did absorb photons, it would likely only absorb a specific wavelength, and not the entire spectrum. Not only would they absorb a specific wavelenght, but the re-emmited photons would likely be a different wavelength than the original photons. And finally, If there was a material that can absorb the entire visual spectrum, some photons would still get through without absorbtion. In this theoretical medium, there would be two waves of photons, the ones that did not get absorbed(traveling at c), and the ones absorbed and re-emitted(those appearing to travel below c). Since no material has ever been found that obeys this two-wave theory, you're absorbtion/re-emmited theory is flawded.(By the way a few materials do absorb and re-emmit photons, the phenomena is called fluorescents). Adam is right, only in space does light travel at c, in all other mediums it is slower. Tom
I was wrong regarding my last post. There are materials that absorb the photons of the entire visual spectrum, those materials are called black. They absorb light and re-emit it as infrared radiation. Green objects absorb all visual light except blue and yellow, while red objects absorb all visual light except red, etc. Transparent materials, like glass, DO NOT absorb any photons. Their atoms require too much energy to push their electrons into higher orbits. Tom
Adam, Regarding your second question, photons are considered particles and waves(or disturbances) at the same time. When photons curve around stars, and when they produce forces when they hit an object, they are considered particles. It has even been found that atoms recoil, like a gun, as they emit photons indicating that they may have mass, therefore they are particles. Under other circumstances, photons obey as if are electromagnetic disturbances propagating through space like a wave on the ocean. So are photons waves or particles? That's a question that hasn't been answered yet. Tom
So are photons waves or particles? That's a question that hasn't been answered yet. Both. Transparent materials, like glass, DO NOT absorb any photons. What do they do then? Do the do they not interact with the photons at all? If that was the case, the photons would travel through the medium unimpeded, meaning they would travel at c no matter what medium they travelled through (as long as it was transparent). You say yourself this does not happen, that the photons travel at a slower speed in a refractive index. So what does happen? Do they just magically know they should be travelling slower?
Adam, A photon is a quantum of the electromagnetic field. It behaves in many ways like a particle. It carries a particular amount of energy and momentum and is localised to a small region of space at any time. To all intents and purposes, it is a particle. Joeblow, You are right that clear glass does not totally absorb photons passing through it. The energy levels in glass absorb light at frequency of UV and above, but not visible light. However, frequencies below the absoption frequencies interact with "virtual" energy levels below the absorption levels in the glass, and those interactions take time. Glass exhibits <b>dispersion</b> effects. It has a slightly different refractive index for different frequencies of light, corresponding to the different energy gaps between the light frequencies and the energy gaps in the glass. What that means is that different colours of light <i>do</i> travel at different speeds through the glass, and this is well described in the literature.
"However, if you start looking in a four-dimensional curved spacetime and use a special "ruler" to measure everything, then they happen to travel straight on." this spacetime reminded me of Aspden who told following:- <<The word 'spacetime' is itself a presumptuous term if one is intent on engaging in a debate about the physics concerned with the nature of the universe. I am reminded by a conversation I once had at Cambridge with Dr. Sciama, the university don who supervised the Ph.D. research of Stephen Hawking. I was seeking to interest Dr. Sciama in my theory of the aether, stressing its quantum connection by its theoretical evaluation of the fine-structure constant and its relevance to gravitation. Nothing came from that conversation other than a comment which stands in my memory. Dr. Sciama said: "We all believe there is an aether but we never use that word; instead, we call it 'spacetime'". I may add that Dr. Sciama and I were both members of Trinity College, Cambridge and were awarded our Ph.D. degrees in the same year, 1954.>> as for all those discussions on photons: there are at least 4 main models in quantum physics for photons which all contradict each other but whom all work in different situations seems we have a long long way to go for finding out a proper theory for that and for all those discussions concerning mass and energy, I find that strange isn't mass 'frozen' energy? further, what is mass? what is energy? science has its limits, it can't tell WHAT something is, just say what it does but I guess that asking WHAT something is, is actually a wrong question (the same position I take in regard to time) can someone give some valuable critical view of this theory of Sansbury? (sorry, but it seems his website is just gone) if you don't know his theory, never mind then
sometin' went wrong here is the full quote this spacetime reminded me of Aspden who told following:- The word 'spacetime' is itself a presumptuous term if one is intent on engaging in a debate about the physics concerned with the nature of the universe. I am reminded by a conversation I once had at Cambridge with Dr. Sciama, the university don who supervised the Ph.D. research of Stephen Hawking. I was seeking to interest Dr. Sciama in my theory of the aether, stressing its quantum connection by its theoretical evaluation of the fine-structure constant and its relevance to gravitation. Nothing came from that conversation other than a comment which stands in my memory. Dr. Sciama said: "We all believe there is an aether but we never use that word; instead, we call it 'spacetime'". I may add that Dr. Sciama and I were both members of Trinity College, Cambridge and were awarded our Ph.D. degrees in the same year, 1954.
