I have a few general questions regarding the properties of a Photon, if anyone can help or knows where I can find the answers? Please Register or Log in to view the hidden image! 1) How is a photon (particle of light) created? 2) Do photons begin to move at the speed of light as soon as they are created, or do they accelerate to the speed of light? 3) Is the photon the only particle which travels at the speed of light? 4) Is the photon the only particle which does not have any mass? 5) Is the photon the only particle which does not experience the effects of time? And is this directly proportional to the speed at which it travels? And are both these qualities the direct result of the photon having no mass (i.e., an automatic outcome of an object having no mass)? 6) How does a Photon capture the image of the object it bounces off, such as the moon? 7) Can a photon be slowed down? And if it can, does it still remain mass less and timeless or do these values change? 8) Could a photon ever travel faster then the speed of light? 9) Can a photon be destroyed, or absorbed, and what process happens to the photon if either of these things can happen? Thanks, Please Register or Log in to view the hidden image!
Hi BIOS424. Welcome to SciForums. I am no quantum mechanics expert, but I will try to answer your questions anyway. Typically when a charged particle like an electron loses energy either by dropping to a lower quantum mechanical energy level or in "brehmasralung" (sp?) radiation. The energy has to go somewhere, so it becomes a photon. Photons always move at the speed of light. All massless particles should move at the speed of light. I know gravitons are supposed to be massless, but they have not yet actually been detected. There might be others besides gravitons and photons. I don't know if any other massless particles have actually been detected though. Things like time and distance don't make a lot of sense for a photon according to special relativity. This is a consequence of having an infinite time dilation and length contraction. An image is not a product of a single photon. The creation of an image is much better understood by considering the wave-like and ray-like behavior of light instead of considering the particle-like behavior of light. Yes, the speed of light is lower in any refractive medium (like glass). This is also better understood as a macroscopic wave property rather than a single-photon particle property. No. Certainly. The photon disappears and its energy and momentum are transfered to the absorbing particle. This is what causes things to warm up in the sun, photovoltaics to work, photo synthesis, etc. -Dale
Could be, but my homework problems were usually much more mathematical and less descriptive. If it is for a class then this kind of material should have been covered explicitly in the lecture. So if he didn't know those answers then he probably has communication problems with the professor. -Dale
Thanks for the reply and the welcome to sciforums, this is however not a homework assignment (I'm actually a 26 year old powder coater, boring job which gives alot of time to think about things). :bugeye: I'm really just trying to understand a little more about photons, as I'm interested in how the information which constructs the universe uses its "aloted energy". I want to know a little more about photons as they exhibit some interesting behaviour particulary the coralation of mass/speed/time. Hypertheticly speaking (and I'm in the realms of si-fi here), but if it were possible to completely remove the mass of an object, say an apple, would the apple also have an infinite time dilation. Also you said that the 'as yet to be detected' graviton should also be massless, but I inderstood it that the graviton is present in all things which have mass and exhibit a gravitational pull. As I say, I'm no science student, so my knowledge in this area is patchy. Tim
No worries. As I told Tom it didn't sound like typical homework problems to me. Well, after removing the mass of an apple you would wind up with something that couldn't really be called an apple any more. But theoretically this is possible. You know the famous E=mc² equation, the foundation of nuclear power and nuclear weapons. If you convert all of an apple's mass to energy then the apple would turn into an enormous number of high energy photons that would radiate in all directions at the speed of light. Each individual photon would have all the usual properties of photons. Gravitons carry energy away from accelerating masses in a similar way as photons carry energy away from accelerating charges. They are undoubtedly pretty common, but the amount of energy they carry is so incredibly low that they are nearly impossible to detect. It's like trying to weigh an elephant to detect a flea, except much harder. -Dale
Just one or two more questions Dale on sort of the same theme, and then I think I'm done for the minute. Are all particles which have mass, by default motionless within space (unless otherwise influenced by another force)? And do all particles which have mass, protons, neutrons, etc, etc and there smaller component parts, experience decay? That is to say a continual movement through time as measured by its changing state. I use the term 'movement through time' loosely here. Is movement through time experienced at the same rate for all motionless particles with mass? Tim
8) Could a photon ever travel faster then the speed of light? Acually, I think it can. c, as a speed of light, is the speed that light travels in vacuum. If I remebmer right, the speed of light can be greater then c( - which is the speed of light Please Register or Log in to view the hidden image! ) And, the speed of light may vary in time. See my thread called "Constants not so constant" for that.
I think you are refering to Newton's First Law. This says that unless a massive particle is influenced by another force it moves in a straight line at a constant speed. If it is motionless it will remain motionless, if it is moving it will keep moving. I think protons neutrons and electrons, the basic components of atoms, are all stable and do not have any measurable decay. Again, I am no quantum mechanics expert so I am not certain of that. Yes. The "movement through time" would be a particle's proper time. A motionless particle has no "time dilation" so its proper time is equal to the coordinate time. -Dale
I'll add a few things. Gluons are particles which are massless, and therefore required by relativity to travel at the speed of light. The trouble with detecting gluons is that they have a property called color, and nature seems to prefer color confinement. We don't know why. As Dale said, it's not a single photon but many of them. However I wouldn't say that the light captures anything. When reflected light reaches your eye, it excites certain receptors that transform the light into an electrical signal, which is processed by your brain. What you see is your brain's interpretation of that electrical signal. Dale said yes, but the answer is no. It is true that the average speed of light moving through a medium with an index of refraction greater than 1 is less than c. But no photon is ever slowed down: Photons are captured (and therefore destroyed) by the atoms in the medium, which exictes the atom. This excited state propagates for a while and then the atom de-excites, emitting another photon. This new photon is created upon de-excitation, and it travels with speed c. There is a subtle distinction to be made here, because the expression "speed of light" could be understood in one of two ways. First, you have a photon and it travels between two points in a certain time interval. You divide the distance traveled by the photon by the time it took to traverse the distance, and that is the speed of the photon. In this sense the answer to your question is, "No, the photon cannot ever travel faster than the speed of light by definition." Second, you have the invariant speed of relativity, which unfortunately was called "speed of light". The fact is that this invariant speed need not ever have been identified with light. Relativity requires an invariant speed, not some special one. This invariant speed happens to be the speed of waves in classical electrodynamics, and any massless particle is required to have that speed in any inertial frame. So in this sense the answer to your question is, "We are as certain that the photon always travels at the speed of light as we are certain that the photon is massless." So now on to the next obvious question: How certain are we that the photon is massless? The currently accepted experimental upper bound on the photon rest energy is <6*10<sup>-17</sup>eV, which corresponds to about 10<sup>-35</sup>kg. That's about 100,000 times less than the mass of an electron! They can indeed be destroyed. In fact, if photons couldn't be destroyed then we would never even know about them! That's because in order to see images or feel the heat from radiation, photons have to be absorbed in (and therefore destroyed by) the various cells of your body. If photons could not be destroyed then we would literally have no way of experiencing them at all.
Bound neutrons will sometimes decay weakly, and this is the reason that some radioactive isotopes are said to be beta-emitters. All free neutrons are unstable, and decay with a mean life of about 15 minutes. Protons and electrons are believed to be stable against any decay, whether they are bound or free.
A minor correction, protons can decay into neutrons when bound in a nucleus. One such process is beta plus decay where a proton decays into a neutron plus a positron plus a neutrino. Such decays are favored when they bring the nucleus into a lower energy state.
It seems to me that there is a boundless supply of clean energy coming in every second of every hour of everyday. How is it that we have hardly any research dedicated to finding new ways to harness the suns energy? Is there a reason that there have been no major breakthroughs in solar panel technology?
Well, all of the research into biofuels is simply a way to harness the sun's energy. So I wouldn't say that there is hardly any research. Photovoltaics are quite a bit less efficient and more dangerous to the environment than photosynthesis. -Dale
If I may, I´ll try to answer that one. Neutrinos have mass, If you ment that. http://www.ps.uci.edu/~superk/announce.html
Is a given particles movement through speed and its movement through time inversely proportional to one another? And if it is, does it suggest that a given piece of information within the universe (such as a photon) has a finite supply of 'energy' for how it can exist. You can move through space, but some of the 'energy' which is used in movement through time has to be pinched, travel faster through space, and you move even slower through time. Tim
Why do massless particles have to travel at the speed of light? Why can't they just "sit there"? Also, hypothetically, if I had a stationairy object with mass, and I suddenly removed its mass, by what mechanism would the object accelerate to c?
