Aki Posted October 9, 2004 Report Share Posted October 9, 2004 Why do gravitons and waves travel at the speed of light? Why not at another speed? Link to comment Share on other sites More sharing options...
Tormod Posted October 9, 2004 Report Share Posted October 9, 2004 I am not aware that gravitons have been discovered yet, but I think they are expected to travel at the speed of light because a) they are massless and B) are predicted to have similar properties to the photon. I assume you mean "gravitational" waves (because obviously not all waves travel at the speed of light) but again the reason would be that gravity and light share some properties, and one of them is speed. I am not a physicist so I assume that there is a better explanation... Link to comment Share on other sites More sharing options...
Bo Posted October 11, 2004 Report Share Posted October 11, 2004 All massless particles travel at the speed of light. and as it turns out that the graviton is massless, it travels at the speed of light.the handwaving argument why massless particles travel with the speed of light:- To accalarate a particle, you need some energy, proportional to it's mass.- Special relativity says that the faster a particle moves,the higher it's 'effective' mass becomes.- at the time you want to give the particle the speed f light: the mass becomes infinite, therefore the energy required to do this is infinite. This basicly means that it is impossible.- so the only way to have a particle move at the speed of light, would mean to give it mass 0. Bo Link to comment Share on other sites More sharing options...
lindagarrette Posted October 11, 2004 Report Share Posted October 11, 2004 Whatever happened to the notion that gravity is a spacial displacement? Why is gravity sometimes referred to as consisting of waves, or particles when it iss not at all similar to light except for its speed. (And why is that?) Light is just the energy generated when electrons move to a higher orbit in theer atomic structure. Right? And according to recent theories, at least the ones that attempt to explain the universe being created by crashing "branes," gravity can cross the void between these phenomena but photons (or light waves) cannot. If there is no graviton, are we comparing macro with micro concepts like apples and oranges? This is all so confusing it's hard to ask about it coherently. A little help, please. I'm still stuck on the concept of mass. I think I've asked about under every topic in this forum. Link to comment Share on other sites More sharing options...
Tormod Posted October 11, 2004 Report Share Posted October 11, 2004 Linda, good questions. I guess one reason why you have to keep asking is that few of us do understand what mass is. Link to comment Share on other sites More sharing options...
Uncle Martin Posted October 12, 2004 Report Share Posted October 12, 2004 Tightly bundled energy? Link to comment Share on other sites More sharing options...
lindagarrette Posted October 12, 2004 Report Share Posted October 12, 2004 Originally posted by: Uncle MartinTightly bundled energy?Granted, mass is potential energy, by definition. I guess the question is, what is <b>matter?</b> matter is composed on particles which are composed of something else. Whenever you jump off a tall building and land on concrete, it isn't your particles smacking intoh the concrte particles, but the electromagnetic fields of energy colliding. Right? Back to gravity and mass. Gravity acts on matter (mass) and on photons which have zero mass at rest. Except that photons are never at rest, so they also have mass. Does this have anything to do with Higgs Bosons? Link to comment Share on other sites More sharing options...
wazzuuup Posted October 13, 2004 Report Share Posted October 13, 2004 How can photons be massless when at rest, since intuitively it seems like nothing can be massless if it is made up of something. How could gravity act upon massless things, I always thought that masses attract each other, so how can massless photons be attracted? Is it correct that when photons move at high speeds, the energy is the mass, so when they are at rest, there is no mass? I am so confused… Link to comment Share on other sites More sharing options...
Tormod Posted October 13, 2004 Report Share Posted October 13, 2004 Energy IS mass. That is the basis of relativity theory. E=mc^2 describes how much energy you get from a given mass multiplied with the speed of light in vacuum squared. So while a particle may be massless the equation is NOT E = zero x c^2. Why? Because if an object has *both* zero energy AND zero mass, then it does not exist. So turn it over and you'll find that m=(c^2)/E. That will never give 0. Link to comment Share on other sites More sharing options...
Freethinker Posted October 13, 2004 Report Share Posted October 13, 2004 Originally posted by: TormodEnergy IS mass. That is the basis of relativity theory. E=mc^2...So turn it over and you'll find that m=(c^2)/E. That will never give 0. Er Tormod E=m(C^2) divide (C^2) out of both sides E/(C^2)=m(C^2)/(C^2) (C^2)/(C^2) cancel and leave E/(C^2)= m or: m=E/(C^2) NOT m=(C^2)/E You shouldn't post math this late at night! :-) Link to comment Share on other sites More sharing options...
Freethinker Posted October 13, 2004 Report Share Posted October 13, 2004 Originally posted by: wazzuuupHow can photons be massless when at rest, But I don't think Tormod answered your question. The problem is it is a very difficult question to answer. Partially because of that old complication "semantics". Language is not well equipt to discuss particle physics. So we use words that are close or at best representitive. The first problem is that there is no such thing as a photon "at rest". It might be said that a photon stops existing if it has no momentum. But even that would not be completely literally correct. When we talk about photons, the best we can describe is a "rest mass". When an observer sees the particle still and with zero speed. In other words, the particle is at rest as far as this observer is concerned. This is Special Relativity. It also deals with the wave/particle duality. It's momentum is a particle function and thus would be when mass is a related issue. So perhaps the question should have been CAN a photon be at rest as a particle? But that answer would be no less complicated! :-) Link to comment Share on other sites More sharing options...
Tormod Posted October 14, 2004 Report Share Posted October 14, 2004 Originally posted by: Freethinker E/(C^2)= m or: m=E/(C^2) NOT m=(C^2)/E You shouldn't post math this late at night! :-) Eeeeh...... I never said maths was my best subject in school...my apologies. The point still stands, though. Mass could reach towards infinity but never be zero. Link to comment Share on other sites More sharing options...
Tormod Posted October 14, 2004 Report Share Posted October 14, 2004 Okay, did some research (talked to one of my colleagues who is an astrophysicist). The photon has no rest mass because it is never at rest. A photon always travels at c which is the speed of light. The energy of the photon is (simply put) calculated by multiplying the Planck length with the photon's frequency. the frequency of a photon is related to the energy level it's originating atom had to get rid of due to electrons changing states in "orbit". One other thing: Massless particles travel at the speed of light because they can. Nothing is slowing them down. Particles with mass CANNOT travel at the speed of light. This is *not* because (as some believe) their mass would become infinite, but because it would require an infinite amount of energy to accelerate the particle to light speed. I wrote "Mass could reach towards infinity but never be zero" above, which then obviously was wrong. Hope this gets some of the cake off my face. Link to comment Share on other sites More sharing options...
Tormod Posted October 14, 2004 Report Share Posted October 14, 2004 ...and this, of course, is excactly what Bo stated before I even started my venture into mathematical explanations. Oh, I'll just shut up now. Link to comment Share on other sites More sharing options...
Freethinker Posted October 14, 2004 Report Share Posted October 14, 2004 Originally posted by: TormodOkay, did some research (talked to one of my colleagues who is an astrophysicist). The photon has no rest mass because it is never at rest. A photon always travels at c which is the speed of light.OK, now I am confused. I sure don't want to argue with an astrophysicist. But c is only c in a vacuum. The "speed of light"/ thus the speed of a photon (I would assume) is not a constant except in a vacuum. It slows thru mass. In fact you have a Hypog News link which discusses slowing the speed of light dramatically Researchers use semiconductors to set speed limit on lighthttp://www.hypography.com/article.cfm?id=34291 It says "31,000 times slower than the 186,000 miles (or 300 million meters) per second that light normally clocks while traveling through a vacuum."Hope this gets some of the cake off my face.We aren't going to let oyu off that easy! :-) Link to comment Share on other sites More sharing options...
Tormod Posted October 14, 2004 Report Share Posted October 14, 2004 I think it wouldn't matter what value c has - the energy is still the Planck length times the frequency of the photon. Ku is careful to point out that the experiments do not violate the laws of physics, which state that the speed of light is a physical constant that cannot be changed. What can be, and has been, changed is the speed with which a light wave's amplitude, also known as the "envelope" of a wave, is transmitted. So they are not changing the frequency of the photon at all, just the amplitude. Link to comment Share on other sites More sharing options...
lindagarrette Posted October 14, 2004 Report Share Posted October 14, 2004 So even though the photon has energy, it cannot have mass, because it is always travelling at the speed of light which is a non possibility for anything with mass. But he equation doesn't work out that way. But a photon can only be have as a particle when it is not in motion (which is impossible) That pretty much clenches the theory that a photon is not matter. Right? Link to comment Share on other sites More sharing options...
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