MooseMan Posted July 10, 2003 #1 Share Posted July 10, 2003 I dont know if there is already a thread about tihs but was wondering if anyone has heard about this? Briefly, it involves literally usuing solar winds to propel a large masted spaceship accross space. Does anyone have any links on this becasue I do not see how it could possibly work - wouldnt the sails be continually fractures by small particles of debris and how the hell would you stop when you get to your destination?! Ok if that made any sense to anyone thats a bonus! Link to comment Share on other sites More sharing options...
Saru Posted July 10, 2003 #2 Share Posted July 10, 2003 Mooseman, Here's a link to an article posted a week or two ago in which a physicist has claimed that Solar Sailing defies the laws of physics. I have to admit I'm not too convinced about Solar Sailing myself. Click Here Brief summary : Although photons do not have mass, they are considered to have momentum, so according to the law of conservation of momentum, the photon loses some of its energy to the sail as it bounces off, giving the sail a shove in the opposite direction. But Thomas Gold from Cornell University in New York says the proponents of solar sailing have forgotten about thermodynamics, the branch of physics governing heat transfer. Link to comment Share on other sites More sharing options...
Aslan Posted July 10, 2003 #3 Share Posted July 10, 2003 I'm sorry to go on, but I just get more and more baffled by this. A photon has momentum but no mass? A photon is an independent thing, existent in physical reality and independent of all other things in physical reality but it has no mass? I accept that you all know a lot more about this than I appear to, but can anyone explain it to me logically, rather than just telling me over and over that photons have no mass and there she lies. And are there any physicists in the field who dispute that photons are massless, and if so has anybody got a link? And on the topic of solar sailing, if solar sailing defies physics doesn't the masslessness of photons itself defy every other physical law? So there's another question. Is it just photons that are considered to be have no mass, or is there anything else. I'm a pain, aren't I. Link to comment Share on other sites More sharing options...
Saru Posted July 10, 2003 #4 Share Posted July 10, 2003 I have to say that in this case I don't really agree with the idea that photons have momentum but no mass. Despite the general consensus that photons have no mass, I am undecided as to whether they do or not, but if it is assumed that they don't, then it shouldn't be possible for them to have momentum, because momentum is calculated by multiplying mass times velocity, and if you have no mass, then you have no momentum. Although photons do not have mass, they are considered to have momentum, so according to the law of conservation of momentum, the photon loses some of its energy to the sail as it bounces off, giving the sail a shove in the opposite direction. I disagree with this because of that reason. If a photon has no mass, how can it possibly have momentum ? Where is the momentum coming from ? Anyhow, here's a couple of links for you on the subject of massless photons : Link to a NASA site on the subject Another from USAToday Link to comment Share on other sites More sharing options...
Aslan Posted July 10, 2003 #5 Share Posted July 10, 2003 Thanks for the links SaRuMaN! I shall study. Link to comment Share on other sites More sharing options...
Aslan Posted July 10, 2003 #6 Share Posted July 10, 2003 Nope. I'm even more baffled now. Photons do not have mass, apparently, according to the present definition of mass. However, an upper limit on photon rest mass has been decided upon. I presume that rest mass is not the same as mass, per se, but what does this mean? As far as I can gather, photons have no mass because they as near as dammit have no mass, which is surely a completely different thing. Am I being stupid here? Link to comment Share on other sites More sharing options...
antares Posted July 10, 2003 #7 Share Posted July 10, 2003 I have to say that in this case I don't really agree with the idea that photons have momentum but no mass. SaRuMan and Aslan, Photons are described by the quantum mechanics not by classical mechanics. The total energy of a particle in the relativistic case is: E^2 = (p*c)^2+(mc^2)^2 (if I am not mistaken) So even if the particle does not have mass it can have momentum since it has energy... Photons does not have mass but have momentum - this is a proven fact. Link to comment Share on other sites More sharing options...
Saru Posted July 10, 2003 #8 Share Posted July 10, 2003 Antares, Unfortunately Quantum Mechanics goes beyond what I've studied of physics, but this strikes me as baring a resemblance to comparing Newtonian mechanics with General Relativity when trying to work with something travelling at relatavistic speeds. Newtonian mechanics breaks down when talking about relativistic speeds, as does classical mechanics it seems when talking about the mass and momentum of photons. Looks like this is something I'm going to need to read up on. Link to comment Share on other sites More sharing options...
antares Posted July 10, 2003 #9 Share Posted July 10, 2003 Yes..., physisists tried to explain the elementary particles with the classical mechanics for a long long time - that's why they introduced the corpuscular-wavy nature of particles. But some things just cannot be explained with that. Then quantum mechanics appeared and postulated that energy is quantized - not continius as it is in the classical physics. Link to comment Share on other sites More sharing options...
Aslan Posted July 10, 2003 #10 Share Posted July 10, 2003 The total energy of a particle in the relativistic case is: E^2 = (p*c)^2+(mc^2)^2 (if I am not mistaken) See, this is where I come unstuck. You might as well quote ancient Sanskrit at me and tell me that's why photons have no mass. Logically I don't understand; in terms of technicalities I'm a babe in arms. Link to comment Share on other sites More sharing options...
antares Posted July 10, 2003 #11 Share Posted July 10, 2003 OK, It's not Sanscrit This equation represent the energy balance - as in the Newtonian mechanics energy is equal to the potential + kinetic energy. Same here but for the quantum case. I know it is hard to imagine that something does not have mass, but the photons are exactly like that, they propagate as energy pack and as soon as reach matter (stars, planets, houses, trees, me, you) they are absorbed as energy. So think of a foton as energy propagating trough space. Is that better And here comes the idea of the Space sailing - photons will transfer their momentum to the spacecraft... Link to comment Share on other sites More sharing options...
Aslan Posted July 10, 2003 #12 Share Posted July 10, 2003 Yes, thank you, Antares, that actually IS better. I'm logically happy with all that, and actually, even to someone as dense as me, it makes sense. My basic quandry, sadly, is still there. I say again, how can something that IS, and is measureable, and can have an effect on other things, have no mass? Surely the basic fact of it's BEING gives it mass. I mean (and I'm probably really off into LaLa land here) it's not the same as kinetic energy, or potential energy, is it. It is a quantifiable and democratic thing that is manifestly apparent to even a baby. And what of my other question? Are there other massless things in the universe? Because if there aren't then surely light isn't considered to violate physical laws purely because it has been made an exception of, and given a whole set of physical laws all it's own - like gliby explaining away someone who can't die as a perfectly natural thing even though their existence goes against every other natural law. I'm going to stop drinking soon. Link to comment Share on other sites More sharing options...
antares Posted July 10, 2003 #13 Share Posted July 10, 2003 I say again, how can something that IS, and is measureable, and can have an effect on other things, have no mass? Hm, OK. It is not necessary for a particle to have a mass in order to be measured, there are other properties that can be measured as well - momentum, charge, etc... It happens to be that photons are more exotic than the protons, neutrons, electrons... And this is because they represent fields rather than matter... ( I am not sure if my explanation is good enough here) Another thing - not just light is "made" of photons, all types of electromagnetic radiation - infrared, ultraviolet, etc. also propagates and reacts with the matter as photons. The difference between the photons of the visible light and the photons of the infrared radiation is just in the frequency... So visible light is not special at all, what makes it special is the human eye - this is the frequency it processes best. I hope that will help you to grasp the concept of photons Link to comment Share on other sites More sharing options...
MooseMan Posted July 12, 2003 Author #14 Share Posted July 12, 2003 My head hurts now! Link to comment Share on other sites More sharing options...
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