Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: MatejBrada on 22/09/2005 20:59:33
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As there are so many questions about light, I would add several of them. Photons have no mass. So as F=m.a, we should be able to stop it (or make it move faster) with zero force. What is wrong in my statement? I wasn't able to find it for quite a time already.
Secondly, If photons have no mass, how come, that gravity has its influecne upon light?
Thirdly, how are photons made? Or are they actually the piece between matter and energy?
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I will try an answer, though I am by no means the final authority on the subject.
1) all tests we have been able to think of indicate no mass.
2) we do stop them, it is called a mirror. (they bounce well)
3) you have to be able to reach out and touch the photons. So far we have found nothing that can touch them, gravity included.
4) As has been said before, photons are formed by electrons and adsorbed by electrons. They are simply carriers of energy between electrons it seems.
5) Light is not bent by gravity, but it can be bent by passing though a gravity well that distorts time, and time distortion will bend light. So will a prism and a defraction grading. These two items bend light more or less by their wavelength. You often hear the argument that gravity bends light, but the theory says it is the time distortion caused by gravity. So gravity does _NOT_ directly act on a photon. There are theories that calculate a moving mass for photons as if gravity were directly operating on photons (F=MA), but the Special Theory of Relativity does not require or say that gravity acts directly on photons.
6) Photons are made when an electron decides to shed energy to become more stable. Offer an electron good company, and a bottle of beer, and they will sit down, kick their feet up, and settle into a lower energy state in a nano-second, and kick out the extra energy as a photon.
David
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In my opinion, lightlines show us the traces of our space-time. It maybe like the case of the Fe particles show the magnetical traces of a magnet.
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Matej,
I visited the Czech Republic with my wife last year, and we loved it. I'm jealous of you living there - access to all those wonderful beers!
Anyway, to add my thoughts to Sparkman's ...
Relativity says matter and energy are equivalent (they can be converted into each other in line with Einstein's equation E=mc^2). So if photons have energy (which of course they do) then they can have gravitational effects just like mass can.
Also, Newton's formula F=m.a is just a special case of the more general form F=dp/dt - i.e. force equals the rate of change of momentum with respect to time (p=momentum, dp=change in momentum, dt=change in time).
For classical material objects, p=m.v (mass x velocity), so dp/dt = d(m.v)/dt = m.dv/dt = m.a (as acceleration, a=dv/dt, is just the rate of change of velocity with respect to time).
However, light still has momentum, even though it doesn't have mass. The momentum of a photon is calculated as p=h/w, where h is Planck's constant and w is the wavelength of the photon's light beam.
So you still need to exert a (non-zero) force to change the momentum of a photon - and, conversely, a photon can still exert a force on another object.
Try doing a web search on "Compton scattering" to find out more about the experiments that were done in the first quarter of the 1900s to prove that photons do actually have momentum and can be used to apply a force to (change the motion of) electrons.
A final word: When trying to understand light, relativity and quantum physics, you need to think in completely different ways, and give up relying on classical concepts like mass (and the conservation of mass). It's best to just focus on energy. Mass is just one form of energy, which can be transformed into other forms of energy (just as heat energy can be transformed into kinetic energy - e.g. a steam engine being used to drive a turbine). And energy can be transformed into mass, of course - photons, for example, can be converted into mass when they transform into virtual electron-positron pairs.
Hope this helps.
Solvay.
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Now, I understand it at least partially. One more question. If you are taking mass as another type of energy, can you use energy in calculations instead of mass and vice versa, or is it impossible?
Thank you for your praise on Czech beer, but actually, I can't enjoy it right now, because I'm studying in Switzerland and the beer here... I would rather not speak about it.
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I don't know that we have a clue as to why these buggers move so fast. This seems to be a "Feature" of our universe that massless entities move at this velocity. They appear to slow down when traveling though other materials than a vacuum, but I understand that the reduction in speed is only that the photons are being adsorbed and reemitted many times as they transverse the material, and the delays in reemission accounts for the delay, and the perceived slower speed.
The experiment that "froze" light just was able to achieve several seconds between adsorption and emission. What is strange with this phenomenon is that the light continues in the same direction with only minor dispersion. Normally, you would think that on reemission the light would be totally randomized in direction.
When a scientist says he/she knows something, it means he/she can describe HOW it works. It does not mean they understand WHY it works. We know the how of light or at least some of it. The why still doesn't make sense.
David
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Solvay 1927
I admit the name got me going for a moment before I realized that you were referring to the fifth Solvay Conference held in Brussels in 1927, at which 29 delegates took part , at least eight of the delegates were already nobel laureates and eventually 17 of the delegates received the nobel prize. The Fifth. Solvay Conference was notable for the debates between Neils Bohr and Albert Einstein . It was at this venue that Bohr presented his complementary theory stating that light is a mixture of both wave and particle properties until the experimenter chooses what to observe in a given experiment. This theory followed from Heisenberg’s statement that the path of a particle comes into existence when we first observe it. The Conference is also notable for the statement made jointly by Heisenberg and Born declaring Quantum Mechanics to be complete and irrevocable. Surely such an unequivocal statement , given the limited amount of knowledge and technical capability available in 1927 , should be open to question in 2005 , yet the basic tenet of wave-particle duality remains inviolate and sacrosanct , no-one can question any of the conclusions or tenets of QM. Isn’t this reminiscent of the Middle Ages where only the clergy knew Latin and served as interpreters of the Bible while the layman ( who had no Latin ) had to accept without question the tenets laid down by the church. In QM the language is highly abstruse mathematics and the tenets have been laid down by the Solvay Conference. Are statements such as are made in the double slit experiment that phenomenon such as disassociation , (i.e., the ability of a particle having mass and dimensions such as the neutron , to be in two places at one and the same time ) acceptable. Yet this is the very heart of QM. The question is are different interpretations possible and if such different interpretations are possible , why can’t they be voiced without attracting severe censure and stigmatization.
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Matej,
I've never been to Switzerland. Based on your comments, maybe I won't bother. [:)]
Regarding your question - my understanding is that you CAN use energy in place of mass depending on what calculations you're doing / what phenomena you're describing. But to be accurate you also have to take momentum into account too, as there's some sort of equivalence between energy and momentum too. (I think - I'm still trying to learn more about this and get my head around it.)
Einstein's E=mc^2 applies to stationary matter with mass. The more complete equation (allowing for kinetic energy arising from movement) is actually:
E^2 = (m^2)(c^4) + (p^2)(c^2)
(where p=momentum, m=mass, etc.)
When there's no momentum/movement, this equation reduces to the usual E=mc^2.
Where there's no mass (e.g. for photons), it reduces to E=pc.
When it's a moving object with mass, you have to use the full equation.
David/"Steve" - I think your points might be better picked up in the new discussion topic McQueen has added re: Copenhagen interpretation.
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P.S. re: the Solvay Conference, I chose the name mainly because I was just bowled over when I first saw that photograph and the list of attendees (Einstein, Bohr, Heisenberg, Pauli, Schrodinger, Dirac, De Broglie, Planck, Curie, Lorentz, Born, etc. etc. etc.)
If anyone's interested, the following website (and the movie clip link in it) are quite unique:
http://www.maxborn.net/index.php?page=filmnews
Solvay.
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Solvay
Thanks for the link , I have edited my post to correct some mistakes . The picture is quite remarkable , almost as fantastic , amazing and improbable as the Picture from Hubble posted in the thread "Big Bang."