Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: jeffreyH on 19/12/2014 11:47:22
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Consider a thought experiment that would not happen in reality. Consider an observer moving in the frame of reference of a photon moving away from a gravitational source. Firstly would the observer detect the change in the nature of the wave of the photon and the resulting change in energy as it travels away? Would the wave of the mass of observer change in such a way that the change in the photon would be undetectable to them?
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As a thought example purely. I don't think so, imagine two photons 'at rest' with each other. Would you expect them to observe each other to 'red shift' leaving a gravitational potential? the idea behind a photon, assuming it to propagate, is its kinetic energy. And that one is assumed to come from its motion, which is 'c'. As a thought that is :)
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Against that you can argue that different photons contain different 'energies', as a blue shifted 'photon', to you observing and measuring it, will contain more 'energy' than a red shifted. And that one is strange as it doesn't matter for this what 'uniform speed' you define yourself at, relative those measurements (actually it does, but let's assume that you're doing this being 'at rest' with your light source, measuring at some 'sink', also at rest with you.). One definition of it is using the idea of 'photons' consisting of 'frequencies', as some 'wave packet'. But that one is a ideal definition, as I understand it going back to the concept of a 'infinite plane wave'. http://amasci.com/miscon/coherenc.html
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that would not happen in reality... an observer moving in the frame of reference of a photon
Einstein says (and the earlier Michelson-Morley experiment demonstrated) that a photon always seems to be travelling at "c" relative to every observer.
So I agree that this could not happen in reality, because no observer can move in the frame of reference of a photon.
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Consider a thought experiment that would not happen in reality. Consider an observer moving in the frame of reference of a photon moving away from a gravitational source. Firstly would the observer detect the change in the nature of the wave of the photon and the resulting change in energy as it travels away? Would the wave of the mass of observer change in such a way that the change in the photon would be undetectable to them?
The reason they can't happen in reality is the same reason that your question cannot be answered. E.g. I assume that you're not asking us to forget the laws of physics in order to answer your question, correct? If so then you're asking us to suppose that something has happened which the laws of physics prohibit, i.e. being at rest in the rest frame of light. If we did that then we'd be violating the law of physics stating that the speed of light is invariant and we'd also be violating Maxwell's equations.
If, instead, you're asking us what happens under certain circumstances, and supposing the laws of physics are invalid, then you can in fact make up any answer that you'd like to since one is just as likely to be correct as another when you're making up the laws of physics as you go along.
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Consider a thought experiment that would not happen in reality. Consider an observer moving in the frame of reference of a photon moving away from a gravitational source. Firstly would the observer detect the change in the nature of the wave of the photon and the resulting change in energy as it travels away? Would the wave of the mass of observer change in such a way that the change in the photon would be undetectable to them?
The reason they can't happen in reality is the same reason that your question cannot be answered. E.g. I assume that you're not asking us to forget the laws of physics in order to answer your question, correct? If so then you're asking us to suppose that something has happened which the laws of physics prohibit, i.e. being at rest in the rest frame of light. If we did that then we'd be violating the law of physics stating that the speed of light is invariant and we'd also be violating Maxwell's equations.
If, instead, you're asking us what happens under certain circumstances, and supposing the laws of physics are invalid, then you can in fact make up any answer that you'd like to since one is just as likely to be correct as another when you're making up the laws of physics as you go along.
Give that man a cigar. Most people will simply argue that the speed of light is invariant but then speculate on what might happen as if the thought experiment were possible. The fact that it IS impossible is the whole point. Without this clarity we simply have endless rambling.
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Leading on from this impossibility we can question whether light has any mass, although not rest mass. To accelerate a mass to light speed we say that we need infinite energy. If this is so then how can we say a photon has any mass? Is there a mass limit so infinitesimally small that when increased relativistically it will not require infinite energy to move it at light speed? Is this the photon mass?
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Leading on from this impossibility we can question whether light has any mass, although not rest mass. To accelerate a mass to light speed we say that we need infinite energy. If this is so then how can we say a photon has any mass?
Because photons are "born" moving at the speed of light so no work is done on them to accelerate them. In fact work cannot even be done on a photon in an inertial frame of reference.
Is there a mass limit so infinitesimally small that when increased relativistically it will not require infinite energy to move it at light speed? Is this the photon mass?
No. The photon mass is the mass a photon has by virtue of its momentum. i.e. m = p/v = p/c.
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Leading on from this impossibility we can question whether light has any mass, although not rest mass. To accelerate a mass to light speed we say that we need infinite energy. If this is so then how can we say a photon has any mass?
Because photons are "born" moving at the speed of light so no work is done on them to accelerate them. In fact work cannot even be done on a photon in an inertial frame of reference.
And of course in order to do work on the photon would require the force to be in the photon's frame of reference. [:o]
Is there a mass limit so infinitesimally small that when increased relativistically it will not require infinite energy to move it at light speed? Is this the photon mass?
No. The photon mass is the mass a photon has by virtue of its momentum. i.e. m = p/v = p/c.
So how do we determine the momentum with zero rest mass? We know v in advance but m would need to be determined. In order to determine p we need m.
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And of course in order to do work on the photon would require the force to be in the photon's frame of reference. [:o]
I said inertial frame because in a non-inertial frame there are gravitational fields present which can do work on photons.
So how do we determine the momentum with zero rest mass?
Remember that its only the rest mass that's zero. Not the mass (aka relativistic mass) itself. As I said above that has the value m = p/c so p = mc. For light and photons E = pc so p = E/c as well.
We know v in advance but m would need to be determined. In order to determine p we need m.
You have to determine what you know about the photon first. If you know the energy then the momentum is p = E/c.
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Now take a look at what Pete has said and then read the section Momentum of the Photon here:
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/relmom.html
From this you will see why Pete has used E/c and why he is right. Pete often gets jumped on for his attempts to put people on the right track. Often by those he is trying to help. Well he is right here isn't he? Without people such as Pete, Clifford, Evan and JP this site would be full of nonsense. Let's try and learn something we might all benefit.
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If your observer was to travel at the speed of light (he would have to turn into radiation to go that fast) he would not be able to observe anything at all because time for him would stop. For this reason alone, he would detect nothing.
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Jeffrey, if you ask a question just to prove that you didn't really meant to ask it? what exactly did you think you were doing there? Setting up a 'honey trap'?
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Jeffrey, if you ask a question just to prove that you didn't really meant to ask it? what exactly did you think you were doing there? Setting up a 'honey trap'?
This forum is unique amongst the physics forums that I have used. Even though I have wandered into speculation I have never been banned. I have been banned for less from other forums. The ethos of a forum such as this is to educate and sometimes entertain and be fun. My original question was a set up to make a point, yes. Why not? Sometimes people come with preconceived ideas and use up bandwidth. I have modified my thinking quite a lot since being here because I have learned things. If it gets clogged up with nonsense there is less opportunity to learn useful things pertaining to physics..
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Good to know Jeffrey :)
It is a rather good forum.
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Now take a look at what Pete has said and then read the section Momentum of the Photon here:
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/relmom.html
From this you will see why Pete has used E/c and why he is right. Pete often gets jumped on for his attempts to put people on the right track. Often by those he is trying to help. Well he is right here isn't he? Without people such as Pete, Clifford, Evan and JP this site would be full of nonsense. Let's try and learn something we might all benefit.
Thanks Jeff. That's very kind of you. :)