We know that light travels at a certain speed, and we theorise that it is constant according to ambience ( vacuum) What I would like to explore with this thread is Why? Why is the photon restricted not only to a maximum speed but to a minimum speed as well? What is it about the photon and/or space itself that restricts this movement? Does space provide resistance to movement of the photon or anything else for that matter? I get the impression that space increases it's resistance to movement depending on velocity etc? It reminds me of that old law stated as OHMS law.......Voltage = Current flow * resistance Is it possible that space could be considered as a conductor and that it's resistance inhibits movement thus generating a negative energy thus holding light steady at it's velocity?
EM waves have to travel at c because if it was travelling faster, according to Maxwell, the oscillating electric and magnetic fields would have to get larger and if it was travelling less than c then the amplitude of these waves would fizzle out over time! According to the law of conservation of energy, these two things cannot happen.
I don't know and maybe I'm out of line with this comment but Theory and math does not dictate the reality. The reality is not concerned about laws and theory. OHMS law does not tell electricity what to do it only explains a uniformity of observations.... I know John I don't need to tell you this but I just felt like saying it ........... We seem to sometimes forget this fact...... To me, just because Maxwell created a mathamatical model that fits and just because relativity makes sense etc does not determine the reality of what we are observing but only attempts to describe it. So therefore your response whilst correct from your perspective is far from adequate from mine. excuse me while I ramble on..... I sort of look at it like this....I give you 10 oranges and ask you to describe them in a theory. decimal, binary, linea, probability, organic, life, perception, monosim, imagination, subjectivity, and so on...but at all times they are just 10 oranges. I ask why is 'c' "c"? and the answer is that it can't be anything else due to an equation so therefore the equation is limiting the photon and not the reality of the photon..... do you see my argument? the universe is not governed by a law of conservation....the law only describes what appears to be the case.....( I am not argueing whether the law is correct or not)
Well what u are really asking there is why the fundamental constants of nature are the way they are. My answer to this is that u are looking at science too subjectively (like the way a lot of religious people do) and that if they weren't that number what would they be??? They would have to be a different number but they would have to be SOME number!!! Otherwise the universe would not exist but then u may ask why does the universe exist anyway?!
Yes I would have to agree with John on that one QQ. Another idea - when you pass a multitude of waves through each other you get a superposition. Now nature does this in an instant but when we try to work it out we have to go through a whole heap of mathematics to get the same result. Surely nature must have an incredible ability to calculate mathematics to work out what it does next - or does it do this my some unknown means? The answer to that is the latter. Nature does not use mathematics to work itself out, rather we use it to 'predict' what nature is going to do. So why is c c? It is because as John said - or because if it weren't c then it would not be light (EM radiation). Maxwell found a good way of reasoning why the speed of light is c by using some fundamental facts about our universe - the permittivity and permeability of free space. These two 'constants' are constant in our part of the universe but no-one knows if they change. They might! - but we have no way to know. If Maxwell did not discover the link between his equations and EM radiation, c would still be c. Why? Well, as I said before it is because if it isn't it isn't!
Not quite. Permittivity is the ability to resist the formation of an electric field. Permeability is the ability to "conduct" a magnetic field. Conductivity is a separate measure. Resistivity is 1/conductivity. Resistance is resistivity x length. Permeability, permittivity, and conductivity are all needed to describe the electromagnetic properties of a material. The values will vary with temperature, frequency, and other conditions. Permittivity is mostly used to describe how good an insulator is. It tells you how strong an electric field an insulator will take before it becomes a conductor. Permeability describes how a material affects magnetic fields. High permeability materials concentrate magnetic field. For instance, iron has a high permeability, so a solenoid with an iron core will produce a much stronger magnetic field than a solenoid with an air core. Together, permittivity and permeability determine the speed of electromagnetic radiation through a material. Note that this speed is not related to conductivity. Conductivity quantifies how well a conductor transports electric charge.
Pete, thanks for that.....I had forgotten those terms and attributes.....so long a go Could I suggest that you are saying that space ( ie. vacuum ) has all these attributes in some form?
Yes. For a vaccuum: Permeability = 4pi × 10<sup>-7</sup> N/A<sup>2</sup> Permittivity = 8.85419 x 10<sup>-12</sup>F/m Conductivity = 0
so for the sake of simplicity it could be stated that space ( vacuum ) may very well govern the speed of light by it's attributes.... as stated by oxymoron......hmmmm....of course in conjunction with the attributes of light or EM. Obviously the answer leads on to the question of how a vacuum can have these attributes but I think this one of those yet to be solved mysteries hey? By the way thanks guys for perservering with me.......
Actually, light itself governs spacetime Please Register or Log in to view the hidden image! But I know what u mean!
Can someone show me how you get the speed of light to be c from maxwell's equations? I know vector calculus (all though i am a little rusty at it).
Put B as equal to curl(A) where A is some vector potential. Substitutre this form of B into Amperes law with the Maxwell correction for time varying electric fields, setting the current to 0 (no sources, vacuum), as well as the new form of E, which will also be in terms of A i.e. E = -dA/dt curl(B)=mu.nu. dE/dt which gives curl(curl(A))=mu.nu.d/dt(-dA/dt) using vector identities gives {grad^2-(mu.nu.(d/dt)^2)}A=0 This is the general form of any wave equation, the speed of propogation of which is identified with c = 1/sqrt(mu.nu) P.S. Sorry about the notation
Well, you can slow light down far below c (look at index of refraction, etc). But if it stops (or slows down a lot) it changes into heat pretty much.
Yep, just change the values of permeability and permittivity. Index of refraction can be derived from these two values: n = √(permittivity.permeability) Permeability doesn't usually change a lot for non-conductors, so this is often approximated by n = √(permittivity) I'm not sure about that... I wouldn't have thought so (that seems related more to absorption than refraction).
Well what u are doing there is slowing down average light so that it appears that photons are moving slower when in fact they still move at the same speed. Average light is slowed down due to absorption and re-emission.
hmmm...interesting stuff. Another question. Is e=mc^2 only for a vacuum? Would it be different, say, under water?
