Does light travel forever?

Discussion in 'Physics & Math' started by drumbeat, Mar 28, 2011.

  1. Pete It's not rocket surgery Moderator

    I don't follow your logic. The steps in your argument seem disconnected.
    I don't see any contradiction between instantaneous emission and a change in photon energy/momentum/frequency/wavelength with source motion.
    GR is perfectly capable of handling non-empty space, so how is this a problem? You seem to be confusing the speed of light through a medium with the value of c in the field equations.

    Have you considered the observable differences between refraction through a gaseous medium and gravitational lensing?

    I think you're on the wrong track here.
    I don't pretend to understand GR past a very vague qualititative level, but I'm pretty sure that the constancy of the speed of light is about local lorentz invariance. It's about a particular speed, c, being the same local speed regardless of the motion of your local reference frame.
    It's not about the speed of light through a medium.
    It's not even about c being constant throughout spacetime.

    It is my understanding that Einstein's field equations are certainly in conflict with Newtonian dynamics locally, because locally (locally enough that spacetime is flat) don't the Field equations simplify to Special Relativity?
    There is no time dilation and length contraction in Newtonian dynamics
  2. Google AdSense Guest Advertisement

    to hide all adverts.
  3. Pete It's not rocket surgery Moderator

    This is where the qualitative understanding that you and I have is insufficient, and a quantitative approach is needed.

    The spectral broadening due to Doppler shift can be calculated.
    The spectral broadening due to changes is atomic energy levels can be calculated.
    The actual spectral broadening can be measured.

    Would you agree that if the measured spectral broadening matched up with that predicted by a doppler shift model and not by an atomic energy shift model, then the doppler shift is probably the better explanation, and vice versa?
  4. Google AdSense Guest Advertisement

    to hide all adverts.
  5. RJBeery Natural Philosopher Valued Senior Member

    My 2 cents here, and perhaps not pertinent, but I think of red-shift in the following way: we cannot say that the emission of a photon is time independent because measuring wavelength is a function of time. In my mind's eye, a red-shifted wavefront reaches its destination at the familiar velocity of c, but the longer wavelength necessitates that detecting that "photon" actually takes marginally longer than if it hadn't been red-shifted. By this logic, detectable emitted light which is red-shifted hasn't been slowed, but rather delayed by an amount proportional to its red-shifted wavelength increase.
  6. Google AdSense Guest Advertisement

    to hide all adverts.
  7. Pete It's not rocket surgery Moderator

    Don't know.
    I can understand (I think) a photon having energy and momentum, but the frequency and wavelength properties cross the curtain into quantum weirdness zone.

    Here be dragons. (I can't read those modern maps

    Please Register or Log in to view the hidden image!

  8. RJBeery Natural Philosopher Valued Senior Member

    Well, I can't wait to take a course on optics because that will answer a lot of questions that I have. Can we detect, say, 3/4ths of an EM wave? I don't think so but I frankly don't know. If the answer is no, then my personal understanding above is only logical (unless we claim that the red-shifted wavefront arrives at the destination faster than c such that the detectable photon's velocity remains unchanged).
  9. OnlyMe Valued Senior Member

    Whew! That's a lot to digest. But I was expecting things to get stirred up...

    You got sitting here with a big grin... No real answers but still...

    I'll have to wait until tomorrow when I can get to my desktop to respond...

    But, both of you RJBeery and Pete made some very good critiques...
  10. Pete It's not rocket surgery Moderator

    Wave model.
    Particle model.
    Quantum model.

    Not to be confused, I think.
  11. kurros Registered Senior Member

    Ok so I haven't read everything in this thread, but there seem to be some odd ideas about photons flying around.
    Can we detect 3/4th of an EM wave? Not sure what you mean by this. You can measure the wavelength of a single photon, you just need something like this:
    i.e. some contraption to absorb the photon and use that energy to create an electrical pulse which can be amplified to something useful. The energy of this pulse is proportional to the energy of the photon so you can compute the photon wavelength from it.
    Maybe you are mixing up classical and quantum ideas, perhaps you can clarify what you mean for me. You can't measure the wavelengh of 3/4 of a photon... well not exactly, you can collide a photon with something and steal 3/4 of it's energy, and if you knew it's wavelength in the first place you could calculate the wavelength that it left with. Though I presume that's not what you are talking about.

    Also an odd concept... photons are absorbed or emitted by atoms effectively instantaneously, independently of their wavelength. If you are measuring an ultra-long wavelength radio photon (say 1km wavelength or more), you don't have to wait for the "whole" photon to "reach" you before you can say you caught it all, the wavelength is a thing associated with the wavefunction of the photon, which tells you something about the positions you are likely to find the photon at. The photon itself is a point particle, i.e. it has no size. So if you hit a radio sensitive screen with a 1km wavelength radio photon, it still only makes a tiny blip on the screen, to whatever the resolution of the screen it.
    Last edited: Mar 30, 2011
  12. RJBeery Natural Philosopher Valued Senior Member

    As I said, I'm only explaining things from my world which very well could be filled with "dragons". One thing I'm sure of, though, is that there is ultimately a single description of reality, one which incorporates (or at least explains) the wave-particle duality of things.

    Here's a question, Kurros: can a black-body oven contain photons whose wavelength is greater than the dimensions of the oven?
  13. RJBeery Natural Philosopher Valued Senior Member

    As a follow-up, if this is what the wavelength means to you then, technically, you should arrive at the same conclusion that I did:
    In other words, both of our descriptions would suggest that a red-shifted photon would (on average, in your case) arrive a bit later than if they were not red-shifted.
  14. Pete It's not rocket surgery Moderator

  15. RJBeery Natural Philosopher Valued Senior Member

    Because if you interpret the wavelength as such
    then a longer wavelength increases the possible locations you are likely to find the photon..right? If photon A is emitted from an inertial source and an identical photon B is emitted simultaneously from another source which is local but receding (relative to the receptor), then Kurros seems to be claiming that we can expect both photons to arrive within a period of time that is a function of the distance between the sources and the receptor with a margin of error equal to the photons' respective wavelengths. If this is true then we must postulate that EITHER the red-shifted photon B has a possibility of arriving sooner than A, or that on average it will arrive later than A.

    Listen Pete, just to be clear, I threw my hat in the ring with the caveat that I haven't taken any optics courses so don't interpret what I'm saying as a demand that I'm right. When Kurros objected to my thoughts I sincerely asked about the black body oven, not knowing the answer. After I researched it and discovered I was right (that photons in a black body oven are limited to those with wavelengths less than the dimensions of the oven), it bolsters my belief that photon emission and absorption is not time independent.
  16. kurros Registered Senior Member

    Hmm, well you are correct in the sense that it is possible to measure the position of a short wavelength photon more precisely than a long wavelength photon.

    If your emitter is spitting out little wavepackets of light, I believe that yes it is the case that the redshift due to the receding source will stretch the wavepacket out, increasing the range of possible arrival times of the photon. If you measure the flight time as starting when the centre of the wavepacket leaves the source (for simplicity; I think it would get tricky to consider how you really know when your source has emitted a photon) and your photons are travelling equal distance, then their average flight time should be the same. The redshifted one will just have a larger spread of arrival times. I mean the flight time has to be distance/c on average, regardless of wavelength.

    As for the blackbody thing, sure, if the box is too small you can't fit certain photons in it. I think this can be understood classically, you don't have to think about individual photons. The box only allows certain modes of electromagnetic field to exist in it. I'm not sure what you were getting at though.
  17. siphra Registered Senior Member

    Ok, FTR, infinite space is one theory, it is not a leading theory. Finite volumetric space is one of the other theories. Space IS something, space reacts to the presence and absence of mass. And there was less space in the past than there is now.

    I don't quite know where you get your physics from, but I think you may be either not understanding it, (Or conversely I am willing to open the possibility that I am not... we can let the forum decide that...) or latching on to one theory and accepting it as fact. I consider the last option to be most likely, like all those 'String and M-theorists' out there.
  18. przyk squishy Valued Senior Member

    Possibility #3: the photon got its positional uncertainty from its source (which blurs the idea of emitting two photons simultaneously). In principle the photon interacts in an instantaneous, point-like fashion. I'm no expert in the current experimental limits, but quantum theory at least puts no lower bound on the time needed to detect or otherwise interact with a given photon.

    Also, the wavelength is not necessarily related to the uncertainty in a photon's position or detection time. A photon with an exact frequency and wavelength also has an exact momentum so in theory it could turn up, with equal probability, anywhere in the universe. You'd have zero probability of detecting it in any finite region of space. We don't produce photons with exact frequencies in the lab. Photons produced in the lab are described by wave packets built out of a superposition of frequency modes clustered around some central frequency. The positional uncertainty associated with a photon wave packet is (for a given wave form, eg. Gaussian) inversely related to its spectral width. In practical cases the positional uncertainty of a photon is a lot larger than its (central) wavelength, though in theory there's nothing preventing it from being the same or even smaller.
  19. OnlyMe Valued Senior Member

    Instead of a line by line response, I thought it better to begin with what I hope is a clearer description the basic perspective of my previous post(s).

    Both General Relativity and Newtonian Gravity are gravitational field theories. Einstein's field equations, reduce to agreement with Newtonian Dynamics at non-relativistic velocities and local distances, including planetary orbits. For all practical applications, even sending satellites to other planets, Newton's formulas are used.

    General Relativity is built on a foundation of the assumed facts and conclusions of Special Relativity, applied to a 4 dimensional Minkowski space-time. (I think Minkowski had been one of his professors.)

    Einstein introduced the assumption of the speed of light as a universal constant near the beginning of his 1905 paper introducing Special Relativity.

    The speed of light had already been experimentally determined for a variety of mediums including "vacuum".

    The assumption of empty space..., in the second paragraph of his 1905 paper introducing the special theory of relativity, Einstein makes reference to unsuccessful attempts to discover any motion of the earth relative to the "light medium". An apparent reference to the Michelson & Morley experiments. Earlier that year he had published a paper on Brownian motion, arguing in favor of the existence of atoms. At the time the only subatomic particle known was the electron. So without the ether, "the light medium" and absent atoms, "a vacuum", empty space was empty and the velocity of light a universal constant.

    The Lorentz Transformations predate Einstein's work. They were developed by Lorentz in an attempt to prove the existence of the Luminiferous Aether by explaining the results of the Michelson-Morley experiments. The speed of light as initially used by Lorentz did refer to the speed of light locally. The Lorentz Transformations are responsible for both length contraction and time dilation, of the special and general theories of relativity, though I believe it was Einstein that first recognized the time dilation aspect.

    The assumption that the speed of light is constant, is a crucial component of length contraction. Length contraction is a critical component of curved space and time dilation. General relativity would be seriously challenged should it ever be discovered that the speed of light is variable or only locally constant.

    As a side note: The Michelson & Morley experiments were designed to detect a change in the interference pattern of a beam of light spit and recombined after traveling in two directions, one in the line of the earths motion and one at 90 degrees to that motion. The experiment was sensitive enough that it should have detected the aether as defined at the time. It was also sensitive enough that had there been a red shift, it to should have been detectable as it would also have affected the interference pattern.
  20. RJBeery Natural Philosopher Valued Senior Member

    While I don't disagree with the bolded words, they are basically the source of our consternation (presuming simultaneous emission is achievable). If this were true, though, then red-shifted photons would frequently arrive sooner than their shorter wavelength counterparts according to your description, certainly in violation of c. Sending information at a velocity in excess of c would simply be a matter of sending enough photons such that "one" of them does so, including the possible "instantaneous transmission" of a sufficiently red-shifted photon! The truth probably does lie somewhere in the cloudiness of "when" the photon was actually emitted, as you said, and przyk later echoed. As the scenario currently stands, however, the only reasonable explanation to me is that the red-shifted photons would arrive later because the process of emission and absorption "takes longer" (i.e. it is a function of time, not instantaneous), even though the photon itself travels at c.
    The point is that the material in a black body oven is certainly capable of absorbing any photon whatsoever, and in my opinion the fact that the oven's dimensions determine which photons it contains directly contradicts this statement:
  21. Pete It's not rocket surgery Moderator

    I still don't think that's right, especially if you're talking about changes in the speed of light as it passes through a medium.
    It seems to me that the field equations would still apply if c was a scalar field.
    But anyway, have you considered experiments that explicitly look for non-local variations in the constant c? What do they show?
    Do you have any reason to think that c is not constant?

    What's the point? Should there have been any red shift?
  22. OnlyMe Valued Senior Member

    Second point first. Good catch. Not sure, but if the Lorentz transformations apply there should have at times during the experiment been a length contraction either in the line of motion or opposed to it. That is what Lorentz was trying to prove. Astronomical redshift is explained by an expansion of space, should not then a contraction affect the wavelength also? But it may not have been something that could be separated from the ordinary motion in space.

    I am sure that AE's field equations could model it if we could accurately define the volume shape and variations in density, but it would take a super computer even after having all of the needed data. AE's field equations are a mean test even for the best mathematicians.

    The earlier reference to variations due to changes in the matter density of space came from earlier references, in the thread. I think Rocks? Questioned the idea of gas clouds affecting the velocity and earlier in the thread there was a link to a report on an experiment that slowed and even claims to have stopped a photon. I believe the slowing down part has been repeated but stopping a photon...? I don't know if it has been repeated and confirmed.

    Both issues raised, for me the question whether we could actually assume an empty space constant speed for light. I don't think that in either case they disprove GR. They just make things more complicated.

    My personal opinion is that before we get to a unified theory will we find that both general relativity and quantum theory are incomplete and perhaps not entirely accurate.
  23. Pete It's not rocket surgery Moderator

    The detector was at rest with the emitter, so no, there should not have been any redshift.
    And no, there shouldn't have been any length contraction of any part of the apparatus in the rest frame of the detector and emitter, because they're all at rest with respect to each other.

    Yes, it's complicated. And yes, it is my understanding that the maths has been done to predict how variations in the value of c (or more specifically, changes in the value of the fine structure constant) would show up in observations.

    I'm not completely sure whether variations in the fine structure constant are consistent with GR or not, but observations show that if there are variations, they are tiny.

    Yes, the matter density of space varies. Why is this a problem for GR?
    Yes, the speed of light varies in different media. No, that doesn't change the value of the c in the field equations.

    Yes, it's complicated.

    Yes, that's my understanding of the mainstream.

Share This Page