Listen Now Download as mp3 from the show What does Falling into a Black Hole Feel Like? QuestionRobert de Vos asked: We know that light is composed of photons that move at 300,000 km per second in a vacuum. If I shine a light though a 5cm sheet of glass, it will slow down and also heat up the glass as it passes through. As it leaves the glass, it then speeds up again, Now where does the energy come from to speed it back up to 300,000 per second?
One could assume that the continuous flow from the source achieves this, but what would happen if one directed a single photon through the glass sheet?
Would it still lose energy to the glass as it passes through and then as it emerges, how could it find the energy (from nowhere) to speed back up to 300,000 km per second?
AnswerWe put this question to Cambridge University physicist Zephyr Penoyre... Zephyr  When it goes into the glass, it’s moving slower. But also, because the frequency has to be the same, the same waves have to be coming into the glass at the same rate as they're coming out, otherwise you’ve lost waves somewhere along the way… Chris  And it’s changed colour. Zephyr  And that’s because wavelength has changed. So, the energy of the light is to do with the wavelength of the light. So, as it slows down, the energy gets higher  because the wavelength has shrunk  so the total amount of energy passing through the glass is exactly the same as it was going through the air, it's just moving at a slightly different speed. Multimedia
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CommentsPhotons never move at other than light speed the apparent slowing down is due to the tortuous path they take being absorbed and reemitted by atoms en route. Re the speed of light in general  I have asked elsewhere on this forum about the scientist who has slowed down light and even stopped it at one point, and why this is not being celebrated/discussed/questioned/debated/explored more. Is this scientist's discovery (Lene Haus) not that important after all? I thought that showing Einstein was wrong would be a big deal. annie123, Fri, 14th Mar 2014
It doesn't. Light which exits the material it's not the same light which enters and which is inside: the em field which exits is generated by the last charges inside the material. At the event horizon of a black hole the gravitational field exhibits a critical density like a 'perfect' medium. Perfect in respect of its ability to stop light. The photon's energy is effectively canceled exactly at the horizon. At this point the electromagnetic and gravitational fields are at equilibrium. Not in the sense of a stationary point but in the balance of energies. This does not help unless observational data can be collected from the immediate environment surrounding a black hole. A collision with the horizon of Sag A* is the best chance of obtaining this data. jeffreyH, Thu, 18th Jun 2015
Reading through the views expressed, there seemed to be three alternative explanations put forward.
I have never read anywhere that wave amplitude is a factor. Do you have any references for that? jeffreyH, Fri, 24th Jul 2015
"Translucent"? it's not the correct term."sinusoidal wave model of a single photon"? It doesn't exist. Maybe you intended something else.No, as others already wrote, in an electromagnetic wave, not only wavelength λ and frequency f are related, but radiation velocity (precisely phase velocity vp) too:
Presuming light has a speed and it is not something in space that is being energized at a speed such as CBMR. Thebox, Sun, 26th Jul 2015
I'm about to start reading up on Hilbert spaces. jeffreyH, Mon, 27th Jul 2015 When you calculate the path and speed of a photon through a block of glass, you must do a calculation at the boundaries of the glass block, because the conditions change (ie glass has a higher refractive index than air), and this affects the speed of the photon. It does not matter whether you are using classical optics or infinitedimensional Hilbert spaces to do the calculation. One thing that is significant about the answers given here is that they either tend to ignore physical facts as in the posts favouring the propagation of light through a substance in accordance with Maxwell’s equations or they give static answers which ignore the dynamics present. Too often (almost exclusively) the emission and absorption of photons by electrons in an atom is treated as a one off case, an electron within the atom absorbs and then emits an electron, this does not fit in with the observed data Firstly the definition of spectroscopy is as follows: Good. Let me know what you grasped (or not). No, if you use classical optics and/or classical electrodynamics you calculate the path, speed, phase, etc, of a light beam or of an em wavefront, not of a photon, because "photon" is a quantistic concept which have to be treated quantistically. Infact in all of these discussions we could better never talk of photons at all; to talk of photons it's a sort of "trend".
Good. Let me know what you grasped (or not).
It has been mentioned above as to an exchange of energies between the light and the medium it is passing through affecting frequency. You are misunderstanding what a photon is and how it travels, if you look on my post TOE fun you will I have demistified it and that includes the slit experiment apparent wavelike effects as well and much more. Aquarius, Tue, 4th Aug 2015 Erm...Nope,sorry I am not misunderstanding the photon or how it travels. What I am doing is extrapolating a notion regarding Planck's h constant with regards to measuring frequency and relating it to quantum light theory...but I'll gladly check out your thread. :) timey, Tue, 4th Aug 2015
As a start you need to read and understand these two pages. As far as I am aware JefferyH, I am not questioning the mathematical structure of the Dirac delta function or of the momentum operator. These are considerations that were formulated after Plancks discovery of quanta and his dirivation of the h constant which is an action of a time or a length, whereas Planck tried, to no avail, to iron out the quantum nature of his findings.
Nope, I'm not confusing these relativity/space time related considerations of proper time or coordinate time. I do have my own ideas on time  but this thread is not my thread, nor is it in the new theories section...therefore for the purposes of this conversation I think it within the remit of accepted physics to say that we have time as a measurement and, as a separate issue, that we observe the occurrence of the phenomenon of time... and that this time is subject to change in the rate of its occurrence due to changes in the gravity field.
JeffreyH :) You wouldn't be the first person to say I have trouble expressing myself. Thank you very much for the positive comment! timey, Wed, 5th Aug 2015 A further point of interest might be found in the consideration of whether or not the changes in the rate of time themselves are delivered in quantum packages... Anyway, since no one wishes to 'engage' in actual discussion about my speculations, I will make this my last post here on the subject. I do monologues in my head all day long. It's boring.
Well yes... The best I can do is reiterate a passage from an earlier post... Bill... I've been giving some thought to your comment on coming up with a sound argument in favour of the quantum world being subject to a different time structure or different time structures than our own, ie: a different rate or different rates in the occurrence of time.
If we move away from photons for a second and consider particles with rest mass we can propose something interesting. I will state straight away that a particle with rest mass, if it could travel at c, cannot possibly have angular momentum. This is because the velocity of angular momentum would be summed to the straight line velocity giving an overall value > c. This reduction of angular momentum should relate directly to time dilation and should also be present in a gravitational field. jeffreyH, Mon, 10th Aug 2015 This brings up the possibility that in the total absence of gravitation the photon would have no angular momentum. jeffreyH, Mon, 10th Aug 2015
But it can't, so every consequence of an incorrect assumption have to be incorrect.This is not true, velocities composition is not a mere sum, relativistically. But lightarrow, what if they could? Do you mean if you can accelerate a massive object to light speed without having to use infinite energy? And what if we all were blue angels with green wings and bright eyes? If physics doesn't count, everything is allowed, isnt'it? Erm, no lightarrow... what I was thinking about (in relation to Jeff's 'experimental speculation') with regards to particles with rest mass  was a situation where a very massive body (black hole) jets very small bodies of mass (particles) into space  potentially at a rate faster than the light speed we experience on earth. This being on the basis  (again in relation to Jeff's 'experimental speculation' on light experiencing no angular momentum in a 0 gravity field)  that the speed of light, in relation to a more massive body of mass than earths, 'may' (speculative) be going faster. This being 'a' logical progression of such speculations...and that these particles in that situation would have no trouble reaching 'our' speed of light, but would not be able to reach the faster speed of light of their own reference frame (this being the black hole), for the same reason that the same particles could not reach the speed of 'our' light in our reference frame of earth.
True enough... However the stark, in your face facts remain that quantum, all these years later is not reconciled with gravity, relativity, all these years later does not 'fully' describe the universe, and Planck's h constant was derived before it was possible to 'measure' time and time dilation with such great accuracy... And LHC, despite its multitude of funding, hasn't really brought the situation much further along atall considering...
But it can't, so every consequence of an incorrect assumption have to be incorrect.This is not true, velocities composition is not a mere sum, relativistically.
No, you are wrong. They exist as numbers, but not only! Quantum description of nature is impossible without them and this suggests they are more deeply "ingrained" with reality.You are making a big mistake. What is real is the result of a measure of an observable; since observables are associated with hermitian operators and these have (mathematically) real eigenvalues, the result is a real number (not a complex one).But, be it a spiral or what you want, in relativity, and I've already wrote it, velocities don't sum up. Well lightarrow, to say so I'm going to reply to your post above even though it isn't directed at me, because that is something I can do, in the hope that Jeff is busy doing some mathematics, which I can't do. Yes you are correct that in relativity it is more complex. Imaginary numbers exist, as numbers; what do you mean with "imaginary concepts"? In physics we can talk of what we know, not of what we don't know. But physics evolves and if now we say that a photon is massless, it doesn't mean "it's established that it will be so forever"! Physics (differently from phylosophy or else) can only describe what is known at the moment; everything else is speculation and not physics.It may turn out everything, so this reasoning is meaningless.If an object moves with velocity v1 with respect to a frame of reference S and S moves at velocity v2 with respect to another frame S' (v1 and v2 be parallel) then the object moves with respect to S' at velocity: Ok, well dark matter (which may have been observed  as yet unproven) and dark energy are imaginary concepts based on mathematical necessity. It is only 'when' we observe their reality that they will come out of the 'theoretical' region. Anything unobserved is an imaginary concept. Any imaginary concept can have a basis in mathematics whether is becomes necessary as a result of mathematical necessity or as a theoretical necessity. For instance we observe redshift and we have theorised that the fact of redshift means that a light source is moving away from us. Although this concept is widely accepted as fact, this being because it is 'the' most logical reasoning in relation to our current understanding, but it is still an imaginary concept nontheless. Other theories concerning redshift have been extrapolated...I really can't be bothered to relate them here :) , suffice to say there are other imaginary concepts attached to the observation of redshift. I could go on...and on :). Firstly Timey I didn't say that light took a spiral path. Secondly you cannot simply use speculation to formulate hypotheses. Most of the discoveries in physics followed on from experimentation where the equations were derived to understand the mechanism at work. Not the other way round. Thirdly don't follow me. I am untrained and this is just a hobby for me. The way I do things is not the correct way as understood by professional physicists. I have studied the areas I need to for those things I am interested in at the time. If you have an interest in quantum physics then try to find out why \sigma_1 n_1 + \sigma_2 n_2 + \sigma_3 n_3 is important and what it means. It is to do with the electron. Also try to find out what <ab><ab>* means. It is to do with probability. These are not difficult and are just linear algebra (matrices). It will surprise you. You will also learn what a complex conjugate is and how it relates to the complex plane. jeffreyH, Fri, 14th Aug 2015 Firstly Jeff, I'm not 'following' you. LOL !!! ...did you think you were guru type material??? To sum up the world of quantum, on a basic level there isn't really a lot to it. Particles have spin, spin can be oriented. If you orient a particle it's spin will remain oriented in that direction. Therefore, we can harness the electron to our purpose. We have to take round the houses methods in order to calculate quantum through probability because p x q does not sum up to the same as q x p, this being because when attempts are made to measure simultaneously a pair of conjugate variables: position and momentum or energy and time, the limitations of these concepts become evident. Look Jeff, having evened my temper with an egg and bacon sandwich, and a cup of tea, I am back to tell you that you are right in the fact that I read your posts. But I also read a lot of other peoples posts too. It's not everybody's posts that I come back to though. Interestingly enough it is for the very reasons you say I should stay away, that I do come back to your posts. My only complaint being that you do not include more written explanation of your intent, direction, and findings in words. I think even a mathematician 'might' agree. timey, Fri, 14th Aug 2015
Which is why I have been using the internet extensively to visit places like Stamford University to study advanced mathematics with the benefit of explanation in words. However...yes... I'm sure that I 'have' missed some of the subtleties of, in particular, the GR field equations. But Jeff, it's one thing to understand these mathematics and quite another to then turn them to one's own purpose... Actually, on reflection I think it fair to tell you that it was the fact that your post suggested that I should further my idea through investigation of the calculation of quantum probability that I found annoying, as this is entirely contrary to the very nature of my suggestion. However it does occur to me (slowly, I admit) that the implications of my notion have perhaps evaded you... timey, Sat, 15th Aug 2015 What notion would that be then? jeffreyH, Sat, 15th Aug 2015
Yes. Heisenberg's uncertainty principle to be precise. timey, Mon, 17th Aug 2015
"The uncertainty discovered by Heisenberg is an intrinsic feature of reality. There could be no improvement, he argued, on the limits set by the size of Planck's constant and enforced by the uncertainty relations in the precision of what is observable in the atomic world. Yeah, I know that all too well as any physicist would, i.e. it's the canonical commutation relation for position and momentum (which are canonically conjugate observables). However it's not Heisenberg's uncertainty principle. It's barely related to it either. The only connection is as I'll explain below. I'm sorry Pete, but I actually did answer your question of "do I mean "equal?"... But it's not true the opposite, infact you can sum up something which is not equal, for example 2+5. In our case we have the operator pq which is not equal to the operator qp. Does it mean they don't sum up? No! Infact they can. Theyr sum is called "anticommutator of p and q" and is a well defined operator. Lol! Are we actually debating the definition and use of words here???
Thanks! Advice taken, 'sum up' has now been banished from my vocabulary with regards to mathematical definition of 'equal'. We could get into a discussion about the term 'equals' in relation to a 'summing up', but you know...perhaps there 'are' better things we could do with our time...
I cannot seem to "quote" you any more... Look Jeff, you haven't responded, which surprises me because logically speaking it is your only move. A move that, it may surprise you, would be received amenably by me.
Oh, you know what Jeff... forgetta'bout'it! ...It's all past tense far as I'm concerned. :) timey, Wed, 26th Aug 2015 Lee Smolin's book "The Trouble with Physics"...page 255256 
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