Jump to content

Banner.jpg.692da9ed9af2eace53e1dc94cfc0e912.jpg

“We don’t really know the speed of light”


Recommended Posts

One way a reflected electromagnetic wave could travel at a different speed is if it travelled through a different medium (gas etc) whilst on it's way back to what it did on it's way there, but that's not really what's being discussed here (same medium there and backuds).

Link to comment
Share on other sites

Physics tends to try to find the simplest possible theories to explain nature. You would need to explain why you think the speed of light should be anisotropic. If we assume it is isotropic and the theories agree with experimental data, then that tends to confirm the assumption that it is isotropic.

Incidentally, my reading of Feynman's book "QED: The strange theory of light and matter" is that there are paths a photon can take in which the speed of light is different, but when you do the path integral over all possibilities, the photon travels through a vacuum at the speed of light we measure (i.e. these other paths make a negligible contribution to the path integral).

Link to comment
Share on other sites

Although it doesn't apply here, the speed of light does vary for the RF carriers being received by your GPS receiver as the carriers make their way through the atmosphere, the time they take does vary depending on what the atmosphere is doing at the time, the different atmospheric layers slightly alter the speed of light in varying amounts.

The RF carriers time of flight measurement is a one way speed of light measurement, it's not a round trip measurement. The GPS satellites take into account the speed of light variations through the atmosphere to allow our little GPS receivers to make as accurate a position calculation as possible. The varying speed of light through our atmosphere is continuously measured by the satellites and ground crew. 

As we don't live in a vacuum, it doesn't apply to the threads topic really, but GPS receivers are doing measurements based on one way electromagnetic wave transmissions and their slightly varying speed.

Link to comment
Share on other sites

Are there any examples of phenomena that are known to display anisotropic properties with respect to their speed and propagation through a medium?

Jim 

Link to comment
Share on other sites

With RF and microwave signal handling a common component is a circulator.  This is a device with 3 or more signal connections (ports).  For a 3-port device, for example, if a signal is sent to port 1 almost all of the signal will come out of port 2 and hardly any out of port 3.  But if a signal is sent to port 2 almost all of the signal with come out of port 3 and hardly any out of port 1.  These devices are often made from material which is anisotropic.  A simpler version of the same thing is a 2 port device called an isolator where almost all the signal input to port 1 flows to port 2 but almost no signal input to port 2 flows to port 1.  

Another example is the ionosphere where two-way radio operators in the 20'th century noticed that occasionally one person could hear the other but not vice versa, which was traced to anisotropies in the ionosphere.

In all those cases, the effective permeability and permittivity of the medium is direction-dependant and so, in general, the speed of light will be direction-dependant for these situations.  

  • Like 1
Link to comment
Share on other sites

2 hours ago, saac said:

Are there any examples of phenomena that are known to display anisotropic properties with respect to their speed and propagation through a medium?

Jim 

Yes biefringent crystals like calcite . Different polarisation of light have different refractive indices.  It is due to their anisotropic crystal structure. Regards Andrew 

Edited by andrew s
Link to comment
Share on other sites

14 minutes ago, andrew s said:

Yes biefringent crystals like calcite . Different polarisation of light have different refractive indices.  It is due to their anisotropic crystal structure. Regards Andrew 

Thanks Andrew. You know I never made that connection before but it makes sense now when you explain it like that. I've seen bifringent crystals under polarised light in the past in context of polarisation effects behaviour  but never thought "why".  Too busy looking looking at the polarisation rules I guess, and the pretty colours.   So would that also be the same process going on with the likes of some transparent plastics and  sellotape doubled up on itself and viewed under polarised light.  Are the plastics/folded sellotape behaving like a biefringent crystal then, perhaps presenting different paths/refractive indices for the light ? In engineering design,  prototype components used to be modelled in transparent plastic then loaded under stress and viewed under polarised light. The resulting coloured patterns would show the internal distribution of the stress, identifying points of potential failure.  I think this technique has been largely replaced now with computer modelling, the software having advanced so much. 

Jim 

Link to comment
Share on other sites

  • 1 month later...
On 18/01/2021 at 21:31, Ouroboros said:

Eh?  How have I not heard of this before?  I came across this today in the article linked below. 

“The speed of light is an assumption, not a certainty. It’s an assumption in that we have never experimentally measured the one-way speed of light. The best we have been able to do, even with our most sophisticated modern technology, is measure the roundtrip speed of light and assume that the speed is the same in both directions.”

We don’t really know the speed of light.

I would be interested to read people’s comments. 

Hi

The issue is as follows, Einsteins original paper states this is  an assumption (by definition) !

'We have not defined a common “time” for A and B, for the latter cannot be defined at all unless we establish by definition that the “time” required by light to travel from A to B equals the “time” it requires to travel from B to A. '

https://www.physics.umd.edu/courses/Phys606/spring_2011/einstein_electrodynamics_of_moving_bodies.pdf (page 3 top para)

I don't think many dispute or believe c is different in different directions, but it's there in black and white, you can't prove it, you assume it. You always need a two way trip, even MM relies on mirrors

There is a good Veritasium on this 

 

 

 

 

 

 

Link to comment
Share on other sites

53 minutes ago, billhinge said:

You always need a two way trip

No you dont. See my suggestion here (and subsequent discussion) which only relies on space being isotropic.

and continued here

Cheers

Robin

Edited by robin_astro
Link to comment
Share on other sites

you are missing the point, sure you can calculate a value for c from other formula such as Maxwells eqns  etc but the question was about measuring directly using distance and time

- refer to Einstein

I can (and most others do) measure the square mass of a neutrino to be a negative number but we choose to say the error on the calculation is just enough to to make it a small positive number and hence not an imaginary mass when square roots are taken. This was my final year grad project study on radioactive decay many years ago, neutrino mass being an easily calculated by product based on conservation of momentum. But it wasn't a direct neutrino mass calculation.

Link to comment
Share on other sites

4 hours ago, billhinge said:

you are missing the point, sure you can calculate a value for c from other formula such as Maxwells eqns 

No I am not. The original hypothesis at the top of the thread  was that it was impossible to measure the speed of light in any one direction. My method (looking for any change in the diffraction pattern produced by a beam of photons sent in different directions) just uses a clock (the frequency of the photon generated by a transition of an electron between two energy levels in an atom),  a yardstick and geometry.  The only formula is v=lamda*f   which identical to that used to calculate the velocity in the return trip experiment (v = 2d/t) except in my experiment the beam is only sent one way. If the velocity of light was direction dependent the wavelength (as measured in the diffraction pattern) would be different. It is not, therefore the conclusion is that either the speed of light is independent of direction,  or  space is not isotropic such that the measurement of distance is in some way directional, (and in some peculiar way, since the diffraction pattern is produced orthogonal to the direction of the light beam)  You are welcome to formulate such an alternative universe and propose how it may be tested.

Cheers

Robin

Edited by robin_astro
Link to comment
Share on other sites

5 hours ago, billhinge said:

I can (and most others do) measure the square mass of a neutrino to be a negative number but we choose to say the error on the calculation is just enough to to make it a small positive number and hence not an imaginary mass when square roots are taken. This was my final year grad project study on radioactive decay many years ago, neutrino mass being an easily calculated by product based on conservation of momentum. But it wasn't a direct neutrino mass calculation.

That is a "straw man" argument, irrelevant to the discussion here. What you are talking about there is experimental uncertainty. It is in principal possible to measure the mass of the neutrino (and possibly will done be at some point in the future) The assertion made at the top of this thread was that it was impossible in principle to determine if the speed of light was the same independent of direction since it could only be measured in a round trip.  I offered a way that it could in be measured one way (in principle and in practise to any degree of precision one chose)

Cheers

Robin

Link to comment
Share on other sites

4 hours ago, robin_astro said:

No I am not. The original hypothesis at the top of the thread  was that it was impossible to measure the speed of light in any one direction. My method (looking for any change in the diffraction pattern produced by a beam of photons sent in different directions) just uses a clock (the frequency of the photon generated by a transition of an electron between two energy levels in an atom),  a yardstick and geometry.  The only formula is v=lamda*f   which identical to that used to calculate the velocity in the return trip experiment (v = 2d/t) except in my experiment the beam is only sent one way. If the velocity of light was direction dependent the wavelength (as measured in the diffraction pattern) would be different. It is not, therefore the conclusion is that either the speed of light is independent of direction,  or  space is not isotropic such that the measurement of distance is in some way directional, (and in some peculiar way, since the diffraction pattern is produced orthogonal to the direction of the light beam)  You are welcome to formulate such an alternative universe and propose how it may be tested.

Cheers

Robin

You are using a 'diffraction grating', which is essentially the same as a mirror at the atomic level. ie photon scattering by atoms hence not a one way trip - refer to Feynman QED, you have chosen to adopt a classic approach which still gives the same answer (nothing wrong with that)

Indeed the photon being diffracted and measured is not even the original one being emitted. 

How does your method differ from the MM expt which also uses mirrors?

I don't say the speed of light 'isn't' isotropic so why do you assert I'm a believer of  alternative universes and physics? Proof and belief in assumption/axioms are subtly different

Link to comment
Share on other sites

 

8 hours ago, billhinge said:

I don't say the speed of light 'isn't' isotropic so why do you assert I'm a believer of  alternative universes and physics? Proof and belief in assumption/axioms are subtly different

Sorry if I implied this, it was not my intention.  I invited you to demonstrate how a universe where the speed of light depends on the direction of measurement could be compatible with the results of my experiment which appears to suggest that either the speed of light is not direction dependent or as Andrew proposed, space is anisotropic (so the distance between two  objects stationary relative to each other depends on the direction of measurement).  Either viewpoint may be valid as Andrew proposed but personally I would then apply Occam's razor and chose  the simplest solution which is space is isotropic and the speed of light is not direction dependent)

The arguments put forward in the videos in the thread to support the impossibility of measuring c using one way time of flight measurements do not involve quantum mechanics. (Once we introduce quantum effects all bets are off 😉) They revolve around the impossibility of two observers agreeing on a common time, (which is correct and a consequence of relativity) and hence assert that any experiment to measure the speed of light must involve a round trip leading to the conclusion that the speed in each direction cannot be independently determined.  There is an important difference however between my experiment and one and two way time of flight measurements. My method uses one clock (The frequency of the light) and a unidirectional light beam (lamp, transmission diffraction grating, camera rigidly mounted in line, a fixed distance apart)

You can see the apparatus I use (an astronomical grism spectrograph with a built in calibration lamp) here 

https://www.shelyak.com/wp-content/uploads/Alpy600Demetra-510x320.png

The question I pose is why would this measure the same wavelength of the light independent of the direction it is pointing in if the speed of light is direction dependent ?

Cheers

Robin

Edited by robin_astro
brackets for clarity
Link to comment
Share on other sites

21 hours ago, robin_astro said:

 

Sorry if I implied this, it was not my intention.  I invited you to demonstrate how a universe where the speed of light depends on the direction of measurement could be compatible with the results of my experiment which appears to suggest that either the speed of light is not direction dependent or as Andrew proposed, space is anisotropic (so the distance between two  objects stationary relative to each other depends on the direction of measurement).  Either viewpoint may be valid as Andrew proposed but personally I would then apply Occam's razor and chose  the simplest solution which is space is isotropic and the speed of light is not direction dependent)

The arguments put forward in the videos in the thread to support the impossibility of measuring c using one way time of flight measurements do not involve quantum mechanics. (Once we introduce quantum effects all bets are off 😉) They revolve around the impossibility of two observers agreeing on a common time, (which is correct and a consequence of relativity) and hence assert that any experiment to measure the speed of light must involve a round trip leading to the conclusion that the speed in each direction cannot be independently determined.  There is an important difference however between my experiment and one and two way time of flight measurements. My method uses one clock (The frequency of the light) and a unidirectional light beam (lamp, transmission diffraction grating, camera rigidly mounted in line, a fixed distance apart)

You can see the apparatus I use (an astronomical grism spectrograph with a built in calibration lamp) here 

https://www.shelyak.com/wp-content/uploads/Alpy600Demetra-510x320.png

The question I pose is why would this measure the same wavelength of the light independent of the direction it is pointing in if the speed of light is direction dependent ?

Cheers

Robin

Thank you, nice website! Let's go back to the beginning. The article states

'The speed of light is an assumption, not a certainty. It’s an assumption in that we have never experimentally measured the one-way speed of light. '

I responded to say, yes, in fact its baked into Einsteins original paper! (I also posted a video that discusses this, the fact that QED, not just Quantum Theory but Quantum Field Theory is not mentioned is not relevant as we were just talking about the challenge of directly measuring A to B vs B to A )

Most reasonable people would agree that the universe is isotropic (you quote Occam's razor which is a reasonable response), but the issue is do you accept that it is isotropic based on faith or do you try to prove it?

The article and my response asserts that the speed of light in one direction can't be measured directly measured.

No one disputes that you can calculate 'a value' from  an equation which includes fundamental parameters but it doesn't demonstrate experimental measurement of the one way speed of light

Your counter-argument seems to be that you have devised an experiment to measure the one way speed of light

My response was, you haven't since you have used a diffraction grating which is just a specialised mirror - these effects are fully described by QED (see 'The strange theory of light and matter' by Feynman)

True, using QED to explain diffraction, reflection and other effects involving light interacting with matter is using a sledge hammer to crack a nut, but Huygens etc are just analogies that give the same results in a way you use Newtonian mechanics rather than general relativity geodesics to describe earthly motion of projectiles. But don't mistake Huygens and Newton for reality. When you know that light is being affected by matter you need to understand mirrors are quantum mechanical in reality! (interestingly QED explains why your telescope secondary doesn't block the star in the centre of your field of view or why holes don't leave gaps in your telescope image - go read reddit or cloudy-nights if you want a laugh at the hand waving arguments). Since we are really in the realm of QED, we know any measurement affects the original state.

So to summarise, your use of a specialised mirror means you haven't measured the 'one-way speed of light. '. I don't dispute the accuracy of your measurements and what you have created with your website etc is highly commendable. Strictly speaking since diffraction gratings are made of atoms you should also try gratings of different materials to see if there is any effect there

I'm sure I won't convince you but it may convince some to dig into this subject further,  🙂 (the feynman book is cheap and part text chapters, part maths, path integrals, probabilities and Feynman diagrams and there are some semi technical videos on the subject)

cheers 

steve

 

Edited by billhinge
Link to comment
Share on other sites

Hi Steve,

No need to patronise me. I am a career hardened  physicist happily living in a quantum mechanical and relativistic universe  😉  

The equipment is just an off the shelf astronomical spectrograph and my measurements were just to test its mechanical stability under different orientations, not to make some profound physical insight. I saw this thread and saw the measurements as an interesting  alternative perspective on the measurement of the speed of light which does not have the difficulties of  round trips or synchronised clocks covered in the article and videos, though it may have other difficulties which we are exploring here (It is clearly a fully one way measurement, no reflection, no return path. The light leaves one end and arrives at the other where the wavelength is measured from the diffraction pattern after being dispersed by the transmission grating. I don't understand why you are still disputing that). 

 According to the measurements, multiplying two independent locally measurable physical properties of electromagnetic radiation, frequency and wavelength gives a constant value, v which  is independent of the orientation.  (The experiment could be made more robust by actually measuring both wavelength and frequency at both ends of the leg rather than assuming the frequency is unchanged. Although this would need two clocks, importantly there is no requirement for them to be synchronised as in the time of flight measurement, just for them to run at the same rate which would be the case for two clocks at rest relative to each other as here, according to special relativity ) 

You offer QED as an explanation as to why this value v may not be the velocity of light and which might be constant even in the case of a universe where the speed of light is different dependent on the direction of travel of the light beam but I have only seen generalisations so far. (The quantum effect of the observer on the measurement and the properties of the grating are spurious here. I don't understand why you would introduce these except to muddy the waters)

I fully expect there is some reason it would give the same result even in a universe where the speed of light is directional  but as an experimental physicist I have the luxury of saying "here are my measurements, explain them 🙂"

Cheers

Robin

 

 

Link to comment
Share on other sites

36 minutes ago, robin_astro said:

Hi Steve,

No need to patronise me. I am a career hardened  physicist happily living in a quantum mechanical and relativistic universe  😉  

The equipment is just an off the shelf astronomical spectrograph and my measurements were just to test its mechanical stability under different orientations, not to make some profound physical insight. I saw this thread and saw the measurements as an interesting  alternative perspective on the measurement of the speed of light which does not have the difficulties of  round trips or synchronised clocks covered in the article and videos, though it may have other difficulties which we are exploring here (It is clearly a fully one way measurement, no reflection, no return path. The light leaves one end and arrives at the other where the wavelength is measured from the diffraction pattern after being dispersed by the transmission grating. I don't understand why you are still disputing that). 

 According to the measurements, multiplying two independent locally measurable physical properties of electromagnetic radiation, frequency and wavelength gives a constant value, v which  is independent of the orientation.  (The experiment could be made more robust by actually measuring both wavelength and frequency at both ends of the leg rather than assuming the frequency is unchanged. Although this would need two clocks, importantly there is no requirement for them to be synchronised as in the time of flight measurement, just for them to run at the same rate which would be the case for two clocks at rest relative to each other as here, according to special relativity ) 

You offer QED as an explanation as to why this value v may not be the velocity of light and which might be constant even in the case of a universe where the speed of light is different dependent on the direction of travel of the light beam but I have only seen generalisations so far. (The quantum effect of the observer on the measurement and the properties of the grating are spurious here. I don't understand why you would introduce these except to muddy the waters)

I fully expect there is some reason it would give the same result even in a universe where the speed of light is directional  but as an experimental physicist I have the luxury of saying "here are my measurements, explain them 🙂"

Cheers

Robin

 

 

How am I patronising you? I don't know your qualifications

I'm just trying to be civil and clear and I'm not just writing to you personally but to all the other readers of this thread 

I thought I was clear,  a spectrogram is a special mirror therefore it fails the test of direct measurement since your system source, mirror, detector.,the same as a traditional 2 direction measurement of light. QED describes the spectrogram part and why you are not measuring a direct A->B measurement

Anyway I've said what I intended, so best call it a day

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.