9 ms·
One thing he sort of implies but doesn't directly state -- which I think a lot of people don't know -- is that it's impossible to measure the "one way" speed of
by empath-nirvana 3y ago
One thing he sort of implies but doesn't directly state -- which I think a lot of people don't know -- is that it's impossible to measure the "one way" speed of light. We can only measure the speed it takes for light to go "there and back". It's possible (but not likely), that light goes faster in one direction than in another direction, and AFAIK, there's no possible way to measure it. You'd think you could do it based on clock synchronization, but clock synchronization itself depends on the assumption that the speed of light is equal both directions.
https://en.wikipedia.org/wiki/One-way_speed_of_light https://en.wikipedia.org/wiki/One-way_speed_of_light
- readams 3y agoAlso a Veritasium video on the subject: https://www.youtube.com/watch?v=pTn6Ewhb27k https://www.youtube.com/watch?v=pTn6Ewhb27k
- andersa 3y agoWhy doesn't it work to have the emitter and sensor together at the same location, synchronize them at that moment, and then move them apart a distance so large the initial delay no longer matters, before running the test? Do we not have accurate enough clock sources to keep the synchronization?
- Tagbert 3y agoMoving the emitters affects the pace of time for them.
- AnimalMuppet 3y agoRight, so move them at identical speeds, with identical acceleration profiles.
- rjp0008 3y agoIf you do this, the clocks will be in the same place, it would have to be opposite acceleration profiles to get them moving away from each other.
- JumpCrisscross 3y ago> move them at identical speeds, with identical acceleration profiles Now do GR.
- AnimalMuppet 3y agoThat's the reason for the identical acceleration profiles. Which is totally obvious, so I suspect that means that I missed your point. Could you clarify?
- JumpCrisscross 3y agoMass and energy curve spacetime. So you could accelerate two clocks identically and still have to correct for nearby mass and energy.
- AnimalMuppet 3y agoWell, sure. I was thinking of stations that are a few tens of km apart on a flat region of Earth, so I don't think that would be much of an issue.
- lazide 3y agoDepends on how many digits of precision
- adgjlsfhk1 3y agothe assumption that time dilation is identical for the same acceleration profiles is equivalent to an assumption of the 1 way speed of light. if you do the full math with a non constant light speed, you find that degree of asymmetry in the 1 way speed of light directly cancels the difference in time dilation
- AnimalMuppet 3y agoHow can that work? I can control both how hard I accelerate, and how far I go with constant velocity. So I can control how much of the clock skew is due to acceleration time dilation. That can't match the speed difference for all possible experimental setups. For experiment 1, say I accelerate the clocks at 0.1 g for 10 seconds, then drive with constant velocity for 1 hour, then decelerate at 0.1 g for 10 seconds. That acceleration time dilation exactly matches the change in propagation time due to the difference in c? Fine, I'll give you that. So for experiment 2, I drive at constant velocity for two hours. I've kept the acceleration time dilation the same as in experiment 1, but doubled the distance. If the change in propagation time matched in experiment 1, it can't match now. Or, for experiment 3, I accelerate at 0.1 g for five seconds, reaching 1/4 of the previous velocity, then drive for two hours. Now the propagation difference is the same as in experiment 1, but the acceleration time dilation is different. So how is this going to come out "you can't tell" in all three experiments?
- AnimalMuppet 3y agoWell, how about this: I have a central facility. In that, I synchronize several clocks. I then slowly move them to satellite facilities in opposite directions. I don't move all the clocks by the same path - some go via triangular routes rather than directly. If all the clocks agree at the satellite facilities, then I have established that space is isotropic for the slow transport of clocks (or at least, it is isotropic for the paths chosen - a skeptic can always device a "sufficiently smart anisotropy" that would appear to be isotropic for the paths chosen). Per the article, that was one of the assumptions that couldn't be trusted, but if we can experimentally establish it, we can trust it. We now have synchronized clocks at the two satellite facilities. (We know they're synchronized because we established that space is anisotropic to the slow transport of clocks, and also because at least some of the clocks were transported with identical profiles in opposite directions.) We can now use time of receipt minus time of transmit to establish the one-way speed of light.
- mecsred 3y agoHow do you measure if the clocks agree or not after you move them? You can try and synchronize all the moved ones at point B, but how do you measure their relative timing to A clocks without relying on the speed of light between A and B.
- floxy 3y agoYou could bring clocks A & B back together again.
- dilyevsky 3y agoAnd by doing this you reversed direction and didn’t actually measure “one way”
- floxy 3y agoSynchronize A & B in one location. Move A & B apart in a careful manner. Send light pulse from A and record time-stamp on A's clock when pulse sent. When pulse is received at B, record time stamp on B's clock. Return clocks A & B together (in a careful manner) to confirm they are still in sync. Compare time stamp between A's transmission, and B's reception. Who knows, maybe when you bring them together, clocks A and B aren't in sync, due to some twin-paradox thing. Maybe you can't be careful enough.
- seiferteric 3y agoIs it possible to use redshift of photons from the sun? Say if you know the hydrogen transition line frequency, then measure very precisely the observed frequency from solar photons, you could calculate the redshift. I suppose this would rely on knowing the mass of the sun already as well.
- Someone 3y agoI think redshift correlates with relative velocity and expansion of the universe, not with distance. https://en.wikipedia.org/wiki/Redshift https://en.wikipedia.org/wiki/Redshift: “The main causes of electromagnetic redshift in astronomy and cosmology are the relative motions of radiation sources, which give rise to the relativistic Doppler effect, and gravitational potentials, which gravitationally redshift escaping radiation. All sufficiently distant light sources show cosmological redshift corresponding to recession speeds proportional to their distances from Earth, a fact known as Hubble's law that implies the universe is expanding.”
- seiferteric 3y agoI thought so as well but recently discovered: https://en.wikipedia.org/wiki/Gravitational_redshift https://en.wikipedia.org/wiki/Gravitational_redshift "gravitational redshift (known as Einstein shift in older literature)[1][2] is the phenomenon that electromagnetic waves or photons travelling out of a gravitational well (seem to) lose energy. This loss of energy corresponds to a decrease in the wave frequency and increase in the wavelength, known more generally as a redshift. "
- gizmo686 3y agoIsn't this essentially essentially the ether theory of light? You can measure the round trip of light along two different axis. If the speed of light was dependent on direction, you would expect these results to differ. It is possible that physics conspires such that the speed of light is direction dependent, but that it averages out if half your path is the exact opposite direction from the other. I think this can be excluded by comparing more complicated paths; although the nessesity for it to form a closed loop might be give physics an unavoidable out if it really wanted to mess with us. There are also theories where the speed of light differs based on direction; but space itself differs in the same way, canceling the effect. These are fundamentally equivelent to a theory where both are constant.
- ForOldHack 3y agoNo, it is not the ether theory of light, and that is a magnificent question: This idea was settled by Mickelson/Morley, who not only measured the round trip in one direction, but measured it in several directions, and found the speed of light to be invariant. For which Albert Einstein received the Nobel Prize for his theory of light. Later, Richard Feynman used first principals to both confirm this for Enistienien physics, and break it for quantum physics. The best reference for this work is not the classic experiment, but on Henry Cavindish's balance, which led to the calculation of G, the gravitation constant to 7 digits of accuracy, based upon the speed of light calculated to 9+ digits of accuracy. The speed of light is invariant: What you the observer actually see, is a frame of reference in space-time, which transforms the space, so that light still travels as fast as it always does, but the space around it is transformed. There have been a few theories of exceptional note: Sir Fred Hoyle solved Einsteins equations for an invariant size of the universe based on a shrinking frame of reference, and found no contradictions. Hence the wimper theory of cosmogony. I count myself as pretty bright, on this subject, able to argue the point rather succinctly, but I never claim to hold a candle and a mirror ( Cavendish ) to Henry Cavendish, nor Sir Fredric Hoyle: You want to get the real brilliance of this total failure: "The Michelson-Morley Experiment (MMX) tried to prove the existence of ether, but they did not observe the movement of interference fringes, which led to the assumption that the speed of light is constant in the inertial reference frame, which is also the theoretical basis of Einstein's special relativity (SR)." It failed to prove the existence of ether. Failed. Richard Feynman also found that for Eisensteinian physics, this was also true from first principals. This is really one of the most brilliant failures in the history of Physics. “Success is the ability to go from failure to failure without losing your enthusiasm” ― Winston Churchill
- whatshisface 3y agoIt's impossible to measure because it has no real existence. The one-way speed of light is as metaphysical of a quantity as the British pound. Numerical speeds can be whatever you want them to be in an arbitrarily curved coordinate system - and the speed of light is defined in the "flattest" one of them.
- MiguelX413 3y agoThe British pound is 0.45359237 kg, no?
- edgyquant 3y agoWhat exactly is a kg?
- MOARDONGZPLZ 3y agoBasically 1,000 grams.
- edgyquant 3y agoWhat is a gram?
- ForOldHack 3y agoA gram is the division of a standard Kilogram (Kg ) into 1000 divisions. It's a poor description to both be circular about it, but the Kg is the standard measure of mass. Look to The definition of the standard kilogram. "Since the revision of the SI on 20 May 2019, we can now compare the gravitational force on an object with an electromagnetic force using a Kibble balance. This allows the kilogram to be defined in term of a fixed numerical value of the Planck constant, a constant which will not change over time." "A Kibble balance is an electromechanical measuring instrument that measures the weight of a test object very precisely by the electric current and voltage needed to produce a compensating force. It is a metrological instrument that can realize the definition of the kilogram unit of mass based on fundamental constants." "One important reason for the change is that Big K is not constant. It has lost around 50 micrograms (about the mass of an eyelash) since it was created. But, frustratingly, when Big K loses mass, it's still exactly one kilogram, per the current definition. When Big K changes, everything else has to adjust."
- x3n0ph3n3 3y agoIf the speed of light were different in one direction, the CMB would not look as uniform as it is.
- ars 3y agoOnly in a closed universe, where light "wraps around". Otherwise it would look exactly the same.
- x3n0ph3n3 3y agoThat doesn't sound right. Please explain.
- ivalm 3y agoTo be fair, there is Doppler shift in cmb, we are moving about 370km/s relative to cmb rest frame. You can possibly imagine a world where some of this asymmetry is from a lorentzian ether.
- x3n0ph3n3 3y agoThat's fair, but highly improbable!
- deleted 3y ago[deleted]
- Kranar 3y agoYou make an excellent point but as it turns out the CMB would still appear uniform even the speed of light was not the same in all directions. The reason is that if the speed of light is different in one direction, then the effect of time dilation will also be different depending on direction as well and these two factors cancel one another out resulting in what will still appear as a uniform CMB. The below paper goes into complete technical details giving an example of the speed of light being c / 2 in one direction and instantaneous in another direction (so that it averages out to c), and how the differing time dilations result an isotropic view of the universe. https://arxiv.org/pdf/2012.12037.pdf https://arxiv.org/pdf/2012.12037.pdf
- deleted 3y ago[deleted]
- cwillu 3y agoI can't find the link offhand, but he's discussed this on another page, or possibly in one of his videos (which would explain why I couldn't find it in 30 seconds).
- LordGrey 3y agoEven light, which travels so fast it takes most races thousands of years to realize that it travels at all, takes time to journey between the stars. -- Douglas Adams
- ivalm 3y agoI think it doesn’t matter, since Lorentz shrinking of ether in non-symmetric speed of light would shrink the faster direction.
- angiosperm 3y ago"You can't measure one-way speed of light" is formal doctrine in relativity cant, and "true" as far as it goes, in that anything you manage to measure makes no difference without reference to an absolute rest frame. It hinges on formally failing to synchronize clocks at a distance from one another, and people make hay about that, rather pointlessly. They are really talking about "speed of causality", not light, which happens to go at the same speed; and one-way speed of causality doesn't mean anything. But the cosmic microwave background defines an absolute rest frame that we happen to be measured going 600km/s against. And you can measure one-way speed relative to that, provided you admit that, yes, you really can synchronize displaced clocks entirely adequately for the purpose, as we do absolutely routinely for GPS satellites in wacky orbits thousands of miles apart. Your measurement had better have the time-to-traverse from you to a clock indicating an extra 600km/s in that direction, and short the same in the other direction, and equal toward clocks placed at right angles to those directions. If it doesn't turn out to match CMB asymmetry, you probably get a Nobel prize. On the subject of the CMB, some of the measurements show a bias exactly aligned with the plane of the solar ecliptic. This is called the "Axis of Evil" in astronomy circles, and is rarely mentioned as it is deeply embarrassing to cosmologists. "Cosmologists are often in error, but never in doubt." -- Lev Landau.
- 00N8 3y agoAlthough we can't directly measure the one way speed, can't we at least demonstrate that the speed of light is the same for various arbitrary directions? I'm imagining sending a very brief & well columnated laser pulse aimed at e.g. Pluto, such that we have to "lead" the shot based on light travel time. If the outward bound light pulse travels slower/faster than expected, it will arrive at the wrong time & miss the planet, won't it? You could repeat the experiment at different times of the Plutonian year to verify other directions, etc.. I think there may be practical problems with this method in terms of creating such a well columnated laser pulse, but in principle couldn't we use it to rule out anisotropic light speeds in several (perhaps most/all) directions?