Francis: but with a circular path (i.e. a 100 meter long fiber optic cable that both starts and ends at point A, where the source and detector are both located) you don’t need to have the source and detector in difference places, which negates the synchronization issue which I understand to be the fundamental problem with the 1-way SOL measurement.
As far as I know, the Anthropic Principle ( the universe exists only because humans do) has been dismissed as a viable, rational, logical explanation of the universe.
Time is a superficial concept the human brain has invented when trying to comprehend the universe.
It looks like that from all our experiments, this is true, that there isn’t a time constant in the universe, we only use this idea to help ourselves understand what’s going on around us.
At this point I’d like to repeat on of my previous comments… I may not know what I’m talking about. 
The construction of most fiber-optic cables relies on a variable index of refraction radially from the center of the conduit. If a laser beam is shined into one end of the cable, it propagates through the cable in a coherent bundle. In time, after a certain distance has been traversed, the beam starts to disperse, or broaden. The photons moving closer to the outer casing of the fiber optic cable are refracted more strongly toward the centerline.
This is also how the beam is made to curve. As the photons proceed around the loop, some approach the outer wall more closely, and are bent back toward the center. Since the entire beam must be kept within the cable, all the photons which entered will have been forced to bend around the loop.
The physical mechanism of refraction depends on the fact that light moves more slowly through material with a higher index of refraction. The consequences of physically distorting the light beam in the fiber optic cable are functionally equivalent to reflecting the beam back to the starting point with mirrors, and constitute an identical situation to a two-way measurement of the speed of light. Hence, Any measurement in which the light follows a closed path is considered a two-way speed measurement.
Sorry, you cannot wiggle out of it. The Universe is conspiring against you, it seems!
It negates synchronisation in a way which doesn’t allow you to answer the one way speed question.
The question is: is the speed in one direction the same as the speed in the opposite direction?
Placing the detector and emitter at the same point requires that you have only answered the two way question. The light is travelling for half the journey one way, and half the journey the other. You have used some mechanism to change the direction, be it reflection or refraction, but the light has changed direction. The fact that it is travelling back is the crux of the problem. The fibre is no different to sending the light on a round trip with a large number of mirrors arranged to make the light path a polygon of arbitrary number of sides. Indeed with a flat polygon you could say that this is a four way speed measure, as you are measuring the aggregate speed in both directions in two dimensions. (You could extend this to 3D as well.)
Gravitation is about all you have left. If you have a sufficiently strong gravitational field that light is curved around to come back to the emitter you have more serious problems on your hands. Mostly that you can’t get anything out from within that field - your are on the event horizon of a black hole. But space is curved so that light thinks that that path is indeed a straight line. We haven’t done this experiment. Partly because we can’t. But in the end, mirrors and/or refraction do not qualify as a straight line.
Here’s the deal—think about Special Relativity for a moment. An observer bound to Clock A sees Clock B moving with uniform (unaccelerated) motion relative to Clock A’s frame of reference. Due to time dilation effects, A determines B is running slow. But in B’s frame of reference, it is A which is running slow.
Go back to the two clocks, at the same location before they take their places at each end of the measured light path. They are perfectly synchronized. A stays put at one end of the path, while B moves to the other end. In order to start moving, however, B , initially at rest with respect to A, must accelerate. When B reaches the other end of the path, it must accelerate again to stop moving. After accelerating, moving, accelerating again, are the two clocks still synchronized?
An accelerated frame of reference will almost certainly cause two previously synchronized clocks to fall out of step.
The exact same thing happens when the light beam in a fiber optic cable is accelerated around a circle to exit the cable at the same location it entered. From the frame of reference of the emitter/detector, the light beam has been continuously accelerated to execute a complete 360° loop. Any observation you may wish to make about the One-Way speed of light has been compromised by the acceleration the light beam has undergone in its journey.
Face it, there’s no easy answer to the problem of measuring the One-Way Speed of Light.
Of course. Because the 100-meter-long fiber optic cable that both starts and ends at point A is a 2-way SOL measurement.
The Anthropic Principle has not been dismissed and is used by many, although not all, physicists, in a variety of forms and definitions.
Time is not superficial but perhaps the deepest of all physical concepts. Again, while a small number of physicists dismiss time in various ways, virtually all are trying grapple with what time means, and many theories exist.
I don’t understand what you mean by a time constant, so all I can say is that while different observers in different reference frames may experience time elapsing differently, that’s not the same as the underlying meaning of time changing. Analogously, the local speed of light may change in different media but a constant exists for a speed maximum.
That’s not the definition of the anthropic principle, though the existence of humans is quite often part of the discussion when the AP comes up. I would never say the universe exists because only humans do, in fact I think that’s pretty dumb and completely illogical.
As for the other comments, I think I now see why my circular-path example is a 2-way SOL measurement. Thanks for the explanations.
As I understand it, neutrinos are detected by the photons emitted when they do happen to interact with matter. So would it be possible/at all beneficial to measure a two way trip - neutrinos outbound, photons return - then compare this to two-way photons on the same trip?
First, neutrino experiments are not conducted in a tunnel - neutrinos rarely interact with matter, so you don’t need a tunnel (and thus no return path for photons which do). Second, the direction of the emitted photon is related to the direction of the incident neutrino. This allows discrimination of random cosmic neutrinos which are always producing hits in the detector from the directed events (along with time synchronisation). Finally, a neutrino event produces a single photon - reflecting that directly back to source and detecting it would take even more luck than spotting a neutrino event in the first place. I calculated (in another thread) that in the two weeks of additional CERN testing that 2 trillion tests generated 27 detectable events. It is pretty rare.
Si
You’re talking about Cherenkov radiation emitted by a particle that is produced by an occasional neutrino interaction with the (big!) volume of the detector. Those photons’ direction is not predictable, so you would have to somehow reflect them in the right direction, then send the faint-to-begin-with light (non-laser, so dispersion would be quite a factor) back through solid earth somehow (since that is how neutrinos get from emitter to the detector) and provide some kind of path for the to- and fro- photons through the earth as well. Quite an undertaking
Sure, it’s almost certainly not a practical idea as stated, but is the theory workable?
Alternatively, can we conduct the experiment in a tunnel and directly race neutrinos vs photons alongside one another? (one way, as we only need know who wins, not by exactly how much)
But how fast would they go if they were on a treadmill?
:dubious:
Unfortunately, we cannot generate neutrinos like photons by flicking a switch. There is a big particle accelerator involved, and physicists only have theories about how much time elapses between the generating beam impact on the target and the neutrino shower. And I don’t think we are yet at the stage where we can build a perfectly (laser beam) straight tunnel for over 100km (for good timing resolution) through the crust with a damn big neutrino detector at the other end.
I think the best evidence for the speed of the neutrino still comes from Supernova 1987a - the neutrino spike started 4 hours before the extrapolated start of the visual supernova, about where supernova theory predicts. But that does not give any insight into the boundary conditions when the neutrinos are produced (which IMHO is the most likely time for odd effects to occur, such as dimensional tunneling).
Si
The other problem with the 1987a neutrinos vis-a-vis the CERN experiment is that the energies are vastly different, and it’s plausible that if there is something that can make neutrinos exceed c, it might depend on energy.
Or, for that matter, it might also be related to propogating through matter instead of through a vacuum.
And there are many, many different forms of the Anthropic Principle. Some of them are universally accepted as just plain common sense, while others are, to say the least, highly controversial.
Yea I agree with that. When I talk about it, i usually say “thinking anthropically” and I think of it as avoiding a logical fallacy… kind of a version of the post hoc fallacy related to small odds (or what seem like small odds).
For example, some people will say it’s an amazing bit of luck, one in a gazillion chance that earth just happened to be the precise distance from the sun to allow liquid water, to be the right temperature, to have a moon and a stable orbit and an atmosphere with enough oxygen to sustain life. Life can only exist on such planets so thinking anthropically, it’s not a one in a gazillion bit of luck, it’s the only sort of place life can exist (as far as we know) and thus that’s where life is found. It’s no surprise, it’s not an amazing feat of beating the odds. So I see it as essentially avoiding the post hoc fallacy when it comes to odds/statistics. I realize some have extended it to things like “the universe only exists because people do” but that’s a moronic mistake and nothing to do with the anthropic principle as I understand it and far as I’m concerned.
Suppose you have 2 clocks with both hooked up to a detector. Detector A records the time of Clock A when it detects Clock B hits 12:00. Then in reverse Detector B records the time of Clock B when it detects Clock A hitting 12:00. Each clock runs and another recording is made when each detector sees the other clock at 1:00.
Assuming each clock is running at the same temporal rate, it is clear that the durations between recordings should be the same for each clock if the speed of light is the same in both directions. If the speed of light is different one way from the other, would the durations between recordings be different?
Leading slightly on from the OP, there’s something which has always bothered me about time synchronization:
Whenever they show an image of some cosmic phenomenon, they’ll always say, “It’s X light years away, therefore we’re seeing it as it was X years ago, not as it is now”.
But what does it mean to say what “now” is at a distant place? Or is there a clearly-defined meaning to this, related to our light-cone?
Another wrench in the works: would we or would we not know if a (relatively) massive gravitational source would crop up some time after the light has passed on it’s way to B, and thereby alter the time on the way back?