Can someone please explain this one-way vs two-way speed of light dichotomy to me in a simple, easy to understand way? I kinda sorta maybe think I have a bit of a clue about what this is really talking about, but if there is someone out there who really knows about this and can explain it well, with a good analogy perhaps, I’d greatly appreciate it. Obviously I’ve read the above wikipedia entry, but I’m left with questions… questions that at this point I’m not even sure how to ask (if that makes any sense). I’d never heard of this one vs two way SOL issue before coming across this. I understand it to deal with physical limits of measurement rather than a practical/theoretical speed distinction (right?).
I don’t claim to understand this level of physics, but I would recommend PBS’s NOVA series: The Fabric of Cosmos that you can view all 4 parts of it online as of Dec 10, 2011.
I don’t know the answer to your question but it has to do with the fact that time is not a constant and each one of “us” has their own individual time-clock that measures the passage of time. I think I kind of comprehended this idea on episode 2.
Now comes the question, who is “us” … does every molecule of “us” have its own time clock?
All questions fall back to: what is time?
Well, we don’t now. But we do know some properties that we attribute to time. One of them is that it’s not a constant so an emitter of a signal is at a different “time” than the receiver of that signal.
It’s very likely I don’t know what I’m talking about
Time is not constant throughout the universe. It is slower at some points and faster at others. It is influenced by gravity, acceleration, even velocity.
You can try to measure the one-way speed of light, but you have no way of knowing if it is accurate since you can’t factor in how much time has changed on the route. However, if you measure the two-way speed, you can cancel out the differences from the way back.
Furthermore, the time at the start and end will be different. If you do not have a way to synchonize, you will wind up with two different measurements: one from the starting location based on its time, and one from the ending location, based on its time. Sending a second beam of light is the only way of synchronizing the clocks.
But wait, you say. What if I synchronize them ahead of time? That won’t work, either. The difference in time changes. You need the synchronization to be nearly simultaneous to get accurate results. The differences may be minute, but at such high speeds, a difference of even a fraction of a second can give horribly inaccurate results.
Now, granted, I am no expert, just someone whose hobby was researching the oddness of relativity since the sixth grade. But I’m 99.9% certain I’ve understood this correctly.
BigT: That’s my understanding of it too, for the most part. Except to me it just seems like it would be easier to measure the 2 way SOL, but one should be able to come up with a way that measures the 1wSOL… some examples of attempts are given, some sound pretty clever and effective. They’re basically saying it’s a law of physics that the 1wSOL cannot be measured and that’s what I don’t get. It just seems a lot easier to measure the 2wSOL because the source and detector are in the same place so it solves the sync problem.
Moreover, since time is not constant at all places in the universe, as you mention, the 2wSOL measures light over a greater distance which must double whatever error there is over the distance you’re measuring. Halving the result won’t get rid of that error.
It seems we’re able to sync the atomic clocks of 24+ GPS satellites pretty effectively from one point.
My first issue is the “impossible law of physics” rather than one being far more convenient to measure than the other.
My second issue is the way they talk about 1wSOL and 2wSOL like they’re really 2 different things. Is the gist of this there are no one way or two way SOLs, but only that to measure the SOL most accurately the light source and detector should be at the same place, so you need a mirror somewhere to reflect the light back? That’s simple enough but they make it out like they’re 2 totally different things and there might be a different speed of one than the other. That’s, along with the “hard” vs “inviolate rule of physics” issue is what baffles, or at least confuses me. Seems someone smarter than me could come up with a way to measure the “1 way speed of light” … seems people have but “no, that really just measured the 2wSOL” … huh?
Naxos: I think time is just a necessary “medium” for existence to exist, to ask what it is, as if it has to be made of some time particle, is a result of not using the anthropic principle. At least that’s always been my view of it.
What about this. Light source at point A, detector at point B, some sort of signal at point C, which is equidistant form points A and B. Setting of the signal at C should reach A and B simultaneously, which causes the light source to start from A and starts the timer at B, which stops when it detects the light from A.
Why wouldn’t that work? It’s like the starter gun at a track race, the racer closet to the gun has a slight advantage over the racer a few feet further away from the gun due to the speed of sound (obviously an advantage negligible to humans for running purposes). So only race 2 people at a time and make sure they’re both equidistant from the starter gun (i.e. the gun is 10 feet behind and between them so they both hear the bang at the same time).
I just realized that the answer to my above proposed 1 way SOL experiment is probably that to measure the distances accurately enough, we’d use a laser measurement, which does use the “2 way speed of light” to reflect the laser back to its source. So the accuracy of the distances between A-C and B-C would be derivative of the “2 way SOL” measurements. Does that make the results of the triangular ABC experiment described above, which seemingly measure the “1 way SOL” invalid? I’ll bet the answer is probably yes. Hmmm.
The other problem is that** you don’t know** that your signal is going to take the same time from C-A as from C-B, even if they are equidistant.
This is one of the reasons why the CERN neutrino experiment is so contentious. It is not possible to reflect neutrinos back to source (it is hard enough to spot them once). They believe that they have synchronised their time sources to allow them to run a 1-way measurement, but the tolerances are so tight it is hard to know if they have in fact done so.
GPS satellites use a synchronised time source, compensate for relativistic effects, and are accurate to a few meters. Not even close for measuring the speed of light. To get better than that, you need differential GPS, where one GPS is fixed and the moving GPS compares itself to the static one. This subtracts out consistent errors in path length. But this requires close contact between the two GPS units, and is not suitable for long distance experiments. It also only gives centimenter resolution.
What about using a circular mirror tunnel vacuum tube (kinda like a fiber optic cable but with no density as it’s a vacuum - obviously the SOL thru a fiber optic cable is slower than the SOL thru a vacuum which is what we want to measure) that is, say, exactly X miles in circumference where the source and detector are both located at the same point? A big loop. Why couldn’t that measure the 1 way SOL?
Anyway, go down to the section of the wikipedia article where it says “Theories in which the one-way speed of light is not equal to the two-way speed” … this is what throws me because they’re talking about it as if it’s more than just a reality of SOL measurement but two actual different things (regardless of whether the theories are right). That’s implied throughout the whole thing, in fact. Therein lies my confusion, I think.
You aren’t so much interested in the speed that light travels, what the problem is, is measuring the speed of causality. Light just happens to travel at that speed.
So, think in terms of the speed at which causation travels. Synchronisation of clocks involves the propagation of causative information. Any thought experiment can be done just by worrying about that. In particular, you want to avoid measuring the speed of causation using any sort of causation. (Which is clearly on the edge of nonsense.)
What you end up with is the issue of simultaneity. Because causation propagates at a finite speed, you end up with the disconcerting problem that you can’t have a distinguished observer. If you have three locations A,B and C, each may see events occur at the other two locations in different orders. Something that makes reasoning about the propagation of light between two locations difficult.
The two-way speed of light is the average speed of light from one point, such as a source, to a mirror and back again. Because the light starts and finishes in the same place only one clock is needed to measure the total time, thus this speed can be experimentally determined independently of any clock synchronization scheme. [FONT=Trebuchet MS]Any measurement in which the light follows a closed path is considered a two-way speed measurement. [Emphasis added—DHMO][/FONT]
The issue of “simultaneity” goes to the very heart of the problem. The discussion over the “One-Way” speed of light is not that it cannot be measured, but that some method must be used to assure that the clocks at each end of the measured path are “synchronized.” To assert that “Setting of the signal at C should reach A and B simultaneously” is assuming that which you are trying to prove, that two clocks, physically distant, are “in synch” with each other.
The first sentence in the article:
The “one-way” speed of light from a source to a detector cannot be measured independently of a convention as to how to synchronize the clocks at the source and the detector. [Emphasis added—DHMO]
This makes it clear that the issue at hand is more one of assuring that two physically separate clocks read the same, and the difficulty is assuring a “convention” which we can trust to ensure synchronicity.
DMHO: “Any measurement in which the light follows a closed path is considered a two-way speed measurement.”
But why? Yea I read that too, but using a circular path where the photon source and detector are both located at the same place in the loop seems to negate the problems of synchronization.
My problem is there’s a lot of ipse dixit in the wikipedia entry and everything else I can find on google about this subject. Far more than ipse dixit than one would ever expect in an entry about science, let alone pure science like physics. The sentence you quoted is a prime example.
Well there’s always going to be gravity in any SOL measurement, straight line or circle. How could you possibly measure the SOL without gravity affecting it?
Why? Because any mechanism which will curve the light path into a closed loop, such that it can be measured at the same point from which it started, is functionally identical to a reflected path. This is true whether the curve is imposed by mirrors, gravity wells, or some other, more esoteric means.
A “One-Way” measurement must, by the definition of the problem, time a light beam’s propagation from [FONT=Trebuchet MS]Here, where it originated, to There, where it is recorded. Here and There must be separated by some measurable distance.[/FONT]
The stumbling block so far has been to define a [FONT=Trebuchet MS]convention by means of which we may be assured of the synchronicity of the clocks at each end of the measured light path. We can have two clocks in close proximity keeping perfect synchronization to the [COLOR=#800080]yoctosecond. We then move them to the ends of the measured light path. Whether one stays stationary at one end while the other moves, or both move to the opposite ends, we must be assured that the act of moving the clocks does not change their synchronization.[/FONT][/COLOR]
The issue, to date, is that we know that movement causes time dilation effects, causing the clocks to lose synchronicity, which we can easily measure when we bring the clocks back together. Are they still synchronized when they are apart? We do not really know. Any means of communicating between the clocks is fundamentally identical with the situation under examination: determining the one-way speed of light between the two measured endpoints.
More to the point, how do you create a closed path for your light without it ever changing the way it’s going? If your light starts off going one way, then a half-circle later, it’s going to be going the opposite way.
Well, you could use a sealed/coated fiber optic cable. The point isn’t that it’s going the other way, the point is you have the light source and detector in the same place so there’s no need to move them apart (see DHMO’s previous post, I think he explains that aspect of it well).
I realize for the “true” SOL we’re looking for the SOL in a vacuum and a fiber optic cable is made of glass, so at best you’d only get the SOL through the medium from which the fiber optic cable is made. But you’d still be measuring the SOL. The fact that it’s a circular path doesn’t make it “two-way” for purposes of this problem.
We should be able to measure the length of a fiber optic cable pretty accurately without having to use light to measure it, thus not relying on a two-way SOL measurement to establish the distance of the test for the one-way SOL measurement which, as I explained above, I would assume would make the one way SOL result derivative and not distinct from the two-way SOL measurement.
Even with a fibre you have to ask the question - why isn’t the light going in a straight line? You can regard a fibre as a very very large number of mirrors (or a very large number of prisms) that deflect the light ever so slightly on its path. A simple two way light path is one mirror (or two prisms/corner cube) so the net question with a fibre remains the same. Rather than being even a simple two way experiment a fibre is a many way experiment. But it isn’t a one way.
