faster than light?

You and the other posters who have pointed out that the condition of
the earth's stopping to spin on its axis is unrealistic are all correct. You
are looking at the scenario as an experienced, educated adults. Middle
schoolers are at the other end of the age, experience, and education
spectrums.

In the students' world of cartoons, high visual impact special effects on
TV and movies, and video games, the idea of the world stopping to spin
is not all that far fetched. The fact that the proposed condition is
impossible should come out of the students input, not the teachers
saying it is so.

In my experience, they will quickly get caught up in trying to identify
the chaos and destruction of the immediate world around them. From
there, it is possible to move gradually towards the overriding larger
issues of the earth being ripped apart.

My approach would be to start small to get the students engaged in
thinking based on their limited experience, outlook, and perspective,
and then move towards the larger implications of the scenario being
impossible.

The students don't need to know the right answers when we start.
Their understanding at the end of the inquiry process is the goal.

Wish me luck.

Visualization Aid: WHat would it look like to have familiar things
slammed together at ~1000 MPH?

 

Actually, it is trickier than that. Even speculating 'miraculous' means, the answer to the question is intertwined with what you mean by among other things 'Earth'. Are the oceans included? Dirt? People? Atmosphere? I consider all those things part of the 'Earth'. If they all stop simultaneously, would you even notice? If people (and only people) are not included in 'Earth', we suddenly have 1000 mph velocity Eastward velocity (relative to the earth surface). Saying the 'Earth' is the solid parts doesn't help either, since there aren't many 'solids' that will remain so under an instantaneous deceleration from 1000 mph or so.

In other words it isn't a simple thought experiment; alternate assumptions on what you mean, will cause more arguments than the actual physics.

 

I believe that both atomic clocks had a ground link via optical fiber that checked on the synchronicity. Since they were linked, they are in the same reference frame. Granted it is not an inertial reference frame.

Both sites could calibrate their clock by observing the same singular event, and calling that the start time.

If, for instance, you hung an atomic clock from a balloon that was equidistant from both sites, and then sent out a time signal in order to calibrate each clock, then it ought to be that both clocks are synchronized. Does that same effect as before,with the GPS satellites moving at 9000 mph relative to the earth, still come into play in this case?

The optical fiber link has the same latency in each direction. By sending a signal at point A to point B, then sending a signal from point B to point A, you can establish the same time at both places within a certain error.

 

Rokeby: I really wonder if an educational purpose is served by asking kids a question which assumes an impossible condition. As I said earlier, what it means for the Earth to stop turning is unclear to the point of being meaningless.

Thought experiments which would lead to vastly different results:

Shove a hypothetical solid bar through the Earth.

Clamp a hypothetical "coconut shell" over the Earth, braking it from the outside.

Miraculously exert just the correct force on every atom within the Earth to stop its motion.

You're going to have a useless discussion as each kid assumes a different mechanism; and since all are impossible you'll be encouraging them to accept the impossible as hypothetically possible. The original question has no answer because the question is essentially meaningless. You are essentially asking them to write a cartoon.

A more useful question would be: Could the Earth suddenly stop turning? What could cause that and what would be the consequences?

More useful yet would be to critique those Roadrunner cartoons, examining all the ways they violate the laws of physics. The question would then be: Why can't this happen? This would encourage critical thinking and teach the principles of physics.

 

Don't lose sight of the big picture. These students are about 12 years old. The point of the question was to understand the concept of inertia. The other thing a clever teacher can do is make it interesting by posing the question in a fashion that engages the class. If the discussion actually got to discussing the points you have, that would be counted as one of the most successful teaching questions ever.

 

I made a first read of the paper issued by the researcher identifying the candidate error. Unfortunately, it assumed complete understanding of how the original experiment was doing the timing and I have not read that. The indications are that a second order relativity error is involved, not the first order that I had understood from the other articles.....but then that's why you try and go to the original sources. This issue may not be closed yet.

The word "synchronicity" must be used very, very carefully. Specifically, the only location where true synchronicity is occurring here is exactly between the two clocks (with the observer and all clocks stationary). At either end clock, the other clock is delayed, not synchronous.

GPS Position and time calculations have two relativity corrections incorporated. The General Relativity correction is to have the satellite cesium frequency standards are set run slower (lower frequency) than a ground based cesium standard. As the earths gravity accelerates the signals, they speed up to the same frequency as the ground standard. (Note-If a GPS satellite clock is returned to earth, it actually has not aged as much as one on that has stayed on the ground.)

The second relativity correction is for the speed of the satellite causing time dilation (time slowing down). This is called the Lorentz Correction. The result of these corrections is for a ground based GPS receiver to calculate a position (Lat, Long, Elev.) to WGS-84 coordinates and time to UTC. Note that all other UTCs observed from all other receivers is delayed by the distance involved. In this case, the only point where there is true "synchronicity" among Ground GPS Receivers would be the center of the earth.

 

This balloon must be suspended higher than 34,000 feet, and held to an equidistant position to within about 3 to 5 feet. Quite impractical, even without considering any weather-induced refraction index mismatch between the two paths.

The latencies won't be the same, once all the propagation delays through the individual transmitter and receiver signal paths are considered. While the total error can be held to very reasonable limits, this must not be assumed. It must be demonstrated with careful review and analysis. I can think of numerous potential mismatch mechanisms, and the experts will be able to add plenty more.

Does GR even have a concept of 'synchronized' clocks at different locations?

 

The balloon was only a hypothetical apparatus. Another one could be a very large tower, etc. We're dealing with line of site issues but you get the point.

In the optical fiber, you can reflect the signal back with a mirror. This gives 2x latency. I'm not going to assume the signal travels at the same speed in both directions, but after the first measurement, you can then send a time signal from point A to point B to check if it is +x later when received. Then send another time signal from point B to point A and see if it is +x later.

If I'm thinking about this correctly, a 1000 megahertz signal should be able to limit the error of latency to a nanosecond.

As for the GR aspect, I don't quite know the answer yet.

 

My point was that the curvature of the earth over this distance prevents any line-of-sight method from a fixed source. The source must be in flight or in orbit, which kills both the equidistant idea and the stationary idea.

This does not properly test the optical receivers, transmitters, and signal paths between fiber and clocks.

This test also excites the receiver with a far weaker signal than when it is used in one-way mode. And unless the link was seriously overdesigned, there is a very good chance of not having enough signal to perform this test 2x test at all.

With plenty of signal processing and good signal to noise ratio, one can measure time intervals far smaller the 1/F.

 

Well one thing is certain is you have to have synchronized clocks in order to measure one-way velocity.

Now I also read that the experimenters used a portable ground base time transfer device. This is an independent verification of that the clocks were synchronized at one time. The ground link was to verify the clocks were synchronized over time.

Basically I'm not disputing that there could have been a mistake, but does it not seem to be accounted for?

 

Or we could use something that travels well through the Earth. How about neutrinos?



Tom

 

My SR is very rusty, and I never sufficiently absorbed GR.

But one of the things I seem to remember is that the very concept of synchronized clocks at different locations, which seems so intuitively obvious in our normal world, falls apart under Relativity.

FL_Prius_Driver digs a bit deeper:

See the rest of his post for more detail. I'm a test and measurement guy, so I'm more familiar with what kinds of assumptions about the electrical test and communications equipment can turn out to be incorrect.

 

* Debunked * !!!

Just a timing problem, no FTL occured. We're talking a difference of 30 nanoseconds, billionths of a second.

Even two atomic clocks have problems being perfectly timed to a single nanosecond, they need to be calibrated together beforehand. Then transported - we're talking Italy & Switzerland. Lots of bumpy roads.

 

Do you see any actual articles claiming proof of debunking?

The ones I'm seeing suggest the FTL results are facing difficulties, but an actual debunking will take more time:

"Autiero thinks that despite the huge interest from the public and the media, the debate will have to play out at the normal pace of science, "which is necessarily slow". The experimental work that was the basis for OPERA's claim took almost six years. "Further developments will be quicker but cannot happen on a few days' timescale," he says."

Finding Puts Brakes on Faster-Than-Light Neutrinos
An independent experiment confirms that subatomic particles have wrong energy spectrum for superluminal travel.

Critics take aim at fast neutrinos
Lack of energy trail suggests finding was miscalculated

 

Rather than debunked, I would say that other results are not consistent with FTL.

Tom

 

Debunked by logic. If it truly was FTL, don't you think it'd be a tad faster than a 30 nanosecond difference? Anyhow, Phil Plait on his blog comments on it, and how it's most likely a timing problem, and experts are working on it.

Of course, how it was all blown out of proportion by the scientifically illiterate "press".

 

Extraordinary claims require extraordinary evidence. In this case, the evidence has turned out to be questionable, if not downright faulty. Further investigation could turn it around again, but at this point, there is no extraordinary evidence for faster-than-light travel. Only one disputed and questionable study. Thus, Relativity stands.

BTW, Cerenkov (sp?) radiation is a fun quirk. Relativity actually specifies that the speed of light in a vacuum is the limiting speed. Light travels slower in a dense medium such as water. In water, relativistic particles travel faster than the light their interaction with the water emits. This does not violate Relativity, but it's pretty cool.

 

A reliance on 'logic' alone, unguided by observation and experiment, would miss a lot of what is now established physics.

The universe doesn't care what I think.

The result was a tad faster than 30ns. It was 60ns.

I agree with the "most likely" part, but using that to declare it already "debunked" is extremely premature.

 

What everyone forgets is that we only accept what we cannot disprove and we keep testing. Science does not prove anything, but is a systematic procedure for exploring. You start with a hypothesis (a testable statement). You do not start with a conclusion.