Fact or Fiction: The Days (and Nights) Are Getting Longer

Forces from afar conspire to put the brakes on our spinning world—ocean tides generated by both the moon and sun’s gravity add 1.7 milliseconds to the length of a day each century, although that figure changes on geologic timescales. The moon is slowly spiraling away from Earth as it drives day-stretching tides, a phenomenon recorded in rocks and fossils that provides clues to the satellite’s origin and ultimate fate. “You’re putting energy into the moon’s orbit and taking it out of the Earth’s spin,” says James Williams, a senior research scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.

Uncertain Principles: Relativity on a Human Scale

Precision clock measurements from NIST, directly observing both gravitational and kinematic time dilation, all without leaving the lab.

You can do this type of experiment with less precise devices, but you’d have to have a more robust apparatus to accumulate the data because you’d need to do a longer experiment. I’ve detailed how commercially available clocks have done the kinematic dilation measurement, though at ~30 m/s rather than 10 m/s, which required an experiment lasting several days. Similarly, one could do an experiment with clocks on different floors of a building over the course of a few weeks and see a gravitational redshift. It’s probably not worth the effort to go out of your way to do so, given the level of confirmation we already have for relativity — GPS works, for example.

The trick here is that these measurements were made in a relatively short period of time, but that’s generally the nature of cutting-edge science like this — it takes a lot of effort to get all the parts to run, and the experiment yields data for a limited time. So Chad is correct in asking whether they really count as “official clocks” — they don’t insofar as timekeeping is concerned. They probably don’t run long enough to really impact a true clock ensemble. The value here is that what’s hard to do today will be easier to do tomorrow, and eventually this (or a competing) kind of technology will become “true” clocks that are more compact and run continuously for extended periods and do have a significant impact on timekeeping.

(I should be listening to similarly-themed talks this week — I was supposed to go the Frequency Control Symposium. But I managed to get sick — not vodka-induced, BTW — as the weekend drew to a close. No way I was getting on a plane at that point. Bleh.)

I linked to a gravitational redshift experiment that was recently published, and have had a chance to read the paper. It’s quite cool. I had been dismayed at the first couple of popular summaries, but this one is pretty good and as I had indicated, the press release is pretty good for a press release. So there’s not a lot to add.

The original experiment was designed to measure gravitational acceleration; the two trajectories have a different potential energy, mgh, and will accumulate a phase difference as compared to the other trajectory. You can think of this as the difference in deBroglie wavelengths, where the difference in momentum leads to a slightly different wavelength, and this gives a phase difference when the atoms recombine. Since the mass and the relative trajectory are known, measuring the phase difference will allow you to determine g.

The special insight presented in this paper was the interpretation in terms of relativity. The energy, rather than being the classical kinetic and potential terms, is mc^2. The “oscillation” that allows for interference is now at a much higher frequency, and the accumulated phase will be gh/c^2, which is the gravitational redshift. To do this, you need to independently know g, which was determined using a corner-cube gravimeter.

There is also a difference in the implications by reinterpreting the results. The first measurement assumes the classical physics is correct — the phase difference is proportional to g is the result of an equation that is assumed to be true at this level of precision — and that there is a phase to measure, i.e. the atoms have a wave nature, which is an early prediction of quantum mechanics. The answer in the form of a value for g only makes sense if we assume these theories are correct. And that’s not really a problem, since we have independently tested those theories many times over the years.

But the other interpretation is a direct test of relativity — the theory predicts an answer which can be directly compared to the result. And that allows one to put limits on how “wrong” this aspect of relativity might be. You add another term onto the time dilation term, with a perturbation expansion (we already know relativity is pretty good and have results from the Vessot rocket experiment, so any deviation has to be small). So we write the time dilation as (1+B)(gh/c^2), and then assume the worst case, that all of the discrepancy in the measurement is not experimental error, but rather a flaw in the theory. And they get a result of B = 7±7 x 10^-9, which is consistent with relativity being correct (B = 0), and limits any problems with this aspect of it to a part in a hundred million.

“A precision measurement of the gravitational redshift by the interference of matter waves,” Müller, Peters and Chu, Nature 463, 926-929

What Is Time? One Physicist Hunts for the Ultimate Theory

Sean Carroll is interviewed by Wired, on the subject of the arrow of time. He should write a book or something.

[W]hy was the entropy ever low to begin with? Why were the papers neatly stacked in the universe? Basically, our observable universe begins around 13.7 billion years ago in a state of exquisite order, exquisitely low entropy. It’s like the universe is a wind-up toy that has been sort of puttering along for the last 13.7 billion years and will eventually wind down to nothing. But why was it ever wound up in the first place? Why was it in such a weird low entropy unusual state?

That is what I’m trying to tackle. I’m trying to understand cosmology, why the big bang had the properties it did. And it’s interesting to think that connects directly to our kitchens and how we can make eggs, how we can remember one direction of time, why causes precede effects, why we are born young and grow older. It’s all because of entropy increasing. It’s all because of conditions of the Big Bang.