Tag Archives: general relativity

Testing Einstein

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We’re coming up on the golden anniversary of some very important experiments that were milestones in confirming relativity and were enabled by a breakthrough in nuclear physics, the Mossbauer effect. Mossbauer’s discovery (published in 1958) of the Mossbauer effect (what were the odds of that happening?) was that nuclei in a lattice had essentially no recoil when emitting gammas, since effectively they shared the mass of the entire sample. Normally, the conservation of momentum from the recoil of a nucleus shifted the gamma’s energy out of resonance, meaning that the gamma would not be reabsorbed by an identical nucleus; even though the recoil from the emission of a 100 keV gamma would only cause a shift of a few thousandths of a eV in the gamma’s energy, this is significantly larger than the width of the transition. However, effectively increasing the mass of the emitter by even a small fraction of Avogadro’s number — which you can do with just a speck of material — all but eliminates that energy shift, and the ground-state nucleus will absorb the photons emitted by the excited state.

This incredible new tool set the stage for several experiments in General and Special Relativity. One is the famous Pound-Rebka experiment that took place at the Jefferson Lab tower at Harvard. The premise of the experiment was that a photon climbing or falling in a potential well would be red- or blueshifted, and this could be compensated for by moving the source; when the Doppler shift canceled the gravitational effect, the photons would be on resonance and be absorbed by the target, but at other speeds would not be absorbed. This would cause a variation in the number of photons striking a detector. The gravitational redshift is small, \(gh/c^2 = 2.45 x10^-15 \) and the Doppler shift necessary to compensate is just \(7 x 10^-7 m/s\) . This sensitivity is doubled by reversing the experiment and looking for the blueshift of a falling photon. The source, Fe-57 (from a decay in Co-57) has a transition at 14.4 keV, and is narrow (about 10^-8 eV) owing to a ~100 ns lifetime.

The simplicity of the basic experiment masks some subtleties of device. The source and absorber needed to be specially prepared; the Co was diffused into a thin Fe sheet so that the source was in a very thin layer near the surface, and for the target a thin layer of Fe was electroplated onto a Be disc. These were vertically separated by 22.5 meters, and to reduce absorption by air, this space was taken by a mylar bag filled with Helium. The source was put on a transducer, i.e. a speaker cone, and oscillated at low frequency. To eliminate thermal effects, since the difference in thermal motion between the source and target materials could shift the nuclei out of resonance, they were stabilized to the same temperature.

The second order Doppler shift resulting from
lattice vibrations required that the temperature
difference between the source and absorber be
controlled or monitored. A difference of 1ºC
would produce a shift as large as that sought, so
the potential difference of a thermocouple with
one junction at the source and the other at the
main absorber was recorded. An identical system
was provided for the monitor channel.

The results agreed to about 10%, and a later experiment by Pound and Snider agreed to 1%

But it doesn’t end there. A lesser-know cousin to this experiment was carried out to observe the frequency shift in a rotating system. Once again using the Mossbauer spectroscopy of Fe-57, the source and target were mounted on the axle and rim of a cylinder, which was then rotated at some speed. In this case, one can look at the effect either by viewing this as a pseudo-gravitational potential or as a kinematic time dilation effect (both approaches, not surprisingly, yield the same answer), with the fractional frequency shift of \(v^2/2c^2 \) . The cylinder was rotated at different speeds and the increase in the counting rate was observed, as the target moved out of resonance with the source due to the frequency shift of the target.

Gravitational Red-Shift in Nuclear Resonance
Phys. Rev. Lett. 3, 439 – 441 (1959)
R. V. Pound and G. A. Rebka, Jr.
(Theory)

Apparent Weight of Photons
Phys. Rev. Lett. 4, 337 – 341 (1960)
R. V. Pound and G. A. Rebka, Jr.
(Experiment)

Measurement of the red shift in an accelerated system using the Mossbauer effect in Fe-57
Phys. Rev. Letters. 4, 165 (1960)
H. J. Hay, J. P. Schiffer, T. E. Cranshaw, and P. A. Egelstaff

Measurement of Relativistic Time Dilatation using the Mössbauer Effect
Nature 198, 1186 – 1187 (22 June 1963)
D. C. Champeney, G. R. Isaak and A. M. Khan

Just Checking

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Possible Abnormality In Fundamental Building Block Of Einstein’s Theory Of Relativity

Physicists at Indiana University have developed a promising new way to identify a possible abnormality in a fundamental building block of Einstein’s theory of relativity known as “Lorentz invariance.” If confirmed, the abnormality would disprove the basic tenet that the laws of physics remain the same for any two objects traveling at a constant speed or rotated relative to one another.
[…]
The new violations change the gravitational properties of objects depending on their motion and composition. Objects on the Earth are always moving differently in different seasons because the Earth revolves around the Sun, so apples could fall faster in some seasons than others. Also, different objects like apples and oranges may fall differently.

I find it amusing that there are a bunch of relativity cranks who claim that relativity is treated as dogma. The reality is that it isn’t all that hard to find scientists devising tests of relativity of various sorts, whether it’s testing the predictions of GR or checking for anomalies such as this.

Of course, thus far whenever someone has devised a clever test like this, we’ve found that relativity is correct.

It's About Time: More NPR Physics Discussions

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A Light Take On The Gravity-Time Relationship

Brian Greene explains the link between gravity and time.

Greene has written a short (less than 40 cardboard pages) new picture book called Icarus at the Edge of Time. It tells the story of a young boy who slips off in a space ship and cruises over to a black hole, only to discover that he’s made a terrible mistake: He forgot one of Einstein’s fundamental observations, which is that time is not the same for everybody everywhere.

[…]

Einstein’s theories posit that as one gets closer to a center of gravity, time will “slow down.” So if you spend the rest of your life closer to the Earth’s center of gravity on 34th Street while I spend the rest of my life at the top of the Empire State Building, time for you will tick a teeny, teeny bit more slowly than time for me.

Einstein meant this not poetically, but literally. If you and I each had a watch, ticking off hundred-billionths of seconds, the watch on your wrist down below on the street would tick fewer times than the watch I was wearing up in the sky. It wouldn’t be a big difference — a few billionths of a second over 20 years — but it would be a real difference. If we decided after several decades to meet and compare watches, we’d see that they would literally differ, that time for the two of us had indeed ticked differently.

via Physics Buzz

Is Gravity Ruining Time?

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I’ve mentioned I’m at a conference — it’s the 7th Symposium of Frequency Standards and Metrology being held near Monterey. It’s a bunch of scientists getting together every ~7 years to discuss the state-of-the art in frequency standards, clocks, and precision measurements, and float ideas for future experiments. The last one was in St. Andrews, Scotland in 2001 (unfortunately it spanned 9/11/2001, which was a bit of a distraction, to say the least.)

There have been a lot of talks that I couldn’t possibly distill into coherent summaries, but I’ll try to do one or two when I get the chance. I’ve got one for now, though, that doesn’t require as much heavy lifting.

Dan Kleppner gave the first talk (Is Gravity Ruining Time?) as a sort of introduction, and gave some perspective on timekeeping, since he has been doing physics from before the development of the hydrogen maser (making him, as he put it, prehistoric). Two main things came out of this talk: an appreciation of a limitation on how we define the second, and a story about I.I. Rabi.

The second is defined as 9,192,631,770 oscillations between the hyperfine states of an unperturbed cesium-133 atom, but this definition does not explicitly mention anything about relativity, of which gravity is a part. It’s basically taken by convention that we use devices at rest on the geoid (an idealized surface of the earth, basically what it would look like without tides) but devices have reached the point where this may not be good enough. The gravitational redshift is given by gh/c^2 near the earth, and this is about a part in 1016 per meter change in height. Clocks need to be adjusted for their altitude/elevation, and this has been necessary for some time; the effect has been measured in the Pound-Rebka experiment and in the rocket launch of a hydrogen maser by Robert Vessot, and is accounted for in GPS and every other satellite carrying a clock. But ground-based clocks are now getting to be good enough to where sub-meter changes in height will need to be taken into account. And since the geoid can only be determined to several cm and it changes with time (and clocks move with respect to the geoid via earth tides of about 30 cm), this will soon become a significant term in the error budgets of frequency standards. So the point of the talk was that gravity is going to take a more prominent role in frequency and time measurements, and may in fact require a redefinition of the second, though it would not impact “everyday” time.

The story he told about Rabi went something like this: Rabi didn’t like writing articles, so there is no formal writeup of his proposal to use an oscillator tuned to a hydrogen transition as a time measurement device — the idea that would eventually become the hydrogen maser and used in other atomic clocks. But in 1945, after he had the Nobel prize, he gave the Richtmyer lecture to the American Association of Physics Teachers on the topic of using a hydrogen magnetic resonance measurement as a potential timekeeping device, and it was written up by the New York Times science correspondent, William Laurence, in an article called ‘Cosmic Pendulum’ For Clock Planned, in which he gives a very basic summary of the principles Rabi had explained. So the cutting-edge science was “formally” proposed in the Times rather than a science journal. In the AAPT’s list of Richtmyer lectures, Rabi’s is one of the few from that era that were not written up and presented in the American Journal of Physics.

(The Times article is here but the archive is paywalled)