This actually has significant practical importance, because it is hoped that using this transition of the thorium nucleus it will be possible to build atomic clocks even better than those using transitions in the spectra of ions or neutral atoms, because the energy levels of the nucleus are less sensitive to any external influences.
While in the best atomic clocks one must use single ions held in electromagnetic traps or a small number of neutral atoms held in an optical lattice with lasers, in both cases in vacuum, because the ions or neutral atoms must not be close to each other, to avoid influences, with thorium 229 it is hoped that a simple solid crystal can be used, because the nuclei will not influence each other.
The ability to use a solid crystal not only simplifies a lot the construction of the atomic clock, but it should enable the use of a greater number of nuclei than the number of ions or atoms used in the current atomic clocks, which would increase the signal to noise ratio, which would require shorter averaging times than today, when the best atomic clocks require averaging over many hours or days for reaching their limits in accuracy, making them useless for the measurement of short time intervals (except for removing the drift caused by aging of whatever clocks are used for short times).
The article points to a use I wouldn't have thought of.
The deeper you go into a gravitational field, the slower time goes. Therefore comparing clocks in different places gives a way to measure gravity. These clocks could be sufficiently precise to find mineral deposits underground from their gravity signature.
Magnetic anomalies also highlight inteesting places for minerals, the issue with both magnetic and gravity fields variations lies with determining the "true" depth to target (medium sized shallow target, or massive deep taget?) which is known as an inversion problem.
Linear inverse problem theory is an extremely powerful tool for solving inverse problems. Much of the information that we currently have on the Earth’s interior is based on linear inverse problems
Despite the success of linear inverse theory, one should be aware that for many practical problems our ability to solve inverse problems is largely confined to the estimation problem.
The problem is that the planet could be hollow and produce the same gravitational measurements on the surface and outside. It needs to be coupled with a model that introduces constraints for the inverse problem to be defined.
Since mining is only concerned with material that's within maybe 0.1% of the distance from the surface to the core, seems like you'd just need to move the sensor around and make sure the signal changes about where you'd expect for a mass of X Kg at a depth of Y meters instead of a supermassive chunk of dense material much deeper. Or, to put it another way, build a grid map of the area and subtract any background signal. Would that not work for some reason?
In practice, that's what would happen. Move around until seeing some larger gravitational pull, likely indicating some deposit. However, formally, this is not correct due to the mere fact that the gravitational force is proportional to 1/R^2, just like a Columb force. Thus, there are infinite numbers of mass distributions that produce the exact same gravitational field on the surface. The planet could be hollow, and we would not know it only from the field measurements.
A practical constraint is mass density, which has maximum and minimum values. We can make a crude approximation that the planet's density is constant, evaluate the field on the surface from the planet's shape and compare it with measurement. This would be more useful, but still, it wouldn't tell us whether there is a combo of water reservoir and a large massive deposit below it.
Most units of measurement are derived from the second, so the more precise our frequency standards, the more precise everything else can be. Things like interferometry and spectroscopy depend directly on very precise frequency standards.
> clocks even better than those using transitions in the spectra of ions or neutral atoms
I'd be interested to know how much more accurate a nuclear-state-transition clock might be than a conventional Caesium or Rubidium clock.
TFA seems to make the point that a nuclear clock would be more resistant to external influences, such as EM radiation, than an atomic clock, and so could be used in experiments where such influences might introduce unwanted uncertainty. But I'd like to know what the claim for greater accuracy is based on, rather than simply greater reliability.
You have the math turned around. Because the nuclear resonance is much more stable and high frequency the Q factor and accuracy of the measurement is higher. With a cesium or rubidium clock it's very difficult to control all the influences on how tightly the nominal resonance is achieved and the Q while impressive is a bit less.
There are some real challenges in realization: this will take optical combs and all sorts of other stuff to really take advantage of.
They also point out that because the thorium atoms can be embedded in a solid, and have motion << the wavelength of the radiation, the emission and absorption are largely recoil-free. This eliminates Doppler broadening. What broadening there could be was below the resolution of their pump beam.
While in the best atomic clocks one must use single ions held in electromagnetic traps or a small number of neutral atoms held in an optical lattice with lasers, in both cases in vacuum, because the ions or neutral atoms must not be close to each other, to avoid influences, with thorium 229 it is hoped that a simple solid crystal can be used, because the nuclei will not influence each other.
The ability to use a solid crystal not only simplifies a lot the construction of the atomic clock, but it should enable the use of a greater number of nuclei than the number of ions or atoms used in the current atomic clocks, which would increase the signal to noise ratio, which would require shorter averaging times than today, when the best atomic clocks require averaging over many hours or days for reaching their limits in accuracy, making them useless for the measurement of short time intervals (except for removing the drift caused by aging of whatever clocks are used for short times).