Neutrons in Demand:Scientists Develop New Method To Probe Quantum Gravity

A brand new method to measure gravity and minute quantizations in a gravitational field that uses neutrons entrapped between two vibrating parallel plates immersed in a gravitational field, has been developed by scientists at the University of Technology, Vienna (TU Vienna). Neutrons have earlier been used for electromagnetic (EM) field measurements, but similar methods are now being used to measure gravity, a force which is 10-36 times (i.e. one in a billion billion billion billion parts) as strong as the EM force.

Neutron Probe for Gravity

The physics

Any field which can be quantized (EM can be quantized; gravity cannot be quantized as yet) contains discrete energy levels, which can be occupied by quantum particles. A particle cannot occupy a space between two successive levels. It may, however, jump (technically, make a transition’) from one quantum state to another, giving off radiation in the process.

A quantum particle in a certain state needs to be excited with just the right amount of energy so that it can make transition to a higher energy state. This process is called resonance’.

For quantizing any field, it has to be bounded in space within some finite range. This is conveniently achieved by limiting the extent of the experimental apparatus between two parallel plates. These plates may even be used to induce transitions, as we will see below.

The method

To probe gravitational fields, neutrons are being confined between two closely spaced parallel plates, which can be vibrated at very precise frequencies. If gravity can, indeed, be quantized, then each of the neutrons sits in one of the energy levels in the gravitational field. By vibrating the plates at a very precise frequency (the resonant’ frequency), just the right amount of energy can be pumped into the system. This energy will then be taken up by the neutrons, which will jump’ to higher quantum levels. By measuring the resonance peaks in the vibrational spectrum, scientists hope to accurately map out the quantum levels in the gravitational field.

Extremely cold neutrons are used instead of atoms or electrons, because they are heavy particles and also uncharged. They are unaffected by EM fluctuations, are nearly non-polarizable and are unaffected by the Casimir force.

Gravity and its Quantization

The problem of trying to quantize gravity started with Einstein, when only the EM force had been quantized. Since then, the weak and the strong forces have been quantized and unified into a single theory. Gravity has survived all attempts of quantization and unification. A primary problem with gravity is that the static space-time background present for the other forces is itself distorted by gravity. (In fact, relativity says that the distortion of space-time is gravity). The results of this experiment might give valuable clues as to the energy scales needed for unification. This may also demonstrate the very limits of possibility of a unified theory (like string theory and its many versions).

Calabi fields
String theory demands extra dimensions, which might be curled up


The experiment is much smaller in scale than the existent LIGO and can be performed in a laboratory. Questions still remain as to how fine the measurements need to be in order to be fully sure of the result.

The experiment also hopes to verify the validity of the equivalence principle (which says that gravitational and inertial masses are exactly the same) at extremely small length and energy scales. This principle is crucial for the correctness of general relativity (GR), and thus will deliver a verdict on the applicability of GR at quantum scales. A more sophisticated version of the experiment might even be used to probe into the nature of dark matter, but that is still some time away.

No one is sure if this will work, but as Pauli said, He who dares, wins.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.