In quantum mechanics, charged particles like electrons can be sensitive to the effects of electromagnetic fields, even when placed in a region where both the electric and the magnetic fields are zero. This is known as the Aharonov-Bohm effect. A new study has shown that something similar happens with gravity.
The findings, reported in the journal Science, are a profoundly quantum mechanics result. In classical physics, a particle's trajectory can be worked out just by measuring the interaction with the local force fields. If the value of the field it’s zero, there is no force on the particle.
This is not the case in quantum mechanics. A particle in a spatial superposition will instead feel the potential energy difference in the field, even if said field is zero on its trajectory. This has been proven for induced magnetic fields in 1960, and researchers believed that it ought to hold true for gravity as well.
Gravity doesn’t play well with quantum mechanics. Our leading gravitational theory – General Relativity – often can’t be reconciled with what quantum mechanics proposed. However, in this case, there is an agreement. This effect is equivalent to the time dilation due to being in a gravitational field.
Time dilation due to gravity is important, constantly employed in correcting stuff like GPS clocks. But measuring that at the quantum level is far from easy.
The team sent rubidium atoms up inside a vacuum tube. Closer to the highest end, there was a kilogram ring mass made of tungsten. Laser pulses were used to split, redirect, and recombine the atomic wave packets, because a crucial reality of quantum mechanics is that particles are waves and waves are particles – and you can really play with this duality in fun ways.
The laser spread these wave packets so that part of them reached higher and some remained lower. But the presence of the ring mass curved space-time just as any object with mass does according to general relativity – not enough for us to feel it, but big enough for the atoms.
“Measuring the effect of gravitational time dilation on matter-wave interference is a major step in the emerging field of gravitational quantum mechanics. Furthermore, the impressive sensitivity achieved in these experiments could be exploited in future measurements of Newton’s gravitational constant, gravimetry applications, and test the universality of free fall,” Dr Albert Roura, from the Institute of Quantum Technologies wrote in a related Perspective on the same issue of Science.
The unification of gravity with the other forces of nature is a key goal of physics research. This might be an important step to doing just that.