Gravity, Quantum Objects, and Violations of the Equivalence Principle
Electrons in a conductor seem to behave differently under gravitational and inertial accelerations, threatening to tear down one of the cornerstones of modern physics.
How do quantum objects respond to gravity? It seems a simple enough question
and yet one that leaves theorists scratching their heads. And so it should. The
analysis to date implies that quantum objects violate the fundamental idea that
gravitational and inertial mass are the same thing, an idea known as the
equivalence principle.
Here’s the thinking as laid out today by Timir Datta at the University of
South Carolina and buddy Ming Yin: In the second decade of the 20th century, a
group at Caltech began to puzzle over the inertial properties of electrons in
conductors. They argued that the trailing end of an accelerating metal rod
would be negatively charged because the electrons would lag behind the
conducting lattice as it sped up. Likewise, they surmised that the
circumference of a rotating disk would also be negatively charged with
electrons flung into the periphery.
Advertisement
By this analysis, the effect of a linear or radial acceleration on a quantum
fluid is the same as it is on a Newtonian fluid, like water in a spinning
bucket. Richard Tolman and others even claimed to have measured this buildup of
charge.
This story is only available to subscribers.
Don’t settle for half the story.
Get paywall-free access to technology news for the here and now.
But according to the equivalence principle, if an acceleration can have this
effect on electrons, so too can a gravitational field.
Here things get a little more complex. Calculating the equilibrium that
occurs when gravity acts on a solid crystal filled with conducting electrons is
no easy task.
It turns out that if the crystal is rigid, then gravity pulls the electrons
downward, creating a tiny buildup of negative charge at the bottom of the
crystal and a small electric field that points down. That’s exactly as the equivalence
principle implies.
If the crystal is deformable, however, gravity has a larger effect on the
lattice than it does on the electrons. In this case, gravity compresses the
lattice, creating a positive charge density toward the bottom of the conductor.
Now the electric field is several orders of magnitude bigger and points in the
opposite direction.
That’s a troubling result because it means that it ought to be possible to
tell the difference between an inertial acceleration and a gravitational one by
measuring the direction of the electric field that builds up. And according to
general relativity, that ain’t possible. Surely general relativity, one of the
cornerstones of modern physics cannot be wrong on this point. So what’s gone
wrong?
One obvious question that is unanswered (at least by Datta and Yin) is why
an inertial acceleration does not compress the crystal lattice in the same way
as a gravitational field, creating the same kind of positive charge density.
The measurements made by Tolman and others suggest that this kind of
compression does not occur.
Advertisement
Unless the measurements are wrong. Could it be that this conundrum arises
only because of a few erroneous measurements?
If so, perhaps it’s time for somebody to try them again.
