Entanglement Makes Quantum Particles Measurably Heavier, Says Quantum Theorist

The discovery is a long sought-after link between the theories of quantum mechanics and general relativity

The two towering achievements of 20th century physics are Einstein’s theory of general relativity and quantum mechanics. Both have fundamentally changed the way we view the universe and our place within it.

And yet they are utterly incompatible: quantum mechanics operates on the tiniest scales while relativity operates on the grandest of scales. Never the twain shall meet; although not for lack of trying on the part of several generations of theorists including Einstein himself.

Now one theorist has shown that an exotic quantum effect called entanglement has a real and measurable influence on a gravitational field— the first time this kind of link has ever been shown.

David Bruschi at the Hebrew University of Jerusalem in Israel says the new result has important implications for quantum mechanics and relativity and may represent an important step towards a long sought after theory that explains them both.

Bruschi’s idea is simple in principle. Physicists have long known that a single quantum particle can exist in two places at the same time. There is a clear quantum correlation called entanglement between these two locations that is well-defined mathematically in quantum mechanics.

Bruschi’s new approach is to formulate the mathematics in the context of relativity. He first makes the mathematical assumption that some perturbation of a gravitational field is possible in these circumstances.

He then goes on to formulate the mathematical properties of this perturbation and how they evolve when the two locations are maximally entangled and when they are not, a state known as maximally mixed.

He finds that the perturbation is zero when the states are maximally mixed. But in the other case— when the two locations are maximally entangled— the perturbation spreads through space over a scale related to the energy of the particle and the coherence time of the entanglement.

This kind of perturbation is mathematically similar to a gravitational wave, albeit on a much smaller scale. It is essentially equivalent to the particle having some additional weight. And that is what makes it potentially detectable.

Don’t hold your breath, however. Bruschi has carried out some back-of-an—envelope calculations of the size of the effect for a quantum particle with the mass of an electron, of the order of 10^-31 kilograms. He says the change in this particle’s weight when it is entangled in two locations is just one part in 10^37. Inconceivably small.

But he points out that there are ways of increasing the effect by using very heavy particles or ones that are travelling at ultra-relativistic velocities. A more promising possibility is to use groups of particles that are all entangled, a phenomenon known as N00N states.

Physicists have already created N00N states with as many as 5 photons so it is not hard to imagine that similar states may be possible with heavier stuff.

The key breakthrough in this paper, though, is not the prediction of the scale at which the effect can be observed but the fact that it exists at all.

Bruschi is optimistic about the future. Using the royal “we” he says: “We believe that our results can help in better understanding the overlap of relativity and quantum theories and, ultimately, in the quest of a theory of quantum gravity.”

Clearly any quantum gravity theory would have to account for the effect that he has described. That’s something that may help to prune the large number of theoretical variations that have emerged to date.

Of course, the first observation of this additional “weight of entanglement” would be a major discovery. So the question of whether it is at all feasible with technology that will be available in the near future is one that will have theorists and experimentalists scratching their heads for the next few months.

Ref: http://arxiv.org/abs/1412.4007 : On The Weight Of Entanglement