Thanks LibWhacker.
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Posted on 01/10/2015 12:41:17 AM PST by LibWhacker
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.
Clearly any quantum gravity theory would have to account for the effect that he has described.
Not really, because this is a theoretician's hypothesis, not an experimental fact. Theoretician's don't have to account for other people's theories.
That’s something that may help to prune the large number of theoretical variations that have emerged to date.
Probably not, since we are orders of magnitudes in energy away from any experimental results that would allow us to significantly reduce the landscape to a manageable size, and another proposed property not yet seen by any experiment doesn't contribute to that at all.
>> Physicists have long known that a single quantum particle can exist in two places at the same time
Even with one eye closed?
What’s interesting about this abstract or short article note is that i find it fairly comprehensible. At this point, it seems to be purely speculative or theoretical. The extra weight has yet to be actually observed. But it is in principle testable if a large enough swarm of entangled particles can be assembled. If observation or measurement collapses entanglement, then observation would reduce the weight of the observed particle. Weight is a force, while mass is a fundamental property of matter. Observation destroying some force would no violate conservation of matter-energy (any more than rendering something weightless by putting it into orbit does). The observed system would be no less massive than the entangled on: it would just exert less force on nearby measuring devices. Question: could we measure the weight of an entangled particle without destroying its entanglement? I.e. does any measurement do the trick, or just some sort of “direct” observation?
I also found it interesting that the author puts it this way: “Physicists have long known that a single quantum particle can exist in two places at the same time.” Every discussion I have ever read refers to entangled pairs of particles. Is this just semantics? Or is there a substantive difference between two separate (albeit intimately linked) particles and a single particle with two (or more) locations?
Yes, and no to both questions. :-)
Thanks LibWhacker.
I love all this stuff but I'm sorry, this sounds a bit like, "it's not important what the facts in evidence are, but the seriousness of the charge".
The reality is we can't KNOW if "it exists at all" UNTIL "the effect can be observed".
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.Being able to detect such a change then is like being able to detect when some plasma thief swipes one kilogram of matter out of a collection of 10 million suns. Inconceivable accuracy.
This implies that the scale is why they're incompatible, and of course that isn't true at all.
Since both theories are well corroborated by observation and experiment, there is obviously a more fundamental "theory of everything" underlying both.
We just don't have a clue what it is. There is some considerable chance that when (if) we come up with the TOE it will change things greatly.
Physicists speak loosely of "forces" in quantum mechanics as a holdover from Newtonian Physics, but in that context what they mean is "fundamental interactions mediated by gauge bosons." There are no actual forces, and no such thing as "weight" in quantum mechanics. The closest you get is something like <dp/dt> which is based on expectation values, not fundamental quantities. So the answer is, there is no direct observation of weight in quantum mechanics.
“Physicists have long known that a single quantum particle can exist in two places at the same time.”
Frankly, I don't really know what the meaning of that statement is in the context of the phenomenon described. An actual "particle" isn't localized. It has a wave function, which exists in a region of some spatial extent. It isn't in "one place" but it's in a heck of a lot "more places" than just two.
It's possible that the author is talking about a bosonic or fermionic system consisting of two entangled particles as if it were a single "particle," but more likely he's just confusing ordinary delocalization [the Uncertainty Principle] with entanglement [Quantum Indistinguishability]. They aren't the same thing.
Omit the statement. It either has nothing to do with entanglement, or it is objectively false. In either case, it lends nothing to the article.
If it can exist in two places at the same time,
why can’t it exist at two times in the same place?
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