Posted by mudil 3 days ago
I found it hard to believe that they accounted for other forces precisely enough that they could attribute the phase change to gravity, but this is beyond me so I trust the result.
At first I thought "they showed that you can measure a particle falling in gravity," which seemed dumb because we already know that particles fall in gravity. But they showed that you can measure a single (aggregate) particle falling in gravity, which is pretty cool because if gravity is quantum then that means that they observed an interaction between the graviton and their rubidium atom.
You don't need evidence to support a premise.
More critically though the study gets into how they used a reference wave packet to establish a stationary baseline for the interferometer. Assuming the experiment is sufficiently isolated to reduce noise below the necessary threshold this can work in principle.
Can you give me some examples? I guess I'm thinking lead barriers or a Bose Einstein condensate but curious what you mean here.
if quantum objects can fall and multiple quantum waves can occupy the same space, then why doesn't everything always collapse into a single point?
Why does it only happen in black holes and outside of that quantum waves instead create emergent systems instead of just collapsing together?
Does high gravitational force nullify emergence in space/time?
Might be a stupid question. I don't study this subject much.
Electrons (as an example) experience Coulomb pressure (charge repulsion), but also a quantum statistical pressure called Fermi Degeneracy pressure related to their kinetic energy, AND ultimately the Pauli Exclusion Principle (Identical Fermions cannot occupy the same state, but higher momentum states take more energy to reach naturally so this creates resistance to collapse). If you want to learn more about this you can get a lot of mileage out of some reading on Fermi-Dirac statistics, the Pauli Exclusion Principle, and degeneracy pressure. Now this is just using electrons as a model, but ultimately all of the above can be overcome by gravity. When it does you still can't have electrons disobeying the rules, but the potential energy barrier to merge electrons and protons into neutrons is overcome. THEN you have neutron degeneracy pressure, and in theory after that you have a black hole (spacetime singularity surrounded by an event horizon.
However... that may not be the case. It is true that observation has confirmed the existence of objects that are so dense and massive they must have an event horizon, but beyond that we have no way of direct observation, right now (even in principle). A lot of people believe this indicates that a singularity doesn't really exist; it's the usual lesson when a singularity appears in your math: your math is wrong. In the end maybe there's another sort of degeneracy pressure from quarks or something even more fundamental like strings that ultimately prevents final collapse to a true singularity.
ed: typos
I assumed they can occupy the same space due to superposition principle, as waves could stack and modulate each other, sort of like wave A and wave B occupying the same space could produce a wave A+B
So a singular point would be the sum of all waves occupying the space.
But maybe gravity or spacetime itself is a recursive function and black holes are functions without a base case and there is no singular point, only non-terminating recursion.
Some ideas to funnel into AI so I can entertain myself hah
In a black hole though … who knows
(He's listed as V. Vedral on the Tardigrade paper you linked, and it's more common to use last names in this context.)
We need physicists to come up with and develop novel ideas to consider no matter how many of them might be entirely wrong.
That does not follow at all.
Damn Deepak Chopra and his ilk of idiots for making any conversation of quantum mechanics and biology tinged with pseudoscience. Hopefully we’ll keep getting experimental evidence as we go that it’s not at all absurd to consider quantum effects in biology.
That said, those effects are going to look nothing like sustained coherence for long periods of time.
> The phase of free fall is predicted in a purely quantum manner to have a dependence m/6 g^2T^3/ℏ + gmzT on the free-fall time T, where m is the mass of the object, g is the gravitational acceleration relative to the surface of Earth, and z in the spatial coordinate in the direction of gravity. This prediction follows the calculated phase accumulated by an object accelerating in a linear potential, and has been made starting from almost one hundred years ago by Darwin, Kennard and others.
Apparently the phase shift is derivable from just adding a linear potential term mgz to the Hamiltonian.
* https://www.youtube.com/@pbsspacetime/search?query=graviton
It doesn't show/prove how general relativity and quantum mechanics interact.
NOTE: The Dirac equation and Quantum Electro Dynamics (QED) unify quantum mechanics and special relativity (non-accelerating frames of reference).
So the remaining piece is either to extend QED/QCD to accelerating frames of reference or to quantize general relativity. That would likely predict the phase shift observed in this experiment.
And more generally the Standard Model, which includes the weak and strong interactions. The SM is a quantum field theory, which, as you say, unifies QM and SR.
> (non-accelerating frames of reference).
No, SR and QFT are not limited to non-accelerating frames. They are limited to small enough regions of spacetime that spacetime curvature is negligible. This experiment is an illustration of that: it compares an accelerated atom with a free-falling atom to show the phase shift between them, and the lab frame in which it is done is accelerated--but the SM and SR work just fine. But the experiment does not show any effects of spacetime curvature.
> the remaining piece is either to extend QED/QCD to accelerating frames of reference
No, that's already done. See above.
> or to quantize general relativity.
That's the big missing piece, yes. We know how to write the QFT of a massless spin-2 field (which is our naive expectation of what a QFT for gravity would look like), and we know that the classical limit of that QFT is the classical GR we have now. But we know that QFT has to be just an effective theory, just like the Standard Model; it can't be the final answer.
> That would likely predict the phase shift observed in this experiment.
The theories we already have (Standard Model + the equivalence principle are all we actually need) are sufficient to predict that. Of course any more comprehensive theory will have to reproduce that prediction, yes.
Thus, if you extend QED/QCD/SM in a similar way (thinking of QED/QCD/SM extensions in terms of acceleration and curved space with the equivalence principle in mind) that may lead to a quantized theory of gravity. -- Sir Roger Penrose has a similar idea/thinking [3].
One of the key challenges with quantizing gravity is in how the terms in the expressions resulting from analyzing the Feynman diagram interactions behave [4] which prevent them being renormalized. For electromagnetism you can formulate the terms using the fine structure constant (via the coulomb potential, ħ, and c) which results in successive terms decreasing in value and thus stabilizing to a single value.
For gravity using Newton's relationship between two masses in a similar way to deriving the fine structure constant you get Gm^2/ħc. Applying E=mc^2 gives GE^2/ħc^5. Using the Planck energy constant gives (E/E_p)^2 for the energy coupling strength. This means that unlike electromagnetism, the successive terms in the Feynman diagram analysis grows exponentially instead of decreasing to 0. Thus, this approach to quantization doesn't work for gravity.
Note: you can still use this to analyze quantum gravitational effects at small energies by evaluating to a given number of terms.
[1] https://www.britannica.com/story/how-albert-einstein-develop...
[2] https://www.ebsco.com/research-starters/physics/equivalence-...
[3] https://www.youtube.com/watch?v=VQM0OtxvZ-Y "We need to 'gravitise' quantum mechanics, not quantise gravity | Roger Penrose | Full interview"
[4] https://www.youtube.com/watch?v=yTEPm5d6mrI "Why Quantum Gravity Doesn't Work"
Quantum field theory in accelerating frames of reference is old hat; poster children like the Unruh effect [1] and Hawking radiation [2] are from the 1970s.
I must say, it's actually quite refreshing to read an article about a science topic that conveys the caveats and limitations of the study. Far too many of these studies get filtered through the news outlet hype-machine
https://www.youtube.com/watch?v=Uey_mUy1vN0
Hopefully we'll see a result in the next decade