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.
Actually, we do need evidence for premises which are not definitions or basic assumptions (axioms). Graviton is still a hypothetical particle and falls under neither of two categories - Definitions or axioms.
The statement "if gravity is quantum, this was an interaction with a graviton" is true even if it ultimately turns out that gravity is not quantum. The statement is about the consequences of a premise, and is not falsified if that premise is false.
The only way this statement could be false would be to have quantum gravity but no particle that mediates this interaction - which doesn't seem plausible almost by definition.
That's quite an exotic claim! If Sound and Temperature can emerge without a sound particle or temperature particle, why is it not plausible for gravity to exists without graviton?
Though, I get it that mainstream view from physicists is Graviton is the most 'likely' cause, if the gravity is proven to be quantized. But even they would have the humility to accept that this is a theory yet to be proven and observed!
A: If gravity is quantum, then (B) there must be some particle-like think that we call a graviton that mediates the interactions, and then seeing a rubidium atom fall must have been an interaction with this particle.
not A: If gravity is not quantum, then they are making no claims about a graviton.
If you think gravity is not quantum, then you go to the "not A" branch, and they make no claims about that branch - so there is no contradiction with their IF.
This is a global issue with modern particle physics - scientists imagine something and then blink, and two decades and a hundred of books had been published while that something is still not present in any test. It would be fine if that happened rarely, but it seems that it's just par for course nowadays.
It's a quiet test of Penrose's idea that gravitational interactions cause objective collapse of superposition.
Maybe there's something subtle in the details which explains why it isn't that, but it certainly looks adjacent to it - although maybe not deliberately?
I was wondering last night: are any of the fundamental forces "blocked" by an intervening object? I assume not, since there is nothing like that in the equations. But that is kind of interesting, since sometimes you hear talk of hypothetical particles like gravitons.
Blocked? No. Greatly attenuated and restricted in what modes can be accommodated? Yes.
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.
They can't exactly occupy the same space due to the Pauli exclusion principle. IIRC that's believed to be the final "barrier" that prevents neutron stars from collapsing into black holes. But this is also getting into the tricky parts of wave particle duality, so the precise details are a bit difficult for me too.
Only cats can violate Pauli's principle. Anybody who had a cat can attest to the fact they can go through walls. Just close a room with a cat inside and - given enough time - the cat will escape.
It's not a stupid question at all, here's the answer:
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.
Thanks! Lots of information for me to learn about.
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
There is a difference between "being in a superposition" and "occupying the same space". The uncertainty principle basically tells you that an electron is never in a defined place - it exists with some probability in many different places (technically it could be at a definite position, but only if it had completely indefinite momentum, and that's not physically meaningful given energy constraints).
Now, say we have an experiment where two different sources each fire one electron in some direction; and say the electrons have the same spin and other properties except for their initial position and momentum. We can meaningfully say that for a certain location between the two sources there is some > 0 probability for either electron to be there, so the amplitude of each electron's wavefunction at that position is > 0. However, that doesn't mean we can ever find both electrons at that same postion at the same time: the individual wavefunctions are just parts of the two-electron system's wavefunction, and, per the Pauli exclusion principle, that one will be 0 for any state of the form "electron A at position x and electron B at position x". So, for any position, you can find either electron there, but never both.
An additional wrinkle is that this only applies for two identical electrons. If the electrons have different spins, then they can actually be found at the same location at the same time. You can have a spin-up and a spin-down electron in the same place at the same time, but not two spin-up electrons. This is the fundamental property of fermions. However, you can have any number of identical photons at the same location - that's the fundamental property of bosons.
The world is made of two types of particle - fermions and bosons. A difference between them is that fermions can not occupy the same quantum state (the Pauli exclusion principle) while bosons can exist as superpositions. Matter is made of electrons, protons and neutrons which are fermions, while the forces are photons, gluons, W , Z and Higgs which are bosons.
We, through our senses perceive a world. This world we try to understand using physics. But these perceptions itself stands on top of consciousness. So consciousness is at least, as real as the world that physics conventionally tries to reason about. It could be even more fundamental, because you can have only consciousness in this universe, but still could sense a whole universe with "things" inside it. But without consciousness, and just a universe with "things", there is no "sensing"...
Point is, physics should include study of consciousness...
There's a difference between making and testing hypotheses, and desperately scrambling to find proof to justify a conclusion made in advance in the face of mounting evidence that said conclusion is false, which is what Penrose is doing regarding quantum consciousness.
Grossly simplifying: he is a dualist, he wants to reconcile his belief in the soul and his physicist's materialist view of biology, so he resorts to putting the source of consciousness in quantum phenomenas, and seeks to prove that they occur in the brain. I disagree that there's an immaterial soul, I disagree that it puppets the material universe through quantum phenomenas, and I disagree that there are important quantum phenomenas in the brain. Most physicists are with me on these.
>source of consciousness in quantum phenomenas, and seeks to prove that they occur in the brain
I think he should look for consciousness as the source of quantum phenomena. Not the other way around. But as he is a physicist, all nails looks like a physics problem.
It is the same situation where people might have struggled to explain the heavenly observation using an earth centric model. Once you invert the wrong premise that you consider as fundamental, the answer, previously so elusive, becomes trivial...
I'm with you regarding opinion on consciousness, but do you think it would be 'better' if there were no dualists and no dualist essays and ideas?
Because I hardly think so.
This is not like alternative medicine where there is harm for people who don't follow the mainstream. And I stress again that Penrose (in the interviews I saw) doesn't actually claim that he absolutely has to be right.
> do you think it would be 'better' if there were no dualists and no dualist essays and ideas?
No of course not, people should be free to discuss and believe in dualism. What I find regrettable, is how often Penrose's name is invoked in an appeal to authority, as if his past (and very valuable) contributions to physics were enough to make him right on everything, including this subject.
Ultimately, I believe dualism is wrong, and I believe I have good reasons to think so. My previous comment was simply my attempt at deconstructing Penrose's argument, participating in the ever-ongoing popular debate.
> My previous comment was simply my attempt at deconstructing Penrose's argument, participating in the ever-ongoing popular debate.
That may be so, but just the mention of Penrose's name in the context of something else caused you to start a whole subthread that had nothing to do with the subject.
It's statistics that emerges with very high probability due to the law of large numbers. If you calculate 1+1=2 with 100% probability, then quantum physics doesn't meaningfully participate in your reasoning, not more than brownian motion.
You know, I pooh poohed his theories. Still do in sum. But there’s a there there that’s building.
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.
The article says that this proves that Einstein's equivalence principle (resulting in relativity) holds in this test of a falling quantum particle (where gravity results in a phase shift in the quantum state).
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.
> The Dirac equation and Quantum Electro Dynamics (QED) unify quantum mechanics and special relativity
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.
My understanding is that 1) SR doesn't consider acceleration (it's an extension of Galilean/uniform motion), and that 2) when Einstein considered acceleration as well as gravity via the equivalence principle which lead to GR [1]. The key insight of the equivalence principle was that the force from gravity (e.g. standing on the Earth) is no different to the observer in their frame of reference to them being in a room in a rocket accelerating at the same rate as gravity [1], [2].
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.
> the remaining piece is either to extend QED/QCD to accelerating frames of reference or to quantize general relativity
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.
From the article: "The result does not unite quantum mechanics and gravity, nor does it show that gravity itself is quantum".
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
I'm way in over my depth here, but does this maybe that the unification of gravity and quantum physics is further out of reach than we might have hoped? Because it would be easier if gravity disappeared at quantum scales - then it could be understood as an emergent property that emerges out of quantum when you move to bigger scales. But now we have to find something that underlies both.
Unfounded speculation, but is it possible that every particle could have a different individual light speed, and the one we know is just the average speed that they have to drop or speed up to, the way a car needs to travel at the same speed as the highway?
By "every particle" do you mean like, every type of particle?
If you mean individual particles, fundamental particles don't really have distinct individual identities (as shown by fermi and bose statistics).
As for types of particles: Well, photons surely move at the speed photons move at.
Special relativity is derived from the assumption/observation that light travels at the same speed in all inertial reference frames, and generally that the laws of physics work the same in any inertial reference frame.
What you are proposing sounds pretty vague and unclear to me, but, is what you are trying to say compatible with this?
Before you figure that out, you will be surprised to discover that it's impossible to tell if the speed of light is the same in both directions, with only the average of a round trip being the speed of light.
I hate this one because it's true Einstein said we cannot measure the speed of light in one direction that is independent of the clocks' synchronization technique being used to measure.
And yet, somehow we changed that to "we don't know the speed of light in one direction!!!" Which is bogus - can you imagine if speed of light was different if you were pointing east vs west? In space, what is even a direction vs another?
Did you even read the wiki you linked? We've done one-way speeds, it's down below.
How do you synchronize the clocks and compute the measurement, without depending on the conclusion?
You run into a similar problem with Michelson-Morley: because the apparatus and the light both get distorted by any motion, you can’t even in principle detect the aether wind with interferometry. You need a non-comoving dynamic source, eg, LIGO with black hole collisions.
Their one-way experiments seem to require similar, ie, we can observe dynamics but cannot measure a static bias.
Seems like they just added a linear gravitational potential term mgz to their hamiltonian and computed the phase change it would induce. They claim they experimentally confirmed the phase change. I didn't think that worked! They also claim its consistent with some kind of GR derivation, but I haven't looked at that.
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.
[1]: https://arxiv.org/pdf/2502.14535
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