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.
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
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.
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.
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.
Here's the [paper][1] on the arxiv.
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
There's no evidence for gravitons.
> ...if gravity is quantum...
You don't need evidence to support a premise.
Their point is measuring gravity's influence on a single particle is not necessarily evidence of the existence of the graviton.
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.
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.
That's a very deep question. In some sense no, in some sense yes. I would say on the deepest level, no, they are not blocked.
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.
https://www.science.org/doi/10.1126/sciadv.aec8045
> Blocked? No. Greatly attenuated and restricted in what modes can be accommodated? Yes.
Can you give me some examples? I guess I'm thinking lead barriers or a Bose Einstein condensate but curious what you mean here.
Your house has walls to attenuate photons.
Casimir plates are a good example, although Faraday cages are probably better known along with Mu-metal.
I am not very well versed in the subject but
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.
If particles were zero size, they would be already black holes. That's the fix introduced by string theory: particles aren't zero size there.
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.
How could you measure that the cat was inside the room?
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.
ed: typos
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
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.
In a black hole though … who knows
Vlatko tends to show up on papers with let's say, big claims. Consider the superconducting qubit/Tardigrade paper https://arxiv.org/abs/2112.07978
Vlatko Vedral (of Oxford) to be clear.
(He's listed as V. Vedral on the Tardigrade paper you linked, and it's more common to use last names in this context.)
There is a GREAT, accessible video about this experiment here:
https://www.youtube.com/watch?v=CfjnTJos_no
Thanks! We'll put that link in the toptext as well.
Any explanation by Roger Penrose is bound to be good
Except when he's rambling about quantum consciousness or other nonsense, which happens a lot nowadays.
Have you actually hear him talk about it? It is very far from rambling and he will be the first to tell you that he is not sure he is right.
We need physicists to come up with and develop novel ideas to consider no matter how many of them might be entirely wrong.
If mind was random, people would miscalculate 1+1 with 50% probability. So earth might be flat, but it's unlikely.
> If mind was random, people would miscalculate 1+1 with 50% probability.
That does not follow at all.
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.
They didn't start the fire. Quantum + biology → Woo has been with us since the 1960s at least.
Mind-bending stuff. Always wondered when we'd start seeing GR effects pop up at the quantum scale, makes sense.
Is it a subtle effect that cannot be explained by Newtonian gravity (i.e. a different potential affecting, V(z) in the Hamiltonian)?
No. From the paper linked above [0]:
> 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.
[0]: https://arxiv.org/pdf/2502.14535
so many great PBS Space Time on this concept
* https://www.youtube.com/@pbsspacetime/search?query=graviton
So do we finally have one unified theory or are we still none the wiser?
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.
[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"
Can one really derive classical GR completely from a QFT of a spin-2 field? Or only a linear approximation?
> 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.
[1] https://en.wikipedia.org/wiki/Unruh_effect
[2] https://en.wikipedia.org/wiki/Hawking_radiation
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
If you'd learn about a proposed experiment to test the quantum nature of gravity, this is a cool video:
https://www.youtube.com/watch?v=Uey_mUy1vN0
Hopefully we'll see a result in the next decade
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.
I have, but no one takes it seriously. But who cares, one needs to answer ones own questions, and not of the entire world.
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?
Probably no, because according to quantum mechanics, photons are indistinguishable in a way that affects experiments.
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.
See: https://en.wikipedia.org/wiki/One-way_speed_of_light
Anyway, reading that should help you answer your own question, or at least give you a lot more questions to ask.
Criticism: It's not literally impossible.
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.
What measurable predictions does this hypothesis make that would differ from the existing models?
Well, yeah. Perhaps if this experiment was expanded for a chain or a filter of cause and effect then it could be shown that one event affects another.
As opposed to current models, where one event does not affect subsequent events?
Good god, sorry I participated in any way. Apparently there’s a quantum physics colloquium I’m disturbing.
I'm sorry, I thought you knew how scientific ideas were discussed, I didn't know you were just trying to bootstrap a new religion.
I thought QM and GR were mathematically incompatible. How do you even perform an experiment without accepting one frame or the other? Sus.
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.