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There's no evidence for 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

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.

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.

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

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.)

Thanks! We'll put that link in the toptext as well.

Any explanation by Roger Penrose is bound to be good

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

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.

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.

I have, but no one takes it seriously. But who cares, one needs to answer ones own questions, and not of the entire world.

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.

What measurable predictions does this hypothesis make that would differ from the existing models?

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.