Ultra-weak gravitational field detected
nature.com
nature.com
Sure, this is Plank mass, 2 \times 10^-8 kg.
Length unit would be obviously PLank length, ditto for Plank time
> 10micrograms is around the scale at which quantum superpositions may be sustainable before decoherence becomes unstoppable
that doesn't make sense
There are no doubt many Noble prizes?
But how would a theoretical graviton explain this effect? Would it interact with the photon to carry over a force? And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?). Or would it interact with space itself?
Until we have a good quantum theory of gravity, we can't explain how it affects the curvature of spacetime at the quantum level.
All the current quantum gravity theories, i.e. loop quantum gravity, string theory, etc... work if you squint just right, otherwise they fall apart really fast. Spacetime has a bunch of episodes on going theories. A very large elephant in the room is, a lot of these ideas don't have a method of testing. And sure, a lot of theories start out without a practical method until decades later where technology can catch up, but we don't even have a theoretical test to start with!
Spacetime recently did an episode touching on some of your thoughts at the classical level and even then it's... relative[0].
Every quantum computing calculation, is a desktop quantum experiment ;)
Non-Gaussianity as a signature of a quantum theory of gravity
https://arxiv.org/abs/2004.01189
Another cool aspect of the optomechanical tabletop device research was that it wasn't entirely fruitless at all. Although the original mission of finding gravity in quantum space may have failed. The behavior exhibited was so extremely non-classical that an explosion of waveguide manipulating metamaterials was born!
Asking how a quantum theory of gravity would explain curvature is then asking someone to give you a completed quantum theory of gravity, and it is not at all clear that the first successful theory will describe the world terms of particles.
However, it can be instructive to consider electromagnetism as as classical theory and then as a quantum theory: waves in fields become enormous showers of particles, and these photons then carry the force that attracts or repels and can sometimes interact with each other.
It needs to produce better experimental results than modeling gravity as the curvature of spacetime. Whether that still describes it in a way that can be mapped to a curvature based explanation is irrelevant. It should for some cases give the same results?
For the theory to be viable it would have to explain why General Relativity is such a good effective theory at solar system scales. One can do this by treating General Relativity as a theory which emerges in some limit of the more fundamental theory of gravitation (or everything). One can compare that with how Newtonian gravitation emerges from General Relativity as one takes c to infinity, or as one takes v^2/c^2 (where c is usually set to one; one can likewise expand in other small parameters) to zero. The first is the basis for the first section of https://en.wikipedia.org/wiki/Post-Newtonian_expansion The second section has more explanatory power: Newtonian gravitation emerges from General Relativistic systems when multipole moments are small and the distance to the sources is large.
The key to the success of General Relativity compared to many of its competitors is that it very straightforwardly produces the Newtonian solution for the well-studied orbits of the Jovian system, while explaining the anomaly in the precession of Mercury's orbit.
One would have to be able to show conditions in the new theory in which General Relativity emerges (and for which General Relativity has observational support), or the theory is not going to get much practical use in astrophysics or GPS-style engineering, even if it appears to get the microscopic details right. Practically[1] nobody -- there are several serious contenders like Loop Quantum Gravity -- who has developed an alternative theory of gravitation (or everything) has been able to show at all that General Relativity emerges from it.
Lastly, because we don't have the technology to probe the quantum gravitation sector directly, but we do have the technology to probe General Relativity very finely (such is the topic of the article), I can't see how a failure to explain General Relativity's results can do anything but undermine quantum gravity candidates.
- --
[1] People have proposed turning various constants into spacetime-dependent functions, and find that such adjustments can solve some things but generally have to vanish practically everywhere, so such theories are not viable. Among these are Palatini and others https://en.wikipedia.org/wiki/F(R)_gravity and Brans-Dicke https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory. These families of theories usually show how General Relativity could emerge, but further investigation almost always shows that the parameters or features that distinguish these theories from General Relativity are incompatible with observation and so must vanish.
there would be more gravitons if the field is stronger. No gravitons in the empty space.
Gravitons are quantas of the always attractive forces. So traveling in the gravitation field, photon constantly absorbs and emits gravitons, more gravitons - more interactions, thus "bending" photon trajectory in the presence of the gravitation.
If you a bit math inclined, you could look at Feynman's lectures on gravitation (Caltech, fall semester of 1962).
They are quite outdated, a lot of mistakes - he was building it as he is preparing lectures, but approach itself is very interesting
He starts with MICROSCOPIC description - lets introduce gravitons, we want bosons which always attract, so no spin 1, only spin 0 or 2. Dropped spin 0 (don't remember why) and start building interaction diagrams form spin 2 bosons like in any standard quantum field theory.
Discussed exactly the topic of "trajectory bending", got to quasi-classic limit and showed that this is Einstein field eqs, got to weak field limit and Newton, ...
And all this (even his mistakes) in beautiful style and logic.
That being said the standard model is very good at predicting outcomes at a small scale. The issue with gravity is things where General relativity applies need to be huge to be observe. If we want to find a quantum model of gravity we need to observe what gravity does at a small scale and try to fit a model within these observations. That’s why this experiment is a step in the right direction. Maybe gravity does “weird” things at a smaller scale… Back to your question we don’t know we just observe :)
To be fair, Apple has been doing this (multiplying things by i) for a while now. Not that they have given any explanation why.
There's not much innovation right now in terms of personal computing devices. It's a solved problem. To show 'progress' on a year to year basis, something has to change. It used to be eSomething. Then iSomething. Now I suppose it's airSomething.
I'm not sure what comes after 'air', ordinalitly speaking.
This is really one of the best comments I have read on an article like this ever. Having had a cursory introduction to quantum physics in math terms and not understanding much of it, I find it impossible to read any sort of popular science literature on the subject. It's always going to be a very vague description with analogies that raise more questions than give answers. That's not to say I don't applaud the effort of quantum physicists to explain it to a broader public- I just think that most of the time they come away thinking it's some sort of black magic rather than solid science.
Anyway the most straightforward way to implement gravitons in this sense doesn't pan out mathematically so we're not even sure if this model is at all accurate.
Unpacking that statement a bit:
- In quantum mechanics there is a procedure called "quantization" that takes in a classical theory and gives you a quantum theory.
- The things (excitations) that follow your classical intuition are waves [*] and when you quantize them, you get excitations that carry discrete quanta of energy (ex. photons / gravitons / <particle name of choice>)
- It's convenient to talk about states with a definite number of particles because of the way the math works out.
- To get back to something that follows your classical intuition (waves) you have to consider superpositions of the states with definite particle number. These particular states are known as "coherent states" and very much look like your normal notion of a classical wave which is something with a definite phase and amplitude
As an example: you can recover something that looks like a classical radio wave in quantum electrodynamics by taking a particular superposition of states with a definite number of photons. Since it's in superposition, the number of photons in the state isn't well defined. By well defined here, I mean if you measure the number of particles, you can get different answers due to Heisenberg uncertainty.
You can even get standing waves in a cavity (like a microwave oven) which is an oscillator and the state has a definite phase and amplitude that you can measure. Everything looks nice and classical but if you look hard enough you find that actually you can't put in an infinitesimally small amount of energy, you can only add it in multiples of some constant (h-bar).
Now replace the electromagnetic field with a field that represents the curvature of spacetime and away you go, your theory works with small excitation quanta (gravitons) but the actual things in everyday life that you see are complicated superpositions of these things [**].
So the geometry (spacetime) is some superposition of states with definite graviton numbers that have no classical description. Much like how the electric field around an electron or an oscillating electromagnetic field inside of a microwave oven is a superposition of states with definite photon number. Sidenote: it’s also easy to talk about a single graviton in isolation sitting on top of that emergent geometry
[*] This is "backwards" from pop-sci / intro physics because we are talking about electromagnetism which shares some superficial similarities with gravity
[**] For some technical reasons this viewpoint doesn't apply to particles which aren't "force-carriers" ex. electrons, protons, etc
https://en.wikipedia.org/wiki/Static_forces_and_virtual-part...
Even though nothing is fully static, it is a way of approximating.
So the theories would presumably have something similar for the static gravity case you are describing.
We have some findings (hey! mass is intimately connected to curvature of spacetime!) and approximations in forms formulae we know, but that's just it.
I think our physics is akin to epicycles before we found actual place of Earth in solar system and shape of planet orbits.
There's a variety of ideas on how to do this but none have really played out.
Correct!
> Maybe I am missing something, but how could a quantum theory of gravity explain curvature of spacetime? […] how would a theoretical graviton explain this effect?
Note that a quantum theory of gravity need not involve gravitons. The latter only appear in approaches that assume that gravity is a more or less ordinary [string] quantum field theory (QFT). Personally, I've never found this very convincing. Neither did Hawking by the way, who once wrote
> It would be rather boring if this were the case. Gravity would be just like any other field. But I believe it is distinctively different, because it shapes the arena in which it acts, unlike other fields which act in a fixed spacetime background.[0]
Anyway, assuming we're indeed looking at a quantum theory of gravity that involves gravitons, let's tackle your other questions:
> Would it interact with the photon to carry over a force?
Yes, the graviton would interact (very weakly) with all particles we know, including itself.
> And if so, how would it "know" the direction of said force (wouldn't it be itself subject to gravity?).
My first, rather theoretical answer would be: This is not a well-defined question. After all, gravity as a force is a Newtonian concept which does not exist in General Relativity. As you already said, in Relativity matter is "guided" by the curvature of spacetime, not by a force. Moreover, in quantum mechanics Newtonian forces don't (really) exist, either, and while people often speak of "forces" as if they were Newtonian, these forces are really mediated by bosons.
My second, more direct answer would be: In two-particle interactions in quantum field theory, the outgoing particles usually leave the scenery at a different angle (and momentum) than their initial one. So the photon might get deflected by interacting with the graviton.
> Or would [the graviton] interact with space itself?
As already mentioned, the graviton would interact with itself in the same way as gravitational waves can interact with one another. As for space(-time) itself, however, note that the aforementioned "theories involving gravitons" usually assume that spacetime itself is flat (i.e., it is your run-of-the-mill Minkowski spacetime) and that gravitons are just another quantum field on that (so-called) background spacetime. As I personally don't like this idea a lot (see above), I can only guess as to whether its proponents really think spacetime is flat or whether they only assume this to simplify things. After all, we have no idea how to do QFT on curved spacetime in a mathematically precise way. (In fact, we don't know how to do QFT on a flat Minkowski background in a mathematically precise way, either.)
[0]: Hawking: The Nature of Space and Time, https://arxiv.org/abs/hep-th/9409195v1
> The findings take us a step nearer to measuring gravitational fields that are so weak that they could enter the quantum regime.
...is overstated. There's a long way between 90 milligrams and quantum gravity.
And in terms of force, the article states that the force involved in that experiment is equivalent to the force of earth gravity on a mass of 10^–15kg, which is probably even closer to the energy of a graviton.
For a lay reader, it would give them an overall concept they can tie the just-read article to, increasing the likelihood of retention.
For an astute reader, yeah, it smells like bullshit.
Well, turns out if you go small enough then you see it's actually individual packets of light with certain amounts of energy where the amount of packets (photons) sent out decreases as you turn down the intensity. So, the total amount of energy outputted still matches the input, but the output is in discrete packets, where each packet can exceed the level of input energy at a specific time. So, on a large scale everything seems fine but as you go to very small energy levels it starts to look different.
I guess the expectation with gravity is that going to very small masses, some different underlying mechanism will appear. Or it may not. So they're trying to find out.
Still, obviously large masses have gravity, so everyone expects that, even if we can't measure it yet, and don't know how to compute it yet, there must be some attraction between particles. There are some theories about it (for example string theory has something that looks like gravity), but since they all predict values much lower than we could hope to measure for now, the problem remains wide open.
However, simply applying Newton's or Einstein's theories of gravity to QM doesn't work. In QM, particles don't have a single position in space and time, each particle is more like a wave with peaks at different points in space at the same time. If you try to compute the gravitational field generated by all these peaks you get nonsense results. So, while we generally believe that there must be some gravitational attraction between particles with mass, we don't know how it would look like.
I think there are also some theories that predict that elementary particles do NOT interact through gravity, that gravity is somehow an emergent phenomenon of a collection of many particles (just like an elementary particle doesn't have a temperature, but a collection of many many particles does have one). I believe this is a pretty fringe theory, but not quite "flat earth" land. Just including it for completeness.
The maths of QM (Schrodinger equation, standard model) currently say that particles don't have gravity. But, they also imply that nothing has gravity - gravity doesn't exist at all according to the standard model. Obviously this is wrong, so we are searching for a theory of Quantum Gravity.
There are various pieces of math that do have gravity and are consistent with standard QM, but each of them has various other problems, so none is universally accepted, and they vary significantly in how they add gravity to the standard model.
That is correct--it has been expected (since, you know, Newton...) but not yet measured at this scale. That is why the headline says "detected" as opposed to theorized.
>What am I missing when they say quantum particles do not fit with gravity.
Quantum mechanics predicts that Newtons description of gravity will break down under certain conditions. We currently do not have the ability to measure this effect, so it is all theoretical at this point.
Yes. Absolutely everything should have a gravitational field.
> What am I missing when they say quantum particles do not fit with gravity.
Under Relativity, mass and energy are equivalent, therefore energy has a gravitational field. If you try to mathematically quantise the gravitational field the way the electromagnetic field was quantised (the latter giving you a photon, the former would be a graviton), then something goes wrong.
This is where my grasp of the physics gets a bit hazy; I think you find any amount of gravity should produce more gravity, and this self-generation blows up to infinity rather than summing to a finite value.
Or it might have something to do with the expected value of a corresponding zero-point gravity field? Or both? I’m not sure, I only do physics for fun.
I keep thinking it’s a gravitational equivalent to the Ultraviolet Catastrophe, but if it was that simple someone would’ve already solved it.
Things like this are obvious in retrospect, but radical for their time.. most of the physics I learned was elegant and well-understood, then particle physics just... didn’t really feel the same way. Tons of gotchas and edge cases, difficulty generalizing problems, and the obvious disconnect between the physics of the small and that of the large...
In time we’ll look back at current iterations of the standard model and our understanding of the physics of the small and wonder how those Neanderthals didn’t just realize that xyz was the key to make everything much simpler and work in harmony... that’ll be a fun day!
Gravity is something like 10^32 power smaller than the next meaningful force. The problem with gravity is this-- At that resolution, we can't distinguish between two theories-- that mass warps space-time, or that there is a particle that mediates the force of gravity (a "graviton"). That is to say, is gravity analog or is it digital? If gravity is digital, there is a particle that is extremely small that transmits its force. If gravity is analog, then it changes the space around us itself and acts exactly the same way.
So we have a question for the ages-- we have 17 to 25 quantum fields (depending on how you count) and, one, analog field, gravity. So, is gravity analog, or is it impossibly small digital field?
For gods sakes is there a physicist out there, please correct me if I'm wrong. :-)
We know that matter attracts other matter, at the macro level. As this article shows, we've been able to detect the gravitational pull of tiny masses (tiny by macro standards). Since the macro world is made out of the quantum world, QM must somehow explain how gravity arises for particles.
Until it does, there is a fundamental piece missing from our understanding of the universe, so any conclusion we draw from QM or GR must be taken with a large grain of salt. You can't say 'matter is excitation in a quantum field, why should it attract', that is exactly the problem: we know it attracts, so if e citations in a quantum field don't attract, it must mean that matter is something else.
Note: mass is generally defined by E=mc^2, where energy is defined by conservation laws.
I guess I get hung up on the quantum field part because I don't understand what exactly a quantum field is in a physical sense. I get that there's a mathematical concept of a quantum field, but does that map to something concrete? Is everything essentially made of nothing?
Whether they are directly describing reality or some kind of approximation remains an open question. I believe that some progress on the measurement problem, when it happens, could help point in this direction.
1. An account for how the world actually works are Newton's laws of motion - they describe how exactly objects interact and how it affects their movement. It is of course an approximation, and only works at certain scales and speeds, but it describes mathematically the phenomena that we can see happen.
2. An alternative account that gives the same values is Hamiltonian mechanics, which describes the system in terms of how its energy changes over time in certain coordinates. They both give the same predictions, and Hamiltonian mechanics is more suited for actually computing the motion of complex systems, but it is much more abstract and doesn't give a clear account of how the system actually works.
Edit: measuring the force of the ISS passing over head would require femtonewton precision for a mass of 100kg. Might be a bit tricky lol
Spread the masses out over orbit, measure net gravity on those (by orbit deviations?), and you should be able to detect changes in gravity from distant objects. With enough spread maaaaybe you could do something with parallax to detect distance too.
Imagine a system like this in 'the expanse' to detect the stealth asteroids hurled towards earth. Or, in general, to detect massive ships that are 'on the float'.
Radio over Gravity !
> The gravitational force is the weakest of these four.
Isn't that distance dependent? At macroscopic distances, the weak and strong interactions are even weaker, no?
For the strong force the situation is the same in a sense, if you had a kg each of quarks of different colors — except you literally can’t have that, because the energy involved in separating a pair of quarks is so large that it actually causes two new quarks to snap into existence and pair off with the two you were trying to separate before you can separate them on anything approaching macroscopic scales [3]. So the strong force is obligatorily screened (camouflaged) at long distances, but only because it is so strong.
The kg-of-protons experiment on the other hand you could do in principle, it would just probably be better if you didn’t.
[0] https://www.feynmanlectures.caltech.edu/II_01.html
[1] https://sieste.wordpress.com/2012/04/22/feynmans-electric-fo...
What if you want to build a fusion reactor? For now neutrons have to be along for the ride but that's only because we can't squish and heat plain protons hard enough.
The EM force between a proton here and at the edge of the visible universe is the same as the gravitational force between two protons at 1000 meters distance. The scale difference in the "hierarchy problem" is truly not graspable by human intuition.
[This PDF](http://web.mit.edu/sahughes/www/8.022/lec01.pdf) starts with some potentially helpful context.
Usually this claim is discussed in connection to the so-called ["hierarchy problem"](https://en.wikipedia.org/wiki/Hierarchy_problem).
*Relative to an external observer.
Special relativity is weird, but it's very much proven by countless experiments.
Scientists, instead of requesting time on Hubble, ask time for the 3DBuilderAndExperimenter, give it a blueprint and inputs/outputs, it runs, returns any results, and is recycled.
We are such machines, so we know they’re possible to make, even though we don’t yet know how to make one on Earth let alone in space.
- send components to ISS
- assemble
- subtly nudge the container to a different orbit
- wait for it to do the experiment and catch when the orbits meet again. Or if that's too hard just wait for results to be transmitted
> Their estimate deviates from the internationally agreed value (see go.nature.com/2bwkrqz) by about 9% — a small amount, given that the experimental uncertainties of their system have not yet not been optimized for precise measurements of G.
is just misleading. We’re pretty sure of the value of the gravitational constant to at least 4-5 significant figures. The experiment is cool but it’s not like it’s going to get us a more accurate value.
A lot of things we think we know and take for granted is actually slightly inaccurate when the scale shifts drastically. That's because we focus on absolutes instead of relative values. Numbers are a human construct, scientists have forgotten that. Because our brain can easily add 2+2 doesn't mean it's a technique that scales infinitely and remains true. Do we have proof of numbers stability through scale or do we just say there's an "infinite" amount of them to make it easier on the explaining?
ps: pi is another of these constants that rule the laws of the universe. We can't really explain why but it does. It's the same principle as for light. Constants are true not matter the scope, because everything is relative. Our numbers can't truly represent that because additions are inherently flawed when the scale shifts. Everything that adds recursively is actually a multiplication array, and thinking of it as an addition is inaccurate.
tldr: our math needs to evolve to follow an OoP logic instead of being stuck at a human scale perspective.
We know the force-distance (field strength) curves of the "chemical bonding" forces[0].
[0] https://en.wikipedia.org/wiki/Nuclear_force#Field_strength