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Quantum Gravity Observed
Lawrence_Bird writes "AIP News is reporting the first observations of quantum gravitational states by researchers in Grenoble using a beam of ultra cold neutrons. This is an incredible observational achievement when you consider the energies involved - order of 1 pico electron volt (10^ -12eV). The full paper is in the 17 Jan Nature."
incredible observational achievement
I see...
Almost completely obscured by several quantum particles, French scientists measured another force believed to represent Enron chairman Ken Lay's sense of right or wrong.
But they admitted they could be mistaken....
There's no mention in the article of the impact or importance of this observation.
Anyone? Anyone?
.sig last updated Jan. 14, 2000
...did these people isolate a signal on the order of 10^-12 eV? My lock-in amplifier will only manage 10^-11.
http://simscience.org/membranes/advanced/essay/qua ntum_grav1.html
...has a pretty interesting explaination of quantum gravity and how it ties in with Einstien's Relativity and quantum mechanics, the two bedrocks of modern physics.
-- Your local friendly mad scientist-in-training
the fixed url.
Brief but nice overview of quantum gravity:
Quantum Gravity @ Dr. Jim Jessen
This really puts the nail in the coffin of General Relativity. We now know for sure that there will have to be an overhaul of that side of the physics.
Being able to observe gravitational effects at such a small scale could be the key to unlocking the unification of our disparate scientific views of the universe.
Imagine being able to manipulate all the forces, not just electro-magnetic. Gravity producing devices operating on electric principles?
This is going to be fun!
Using the holy grail of OSes...
It's great that they detected somthing so weak like gravity at a quantum level. This may finally help us understand what is it's like in a black hole.
If you don't understand any of my sayings, come to me in private and I shall take you in my German mouth.
I wonder what effect these observations will have on superstring theory, which is supposed to combine the physics of the micro-microscopic world (quantum physics) with the physics of the gigantic universe (general relativity), two branches of study that couldn't previously be combined due to huge inconsistancies in the math.
Superstring theory was supposed to have some profound effect on the theory of gravity, last I remember, but then, I haven't read up on it in a year or so, and there have probably been big developments since.
Thank you. I was just about to post asking if anyone had a copy of "Quantum Gravity for Dummies".
Great, now Hungry Minds is going to sue me for Copywrite infringement.
The sole purpose of the Internet is to get porn and bomb making plans into the hands of children.
the first thing i did when i saw that headline was make a quick mental check that it wasn't the first of april
if the results stand up, this could very well be the first major steps in what would easily be one of the greatest scientific achievements of the 21st century: the completion of einstein's dream of a grand unified theory
quantum gravity (when fully understood) will be the last step at showing the four fundamental forces of nature are in fact driven by a unified underlying principal, that on some level, they are the same.
various other people have posted good links for explanations.
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So if I understand quantum gravity correctly, it is possible for a neutron to stand still for a while, and to suddenly start falling at 1.7 cm/sec? So the way Wile E. Coyote is falling off a cliff isn't completely *wrong*, it is in fact a kid's first introduction to quantum phenomena.
Instead, the neutron is in a quantum state in a potential well. The fact that the potential well is due to gravity, rather than electrical or some other force, has nothing to do with the quantum nature of gravity itself.
Quantum gravity would be if the gravitational force itself were quantized, rather than the neutron state.
That doesn't mean that it isn't a great achievement in a difficult experimental field, which can be used to test fundamental physics including theories of gravity. It merely means that the /. headline is misleading.
This is not what a quantum gravity researcher would call "a test of quantum gravity", insofar as it does not demonstrate that the gravitational field is quantized. What this is, is a test of the effects on quantum matter of a classical gravitational field. In other words, as the Nature article says, it shows that gravity "can have a quantum effect" on other particles. But it does not show that gravity itself is quantized.
If you have a classical potential well, such as that due to a Maxwellian electric field, or a Newtonian or general relativistic gravitational field, a matter particle in that potential well will exhibit quantization of energy, momentum, etc. As the article says, this happens when the well is confining (when you don't have enough energy to escape the well).
An example is the energy levels of an electron in the electric field of an atomic nucleus, the standard orbitals you get when you solve the ordinary Schroedinger equation. Note that this assumes a potential due to an ordinary, classical electric field.
There are atomic effects due to the quantized electromagnetic field, like the quantum electrodynamics (QED) corrections to the Lamb shift coming from vacuum pair production. They crop up when you assume that the electromagnetic field is made up of individual quanta (photons). These effects are much smaller than the dominant, lowest-order classical effect.
So, what these researchers have done is demonstrated that a classical gravitational potential well can lead to quantized observables for matter, like the electronic orbitals of an atom. This is interesting by itself, because the gravitational field is so weak that the Earth's gravitational potential well is relatively much more "shallow" than the electric potential well of an atomic nucleus, as far as the strengths of the forces are concerned.
However, they have not shown that the gravitational field is itself quantized, any more than the quantized orbitals of electrons demonstrate the electromagnetic field is quantized. So they have not provided evidence for quantum gravity (i.e., a quantized gravitational field), any more than Bohr's law for atomic energy transitions provides evidence for QED.
True tests of quantum gravity are much harder than even this difficult experiment. To read about some proposals, try this paper on Planck scale phenomenology by Amelino-Camelia. (You can also see some of his other papers.)
Forgive me for being an amateur, but all they are saying here is that some scientists got some neutrons to display observable QM behavior in response to gravity. Quantum gravity as theorized requires a particle to bear the force (gravitons). If they had discovered gravitons interacting with the neutrons this would have been an epoch-making discovery. What we have here is a "stunning observational achievment" but to say we're all just going to pack up GR and move on to the next level is a bit premature.
God said it, Euler discovered it. The sig is in reference to my belief that if there *is* evidence for intellegent creation then it is not in complexity, such as the apparent "design" of evolutionary processes in biology, but in the fundamental preposterously beautify simplicity of mathematics and perhaps physics. The fact that those five numbers are so elegantly related simply boggles my mind. I've done a lot of independant study and am now well into a college level math track. I can prove the statement, but I can't hope to understand it.
In Capitalist America, bank robs you!
Why can't we use the same idea De Broglie used to explain quantized electron orbits, and apply it to orbiting macroscopic objects? You know, like the Rydberg constant but for gravitationally bound systems instead of Coulombic ones.
E=1/2*m*v^2-G*M*m/r, do the usual substitution for v, rewrite r in terms of n*lambda, where lambda=h/p, and solve for E? Would the resulting energy transitions correspond to the energy of the radiated gravitons? Since gravitons have zero mass, they should obey E=hf, and we can take the limit as n->infinity. Will f reduce to the orbital frequency, as in the case of Coulombic bound systems?
Or am I on crack?
Just to clarify, what's being talked about here is not what physicists usually refer to as 'quantum gravity'. Quantum gravitational effects are relevant at *extremely* large energies, much larger than the energy scales that characterize the processes that we associate with typical particle physics phenomena. It is very unlikely that we will learn much of anything about quantum gravity by looking at such low energy processes as the ones described in this story. There are some scenarios that bring down the scale that characterize quantum gravity to something on the order of TeV, but those are speculative.
Furthermore, learning about quantum gravity *does not* mean that we toss General Relativity. Regardless of what kind of physics goes on at the Planck scale, GR is absurdly accurate over a tremendous range of energies, much more so than we have any right to expect. For instance, even if we develop a consistent theory of Quantum Gravity you'd never use it to explain how the orbit of Mercury differs from the predictions of Newtonian
celestial mechanics, GR does this with as much precision as we'll ever be able to measure.
The results of the experiment in this story, while they may have to do with quantum mechanics in an external gravitational potential, are not the result of quantum gravity effects.
However, they really don't tell us anything about gravitons, any more than the Schroedinger equation for an atomic electron tells us about photons. All it really says is that "energy is lost somehow". It doesn't let us derive the detailed properties of whatever it's being lost to, not even their masslessness. To do that, you have to actually quantize the gravitational field, and get gravitons. It's analogous to going from QM to QED. Your thought experiment is purely QM, not on the level of quantum field theory. So it can't tell us about gravitons.
The /. title is wrong. The experiment had merely observed the quantization of neutron momentum in the external gravitation field. The gravitation in that model (the external field approximation) is a purely classical (non-quantum) potential, i.e. it afects the quantum particle (neutron) but it is not affected by the particle. To detect quantum gravity one would need an experiment that detects quantization of that field (e.g. particle-like aspect of the gravitational field, the same way that photons are manifestation of the quantization of the EM field).
Quantum falling was first used to measure the charge of the electron, where charged balls fell in gravity against a field. No-one knew at the time that it was the electron that was doing it.
The other amusing thing is the diversity of units, none of which are "SI": cm/s, electron volts, rather than m/s, J.
OS/2 - because choice is a terrible thing to waste.
Thanks for your reply. One more question, if I may: if we have an isolated gravitationally bound system, and that system moves to a lower energy state, how do we account for the change in angular momentum of the system?
Forget about antigravity boot, warp travel, and the like. I think the most important discovery is that those ultra cold particles they are shooting are probably responisble for that icy-cold spot in my bed. Seems to happen at about 3:00AM PST, which is probably about the time they get around to firing up the equipment in Grenoble, after their espresso and pastries.
I meant 'account for' as in conservation of angular momentum. If the system moves to a lower energy state, its angular momentum will change -- so what else in the universe also changes angular momentum in order to make the total change zero?
So I'm guessing the picture in your head is something like a big sun, and there's the earth going around the sun in circles. And then over time, you observe that the earth's orbit moves closer to the sun. Is there a loss of angular momentum here, or does the earth somehow speed up to compensate for it? Well, we can check it-
.x. v) for angular momentum. And noting that GM/r^2 = v^2/r, we can arrive at an expression for the angular momentum as a function of distance to the sun.
By some quick calculations, assuming circular orbits, you write L = m (r
L(r) = m (GMr)^(1/2)
So you realize that as the earth moves closer in, it's orbital angular momentum is dropping as sqrt(r). Is this a loss in angular momentum? Sure, but it has to go somewhere.
I'm no planetary physicist, but I'll point to the example of the moon Io orbiting Jupiter. There is also a loss of orbital angular momentum, and that gets transferred into tidal forces that stretch and compress Io, which is thought to be the source of Io's volcanic activity. So we should have to account for the change in angular momentum by looking at the internal degrees of freedom in the massive objects, like axial rotation of Sun and Earth objects. In other words, the internal rotation of the massive objects will get bumped up if orbits start to decay... That is, if we only have Newtonian physics.
By perhaps you're thinking of something more exotic. Let's say that instead of massive objects, these are totally point-like objects, so that you can't transfer angular momentum to them. And then you also observe that orbits decay! So what would cause these orbits to decay? Well, I've forgotten most of my general relativity now, but I think accelerating masses generate gravitons, so angular momentum can be carried off by gravitons too! This, I think, is a very small effect that'll usually get washed out by the other mundane business I mention above.
The GR gravitational wave decay was thought to be observed in some binary neutron star system. Sorry, I don't have a reference for it.
Yes, I had thought of that. Is this a good argument for the existence of gravitons? If they didn't exist, what else would carry away the angular momentum?
I have a feeling the the Quantum Gravity people need to team up with the Dark Energy people, because I suspect they are tackling the same issue. Case in point: Dark Energy is thought to have a "negative pressure" (i.e. the less dense, the more pressure), which is similar to the way "gravitons" work (as the more of them that strike an object, that is to say, the greater density, the less the pressure keeping two objects apart). Also, somehow, mass never seems to run out of gravitons. Stars eventually run out of photons, but gravitons never stop. What happens to all these hojillion gravitons? They can't ALL be absorbed by matter, can they? If they had even a nutrino's nutrino worth of mass, they could easily make up all the dark matter in the universe. Some food for thought...
One other thing, I wonder if there is a such thing as a gravatic black hole. Something so powerfully repulsive that gravitons cannot escape...
"Your superior intellect is no match for our puny weapons!"
To reiterate previous posts, this is just standard quantum mechanics with gravity thrown in. Not quantum gravity! Something quantum gravity- related would involve observing gravitons or something sensational like that.
But there have been older experiments which involve quantum mechanics and gravity. For example, Colella + Overhauser + Werner wrote "Observation of Gravitationally Induced Quantum Interference," Phys. Rev. Lett. 34, 1472 (1975). For any budding physicist, you can check chapter 2 of Sakurai.
For non-physicists, the experiment involves the idea of Feynman path integrals, which is a beautiful, but normal quantum mechanics, idea. Roughly, it says that a quantum wave of particles (let's say, neutrons!) traveling through some potential (let's say, a gravity potential!) will acquire a phase. Now, to pick up this phase, we can combine it with another wave of particles which DIDN'T go through the same path and see if there's interference effects. The result was "yes it does." Thus establishing the applicability of quantum mechanics to regular old gravitational wells.
Now, in this recent Nesvizhevsky et al. paper in Nature, the results are exciting because the authors picked up bound states in a gravitational well, just as one would pick up bound states in a nucleon well (gives us atoms and orbitals and that stuff.)
I'm not a particle physicist, so I got this question. My question is what happens you a neutron makes a transition from one bound state to another? In the atom, you can spontaneously emit a photon and cause a transition, which sometimes comes out in the visible regime so you can see color. Like when you burn cobalt and it turns blue (well, I don't know whether it's really blue or not.) So if a neutron in the Nesvizhevsky experiment made a transition from one height to a lower height, it's gotta be emitting gravitons, right? Or should I wait till the development of Quantum Gravity for an answer?
I see you have accepted questions from the public at large and done a fine job in answering them so here's mine....
This isn't GR, but it's at least associated with Da Man himself. Say you cool a cloud of radioactive atoms into a Bose-Einstein condensate and hold the condensate together for a period longer than the half life (or more accurately, a period long enough to where there is an overwhelmingly high probability that one radioactive decay would sponaneously occur). What happens? If the wave functions of the atoms all merge into a single wave function (admittedly a QM situation, not a GR one) then when the BEC is warmed, how is it "decided" which atom underwent decay? Maybe you could float this around your physics dept and see what the concensus is....
I only have a BS in Physics, marriage, kids, divorce and a job got in the way of my PhD, but I have the requisite curiosity in abundance, since these are such amazing times in which we live....
I see that you know the stuff, and you are happy to answer people's questions on slashdot.
Would it be too much to ask you to drop on wikipedia and add some of this knowledge to the physics section?
A fellow physicist and wikipedian.
When his defense asked, "Which computer has Jon Johansen trespassed upon?" the answer was: "His own."
This actually spawned 3 papers in the current Nature. Viewing the first two requires that your institution be subscribed, but the third one is for all to read.
Timeo idiotikOS et dona ferentes
A reference in the article about the equivalence principle reminded me that Einstein stated that there's no experiment that would enable an observer in a constantly accelerating, windowless vehicle to determine that they weren't stationary in a gravitional field.
I have never heard why a tidal force experiment wouldn't distinquish between the two cases. What is happening in the accelerating vehicle that mimics gravitational tides?
Put another way, if you're standing next to and perpendicular to a black hole, your feet are going to be ripped away from you. If you're standing in an accelerating elevator with an equivalent g force, what's ripping you to shreds? Aren't you getting scrunched instead?
Now that I've read most of the thread, I've gotta ask:
Anyone else feel as dumb as me?
"Sometimes a woman is a kind of religion, she can save your soul & set you free from all your sins" - Bad Examples
First, here's a link to the original article in Nature, where you can download the paper in PDF format.
Secondly, the electrical analogy is an excellent one. Basically, quantum theory started in 1900 with Planck postulating that atoms radiate energy (light, heat) only in discrete quantities. He used this as a "mathematical trick" to derive the spectrum of black-body radiation. (However, he didn't believe his "trick" was true in any literal sense until much later, about 1913). Then in 1905 Einstein postulated the existence of photons, and used them to explain the photoelectrical effect. I'll briefly explain what that is:
When you shine light on a metal plate, it can free electrons from the metal, which can then fly a short distance to a second plate and produce an electric current. What happens is that the electrons in the metal absorb some light and use this energy to break free from the metal (they need a certain threshold energy for this). Any additional energy they have left is then invested in their movement. According to the wave theory of light, the brighter the light you shine on the plate, the more energy the electrons absorb, and the more of them should be able to break free. But, that's not what happens. If you shine a very bright red light at the plate, you don't get any electrons, but a faint blue light, even if it contains much less energy in total, will liberate plenty of electrons. Einstein's explanation was that the photons of red light, having a longer wavelength, each contain less energy. If the light is very bright then you might have LOTS of photons, but each photon only has a relatively low energy. Now, typically, the probability that a given electron is hit by a photon is quite small. This means that those (lucky few) electrons that do aborb a photon will generally only absorb one, not more. If this is a red light photon, then this energy is simply not enough to break free of the metal, so there's no photo effect. But if you shine blue light at the plate, then each photon carries enough energy to liberate an electron, which is why you expect the effect to work with blue light. If you make the light brighter, then there are more photons, hence more electrons are released. But they each still have the same amount of energy. Incidentally, this is what Einstein got his Nobel prize for, not relativity.
Now for the analogy. What has been done in the Grenoble experiment is to confirm the analogue of Planck's result. So we now know (as we had guessed for a long time) that gravitational energy, at least in bound states, comes in discrete quantities. This does not yet imply the existence of gravitons, which would be analogous to photons. So the next experiment we would need is a gravitational version of the photo effect:
Imagine a system in which neutrons are bound in some state and need a little tug to be freed (I have no idea how to bind a neutron in a state such that such a weak tug could pull it free - remember that all other forces are SO much stronger than gravity). Then maybe we could see them pulled free by gravity, and notice the strange effect, that if we increase the gravitational field (by moving a large object near to it - with the experiment done in zero gee) we can pull free MORE neutrons, but each liberated neutron still starts off with the same energy (i.e. speed).
Anybody have any ideas for such a setup? Maybe we should study neutrons orbiting a small lead ball in a zero gee?
"...Look on my works, ye mighty, and despair!"
From the slapdowns by the informed set here, I get the feeling that this is showing quantization in the motion of the neutron, which proves about zip about the forces acting on it. I'm not even sure about whether it's the velocity or the acceleration that's quantised, but either way it's only a very tenuous suggestion (at best) that the gravitons acting on it might be quantized.
What strikes me is the comparison with computer models. I used to work on physics engines for games along with a maths geek who was most disgruntled at the dreadful granularity that we had to work with (double precision floats, how primitive!). He was horrified to discover that such engines often use a dt timestep to do things like (v += a * dt), and to be fair, at 30fps, this requires a little fudging to keep orbits circular or whatever.
So articles like this give me a fuzzy flow, because they intimate that reality is granular. More than a double precision float, or a 33ms timestep, sure, but only by degree. If my poor head is getting this right, the universe seems pixellated at a very fine level, so all us games developers need to do to model it accurately is to get our frame rates way up and our dt's way down. There's a goal to aim for. ;-)
If you were blocking sigs, you wouldn't have to read this.
The "quantum gravity" that Mssrs. Hawking, Thorne, etc. are looking for (and is likely to revolutionize both science and technology in many fundamental ways) is not this. What they're looking for is large-scale manifestation of real quantum gravity particles... in the form of gravity waves.
What this experiment measured was the small-scale effect of VIRTUAL quantum gravity particles. The particles themselves were still not detectable.
Why all the hub-bub? Because now that virtual quantum gravity particles are being characterized, it might be easier to build dectectors for real particles.
Or to find out *some* data about real particles from this data. I doubt we'll see a full characterization, however.
I am disrespectful to dirt! Can you see that I am serious?!
The experiment treats the Earth's gravity well as just another semiclassical potential well. You could get the same effect with protons and a very, very weak electric field (for example).
Not to belittle the experiment -- it's groundbreaking and interesting., and I (for one) can't wait to see a semiclassical quantum verification of the equivalence principle.
It's just not "quantum gravity" in the sense one might naturally think.
fine, but my point is that you do not need experimental data to push forward a theory of the interations of a graviton and normal matter. so could you not just assume that they are what is being radiated so you can make a function? then once your function is made, see how well it predicts events, if it does, then you have a theory that has a good chance of being correct.
when SR and GR were writen, there was no experimental proof that either were correct, just that it could accuratly explain events that could not be explained by classical physics.
I am the Alpha and the Omega-3
Where does the energy go, and what takes part in lowering the energy state? Then you know where to search for the 'lost' angular momentum.
The question is, what is all part of your system, and where/in which form will the energy go, if you are refering to energy-loss due to tidal forces: while the tidal-forces the moon exerts on the earth are slowing down the earth rotation, earth and moon are in fact moving farther apart to account for the angular momentum. So as the moon takes part in slowing down earths rotation it takes up the angular momentum (and some of the energy).
If you imagine your system as some kind of ideal particles (like a hydrogen atom), then moving to a lower energy state can only happen by sending out radiation of some sort (in the Hydrogen-atom electromagnetic radiation). So you no longer have an isolated System, the radiation is carrying not only the energy, but also the angular momentum away.
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