Quantum Gravity Observed 224
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."
Yep (Score:2, Funny)
I see...
Also observed by the microscope... (Score:2, Offtopic)
But they admitted they could be mistaken....
Re:Also observed by the microscope... (Score:1, Offtopic)
Impact (Score:2, Interesting)
There's no mention in the article of the impact or importance of this observation.
Anyone? Anyone?
Re:Impact (Score:1)
Beam me up [Re:Impact] (Score:2, Funny)
Re:Impact (Score:3, Informative)
Re:Impact (Score:3, Informative)
To use the electrical analogy, the existence of discrete atomic spectra did not prove the existence of photons. It only proved that atoms can only emit and absorb electromagnetic energy in discrete amounts. To actually prove the existence of photons, in quantum electrodynamics, required more subtle work. I think the first airtight experimental proof was in spontaneous emission of photons, actually.
Re:Impact (Score:1)
Analogue of the photoelectrical effect? (Score:5, Interesting)
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?
Re:Impact (Score:2)
Re:Impact (Score:2, Informative)
It would be a Bad Thing (tm) if quantum mechanics were inconsistant on a macroscopic scale *phew*.
how on earth... (Score:3, Funny)
Re:how on earth... (Score:1, Redundant)
It gets better:
The next step is to use a more intense beam and an enclosure mirrored on all sides (the energy resolution improves the longer the neutrons spend in the device). An energy resolution as sharp as 10-18 eV is expected
The Gardener
They used an old trick (Score:5, Interesting)
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.
Re:They used an old trick (Score:1)
Re:They used an old trick (Score:2, Interesting)
MKS: physics
English: engineering
CGS: astrophysics
A physicist will say 9.8 m/s^2, but an astrophysicist will say 2x10^33 grams. There's then the adage of "if you see things in a schematic like 10 cm x 50 cm, it's never been built".
Physics does typically use MKS - it's astrophysics that's on the CGS trip. There are reasons for it (What's the magnetic field of the earth? Bout a gauss - tesla are WAY too huge to be convenient except for certain wacky guys).
As for the reason for electrostatic units (e.s.u.), that's simple: they're not a holdover for anything, it's just that when you're not talking about things that are measured in a lab (like volts, amps) you might as well use convenient math, and for e.s.u., the Coloumb force is just qq/r^2. For astrophysics, that's the easiest way to do it.
There will always be a split in the physics community, though, between theorists and experimentalists. Keeping track of constants is a pain in theory - the math is hard enough already without shoving constants in front of everything, and the constants really confuse a lot of the underlying structure, as well. Therefore, theorists will always use natural units, with everything set to 1, basically (Heaviside-Lorentz units for electric charge: set epsilon-naught to 1, mu-naught to 1, so the speed of light is 1). Unfortunately, those numbers are ridiculously inconvenient for actually DOING any physics, since we don't live on a subatomic scale, so real physicists (er... experimentalists
Re:They used an old trick (Score:2)
F=QQ/4pi r^2.
The unit of charge is 1/(2sqrt(pi)) esu.
It's not hard to give CGS units names: Kennelly did this in 1904 {ab-, stat-}, but in practice, scientists do not use units in calculations, and this is why the prefixes (or any naming convention unit), ever caught on.
People who actually do the experimental work will invent units suitable for the task at hand. Refering to any decent units dictionary will dispel any contary belief.
Re:They used an old trick (Score:2)
You can make the speed of light 1 in any system you want, just by adjusting mu-naught (so long as it remains free), but that makes the Maxwell equations ucky.
Re:They used an old trick (Score:1)
Re:They used an old trick (Score:2)
Formerly, the practical electric units were additionals like miles and hours. That selected emu have names (Oersted, Gauss, Maxwell), and the absence of names for more obvious measures (eg charge), is the give-away here. Basically it was cgs + Practical units + some named cgs units.
Re:They used an old trick (Score:2, Informative)
Re:They used an old trick (Score:2)
Anything that is formed with an SI prefix and an SI unit is obviously SI, so CGS has to be SI. I also know that the weird CGS equivalents for certain MKS names (i.e. Gauss vs. tesla, dyne vs. newton) are SI as well, so there's no problem there either.
SI is really quite inclusive. Wasn't there something a while back that said that English units are now SI as well, so "English" is just a system choice? (including English units simply means defining a conversion: 2.54 cm/inch, for instance)
Re:They used an old trick (Score:2)
I don't think so, since cgs uses things like "ergs" for energy, to name but one minor difference.
In actual fact, while essentially all other measurements systems are based or defined on the SI system, the SI system itself has a set of units and the like which are set by various bodies after much arguement.
Just because you or I decide to start talking about fleemes and microfleems defined in terms of SI units does not make our system of fleeme measurement part of the SI.
For a simpler introduction to Quantum Gravity... (Score:5, Informative)
Re:For a simpler introduction to Quantum Gravity.. (Score:5, Informative)
Re:For a simpler introduction to Quantum Gravity.. (Score:2)
Great, now Hungry Minds is going to sue me for Copywrite infringement.
This is great news! (Score:1)
Brief Quantum Gravity Info... (Score:5, Informative)
Brief but nice overview of quantum gravity:
Quantum Gravity [drjimjessen.com] @ Dr. Jim Jessen
Let me be the first to say it. (Score:2, Insightful)
Re:Let me be the first to say it. (Score:1)
I don't think it's quite as drastic as that. Good old General Relativity has still been tried, tested and confirmed time and time again on a large scale.
We might get some additions to the theory, but the fundamental observations made and the underlying maths will still hold, I think.
Re:Let me be the first to say it. (Score:3)
The implications are enormous... (Score:4, Funny)
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!
Re:The implications are enormous... (Score:2)
Yah, that's what they all said... until some dummy switched off the planet's gravity. Whoopsie!
Re:The implications are enormous... (Score:5, Funny)
Re:The implications are enormous... (Score:1)
But once they did understand, it weighed heavily on their minds.
It was not a superposition to be in.
This is pretty amazing (Score:2, Insightful)
Strings & gravity (Score:5, Interesting)
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.
Re:Strings & gravity (Score:5, Informative)
Absolutely none. String theory contains a theory of quantum gravity. But as pointed out correctly by the Anonymous Coward above, this discovery is not a discovery of "quantum gravity", as the term is usually used. They have discovered quantization of neutron orbits in the classical gravitational potential, analagous to the quantization of electron orbits around a proton. (You know, the s,p,d,f energy levels from chemistry?) At low energies (and these are VERY low energies), our classical picture of gravity is extremely accurate, and there's not a graviton in sight. Experimental proof that the graviton exists would be proof of quantum gravity.
Discovering quantum gravity is much, much harder. The energy scale at which quantum gravity becomes important is 10^19 GeV (note 1 GeV/c^2=mass of proton). The accelerators we're building now are 2000 GeV. We won't get to 10^19 in our lifetimes, if ever. There has been a flurry of papers recently saying that we might see quantum gravity at current or near-term accelerators, but they do this by invoking extra dimensions. In other words, curled up extra space-time dimensions that are as big as 100um, and only gravity propegates in the extra dimensions. This has the effect of lowering the energy scale at which gravity becomes important, so that we might be able to see it.
But if that 10^19 figure is really correct, we ain't gonna see quantum gravity anytime soon. Unfortunately...
--Bob
Re:Strings & gravity (Score:2)
Probably the same effect as Schrodinger's Box had on his cat.
http://www-groups.dcs.st-andrews.ac.uk/~history
Re:Strings & gravity (Score:5, Informative)
The Higgs Boson is part of the Standard Model, which is a Quantum Field Theory. (Note the "Quantum" in "Quantum Field Theory") It is not a classical theory. Perhaps you meant "Standard Model" rather than "Classical Theory". The word "Classical" to a physicist means "non-quantum".
As Schrodinger might say, there's more than one way to skin a cat, and there are is more than one way to give mass to the W and Z bosons (which is why we want the higgs). It has been proposed, for instance, that there are several higgs', separated in energy by about 10GeV. This could be responsible for the experimental evidince seen at LEP2 just before it was shut down. One thing is clear, however. Whether or not we find the higgs boson, the LHC (next accelerator at CERN) must find new phenomena. Some of our calculations simply don't make sense as you increase the energy. The higgs is a good, simple solution to part of the problem, but that doesn't mean it's necessarily correct. We theorists like to pretend we know what we'll see (SUSY/Extra Dimensions/Higgs) but I like to keep my mind open and hope that we'll be surprised by what we see at the LHC.
--Bob
Re:Strings & gravity (Score:2)
Someone else already pointed out the fact that "classical" to a physicist doesn't mean what you think it means, so I won't belabor that point.
However, I am going to object to your statement that "the theory falls apart with the Higgs". There is no truth to that whatsoever. If there is no standard Higgs, the Standard Model is toast - but not the Quantum Field Theory (the theory part, not the model part) that the model is based on. There are numerous alternatives to the Standard Higgs sector (SUSY, extra dimensions, dynamical models, dimensional deconstruction, etc.) that we theorists are speculating about, and there are probably other alternatives that we haven't thought of yet.
And even if String Theory is the fundamental theory describing nature, it still doesn't mean that there is anything wrong with the low energy limit that is the Standard Model; after all, we still use Newtonian mechanics where that is appropriate, and we don't try to how apples fall to earth using quantum gravity.
i'm amazed (Score:4, Insightful)
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.
Re:i'm amazed (Score:1)
Re:i'm amazed (offtopic, don't mod) (Score:2)
Basis for cartoon gravity (Score:5, Funny)
Re:Basis for cartoon gravity (Score:2)
Your proposition about standing still is correct. I'm not sure about the speed it falls at, but a collision between a graviton and a neutron might cause that sort of speed in the direction the graviton arrives from. The graviton might also be scattered out in another direction as well.
Re:Basis for cartoon gravity (Score:2, Informative)
Going back to the boundard conditions issue, this is how the experiment works. There is an absorption plate which essential determines the width of the channel. Classically a few neutrons will get through even the narrowest of channels, but quantum mechanically it has to be wider than the wavelength of the relevant particle. The curious thing about this experiment is that the channel is much wider (15 microns) than the neutron wavelength (0.01 micron) and visible light (0.6 micron) but the visible light gets through while the neutron does not! A straighforward explaination is to include the gravitational interaction quantum mechanically; then you get a neutron-graviton quasiparticle with a much longer wavelength that cannot fit through the slit. However, as the mass of light is darn small it couples very weakly and goes through essentially unchanged. The neutron, on the other hand, is sufficiently massive to cause a "strong" coupling and thus doesn't get through.
What about creating gravity? (Score:1)
Maybe all of the research into slowing light down might make this effect measurable...
Quantum? (Score:1)
Yum. (Score:1)
AIP News is reporting the first observations of quantum gravitational states by researchers in Grenoble using a beam of ultra cold neurons.
These "quantum gravitational states" sound trippy. What are these "researchers" on and where can I get some?
Nobel Prize Material (Score:1)
Not 'Quantum gravity' (Score:5, Insightful)
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.
Re:Not 'Quantum gravity' (Score:5, Informative)
The experiment is basically the same one that discovered the electron (with a few details changed).
In essence, if you select a mass small enough, you may be able to observe its interaction with individual quanta. What these people did was to slow the neutron down so much that they could see it fall under gravity. Their idea was that in stead of falling in a parabola, you should be able to see the polygon sides as the individual quanta hit, or downwards speeds quantised at different multiples of a base. It is this second element that they observed.
Since in the past, this yielded the experimental evidence for the electron, here it yields what could be experimental data for the graviton.
Re:Not 'Quantum gravity' (Score:1, Insightful)
Re:Not 'Quantum gravity' (Score:5, Interesting)
Re:Not 'Quantum gravity' (Score:1, Informative)
Re:Not 'Quantum gravity' (Score:2)
Re:Not 'Quantum gravity' (Score:2, Informative)
I should add that I also agree with Viadd that it is an impressive experimental feat anyway.
Svein.
Re:Not 'Quantum gravity' (Score:2)
One has to be careful with one's terminology. Photons have zero REST mass. Since there is an equivelnce between mass and energy, a bunch of photons gathered together (as I understand things) would in fact produce a gravitational field.
Re:Not 'Quantum gravity' (Score:2)
Well, the mass/energy equivalence has a farily large body of experimental measurements to back it up, and there are a lot of theoretical results that depend on this equivalence specifically in the case of the photon, and those calculations do show agreement with real world measurements.
But yes, it is true that nobody has taken a "jar full of photons" and directly measured the gravitational field due to those photons.
Very misleading, not "proof of quantum gravity"! (Score:5, Insightful)
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 [arxiv.org]. (You can also see some of his other papers [arxiv.org].)
Re:Very misleading, not "proof of quantum gravity" (Score:2)
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?
Re:Very misleading, not "proof of quantum gravity" (Score:4, Interesting)
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.
Re:Very misleading, not "proof of quantum gravity" (Score:2)
Re:Very misleading, not "proof of quantum gravity" (Score:2, Informative)
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.
Re:Very misleading, not "proof of quantum gravity" (Score:2)
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.
Re:Very misleading, not "proof of quantum gravity" (Score:2)
Re:Very misleading, not "proof of quantum gravity" (Score:2)
Re:Very misleading, not "proof of quantum gravity" (Score:1)
this would at the very least give people an idea of how a graviton would act and then allow people to look for those situations in order to observe the graviton.
you don't realy need evidence to make a theory, just show that your equasions acuratly predict events in the universe.
Re:Very misleading, not "proof of quantum gravity" (Score:2)
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.
Re:Very misleading, not "proof of quantum gravity" (Score:4, Interesting)
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....
Wikipedia. (Score:2)
Would it be too much to ask you to drop on wikipedia [wikipedia.com] and add some of this knowledge to the physics section?
A fellow physicist and wikipedian.
I don't get it - where are the Gravitons? (Score:3, Insightful)
Re:I don't get it - where are the Gravitons? (Score:1)
Quantum Gravity? Not here... (Score:5, Insightful)
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.
Re:Quantum Gravity? Not here... (Score:2)
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.
How, exactly, do you know that to be true if we aren't currently able to measure that accurately? Not that I'm disagreeing, but as one who obviously considers himself to be a scientist, I would think you would be a little more wary of making unproven statements like that... one must keep an open mind if progress is to continue
That's not Quantum Gravity (Score:4, Insightful)
Re:That's not Quantum Gravity (Score:2)
It is quite possible that it may be a spectral effect similar to ionisation, but if the spectra proves to be regularly placed spikes, then it is better explained as a neutron hit by 1, 2, 3, 4, of the gravitons that make up the earth's gravitional field.
The same experiment done at midday could randomly reveal neutrons hit by gravitons from the Sun, and therefore falling upward!!!
This just goes to show... (Score:1)
In other news (Score:1, Offtopic)
Quantum Gravity and Dark Energy (Score:3, Interesting)
One other thing, I wonder if there is a such thing as a gravatic black hole. Something so powerfully repulsive that gravitons cannot escape...
Re:Quantum Gravity and Dark Energy (Score:2)
Re:Quantum Gravity and Dark Energy (Score:2)
Since we are in la-la land in terms of formulating theories, that's not true in general. We can have all sorts of crazy things where gravitons and photons form different causal structures, like for example dimetric theories. Then gravitons and photons will have different "event horizons" as defined by different geodesics formed by the different metric structures of both. For example.
Sorry to nitpick here, but you touched something closed to my heart
Other gravity + QM experiments done before. (Score:5, Interesting)
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?
Nature Papers (Score:2)
Slightly OT ... (Score:2)
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?
Re:Slightly OT ... (Score:2)
Hmmm.... (Score:2)
Anyone else feel as dumb as me?
Comparison to computer modelling (Score:3, Insightful)
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. ;-)
Re:Comparison to computer modelling (Score:2)
Yes and no. A good analogy is with the notes you can play on a guitar string. You can play the fundamental or various higher harmonics but you can't play the notes in between without changing the length. But you can play various blends of different harmonics. So there's still a continuous infinity of different sounds you can produce. Well QM's a bit like that. It's very different from being pixellated.
Re:Comparison to computer modelling (Score:2)
A grinding sound emerges from my brain as I wonder how a universe that emerged from a singularity and which expressed quantized effects from the first instant could be viewed as having a background continuum. There's all those little wavicles starting at one point and making their quantum hops and skips around. At the very, very finest level, isn't there a finite number of positions they can be in? I'm thinking more integer positions than floats.
Sorry, I'm rambling. Don't mind me. ;-)
Re:Comparison to computer modelling (Score:2)
Well here's the interesting thing about 3D QG (or at least one type of 3D QG): you can express it as a bunch of partial differential equations which means it's basically about continuous fields that aren't discrete in any way. But you can also write it in a completely discrete way where you do calculations on a kind of spacetime tesselated into (4 dimensional) tetrahedra. Guess what? They are exactly equivalent. So it's your pick whether to think about it being 'pixellated' or not - both views are correct.
Some info on this can be found here: http://math.ucr.edu/home/baez/week19.html/ [ucr.edu]
Not really quantum gravity... (Score:3, Informative)
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.
Not quantum gravity; semiclassical experiment! (Score:3, Informative)
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.
Re: Quantum Gravity Observed (Score:1)
Re: Quantum Gravity Observed (Score:1)
Re: Quantum Gravity Observed (Score:2, Funny)
Re:Quantum gravity (Score:1)
Re:big accomplishment (Score:1)