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Using Neutrons To Precisely Test Newton's Law of Gravity
NotSanguine writes with this excerpt from the BBC:
"The neutrons are shot between two parallel plates, one above another and separated by about 25 micrometres — half a hair's width. The upper plate absorbs neutrons, and the lower plate reflects them. As they pass through, they trace out an arc, just like a thrown ball falling due to gravity. ... The new work by the ILL team has added what is known as a piezoelectric resonator to the bottom plate; its purpose is to jiggle the bottom plate at a very particular frequency. The researchers found that as they changed the bottom plate's vibration frequency, there were distinct dips in the number of neutrons detected outside the plates — particular, well-spaced 'resonant' frequencies that the neutrons were inclined to absorb. These frequencies, then, are the gravitational quantum states of neutrons, essentially having energy bounced into them by the bottom plate, and the researchers were able for the first time to force the neutrons from one quantum state to another. The differences in the frequencies — which are proportional to energy — of each of these transitions will be an incredibly sensitive test of gravity at the microscopic scale."
the bible doesn't talk about neutrons
intellectual property law is philosophically incoherent. it is your moral duty to ignore it or sabotage it
Wow, my physics courses apparently forgot to mention that Newton's Law of Gravity had anything to say about the quantum states of neutrons. In fact, I was taught it's not a law; it's a falsified hypothesis.
Newton's Law of Gravity can be seen as an approximation of Einstein's theory. We have to be careful when we speak of "falsified". We haven't discovered that gravity is proportional to 1/r, or that gravity isn't attractive but repulsive. We have discovered that Einstein's models are better predictors of experimental results. We can still us Newton's models to send humans to the Moon. But Newton's model makes no sense when asking questions such as "what would happen to the Earth if the Sun suddenly disappeared. It doesn't predict the bending of light, nor does it properly describe certain orbital phenomenon.
This and no other is the root from which a tyrant springs; when first he appears as a protector - Plato (423 to 327 BC)
If you bother to read the article, you see that they are trying to see whether good old Newtonian gravity is a good approximation at extremely small length and mass scales (scales where the additional accuracy provided by general relativity is unnecessary). They're trying to see if when you make the experiment this sensitive, do you see some kind of quantum effect. The answer so far seems to be no. Yes, the neutrons behave in a quantum mechanical way. The question is, do they behave as you'd predict if Newton's/Einstein's gravity is true, or do they do something unexpected? This has nothing to do with Newton vs. Einstein.
I disagree with the claim that it is falsified. All theories in physics come with two sets of conditions: the bounds in magnitude and the bounds in resolution. Newton's theory came with well-defined bounds - those of classical phenomena. You cannot extrapolate beyond those bounds and claim you are still working with the theory because the theory isn't defined beyond those bounds. Nor can you interpolate to the quantum level for the same reason - the theory isn't defined there.
Relativity didn't replace Newton's theory, it supplemented it. In computing terms, it's a third-party module you can add on. When you install the Relativity dynamic library, the combined theory applies to a much larger range of phenomena.
The only time a theory will actually be falsified is if QM's gravity or relativity can be patched to work within the other's range. They are contradictory and you cannot load both modules into Newtonian physics at the same time. Only one of these two will stand the test of time, the other will die. Whichever one wins will then merge with Newtonian mechanics to produce a universal law of gravity.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
I thought the same when I read this piece of news yesterday. Journalists like to fill their sentences with words that sound appropriate to them: "[Newton's Law of] Gravity", "8.1 magnitude [in Richter's scale]"... and often they make mistakes.
The state you are in while your HEAD is detached... - wait, what?
It's a citation. Nobody said it has to be easy to get. If you think the article submitter is actually a shill for Nature trying to drum up funds by getting a bunch of Slashdotters to pay $18 for a copy of the article, well, you're a new kind of crazy I haven't seen before.
According to the second link, the resonance was at about 705 Hz.
The plate would have to be fairly wide and floppy to resonate at that frequency, but then it wouldn't have much gravitational attraction, so the plate is probably thick and chunky, and not very flexible. And it would have to be made of rubber to have any body resonance at that frequency.
It doesn't predict the bending of light
Err...sorry. I should have said it doesn't predict the bending of light in gravitational fields.
This and no other is the root from which a tyrant springs; when first he appears as a protector - Plato (423 to 327 BC)
You see, cars are attracted to other cars and there are some grooves on the road so a car usually stays jammed into one of them. While there it is both morphed a corresponding Transformer (the transfomer depending of the groove and the car) and the car but if you look at it you may see a car or you may see a transformer ex: Megateron. I hope that this clarify any doubts you had about gravitational quantum transformers.
Jehovah be praised, Oracle was not selected
forgive me for this stupid question, but if neutrons have 0 charge, by what means does the upper plate attract them and the bottom plate repel them? Shouldn't the neutrons just ignore the presence of the plates and fall toward the center of the earth, aka down? or do we already have anti-gravity technology that i am not aware of
I didn't RTFA, but the "charge" for gravity is called "mass", which neutrons have, and the earth's gravity could be neutralized by setting up the plates vertically, so any movement of the neutrons towards the earth's center wouldn't coincide with a movement towards one of the plates. Oh, and both plates may attract the neutrons -- the summary only said that once the neutrons reached the plate surface, one of the plates would absorb the neutrons, and the other plate would reflect them.
Here are a few paragraphs of the original article:
Spectroscopy is a method typically used to assess an unknown quantity of energy by means of a frequency measurement. In many problems, resonance techniques1, 2 enable high-precision measurements, but the observables have generally been restricted to electromagnetic interactions. Here we report the application of resonance spectroscopy to gravity. In contrast to previous resonance methods, the quantum mechanical transition is driven by an oscillating field that does not directly couple an electromagnetic charge or moment to an electromagnetic field. Instead, we observe transitions between gravitational quantum states when the wave packet of an ultra-cold neutron couples to the modulation of a hard surface as the driving force. The experiments have the potential to test the equivalence principle3 and Newton’s gravity law at the micrometre scale
Generally, a quantum mechanical system that is described by two states can be understood in analogy to a spin-1/2 system, where the time development is described by the Bloch equations, assuming two states of a fictitious spin in the multiplet, similar to spin-up and spin-down states. In magnetic resonance of a standard spin-1/2 system, the energy splitting results in the precession of the related magnetic moment in the magnetic field. Transitions between the two states are driven by a transverse magnetic radio frequency field. Similar concepts can be applied to any driven two-level system, for example in optical transitions with light fields. Variations are inherently connected to high-precision measurements such as atomic clocks6, atom interferometry7, nuclear magnetic resonance8, quantum metrology9 and the related spin-echo technique10. The sensitivity reached so far11 in the search for the electric dipole moment of the neutron is 6.8×1022eV, or one Bohr rotation every six days.
In this Letter, we demonstrate that energy eigenstates in the gravity potential of the earth can be probed using a new resonance-spectroscopy technique, using neutrons bounced off a horizontal mirror. This spectroscopy technique has in common the property that a quantum-system is coupled to an external resonator. Quantum mechanical transitions with a characteristic energy exchange between the coupling and the energy-levels are observed on resonance. A novelty of this work is the fact that the quantum mechanical transition is driven by an oscillating field that does not directly couple an electromagnetic charge or moment to an electromagnetic field. Instead, we observe energy transfer on resonance that is based on gravity-quantum states coupled to a modulator. We have named this technique gravity resonance spectroscopy, because the energy difference between these states has a one-to-one correspondence to the frequency of the modulator, in analogy to the nuclear magnetic resonance technique, where the energy splitting of a magnetic moment in an outer magnetic field is related to the frequency of a radio-frequency field. This is possible because of the feature of the quantum bouncing ball12, 13 that the levels are not equidistant in energy. The linear gravity potential leads to measured14, 15, 16 discrete non-equidistant energy eigenstates |nright fence. A combination of any two states can therefore be treated as a two-level system, as each transition can be addressed by its unique energy splitting or, in our case, by vibrating the mirror mechanically at the appropriate frequency. It has also been proposed to realize transitions between gravitational quantum states by means of oscillating magnetic gradient fields17. The physics behind these transitions is related to earlier studies of energy transfer where matter waves bounce off a vibrating mirror18, 19 or a time-dependent crystal20, 21. In the latter case the transitions are between continuum states, in the quantum bouncer the transitions are between discrete eigenstates. Optical dipole traps of atoms are reviewed in ref. 22.
This and no other is the root from which a tyrant springs; when first he appears as a protector - Plato (423 to 327 BC)
Not to mention that the majority of people wanting to read the paper itself (rather than the abstract and BBC summary) are likely to have institutional access anyway.
According to the links, one plate is smooth, and the other is rough. so a neutron will glide over the smooth plate or be scattered at small angles but if it hits the rough plate it will be scattered more, on average. Why the difference? So that they have a different effect and you can tell if perturbing their course causes more to hit the smooth plate or the rough one.
The neutron's course can be perturbed by gravity. In the steady state, this means the neutron just drops in a parabolic arc following gravity, which at these length scales (microns) can be more determined by massive nearby objects (1/r^2 is huge) than by the distant center of the Earth (1/r^2 is tiny). (You might even get a setup where the top plate gravity is equal and opposite to the Earth's gravity, for objects that are close enough.)
Moving one plate nearer or farther away makes the arc change shape, changing how many neutrons are scattered for a given beam intensity and launch angle. Moving the plate in an oscillating motion at a given magnitude should give you an oscillating scattering measurement with a fairly constant magnitude. You would expect the number of neutrons scattered to be irrelevant of the frequency, when averaged over many cycles of the oscillation, if you considered gravity to be purely Newtonian (i.e., Newtonian gravity, f = GmM/r^2, is monotonic with changes in r, even when r is changing with time).
But they don't see that. They see distinct frequencies of plate oscillation that result in bumps or sharp bends in the average scattering.
That says they're seeing non-monotonic, quantized, time-dependent effects that Einsteinian gravity suggests.
Actually it does, but by half the amount predicted by general relativity. This was known to Cavendish in the late eighteenth century.
http://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun
Slashdot: news for Apple. Stuff that Apple.
>> well, you're a new kind of crazy I haven't seen before.
I'm offering an article on the taxonomy of crazy for only $18, if you're interested.
Yes, we obviously need to be worried that the large team of scientists and engineers who designed and built this experiment have overlooked the most basic principles of freshman physics and mechanical design. Good thing we have the keen intelligence of Slashdot science critics to catch all these subtle flaws that would otherwise slip by the reviewers at Nature un-noticed. Should we also worry that the scientists are all part of the government conspiracy to cover up the true Time Cube four-side harmony perfection of gravity symmetry?
1. The neutrons hit the nuclei of the atoms in the plate.
2. When the nuclei are lined up nicely the neutrons are absorbed or glance-off in a more regular fashion; when the nuclei are in lumpy bumps, they are absorbed or glance off in more random fashion.
3. The piezo is only used as a motor to move the plate. The fact that it's piezoelectric is irrelevant and should have been left out. It could have been a servo or a twisted rubber band.
The energy required to make it happen would kill us all probably by turning all the inner rocky planets to molten balls of slag.
The way that the lower/upper plates "repel/attract" the neutrons is not to be due to familiar forces (e.g. electromagnetic, gravitational, weak, strong), but rather due to quantum scattering effects from the bulk of nuclei in the plate material (which can be either attractive or repulsive, depending on material composition) based on the Fermi exclusion principle (identical fermions, such as neutrons, cannot occupy the same quantum state, resulting in effective forces between them not caused by any other forces). While the statement (as is often true of science journalism for the general public) is unclear and confusing, it is somewhat true in the sense that the neutrons are not interacting through a mechanism that would show up on a list of "forces of nature".
You don't need to know what the mass of light is, you only need to treat it as a classical particle traveling at 3e8 m/s. From classical mechanics, objects follow the same trajectory in a gravitational field regardless of mass (different orbits depend only on different initial positions/velocities); a beam of light can be treated just like a very fast moving comet. The mass of the light would only be important if you were trying to calculate the reverse effect of how much a passing light beam would move a planet/star as it passed. The fact that light travels at a finite speed has been known for a long time.
Newton's theory came with well-defined bounds - those of classical phenomena.
I'm not sure that's correct. In Newton's time the word "classical" in the sense of either non-relativistic or non-quantum didn't exist. Gravity and the other mechanical forces are continuous and space is three-dimensional with independent time dimension in Newton's model. The entire idea of "bounds" in terms of relative speed and size where Newtonian mechanics don't apply was invented much later (both pretty much by Einstein) and would likely have seemed ridiculous to Newton.
The interesting thing is that Einstein's two brainchildren - quantised photons and general relativity - don't play well together.
You are not a brain: http://books.google.com/books?id=2oV61CeDx-YC
Two things. First: Newton knew that all objects (at least, all objects we've tested) accelerate toward the earth with a certain acceleration regardless of mass, depending only on their distance from the earth. If the law of gravitation is universal, then why wouldn't light also experience the same acceleration? Assuming that massless particles are an exception goes against Occam's Razor. Only if we observe that light does not deflect would we conclude our theory was wrong. Newton was unable to perform this experiment.
Second: Why would Newton automatically assume that light did not have mass? It seems perfectly obvious today, but is it obvious because everyone knows it or because it's obvious? I don't think it's obvious.
This is a terrible apology for Newton's theory of gravity. Einstein's relativity isn't an "add on" module. It completely subsumes Newton's theory, and shows that is just a very good approximation at ordinary scales.
This in no way diminishes Newton's accomplishment, or even usefulness. However, we can say that his theory of gravity has been falsified.
1.
Atominstitut, Technische Universität Wien, Stadionallee 2, 1020 Vienna, Austria
* Tobias Jenke,
* Hartmut Lemmel &
* Hartmut Abele
2.
Institut Laue-Langevin, 6, Rue Jules Horowitz, 38042 Grenoble Cedex 9, France
* Peter Geltenbort &
* Hartmut Lemmel
3.
E18, Physikdepartment, Technische Universität München, 85748 Garching, Germany
* Hartmut Abele
4.
Physikalisches Institut, Universität Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany
* Hartmut Abele
The idea of physical theories being bounded was actually quite popular in Newton's time - hence the phrasing of Hooke's Law, the approximation of the motion of pendulums, etc. The laws were all quite specifically written in the form "provided these preconditions are true, this result WILL apply". Newton didn't change this in the slightest.
Indeed, that should be obvious from the description of gravitational attraction. Object X is pulled towards object Y because of the mass of object Y alone. However, it is equally true, by the same law, that object Y is pulled towards object X because of object X's mass. They are both pulled towards a common point. Newton was quite aware of this. If you jump up, you do not fall to the Earth, both you and the Earth move towards a common point. Because your mass is insignificant compared to that of the Earth (well, depending on how addicted you are to fast food), you can treat the Earth as in effect a stationary object.
Thus, although Newton's laws require you to apply the laws in both directions to get the "exact" result of his theory, you won't ever find that done in practice. By the time you get to objects of comparable size that can orbit each other you have to start factoring in relativity anyway. Even Newton never really considered the common center of gravity - he expressly confined himself to objects of significantly different mass such that the common center of mass is approximately the same as the center of mass of the larger object. There was never any attempt by Newton to extrapolate outside of that boundary.
There was an unwritten limitation also imposed on it. The addition law of vectors had to apply. This was a geometric property and there were certainly geometries known at the time in which the addition law did NOT hold. Thus, Newtonian Mechanics has the implicit precondition that you are working in a valid geometry. All QM and GR/SR geometries are invalid EXCEPT for special cases when both these theories reduce quite nicely to classical Newtonian Mechanics.
It's a small world and it smells funny; I'd buy another if it wasn't for the money; Take back what I paid (SoM)
Zeroes are troublesome in physics. If a light particle has zero mass, it would mean that any force at all would cause an infinite acceleration. Since it was known that light travelled at finite speeds, it's reasonable to assume that light particles did have a mass (of course, classically there's no such thing as rest mass)
Photons have mass, because they have energy. Furthermore, that photons have zero rest mass is still only an assumption in most models (ref)
Firstly you are wrong in saying that a photon has no mass. It has no rest mass, but it certainly has energy, and even momentum (hv/c). Force is not just m*a, but even more correctly described as the change of momentum dp/dt. Gravity is a force and therefore it causes a change of momentum.
There are many papers that speculate that is light is a particular, what must be the effect of gravity upon it, dating from several years before Cavendish even.
Here is a citation of a letter from John Michell, 1783, even predicting what we call black holes today:
http://books.google.pl/books?id=XpyvPTRwLoQC&pg=PA368&lpg=PA368&dq=Cavendish's+paper+on+gravity+and+light&source=bl&ots=EXGBWCHeYy&sig=g4vE442GKMsAtLsa0yH5ehiUIh8&hl=pl&ei=UEStTf2TGoPxsgaZ1c3XDA&sa=X&oi=book_result&ct=result&resnum=3&ved=0CCsQ6AEwAg#v=onepage&q=light%20gravity&f=false
The idea of photons, even if the name is recent in origin, is as old as light itself. Even Aristotle tried to reconcile several theories of light as being a "streaming substance" (a substance carrying some sort of energy, see de Anima), or as a "ray". The observation of motion of dust particules in a sunlight room does not date from last century.
The more you read the works of the ancient physicists, you will discover that they certainly were not people "without any clue", but often trying to wrap their minds around some serious observations. Whether they succeeded or not is another question, but they certainly had some clue.
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But Newton's model makes no sense when asking questions such as "what would happen to the Earth if the Sun suddenly disappeared. It doesn't predict the bending of light, nor does it properly describe certain orbital phenomenon.
Actually, Newton's Gravitational laws _DID_ predict the bending of light by the Sun, but by a different amount!
There is a factor of 2 difference (can't remember which predicted the greater bending!).
I think if Newton thought photons had 0 mass he'd have found it impossible to apply GMm/r^2 to them without getting the answer f=0 and deducing that photons did not curve in a gravity field.
Remember also that Newton knew calculus, having invented much of it, which means he could have thought about it in terms of limits. F = ma = GMm/r^2. Take the limit of both sides as m goes to zero, and you get a = GM/r^2. No problem there. The burden is actually to come up with a reason why light wouldn't behave the same way as anything else.