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Using Neutrons To Precisely Test Newton's Law of Gravity 123

Posted by Soulskill
from the attempting-the-full-isaac dept.
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."
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Using Neutrons To Precisely Test Newton's Law of Gravity

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  • Newton's (Score:2, Informative)

    by Dr. Spork (142693) on Monday April 18, 2011 @05:56PM (#35861640)
    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.
  • Re:Newton's (Score:5, Informative)

    by catchblue22 (1004569) on Monday April 18, 2011 @06:08PM (#35861780) Homepage

    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.

  • Re:Newton's (Score:5, Informative)

    by pclminion (145572) on Monday April 18, 2011 @06:10PM (#35861804)
    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.
  • by catchblue22 (1004569) on Monday April 18, 2011 @06:39PM (#35862054) Homepage

    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.

  • Re:ummm (Score:5, Informative)

    by blair1q (305137) on Monday April 18, 2011 @06:44PM (#35862086) Journal

    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.

  • Re:Newton's (Score:4, Informative)

    by femtobyte (710429) on Monday April 18, 2011 @08:27PM (#35863154)

    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.

  • Re:Newton's (Score:4, Informative)

    by Woek (161635) on Tuesday April 19, 2011 @02:59AM (#35865372)

    Photons have mass, because they have energy. Furthermore, that photons have zero rest mass is still only an assumption in most models (ref [ucr.edu])

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