Physicists Aim To Test Loop Quantum Gravity Using Black Holes

Both have been criticized for the lack of any experimental data. Now, one might be made or broken by a test. We are talking about the two theories of quantum gravity String Theory and Loop Quantum Gravity – and the latter is all set to be tested. A French team intends to test the correctness of Loop Quantum Gravity (LQG) by simulating the expected signatures of the evaporation of a black hole.

An impression of an evaporating Black Hole (Courtesy: CernCourier)

The Difficult Job

The main problem in testing these two theories lies in the fact that one has to probe Planck scale physics and the high energy associated with that. However, Monte Carlo simulations show the way out, or so thinks the team of A.Barrau, T. Cailleteau, X.Cao, J.Diaz-Polo and J.Grain, whose paper appeared in Physical Review Letters.

The plan is daunting. They plan to observe black holes evaporating via the Hawking radiation mechanism and this should reveal critical signatures, if LQG is true. Also, bigger black holes and smaller ones give different signatures. The paper presents the simulations that they have done and what signatures in the radiation pattern they expect to see.

The tough job comes after that. LQG predictions indicate features in the black hole evaporation spectrum, but in order to get that one needs to look at a large number of black holes, so that the energy reconstruction can be done with as little error as possible.

Barrau’s team is also looking at signatures of LQG in the cosmic background radiation. But, that’s another story.

NASA Celebrates New Year By Putting Moon Gravity Probes In Orbit

The New Year got off to a great start for NASA as it managed to put the twin GRAIL probes in an orbit around the Moon. They are now set to beam each other radio signals, which will keep them synchronized in orbit and the distance from one probe to the other can be known to an error margin or a micron!

The GRAIL probes, before they were launched (Courtesy: NASA)

Measuring the gravity of the situation

The GRAIL probes are designed to accurately measure the Moon’s gravitational field strength. The technique is pretty simple really! When one of the probes passes close to a lunar region, which has high density, it will feel a greater gravitational pull. This will suddenly accelerate the probe and the distance between the two probes will decrease. This is how the density map of the moon can be prepared. Of course, it’s never as easy as this, is it?

Why map the moon anyway? The gravity map of Earth’s closest satellite can give us a good map of the composition of the moon. One of the mysteries that might be solved is why the two faces of the Moon look so very different; one is rolling flatlands, without any craters, while the other one is puckered with numerous craters.

The GRAIL probes will now descend and sink lower into its orbit, getting closer to the Moon’s surface. The data acquisition is supposed to start early March, when the probe is 34 miles above the lunar surface.

Getting young minds into the project

One of the best things about this mission is the attempt to get students into this thing. As Phil Plait writes in his blog, Bad Astronomy, there will be four cameras on each of the probes, called MoonKAM. These will get high resolution photos of the lunar surface and these can be used by middle school students. They may even request NASA to fly the probes over a particular area on the Moon. As Phil Plait puts it:

That’s very cool! … I’ll bet it’ll be an experience they’ll remember their whole lives.

I’m sure too! Being part of science is sometimes being like a detective, without the dirty work!

Dark Matter an Illusion, Can Be Explained by Gravitational Polarization of Quantum Vacuum, Says CERN Scientist

Reality, aren’t thou a heartless beast! A scientist from CERN, Dragan Slavkov Hajdukovic, has claimed that dark matter may be an illusion and that its supposed effect can be explained away by more known kinds of particles simple matter particles and their corresponding anti-particles.

Option 1: Dark Matter

A startling observation in astronomy set out the hunt for an unknown form of matter, just because the gravitating effect of such hypothetical matter could explain the observations. Galactic arms were seen to be moving too rapidly for them to stay attached to the rest of the galaxy, if Newton’s law of gravitation (or even Einstein’s General Relativity) is to hold. Scientists could save the situation by postulating the presence of a large amount of non-luminous or dark matter, interacting with the rest of the Universe only by the gravitational force. No candidate for this kind of matter was proposed and people have come up with various conjectures and models. Right now, we know nothing about the composition of dark matter. It is said that dark matter comprises 23% of the known matter of the Universe. 73% is dark energy and only the rest 4% is normal matter as we know it.

The relative composition of the known Universe

Option 2: Modified Newtonian Dynamics

There was another way to resolve the puzzle modify Newton’s law of gravity. This new school of thought, called MOND or MOdified Newtonian Dynamics, said that the power law dependence of the gravitational force on distance (which is inverse-square for Newton’s law) depends on acceleration. MOND doesn’t need any dark matter. The problem with MOND is that it doesn’t seem to work at all scales of acceleration. Inherent inconsistencies prevent MOND from taking over from Newton’s gravity law.

Option 3: Gravitational polarization of Quantum Vacuum

Now, Hajdukovic postulates another possibility that space itself might be repulsive. He says that if particles and anti-particles were to repel each other gravitationally, rather than universally attract, then it would solve the problem. Vacuum could be polarized, i.e. particles and anti-particles could be created at will, governed by the Heisenberg’s Uncertainty Principle. Now, if these were to act as gravitational dipoles that repel, then the problem of amplifying gravitational fields would be solved. He gives an example of a dielectric slab inserted inside a parallel plate capacitor as an example. In this case, the electric field within the dielectric would decrease. Hajdukovic argues that, if unlike charges repelled instead of attracting, the field would increase in strength. The same, he concludes, is true with gravity.

The Gravitational Dipole (Courtesy: Wikipedia Commons)

Hajdukovic put it precisely:

Concerning gravity, mainstream physics assumes that there is only one gravitational charge (identified with the inertial mass) while I have assumed that, as in the case of electromagnetic interactions, there are two gravitational charges: positive gravitational charge for matter and negative gravitational charge for antimatter

As alien as that might sound to a reader versed in Physics, mainstream labs are actually working to find out whether anti-matter repels matter gravitationally. Primary amongst these is the AEGIS experiment at CERN.

Closing Remarks

The proposal also has a very pleasing symmetry. Gravitational force is unlike the electromagnetic force, since electromagnetic forces both repel (like charges or magnetic poles) and attract (unlike charges/magnetic poles). If Hajdukovic is right, then even gravity will have a repulsive component, but this might mean that we have to take another look at Einstein’s equations of General Relativity.

There are many issues, however. Things like gravitational lensing is explained best by assuming dark matter than without it.

Hajdukovic published his paper Is dark matter an illusion created by the gravitational polarization of the quantum vacuum?in the journal Astrophysics and Space Science.

CERN’s New Einstein Observatory To Detect Gravitational Waves

CERN’s at it again, but it’s not particle physics. Einstein’s also at it again, but this time, it isn’t the famed grizzly haired scientist. A group of European scientists working with CERN will soon propose a design for a telescope the Einstein Observatory  – which will be much better than any other known telescope of its kind. The catch: This one will detect gravitational waves rather than optical radiation or radio waves.

What is the Einstein Observatory

The Einstein Observatory (EO) is a ‘third-generation’ gravitational wave detector and it is designed to be at least a 100 times more sensitive that its existing predecessors. The principle of detection is simple and classic. The arms of the Observatory, each several kilometers long and each being a laser beam will shrink or expand ever so slightly if a gravitational wave passes. This will cause a change in the interference pattern in a central photo-detector. Let’s look at this in more detail.

Einstein’s theory of General Relativity predicts that gravitational energy, stored in gravitational fields, should be released as waves, just like energy in electromagnetic fields is released by electromagnetic waves (which we call light). The problem is that, unlike light, the energy of a gravitational wave is so small that if a typical one passes by earth right now, the earth will shrink and then expand by the breadth of a proton which is much much smaller than even an atom. Detecting such small perturbations is a huge challenge that has so far been unconquered. Relativity predicts that gravitational waves of comparatively large magnitude are emitted by violent cosmic events, like merging of black holes, or fusing of neutron stars, or even supernova explosions. These will be the typical gravitational waves scientists hope to detect with EO. The success of Einstein’s theory has been such that no one doubts the existence of gravitational waves, even though one hasn’t been detected inspite of dedicated search.

Gravity waves are generated by violent cosmic event, like neutron stars merging (An artist's impression)

What EO intends to do is this: there is a particular way two beams of light interfere with each other.

Apparatus for the MM experiment

They form a well-known pattern called an interference pattern (you might see these patterns when water waves interfere). A slight shift in the path a beam of light travels will disturb these patterns. The process is extremely sensitive – and if the beams travel a long distance before interfering, the sensitivity increases. (For science buffs: This is the same principle first used by Michelson in his famous experiment for measuring the speed of light and later, the most famous ‘failed’ experiment in history. This failed experiment, known simply as Michelson-Morley experiment, aimed to detect a change in the speed of light in different directions so as to confirm the aether hypothesis. None was detected. Einstein would later build his Special Theory of Relativity around this result.)

More on the EO

The EO will be housed 100 to 200 meters below ground, in order to minimize the seismic activity of the ground and its effect on the telescope. The EO will be extremely sensitive in the range 1 Hz to 10 kHz, which is the frequency band for the gravitational waves. The Einstein Observatory will lead a scientific revolution, is what Michele Punturo, scientific coordinator of the design, says. The data from the EO will be corroborated and complemented by data from various gamma-ray and X-ray telescopes.

The proposed design of the Einstein Observatory

The EO is actually two interferometers one to detect gravitational wave signals from 2-40 Hz and the other to detect till 10 kHz. This is required, since detecting at low frequencies is a very difficult job and needs dedicated instruments tuned for doing only that.

EO will hope to improve upon existing gravitational wave telescopes like LIGO, Virgo and TAMA (all first generation), and even Advanced LIGO and Advanced Virgo (second generation). The design will be presented at European Gravitational Observatory site in Pisa, Italy.

It is of utmost importance to the progress of cosmology that the telescope, like the illustrious scientist it is named after, becomes as successful as his theories.

Neutrons in Demand:Scientists Develop New Method To Probe Quantum Gravity

A brand new method to measure gravity and minute quantizations in a gravitational field that uses neutrons entrapped between two vibrating parallel plates immersed in a gravitational field, has been developed by scientists at the University of Technology, Vienna (TU Vienna). Neutrons have earlier been used for electromagnetic (EM) field measurements, but similar methods are now being used to measure gravity, a force which is 10-36 times (i.e. one in a billion billion billion billion parts) as strong as the EM force.

Neutron Probe for Gravity

The physics

Any field which can be quantized (EM can be quantized; gravity cannot be quantized as yet) contains discrete energy levels, which can be occupied by quantum particles. A particle cannot occupy a space between two successive levels. It may, however, jump (technically, make a transition’) from one quantum state to another, giving off radiation in the process.

A quantum particle in a certain state needs to be excited with just the right amount of energy so that it can make transition to a higher energy state. This process is called resonance’.

For quantizing any field, it has to be bounded in space within some finite range. This is conveniently achieved by limiting the extent of the experimental apparatus between two parallel plates. These plates may even be used to induce transitions, as we will see below.

The method

To probe gravitational fields, neutrons are being confined between two closely spaced parallel plates, which can be vibrated at very precise frequencies. If gravity can, indeed, be quantized, then each of the neutrons sits in one of the energy levels in the gravitational field. By vibrating the plates at a very precise frequency (the resonant’ frequency), just the right amount of energy can be pumped into the system. This energy will then be taken up by the neutrons, which will jump’ to higher quantum levels. By measuring the resonance peaks in the vibrational spectrum, scientists hope to accurately map out the quantum levels in the gravitational field.

Extremely cold neutrons are used instead of atoms or electrons, because they are heavy particles and also uncharged. They are unaffected by EM fluctuations, are nearly non-polarizable and are unaffected by the Casimir force.

Gravity and its Quantization

The problem of trying to quantize gravity started with Einstein, when only the EM force had been quantized. Since then, the weak and the strong forces have been quantized and unified into a single theory. Gravity has survived all attempts of quantization and unification. A primary problem with gravity is that the static space-time background present for the other forces is itself distorted by gravity. (In fact, relativity says that the distortion of space-time is gravity). The results of this experiment might give valuable clues as to the energy scales needed for unification. This may also demonstrate the very limits of possibility of a unified theory (like string theory and its many versions).

Calabi fields
String theory demands extra dimensions, which might be curled up


The experiment is much smaller in scale than the existent LIGO and can be performed in a laboratory. Questions still remain as to how fine the measurements need to be in order to be fully sure of the result.

The experiment also hopes to verify the validity of the equivalence principle (which says that gravitational and inertial masses are exactly the same) at extremely small length and energy scales. This principle is crucial for the correctness of general relativity (GR), and thus will deliver a verdict on the applicability of GR at quantum scales. A more sophisticated version of the experiment might even be used to probe into the nature of dark matter, but that is still some time away.

No one is sure if this will work, but as Pauli said, He who dares, wins.