Gravity is an extremely weak force. Consider two spheres that are close together, each with one kilogram of mass and one coulomb of electric charge, i.e., one unit each of charge and mass in Standard International Units. There will be electrical repulsion pushing them apart and gravitational attraction pulling them together, but which is bigger? It’s no contest: the electric force between these spheres is 1.35 x 1020 times stronger than the gravitational force. But perhaps this difference is so large because Standard International Units depend on some rather arbitrary choices on the size of units. What about fundamental particles?
OK, if we separate a pair of electrons by, say, a nuclear diameter, how big are the forces? Here the difference is even worse. The electric force between these electrons is 2.40 x 1043 times bigger than the gravitational force. In other words, electricity is almost a trillion-trillion-trillion-trillion-trillion times stronger than gravity.
The gravitational force is so weak that, from one perspective, it is amazing that we have noticed it at all. It is only able to become a strong influence on our existence because it is always attractive and cumulative. All of the atoms in the Earth conspire gravitationally to pull us toward the Earth’s center, giving us weight, while the electrical forces of the electrons and nuclei of these atoms have opposite electrical charges and cancel each other out, so we experience no “electrical weight” from the Earth.
If you wiggle a charged particle like an electron or a proton, it makes electromagnetic waves. This is how radio transmission antennas work. Similarly, if you wiggle a mass, it makes gravity waves. But because of the intrinsic weakness of gravity, it is very difficult to observe the waves that gravity produces. Nevertheless, they are there, and we would like to be able to directly detect them. Gravity waves have been observed indirectly as an energy-loss mechanism in the spin-down of a binary pulsar (two co-orbiting neutron stars), but there have been, as yet, no direct gravity wave observations.
This is an important problem, and the U. S. National Science Foundation has invested heavily to promote the detection of gravity waves. The NSF’s LIGO gravity-wave detectors are large L-shaped interferometers 4.0 kilometers on a side located in the states of Washington and Louisiana (see my AV column “Gravity Waves and LIGO” in the April-1998 issue of Analog). They constitute a major attempt to detect gravity waves of astrophysical interest that may be produced, for example, by co-orbiting neutron stars that are spiraling down into a merger collision. After almost a decade of operation, LIGO has made impressive strides in reducing noise and improving its sensitivity. But unfortunately, LIGO has produced no convincing evidence that such gravity waves have been detected.
The work on LIGO will continue with improved sensitivity in the next few years, perhaps achieving the first gravity wave detections. There are also competing projects in Germany, Italy, and Japan that are also looking for gravity waves with ever-improving sensitivity. In the longer term, the European Space Agency’s LISA project, which will use three satellites orbiting the Earth in orbits that form a giant triangle five million kilometers on a side, will have much greater sensitivity. LISA is scheduled for launch in 2018. Because of its much greater sensitivity, LISA is almost guaranteed to detect gravity waves of astrophysical interest.
However, we may not have to wait until 2018. There is an interesting new innovation that could simplify gravity wave detection and might even make it possible to communicate by generating and detecting gravity waves. This is the work of Prof. Raymond Chiao and his group at the University of California at Merced.
In 1913, Robert A. Millikan published a paper describing a definitive measurement of the charge of the electron. He used tiny electrically-charged oil drops on which the downward pull of gravity was carefully balanced by an upward electrical force. Chiao proposes to create a similar situation that uses pairs of “Millikan oil drops” made of superfluid liquid helium, each with one electron charge and a mass of about 1.9 micrograms. These would be trapped in a magnetic field and held in a delicate balance between gravitational attraction and electrical repulsion.
Chiao ahs shown that, assuming that the charge and mass of each drop can be assumed to move together as a unit (like a fundamental particle) because of quantum effects, a pair of such drops will vibrate with equal amplitudes in response to quadrupole electrical waves and quadrupole gravitational waves of the same strength. Conversely. when such droplet-pairs are caused to be accelerated against one another they should produce equal amounts of quadrupole electrical and gravitational waves. Here, “quadrupole” means the radiation made by pairs with the same electric charges vibrated against one another with a changing distance of separation. Quadrupole radiation has a different emission pattern and a lower strength than the more familiar “dipole” radiation made by vibrating objects of opposite charge against one another. Gravity waves are required to be quadrupole radiation because there are no negative gravitational masses.
If the masses of Chiao’s droplets are about 1.9 micrograms, the electrical and gravitational forces between the drops will be precisely equal and opposite, and the gravity wave response of the drops will be equal to its electrical wave response. If the mass of such drops is one Planck mass (about 22 micrograms), the gravitational response of the drops is 137 times larger than the electrical response. In this case, the system is placed in a situation where quantum mechanical effects are to be expected. The quantum-mechanical effects are important because the charge and mass of a given droplet must move together as a unit (like a fundamental particle), or else the electricity/gravity equivalence is broken. Chiao proposes to levitate such drops in a superconducting magnetic trap at ultra-low milli-kelvin temperatures and use them as transducers between gravitational and electromagnetic radiation.
He suggests that electrically driving pairs of such drops by scattering microwaves from them should produce gravity waves of the same frequency as the microwaves, and that when illuminated with the resulting gravitational radiation, the drops should produce electromagnetic waves of the same microwave frequency. Using this viewpoint as a model, he calculates the probability of “scattering” microwaves into gravity waves and vice versa. Chiao concludes that, provided the drops are separated by a distance comparable to the wavelength of the waves, the probability is large enough to be well within the range of experimental measurements. This suggests an experiment similar to that of Heinrich Hertz, in which he produced and detected radio waves across a distance of a few meters. At UC Merced, Chiao and his co-workers are currently building Hertz-like experimental apparatus to demonstrate generation and detection of gravity waves.
This work points to at least two applications. The first is the implementation of a “gravitational radio”, a device that can send and receive signals in the microwave frequency domain using gravity waves. The other is a plan to attempt the detection of primordial gravity waves left over from the early stages of the Big Bang. Let us consider these one at a time.
Electrical waves (radio waves, light, x-rays, gamma rays) interact strongly with matter, while gravity waves pass through matter almost as if it was not present. Thus, an ensemble of levitated charged drops on one side of the Earth might be able to transmit gravity wave signals right through the Earth, to be detected on the other side by a similar ensemble of levitated charged drops. The U. S. Navy is very interested in communicating with deeply submerged submarines, and microwave-frequency gravity waves might be an ideal medium for doing this, if this generation/detection technique works. Reliable through-the-Earth transmission might eliminate the need for the very expensive communication satellites. Moreover, secret or private messages sent by gravity waves could only be detected with levitated charged drop receivers tuned to the correct wavelengths, providing, at least for a time, a non-interceptible message channel.
Chaio and his group are beginning such experiments at the University of California at Merced. They are placing a transmitter and receiver close together but carefully screened with Faraday cages to suppress electromagnetic waves.
The other application perhaps requires some explanation. In the early stages of the Big Bang, the Standard Model with inflation predicts that a large amount of gravitational radiation was created, with frequencies ranging from 10-18 Hz to 1010 Hz. The ekpyrotic model of Steinhart and Turock (see my AV Column “The New Recycling Universe” in the November-2002 issue of Analog), on the other hand, predict that when extra-dimensional branes clap together to start the Big Bang, there is considerably less gravitational radiation, with what there is concentrated at frequencies between 1.0 and 1010 Hz. There is a third pre-Big-Bang model that predicts even more gravitational radiation than inflation, and that radiation is concentrated at frequencies between 10-5 and 1015 Hz.
With presently available technology, distinguishing between these models is very difficult, because gravitational radiation has not, at this writing, ever been directly detected. The cosmic gravitational radiation is considerably weaker than that from merging neutron stars and would be even harder to detect. There is some hope that the primordial gravity waves may be indirectly detected because they “write” on the primordial electromagnetic waves that were created later in the Big Bang. Therefore, second-generation probes of the cosmic microwave background radiation may be able to observe correlated structures in the polarization of the microwaves arising from very low frequency (10-18 Hz) gravity waves produced in the era of Big Bang inflation. However, these are very difficult measurements, and no such observations have as yet been reported.
On the other hand, if Chiao’s technique can be made to work, it offers the possibility of direct detection of primordial gravity waves. Direct detection of cosmic gravity waves in the GHz region with the levitated charged drops of Chiao would be an extremely important measurement. It could distinguish between the rival models of the early Big Bang, falsifying some models and supporting others.
Professor Chaio has obtained some funding for this research, and he and his group at the University of |California at Merced are actively pursuing establishment of the connection between electromagnetic and gravitational waves, so there should be some results in the near future. If the phenomenon exists, its observation would represent a major breakthrough in gravitational physics. Watch this column for further developments.
Raymond Y. Chiao, “Proposed Observations of Gravity Waves from the Early Universe via ‘Millican Oil Drops’”, arXiv preprint 0606118 v2 [gr-qc], International Journal of Modern Physics D16, 2309-2318 (2007).
Raymond Y. Chiao, “New directions for gravitational wave physics via ‘Millikan oil drops’”, arXiv preprint 0904.3956v2 [gr-qc] 29 Apr 2009, to be published in Visions of Discovery: New Light on Physics, Cosmology, and Consciousness, Cambridge University Press.
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