The graviton remains a phantasm

The experimental detection of gravitational waves this spring confirmed with much fanfare Einstein's General Theory of Relativity. Until the phenomenon of gravitation is fully understood, however, physics has a Herculean task before it. A giant next step is the LISA experiment, which is being carried out with participation of the University and ETH Zurich.

Two Black Holes Merge into One
Image: SXS

Sometimes history repeats itself. So it was in February of this year, when researchers from the LIGO (Interferometer Gravitational Wave Observatory Laser) collaboration announced the experimental detection of gravitational waves. The achievement harked back to 2012, when the research teams of the ATLAS and CMS experiments at CERN announced the discovery of the Higgs boson. In both cases, physicists succeeded after decades of work in finally obtaining the experimental evidence that validated physical phenomenon. Both cases required an elaborate and expensive meter to capture a very weak signal. And in both cases, experimental physicists were able to verify theories posited a long time ago: the Higgs boson discovery confirmed a mechanism that Peter Higgs and other theoretical physicists had postulated nearly 50 years earlier. In the case of gravitational waves, their detection confirmed a 100-year-old prediction made by Albert Einstein's general theory of relativity.

LIGO Measures a Minimum Length Difference

Scientists had measured signals from each of the two LIGO detectors in autumn of 2015. From this they were able to read with stupendous accuracy how two black holes merged 1.3 billion years ago: The black holes with diameters of about 200 km were about 350 km apart; they fell into in a spiraling motion in succession and then fused together. Since the black holes were traveling at approximately one-tenth the speed of light, this process took only about half a second—just as long as the signal received by the two LIGO detectors. When black holes merge, they emit three solar masses of energy in the form of gravitational waves. These waves then reached, after a 1.3 billion year long journey, the LIGO detector in Livingston, Louisiana in the Southeastern USA - and seven milliseconds later the detector on the U.S. western coast in Hanford, Washington on September 14, 2015 at 9:50:45 am. The LIGO researchers recognized the gravitational waves because they slightly compressed and then accordingly stretched out the 4 km long measuring arms of the detectors. The difference between compression and extension was 4 x 10^(-18) m, less than a thousandth of a proton’s diameter. The difference was caused by a curvature of space-time.

No Mediating Particles in Sight

Gravitation is – according to the unanimous view of modern physics - one of the four fundamental forces of nature that include electromagnetic, strong and weak forces. For the latter three forces, we now know that they are mediated by particles, namely photons (electromagnetic forces), gluons (strong forces) or by the W and Z bosons (weak forces). For graviational forces, physicists have also postulated a facilitating particle, the graviton. This particle, however, has not yet been found. Will the LISA experiment give clues as to the whereabouts of this particle? “No, the graviton was not discovered,” says Philippe Jetzer, professor of theoretical physics at the University of Zurich and a leading Swiss gravity researcher, with no ifs and buts. Are gravitational waves then—unlike light waves that can be described as particles (photons)—thus not associated with particles? Philippe Jetzer shakes his head: “A gravitational wave is a curvature of spacetime. If this wave had particle properties, what would that mean? The wave-particle dualism, as we know it from quantum mechanics of light, does not make sense for gravitation. A deeper insight could only be achieved by a quantum mechanical formulation of gravitation. “No-one has so far succeeded at this,” says Jetzer.

Stimulus for Astroparticle Physics

The proof of the Higgs boson at CERN four years ago was a great success of particle physics. Compared to that discovery, the recent detection of gravitational waves won’t “directly” take particle physics forward, says Philippe Jetzer. “Of course, LIGO has triumphantly confirmed General Relativity (GR) and thus a cornerstone of fundamental physics.” However, Jetzer is convinced that findings from gravitational research might stimulate future astroparticle physics research. For example, to answer important questions about dark matter and dark energy, which occupy the universe in addition the matter known to us.

LISA follows LIGO

On one aspect, gravitational research and particle physics face exactly the same challenge: to gain new insights in the future, they must rely on still more sensitive measuring instruments. Particle physicists are intensely debating a new accelerator that could replace the Large Hadron Collider (LHC) at CERN in the mid-2030, after its scheduled decommissioning. The gravitational researchers have very concrete plans for when the time comes: they plan in 2034 to launch a new experiment with LISA, which they have been preparing since 2003 and that will be much more sensitive to gravitational waves than LIGO. With LISA, the convergence of two black holes could be observed not only during the last half second before the merger, but already seconds, minutes or even hours before. Since LISA can detect long waves with a frequency of 1 Hz to 1/100'000 Hz (LIGO: 10 - 10 kHz), the experiment could help researchers observe fusions of so-called supermassive black holes (millions or even billions of solar masses). At best, this could confirm an important postulate of modern cosmology: namely that in the center of each galaxy is a supermassive black hole.

Swiss Contributions to LISA Pathfinder

While the LIGO detectors operate on American soil, the LISA-detectors will operate in outer space. To test the LISA experiment on a small scale, the European Space Agency launched the LISA Pathfinder into space last December. From spring until the end of 2016, scientists will gain experience for building the actual LISA experiments through the LISA Pathfinder experiments. Philippe Jetzer has accompanied this experiment as a member of the science team, while his ETH colleague and geophysicist Prof. Domenico Giardini has made technical contributions to the project (front-end electronics, control of the nozzle). Until LISA can measure gravitational waves, years of development are still necessary: Unlike the LIGO detector in which the the measuring arms for the detection of differences in length due to the curvature of space-time are 4km long, LISA will have measuring arms of 1 to 5 million km of interplanetary space. Philippe Jetzer is now 58 years old, in 2034 he will be retired. Nevertheless, he hopes to participate in the new large gravitational experiment in one form or another: "We have worked well with LISA. Maybe we can speed up the experiment?, who knows?!”

Author: Benedikt Vogel

  • Two Black Holes Merge into One
  • The signal from the merger of two black holes, as it had been received by the two LIGO detectors in September, 2015.
  • Visualization of LISA Pathfinder in space.
  • Two Black Holes Merge into OneImage: SXS1/3
  • The signal from the merger of two black holes, as it had been received by the two LIGO detectors in September, 2015.Image: Physical Review Letters2/3
  • Visualization of LISA Pathfinder in space.Image: ESA C.Carreau3/3

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