Even Einstein Could Not Have Imagined Technology Used to Directly Detect Gravitational Waves

The director of the Max Planck Institute for Gravitational Physics talks about the news this week of indirect evidence for gravitational waves and his work to obtain direct evidence
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Credit: Copyright BICEP2 Collaboration 2014

Physicists were thrilled this week at news of strong evidence for gravitational waves, perturbations of the early universe that confirm it expanded rapidly following the big bang during a period called inflation.

To gain a different perspective on these findings, Carsten Koenneker of Scientific American’s German-language edition Spektrum interviewed Bruce Allen, director of the Max Planck Institute for Gravitational Physics, about the BICEP2 experiment that detected these long-sought waves.

You study gravitational waves. How important are the new findings from BICEP2 for physics?

These are very important, because they provide "missing evidence" for an inflationary phase in the early history of the universe. Also important: This gravitational-wave evidence is stronger than many of us, including me, had expected. It demonstrates how gravitational waves let us "look at" things that we cannot see in other ways.

Why did it take 100 years for us to detect such a strong signal of these waves, which were predicted long ago by Einstein in his theory of general relativity?

The BICEP results are indirect evidence for the existence of gravitational waves. There is other indirect evidence, for example the decay of the orbit of the binary pulsar PSR 1913+16 or of the double pulsar PSR J0737-3039. In the coming years we expect to make the first direct observations with the LIGO, VIRGO, GEO600 and KAGRA experiments. These are possible because in the 98 years since Einstein's 1916 prediction there has been a lot of technological progress in lasers, precision optics, electronic control systems, and computers and data analysis. Einstein could not have imagined that, 100 years ago!

Some commentators compare this result with the discovery of the Higgs boson. Do you agree?

Not quite. For me, the important thing is not the confirmation of the existence of gravitational waves. The important thing is that this is strong evidence supporting an inflationary early history in the universe. If this gravitational wave background had not been found, it would have been similar to the Higgs boson not being found: It would have forced us to reconsider this very well-studied model of the early universe.

If these results are confirmed, what would this mean for theoretical physics?

There are an enormous number of different inflationary models. If these results are confirmed, they will rule out a large number of these models. This is very healthy for theoretical physics!

You conduct gravitational wave experiments within the GEO600 experiment in Hannover together with colleagues from Glasgow and Cardiff. What are the differences between your work and BICEP2?

The LIGO, VIRGO and GEO600 experiments are looking for gravitational waves that are passing by Earth right now. These come from neutron stars and black holes that exist in our universe now—today. The BICEP experiment is observing the effects of gravitational waves from almost 14 billion years ago, before stars and planets even formed. The BICEP experiment is looking at very-low-frequency gravitational waves (80 cycles in 14 billion years). The LIGO, VIRGO and GEO600 experiments are looking for gravitational waves with frequencies of hundreds of hertz. The sources are very different!

Do you think that you might directly confirm the existence of gravitational waves with GEO600 independently from BICEP2?

Yes. In the coming years LIGO, VIRGO and GEO are expected to make direct detections of gravitational waves as they fly by Earth. This is different than the indirect detections (the signs of gravitational waves) being seen by BICEP2 in the cosmic background radiation.

This article is reproduced with permission from the magazine Spektrum. The article was first published on March 18, 2014.

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