The director of the Max Planck Institute for Gravitational Physics talks about the news this week of indirect evidence for gravitational ...
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 editionSpektrum 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!
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