Astronomy

ASTROCONFIDENTIAL ASTRONEWS
BY
ERIC BETZ
FIVE QUESTIONS WITH JOE TAYLOR
LIGO rocked the scientific world by detecting gravitational
waves — a phenomenon chased by astronomers
since the 1960s. However, LIGO isn’t the first
real-world evidence for gravitational waves.
Astronomer Joseph Taylor claimed that prize
back in 1978, when he showed a pair of pulsars —
rapidly spinning neutron stars — was spiraling in
toward each other, emitting gravitational radiation.
These pulsars are dense stellar corpses with masses
larger than the Sun, packed into a sphere the size of
New York City.
That find earned Taylor a ticket to Stockholm with
his former graduate student Russell Hulse. The pair
won the 1993 Nobel Prize in physics. Their binary
pulsar, designated PSR B1913+16, was a crucial test of
general relativity because its tightening orbit allowed
them to measure how much energy was leaking out
in the form of invisible gravitational waves.
THE NOBEL FOUNDATION
Q: Pulsars are now relatively
well-known astrophysical
phenomena, but they were
exotic when you started
your career. Can you take
us back to what it was like
in the late 1960s and ’70s
when pulsars were first
being discovered?
A: The announcement of
the discovery of pulsars in
February 1968 was a big surprise.
It was a major departure
in that the type of signal was
something that varies rapidly.
Radio astronomy was young at
the time anyway, but all of the
known sources were essentially
constant in amplitude.
It’s fair to say that radio
astronomers everywhere
were at first skeptical and
then wanted to do what they
could to follow up on the
original observations made
in Cambridge, England. I had
just passed my final oral
examination as a Ph.D. student,
and I was hired on as a
postdoctoral fellow at Harvard
Observatory with more or less
a blank slate to work on whatever
I wanted to.
Q: Was it immediately
obvious that PSR B1913+16
was a binary pulsar when
you found it in your survey
at the Arecibo Observatory
several years later?
A: Immediately we thought it
must be not be a pulsar at all;
it must be interference because
its period is not constant and
all the rest of them were very
constant. It turned out to be
pretty easy to show it was not
interference because if we
turned the antenna by a little
bit, the signal would go away.
So, it was coming from the
sky and it clearly had many
of the signals of a pulsar, but
it was faster than almost any
pulsar at the time. The only
one faster was the one in the
Crab Nebula, so it was of special
interest for that reason,
but it was of particular interest
because its period changed.
After several weeks, we knew
we had found a pulsar orbiting
another star. It was clear that
you could calculate the velocity;
it was about one-tenth of 1 percent
the speed of light. So, it
was a relativistic system.
Q: What did this pulsar
system ultimately teach us
about relativity?
A: What turned out to be the
most important prediction is
that a system like this should
lose energy gradually in the
form of gravitational waves.
Those waves should be carrying
away orbital energy and
orbital momentum, and the
orbit should gradually evolve.
Q: What was the state of
gravitational wave research
at the time? Had other
attempts like this been done?
A: No. It was a very different
environment at the time. It
wasn’t even by any means an
agreed-upon conclusion that
general relativity necessarily
accommodates gravitational
radiation. Even Einstein had
STELLAR CORPSE. Pulsars rotate so fast that even a “mountain” rising centimeters
from the surface could create gravitational waves that astronomers might soon
be able to detect on Earth. ASTRONOMY: ROEN KELLY
some doubts about that over his
life. There were several papers
published in the teens — 1916,
1918 — describing gravitational
radiation as a part of relativity
theory, but in the 1930s, even
Einstein had doubts.
Q: So, even by the 1970s, it
wasn’t clear that gravitational
waves should exist?
A: I think the weight of
opinion in physics was that
gravitational radiation is a phenomenon
that almost certainly
exists, but that it is going to be
very hard to detect, if ever. Of
course, it’s hard to predict what
technology changes there are
going to be.
But at the level of technology
in the ’50s or ’60s — maybe
even the ’70s — there was no
way to see how that kind of
thing could be detected.
That’s the time when Joseph
Weber started experimentally
putting together an apparatus
that he hoped might enable
him to do this. So, there was
serious interest, but some skepticism
about whether or not it
would be possible.
And there was still some
theoretical skepticism about
whether we correctly understood
what the complicated
non-linear theory of relativity
predicts. There were serious
papers being published questioning
whether gravitational
waves should exist.
Editor’s note: This interview
has been edited for clarity and
space.
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