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Some incredibly exciting news today.
Gravitational waves have been directly detected
for the very first time.
The Advanced LIGO Observatory has
seen the spacetime ripples caused by black holes
at the moment of merger.
The existence of these waves is the last major prediction
of Einstein's theory of general relativity
to be directly verified a century after the theory was
first developed.
It opens a whole new window through which we can now
observe our amazing universe.
This is one of the most important observations
of the century.
On September 18, the Laser Interferometer
Gravitational-Wave Observatory switched back
on after three years of upgrades.
The new, advanced LIGO is around 10 times more sensitive.
And even conservative estimates predicted
that it should certainly detect the passage
of gravitational waves, of ripples
in the fabric of spacetime caused
by extreme gravitational events in the distant universe.
Almost as soon as it was switched on, a signal was seen.
The LIGO team kept very quiet and continued observing.
LIGO has been cautious.
And when here at "Space Time" we realized this was probably
the real deal, we were too.
However, soon after the detection,
we put together a video explaining
what gravitational waves are, how they're formed,
and exactly how advanced LIGO detects them.
It's right here.
This is a great time for you to watch it
if you haven't already.
That was four months ago.
Since then, ripples from mergers of black hole
pairs in distant galaxies have changed the shape of spacetime
here on Earth.
Indeed, across the Milky Way.
And LIGO has seen them.
Today I want to talk about what exactly was seen,
what it tells us, and what we can expect for the future.
Any orbiting pair of massive objects
generates gravitational waves.
But only extremely massive objects,
through orbiting extremely close together,
produce gravitational waves strong enough for us
to detect, at the moment.
Now, LIGO is sensitive to pairs of stellar mass black holes
and/or neutron stars.
In both cases, these are the collapsed core
of a dead star, stellar remnants.
Now these stellar remnants are sometimes found in pairs,
typically when the original stars were also a binary pair
in orbit around each other.
These super dense objects produce
serious gravitational radiation.
But gravitational waves carry energy,
which is sapped from the orbital energy of the system.
As a result, the stellar remnants
will gradually spiral in towards each other.
Now we've seen this slow orbital decay in binary neutron stars.
The amount of energy being lost in these systems
is exactly what we predict it should be if caused
by gravitational radiation.
And so this was a very convincing but indirect
verification of gravitational waves.
However, the waves produced when these stellar cores are still
distant from each other are far too weak,
and have too low of frequency for LIGO
to directly detect them.
But as the cores get closer, the gravitational radiation
becomes extremely intense.
The in spiral speeds up exponentially.
And the last phase of the merger takes only a few minutes.
In those few minutes, the merging black holes or neutron
stars produce such strong ripples
in the fabric of spacetime that LIGO can see them out
through vast distances.
In the case of merging black holes,
to five billion light years.
Now this is really an insane distance,
but that's just as well.
These events are incredibly rare.
It's expected that an observable merger of two black holes
will happen only once every 10,000 years in any given
galaxy.
So we need to watch a lot of galaxies.
Advanced LIGO can feel the ripples produced
by merging black holes through a volume of space
equal to about 0.1% of the observable universe.
That means we're watching millions of galaxies.
In the case of merging neutron stars,
it's a much smaller distance, and so none
have been seen yet, although it will happen eventually.
We'll also spot supernova explosions
that produce neutron stars.
Stand by for that.
Now these mergers have very distinct signals.
They cause actual physical distances to change.
Spacetime is stretched and squeezed as the wave passes by.
These oscillations have a frequency,
number of stretches and squeezes per second,
that matches the rate at which the black holes were orbiting
each other just before merger.
And we're talking up to 1,000 orbits per second.
OK.
So we saw some black holes merge.
That's pretty cool.
But what can we do with that?
Well, everything else aside, it's
a great vindication for Einstein's general relativity.
See, gravitational waves are inevitable if the theory
is correct.
And they were the last major prediction not directly
verified.
If they weren't seen, it'd be a huge problem.
OK.
So Einstein wins again.
Except that now we have the capacity
to do detailed tests of general relativity
in a regime previously unavailable,
in the regime of extreme gravity.
If the merger signals deviate significantly from the expected
signal, then it may be a clue that the theory is incomplete,
or even point to a new, deeper theory.
Now as far as I know, there's no indication
of that from the detected signals.
However, physicists will be all over this data,
looking for holes in the theory for some time.
These observations are going to tell us
a ton about how black holes grow and about the physics
of black holes themselves.
The signals are expected to carry information
about the strange warping of space
in the region of the event horizon.
Before long, we will observe one of these mergers simultaneously
using multiple telescopes that span
the electromagnetic spectrum from radio to visible to X-ray.
And that observation will break open
so many mysteries and no doubt reveal further new mysteries.
Now LIGO is sensitive to gravitational waves
at frequencies produced by merging black holes and neutron
stars, as well as the formation of neutron stars and supernova
explosions.
And potentially, even the actual spin of neutron stars.
However, now that we know that these things are detectable,
it opens up an entirely new spectrum
for observing the universe.
The upcoming Evolved Laser Interferometer Space Antenna--
eLISA-- will see an entirely different part
of this spectrum, much lower frequencies,
opening the possibility to observe
completely new phenomena.
It'll see the slow ringing of binary white dwarf stars
in our own galaxy, as well as the final dance
of pairs of truly gigantic, supermassive black holes
just before they merge in the cores of galaxies.
It's hard to understate this.
This is a really, really big deal,
and it marks the beginning of the era of gravitational wave
astronomy.
It's a new window on the universe
that will reveal phenomena and physics that we never expected.
We'll keep you up to date in future episodes of "Space
Time."
In a recent episode, we heated up
the debate on the question of settlement of Venus versus Mars
by looking at some nice ways to die on both planets.
You guys had some great questions.
TheRealBileth asks, wouldn't Mars' low gravity
mess up bones and muscles?
Now this is a really good point that I didn't mention.
In terms of a long-term colony, Venusian cloud cities
are going to be much healthier.
Astronauts on the International Space Station
lose bone mass at 1% to 1.5% per month,
and lose muscle mass also.
It wouldn't be so bad on Mars, but a long-term stay
may be very unhealthy.
Venus' surface, or even cloud city gravity,
is 90% that of Earth's.
And so the bone and muscle degradation would be minimal.
Laura Lorca wants to know exactly how
to build a cloud city.
Short answer.
Balloons.
In fact, a giant balloon filled with atmospheric pressure air
would float nicely at around 50 kilometers.
So think a huge dome filled with a breathable atmospheric
pressure air, or an even larger balloon base.
It could be tethered to the surface with heat
and pressure-resistant cables, carbon inner tube cables
perhaps.
NASA is already thinking about small floating missions.
Check out its HAVOC program in the link below.
Fabio Brady would like to know whether Martian sandstorms are
really life threatening.
So those sandstorms may be huge, but they're really more dust
storms than sandstorms.
The atmospheric pressure is so thin
that it doesn't generate enough force to move big stuff.
Not large grains of sand, and certainly not people.
This was a bit of creative liberty
taken by the otherwise really awesome movie "The Martian."
Astrophysicist Andy Howell does a great job detailing the sites
of "The Martian" in this video.
His channel is really excellent.
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