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Thanks to brilliant.org for supporting PBS Digital Studios.
Now that gravitational waves are definitely a thing, it's time
to think about some of the crazy things
we can figure out with them.
In some cases, we're going to need a gravitational wave
observatory the size of a galaxy.
In fact, we've already built one.
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We are at the cusp of a golden age of gravitational wave
astronomy.
We've already talked about the Laser Interferometer
Gravitational-Wave Observatory, LIGO, and the first discovery
of gravitational waves here.
Those videos came out almost two years ago.
And a lot has happened since then.
But first, an update on LIGO.
In its two and a half years of operation,
LIGO has observed five certain black hole-black hole mergers.
These events have been consistently surprising.
The black holes in question were enormous.
In three cases, both members of the black hole binary pair
were well over 20 times the mass of the sun.
And in the other cases, they were 30 plus solar masses.
But if these black holes formed in the deaths of massive stars,
which we think they must, then they
should weigh in at between 5 and 15 solid masses, 20 at most.
It's hard to imagine black holes forming bigger
than this or both of a binary pair growing
this large after formation.
Researchers are spending lots of brainpower
trying to come up with new and creative solutions
to explain this anomaly.
Some are trying to adjust stellar evolution
models to allow for the formation of more
massive black holes.
Others are calculating whether these black holes may
have grown inside globular clusters, where
the stellar density is so high that we expect
lots of black hole mergers.
There's also some more creative speculation.
Perhaps we've observed the merging
of primordial black holes formed in the instant
after the Big Bang.
Or perhaps they grew after being swept up
in a quasar accretion disk.
I actually did some work on that last one.
Yet, everyone wants in on the gravitational wave game.
A more recent paper has a very different explanation
for the apparent large black hole masses.
Perhaps these gravitational waves signals
were amplified by another phenomenon predicted
by Einstein's general relativity,
gravitational lensing.
The paths of gravitational waves should also
be warped by intervening gravitational fields which
can amplify the signal and stretch out the wavelengths.
That could make a binary merger in the 5 to 15
solar mass range look like a much more massive merger.
Of course, the really big recent news
was the observation of a neutron star-neutron star merger.
We covered that in detail here when the detection was still
a rumor.
Yet, that one turned out to be true.
For the first time, the event behind a gravitational wave
signal was also seen in light.
First, as a gamma ray burst.
But then also across the electromagnetic spectrum.
Observatories across the planet and in orbit around the planet
swiveled to watch the afterglow of this collision.
Besides what they tell us about neutron stars,
more observations like this should
allow us to figure out where the gravitational wave
signals are often also gravitationally lensed.
Virgo, the Italian-based gravitational wave observatory,
was online for the neutron star merger,
and was extremely important in narrowing down its origin.
More are coming.
LIGO-India, or IndIGO, will join the LIGO network,
aiming to begin operations in 2024.
So, LIGO is great and all.
But with its four kilometer arm length,
it's only sensitive to a frequency range from around 1
hertz to 10 kilohertz.
That roughly corresponds to the frequency of the binary orbits
just before merger, which for black holes and neutron stars
clocks in at a few to maybe 1,000 orbits per second
in the last instant.
The more massive the binary, the lower the frequency.
To catch the merger of the million to billion
solar mass black holes, supermassive black holes that
live in the centers of galaxies, we
need to observe gravitational waves in the 0.1 million
hertz to 0.1 hertz range.
That would require an interferometer
with arms of millions of kilometers in length.
Sure, why not?
That's LISA, the Laser Interferometer Space Antenna.
This spacecraft by the European Space Agency
is scheduled for a 2034 launch.
The three components of this craft
will trail behind the earth in an orbit around the sun.
Its arms will be 2.5 million kilometers in length.
That's roughly 10 times the distance
from the Earth to the moon.
It has to be that big to listen to the 0.1 millihertz
to 0.1 hertz range.
It will see those merging supermassive black holes,
as well as the faint hum of thousands
of binary pairs of white dwarfs, neutron stars, and black holes
long before they merger.
LISA Pathfinder, launched at the end of 2015,
was a mission design as a proof of concept for the technology
involved in the LISA mission.
And it was a rousing success, demonstrating
the feasibility of the mission.
The universe is flooded with space-time ripples.
We expect a faint gravitational wave background
buzz from an earlier epoch of the universe in which binary
supermassive black holes were common, or from cosmic strings,
if they exist.
Or even from the instant after the Big Bang,
when the speculative graviton decoupled
from the other fundamental forces.
LISA may detect some of this.
But much of this gravitational wave background
will have wavelengths as long as many light years.
That's beyond any gravitational wave interferometer
that we could ever physically construct.
We'd need a network of perfect timing devices scattered
across the galaxy.
Oh, wait.
We already have one.
Remember pulsars, neutron stars with jets
that sweep past the Earth as the star
processes, resulting in a sequence of flashes more
regular than an atomic clock.
We're already using these to study the gravitational wave
background at the 1 to 100 nanohertz range.
The international pulsar timing array
is a massive effort spanning many universities and radio
observatories around the world.
It monitors dozens of the fastest rotating
pulsars, millisecond pulsars, spanning
many thousands of light years.
It looks for shifts in the time of arrival of their signals.
Such shifts could indicate tiny fluctuations
in the fabric of space within the pulsar array volume
due to the passage of impossibly vast gravitational waves.
This galaxy scale observatory is already in operation
and has placed valuable limits on the amplitude
of the gravitational wave background.
More time and more data will hopefully yield a detection.
This may be the only way we can look directly at the instant
after the Big Bang.
Some scientists are even trying to see how gravitational waves
should interact with stars.
Gravitational radiation should have physical effects as shown
by the sticky bead argument first presented
by Richard Feynman.
He came up with a thought experiment
of a simple gravitational wave detector, a rod with two
sliding beads.
As a gravitational wave passes by,
the beads are free to follow the expansion and contraction
of space while the rod resists due to the atomic forces
between its atoms.
But as they move along the rod, these beads
generate friction and thus heat.
That heat energy comes from the gravitational wave.
This thought experiment isn't practical,
but it demonstrates that in the right circumstances
gravitational waves should be able to dump
some of their energy into matter, for example,
into stars.
Stars oscillate at particular frequencies.
We spoke about this in our episode on asteroseismology.
If a gravitation wave frequency matches
the natural resonant frequency of a star,
oscillations can be set up inside the star.
This generates an internal friction,
heating up the stellar interior.
The star should then brighten.
And it may be possible to observe this effect
in the dense star fields of galactic cores
if those galaxies also contain binary supermassive black holes
that are generating gravitational waves.
A similar effect may cause white dwarf stars in binary orbits
to explode as they absorb gravitational radiation
from their own orbits.
Links to all of the programs and papers I mentioned
are in the description.
We couldn't talk about everything.
There's a lot happening.
Gravitational wave astronomy is currently in a gold rush.
Questions long unanswered are now being resolved,
and new mysteries open up.
There are going to be some extremely important and also
entirely unexpected discoveries as we
learn to decode the incredible wealth of information carried
to us in the rippling fabric of space-time.
Before Einstein was able to derive his general theory
of relativity, which predicted the existence
of gravitational waves, he had to master it
precursor, Newtonian gravity.
He needed to understand Newton's theory of gravity
well enough to see where it failed.
If you haven't already done some serious university-level
physics, then mastering Newtonian gravity
is also your next step toward understanding
Einstein's view of space-time.
Happily, you don't need to do a physics degree.
Our friends at brilliant.org have
created a series of courses that will guide you step-by-step
from the basics of classical mechanics
through the intricacies of Newtonian gravity.
All of brilliant.org's many courses on math and physics
use the best pedagogical practices.
They focus on developing deep conceptual understanding
through solving real problems.
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As always, a huge thank you to everyone
who supports us on Patreon.
Even the smallest contributions really do help a lot.
And this week, we want to give an extra huge thanks
to Shannon Catalono.
Shannon, you've been one of our most stalwart supporters
since the beginning.
And you always ask the best questions
on the space-time hangout YouTube stream.
We wanted to invite you to join the full live call next time.
We hope you'll join us.
OK.
So, last week's episode was a mega collaboration
with PBS "Eons" and "It's OK to Be Smart,"
tackling the giant question, what is the origin of life?
Our episode was on the physics of life.
Let's discuss.
Many of you noticed that we showed a red blood
cell with DNA and pointed out that adult red blood
cells don't have DNA.
Yeah, that was actually a martian red blood cell.
They totally have DNA.
OK, whatever.
I'm not a biologist.
On the other hand, Johannsen, of "It's
OK to Be Smart," who was collaborating on that episode,
he is a biologist.
And I want to stress that he did not see that animation
before we posted.
If he had, he would have been as outraged as you guys were.
Bose-Einstein asks, is life causing the universe
to reach high entropy faster than if life didn't exist?
Well, that's right, it is.
We are, from an astronomical perspective.
Life reduces the Earth's albedo, it's reflectivity.
Without life, Earth would reflect a higher intensity
spectrum closer to the 6,000 Kelvin thermal spectrum
it receives from the sun.
As is, Earth, with the help of its absorbing biosphere,
reprocesses a lot of that light and radiates something
closer to a 300 Kelvin thermal spectrum.
Plant a tree, and you accelerate the heat death of the universe.
But you decrease entropy locally.
So you should selfishly plant that tree anyway.
Slimthrall suggests a flaw in the idea
that life is a mechanism for increasing entropy.
The suggestion is that, if this were true,
then life should be much more common, more common than not
wherever conditions are vaguely favorable.
The problem with your problem, Slimthrall,
is that entropically favorable reactions still
require suitable conditions.
By analogy, energetically favorable reactions
sometimes also require special conditions.
For example, it's energetically favorable for liquid water
to freeze below a certain temperature.
But it doesn't happen unless it is a solid nucleus for that
first ice crystal to attach to.
Sometimes the conditions for a given path to low energy
or to high entropy are unusual.
That appears to be the case with life.
Michael always wanted to be a supervillain,
and now takes comfort that his mere existence is helping
to unravel the universe.
Well, thanks for joining us, Captain Entropy.
You are by far the laziest supervillain ever.