Hi there, physics fans.
In our last episode, we talked about one of the most mind-bending phenomena in the universe
– a black hole.
But we only had so much time and there’s so much more to say.
I didn’t tell you about how black holes mess with time or how they bend space.
In fact, I didn’t tell you about a bunch of things.
That’s OK because they are great topics for this episode of Subatomic Stories.In episode
13, I talked about some of the consequences of general relativity on the nature of space.
In general relativity, gravity isn’t a force in the traditional sense – rather it is
the bending of space.
I mentioned Arthur’s Eddington’s eclipse expedition in 1919, which showed that the
prediction was true and catapulted Einstein to stratospheric fame.
Most observations of the bending of space are pretty minor and are look largely indistinguishable
from the predictions of Newton, but there is one that very different.
This is the prediction that rapidly moving objects can cause ripples space.
The phrase for these ripples is gravitational waves.
These spatial distortions mean that the moving bodies will lose energy.
It’s not all that different from hot objects losing energy by emitting light.
The first data proving that gravitational waves are real was the 1974 observation by
Russell Hulse and Joseph Taylor of two neutron stars, locked in a tight orbit.
These researchers saw that the amount of time it took for the two stars to orbit one another
was steadily decreasing.
That’s because the two stars were losing energy to gravitational radiation.
Hulse and Taylor shared the 1993 Nobel Prize for their work.
But Hulse and Taylor’s work was indirect evidence.
It would be nice to observe gravitational waves here on Earth.
That happened in 2015.
Literally, a long time ago, in a galaxy far, far, away, two black holes orbited one another,
much like Hulse and Taylor’s neutron stars.
One was about 35 times the mass of the sun; the other was 30 times.
Being black holes, they were invisible.
For eons, they orbited one another, emitting gravitational waves, and drawing ever closer
to one another.
They orbited faster and faster.
Then they merged.
The final process took two tenths of a second.
Two black holes with a combined mass of 65 solar masses became a single black hole with
a mass of 62 solar masses.
The missing 3 solar masses of energy was emitted by gravitational radiation.
For that fraction of a second, the gravitational energy emitted by the merging black holes
was about 50 times more energy than all of the light emitted by every glowing thing in
the universe.
Wow.
As in, like, holy cow.
Those gravitational waves roared across the universe, getting ever smaller because the
energy spread out in a sphere that expanded at the speed of light.
Then, about 1.4 billion years later, they washed over the Earth.
Two large scientific instruments, both in the U.S., one in Louisiana and one in Washington
state, used powerful lasers to detect the tiny distortion of space.
The instruments are 2.5 miles (or 4 kilometers) in size, and that distance changed by less
than one-one thousandth the diameter of a proton.
Proportionally, that’s equivalent to measuring the distance from the Sun to the nearest star
with a precision of the width of a human hair.
The experiment that accomplished it is called LIGO, short for Laser Interferometer Gravitational-Wave
Observatory.
It’s an amazing facility.
Look it up.
The observation of gravitational waves was certainly an awesome technical achievement
and it’s amazing that they could determine such incredibly tiny ripples in space, but
the experimenters weren’t done.
In fact, they’ve detected lots of gravitational waves.
But one detection totally pegged the cool-o-meter.
In 2017, the LIGO experiment detected another passage of gravitational waves.
This time, a detector in Italy called VIRGO joined the two American detectors.
Together, they could identify very precisely the direction the gravitational waves were
coming from.
This allowed telescopes sensitive to electromagnetic radiation to look in the same direction.
Gamma ray telescopes observed gamma radiation just 1.7 seconds after the gravitational waves
arrived.
Astronomers knew that the burst of gamma ray radiation came from the merging of two neutron
stars.
They identified the galaxy housing these neutron stars and it was a giant lenticular galaxy
called NGC 4993.
NGC 4993 is located about 130 million lightyears away.
So, let’s do some math.
Gravity and light travelled 130 million lightyears and arrived within 1.7 seconds of one another.
We know that light travels at the speed of light, which means it took 130 million years
to get here.
That’s about 4 times ten to the 15 seconds.
Compare that to the 1.7 seconds delay between the arrival of gravitational waves and gamma
rays and you’ve measured the speed of gravity.
It is equal to the speed of light to better than one part in a quadrillion.
That’s just simply incredible.
It’s a direct measurement, and there is no arguing it away.
Gravity moves at the speed of light.
Einstein postulated that, but now we have firm data.
Just amazing…
Hmmm…I seemed to have wandered a bit away from black holes, but the capability of being
able to see the ripples in space that we call gravitational waves is just too cool a scientific
achievement not to talk about.
Of course, that means that I only talked about how black holes or neutron stars can distort
space.
I didn’t talk about how they distort time.
Gotta leave something for the next episode, I suppose.OK, so let’s take a look at this
week’s questions.
You know, I think that questions are my favorite part of the Subatomic Stories series and there
were some good ones this week.
Warlok says I should use my physicist voice more often.
Perhaps I should, but as I told acolyte Ben Parker, with great power comes great responsibility.
I have to use this power responsibly.
It’s probably worth watching my long form video on Time Crystals to see what can go
wrong if the power is used frivolously.
The link is in the description.
Paul Hewitt notes that Kip Thorne has said that inside a black hole there is only warped
spacetime and asks for a comment.
Hi Paul.
Well first, let me say that Kip is a really smart guy and a recent Nobelist, so he should
be listened to respectfully.
But, also respectfully, I think I disagree.
Or, more correctly, I think it’s hasty to say anything with certainty.
Here’s what we know and what we don’t.
First, the inside of a black hole is hidden by an event horizon, which means that it is
impossible to see what is really going on.
Thorne’s statement is entirely a theoretical one, derived from the Einstein’s theory
of general relativity.
As an experimentalist, I’m cautious of any purely theoretical statement.
Second, we know that general relativity fails in the subatomic world, so why should we completely
believe its predictions?
We’re going to need a theory of quantum gravity to be sure.
However, with all of that said, we can be pretty sure that whatever is going on inside
a black hole isn’t just squashed matter.
We already know that, at the subatomic level, mass as we know it is really just energy.
I made a video about precisely that.
It is entirely reasonable to imagine that inside a black hole, what exists is just energy.
And, according to relativity, concentrated energy distorts space and time, which is what
Thorne said.
While Thorne’s statement was based on his much deeper understanding of the theory, what
I’ve said here is the central origin of his statement.
However, with the event horizon, it may forever be impossible to make any definitive statement
on the insides of a black hole.
MAFLSTAR asks if small black holes could be dark matter.
The answer to that question is both yes and no.
Back in the 1980s there was an idea that black holes, brown dwarfs, or planets untethered
to stars could make up dark matter and there were experiments designed to look for them.
Together these objects are called MACHOs, short for MAssive Compact Halo Objects.
Searches for MACHOs involved watching distant stars, looking to see MACHOs pass between
the star and the telescope.
You’d think that the passage would dim the distant star, but, because of the gravitational
lensing of general relativity, instead it brightens the image of distant stars.
There are many details, but basically astronomers looked at stars, hoping to see the characteristic
brightening.
They saw some, but not enough to be dark matter.
Once the analysis was complete, astronomers ruled out black holes and other MACHOs in
the range of about a third the mass of the moon to about a hundred solar masses as the
cause of dark matter.
Accordingly, if dark matter is due to black holes, they’ll have to be very light and
that just seems very improbable.
So astronomers have pretty much ruled out the idea.
Brian Guzik asks if everything inside the event horizon of the black hole be considered
the singularity.
Hi Brian.
Well, my answer about Kip Thorne answered a lot of this.
It’s unlikely that there are layers of matter inside a black hole.
But the singularity is a mathematical thing.
It’s the theoretical abstraction with zero size at the center of the black hole.
That’s what singularity means.
So no.
Null null asks me to reconcile how I can say that a pointlike singularity isn’t real,
while particle physicists say that elementary particles are point objects.
Hi Null.
Yeah, that’s a curiosity.
It boils down to shortcuts in how scientist describe things to a non-scientist audience,
combined with some real differences between general relativity and quantum mechanics.
General relativity is a classical theory.
In it, zero size means zero size.
It’s easy to imagine.
However when talking about particle physics, we’re talking quantum mechanics.
There, we mean something different.
For one thing we have to embrace the wave/particle duality.
Particles are also waves and they therefore have wavelengths.
Wavelengths imply a size.
Thus even if we say a particle like an electron has zero size, it has an associated wavelength
that isn’t zero size.
In addition, in episode 9, I talked about the quantum foam, in which a nominally pointlike
particle like the electron is surrounded by a bath of virtual particles that constantly
appear and disappear.
This behavior persists over a volume of non-zero size.
So, this is another measure of electron size.
Finally, there is the question of whether the particle has any constituents.
For instance, on human scales, we can consider the proton to have zero size, but we know
it has a radius of about a femtometer and it contains quarks and gluons.
Thus, when you look at it close enough, a proton is most clearly not a point particle.
So, if you put it all together, in relativity, a singularity just means zero size.
For particles, it means an object containing no constituents and with no intrinsic size,
but you must also take into account the wavelength of the particle, plus the surrounding cloud
of virtual particles.
The term “zero size” means very different things in the two different theories.
Good question.
Andy is wonderful asks why black holes had to be non-rotating for Schwarzschild to calculate
the event horizon.
Hi Andy.
Finally, an easy question.
He made the simplification because it made the math easy.
Real black holes are more complex.
They rotate, have charge, all sorts of things.
Schwarzschild did his calculation in 1916.
Calculating the properties of an electrically charged, non-rotating black hole evolved over
the period of 1916 to 1921.
The exact solution for an uncharged, rotating black hole wasn’t worked out until 1963
by New Zealand mathematician Roy Kerr.
And solution for a charged and rotating black hole didn’t happen until 1965.
So, the short answer is that Schwarzschild picked the low hanging fruit.
I always said he was smart.
Do the easier things first.
In a previous episode, I mentioned the valuable commenting by Dr. Deuteron.
In this episode, I’d like to call out Narf Whals who contributed many good and substantive
answers.
So I’m going to make a Hall of Heroes, which are the names of commenters who have shown
themselves to be valuable contributors.
This doesn’t mean that I endorse these individuals or everything they say – indeed, I don’t
know them.
But as long as they continue to add valuable information to the conversation, I’d like
to recognize them as people whose answers are worth considering carefully.
The hall will grow as others are identified and my apologies for overlooking earlier commenters
worth of the hall of heroes.
I think that’s all the time we have for questions today.
Please like, subscribe, and share, because lots of people like questions and science.
And, by science, I mean physics, of course, because you know where I’m going with this.
Say it with me “Even at home, physics is everything.”