Astronomers have announced the first-ever successful test of Einstein's theory of general
relativity using a supermassive black hole at the center of our galaxy.
And we're going to learn more about it, starting right now.
Hi everyone, welcome back to Launch Pad Astronomy, I'm Christian Ready, your friendly neighborhood
astronomer, and if this is your first time here, please make sure to hit that Subscribe
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Now, the center of our galaxy lies about 26,000 light-years in the direction of Sagittarius.
At the very center is a 4 million solar mass black hole.
That's a supermassive black hole, and indeed, all galaxies have black holes like these at
their centers.
This object is called Sagittarius A-star, or Sag A*, and it's surrounded by a swarm
of nearby bright stars.
Astronomers used this black hole as kind of a laboratory to test Einstein's theory of
general relativity.
In a nutshell, general relativity simply says that matter bends spacetime and spacetime
tells matter how to move.
So this distortion in spacetime is what we call "gravity".
[cicadas singing loudly] Now this is a very different idea of gravity that was first proposed
by Isaac Newton in his classical theory of gravitation.
Essentially, Newton says that gravity is a force between two objects that have any kind
of mass.
So which idea is correct, Einstein or Newton?
Well, the short answer is Einstein because we've tested his theory over and over again
since he first proposed it 100 years ago.
However, we've never been able to test it directly against a supermassive black hole,
until now.
To make this test, astronomers followed a particular star called S2.
S2 completes one orbit every 16 years.
In May of 2018, S2 came within 20 billion kilometers of the black hole.
It reached speeds of 20 million km/h, that's about 3% the speed of light, so this thing
was really hauling the mail.
And it was this close passage that made for a perfect opportunity to test general relativity.
But to make these measurements, astronomers needed a very large telescope, so they used...the
Very Large Telescope.
The VLT consists of four 8-meter Unit Telescopes.
Normally, these telescopes operate independently of one another, so they look at different
things in the sky at any given time.
But on this occasion, all four Unit Telescopes looked to the center of our galaxy at the
same time.
Their light was combined into a special instrument called GRAVITY, which combined their light
to form a single image using a technique called Interferometry.
This allowed VLT to make make measurements 20 times sharper than the individual telescopes
would have made by themselves.
Now it's important to understand just how precise these measurements are.
It's like being able to watch a soccer ball being kicked around a soccer field in a soccer
stadium on the Moon.
So yeah, that precise.
The GRAVITY instrument worked with two other instruments at VLT to measure S2's gravitational
redshift.
As the star passed close to the black hole, it's light was stretched to longer and redder
wavelengths as the light worked to escape the gravitational pull of the black hole.
Hence, the name "gravitational redshift".
The effect is predicted by Einstein's theory of general relativity and not by Newton's
classical theory of gravity.
When they made measurements of S2 as it went by the black hole, they examined its redshift,
and sure enough, it matched Einstein's predicted values perfectly!
These observations were 26 years in the making.
It took that time for for the astronomical techniques and instrumentation to be developed
in order to put general relativity to this test.
And sure enough, it passed with redshifted colors.
See what I did there?
[whispers] Redshifted...all right.
But this is more than just a test of general relativity, this is a new way for us to probe
the supermassive black hole at the center of our galaxy.
It also helps us to understand how this black hole evolved, and how our galaxy evolved with
it.
And speaking of supermassive black holes, did you know that we've actually detected
neutrinos coming from these things?
Holy cow!
There's a video for that you oughta check out as well!
And if you want to join me on this journey through this amazing universe of ours, and
figure out how it works, make sure that you subscribe and ring that notification bell
so you don't miss out on any new videos.
Thank you again for watching, and until next time, keep watching the skies.
[BEEP] The cicadas...the cicadas are all...buzzing right now, they're doing the equivalent, the
cicada equivalent of "hey girl".
[BEEP] ...that's like being able to watch a soccer ball as it's kicked around a soccer
field in a stadium on the Moon.
Oh yeah, it's precise!
[BEEP]