Gravitational waves are ripples in the fabric of spacetime,
predicted by Einstein’s laws of general relativity,
but they are incredibly difficult to detect.
To see them，you need a detector that can accurately measure distances
10,000 times smaller than a proton.
That’s like trying to measure the distance from our Sun
to the nearest star to accuracy of the width human hair.
But we have a technology on Earth that can do that: ALIGO.
The Advanced Laser Interferometer Gravitational-wave Observatory,
and back in November 2015, on a Monday morning,
LIGO detected the first gravitational wave that humans have ever directly observed.
Where they came from and what this means for space science
is nothing short of mind blowing!
This is the story.
A long long time ago, in a galaxy far far away.
我不是开玩笑 13亿年前 离我们13亿光年的地方
I’m not kidding, 1.3 billion years ago and 1.3 billion light years away,
two black holes were stuck in a perilous orbit around one another,getting closer and closer.
黑洞非常神奇 它们的引力 它们弯曲时空的能力
Black holes are incredible objects, the pull of their gravity – the amount they bend spacetime
– is so strong that no light can escape them.
No one knows what exists in the centre of a black hole
as normal physics completely breaks down.
What we do know is that they are infinitely dense.
One of these orbiting black holes was 29 times the mass of the Sun
and the other was 36 times the mass of the Sun,
but they were only about 200km wide.
Which is tiny in comparison to the Sun which is over a million kilometres wide!
And these black holes were orbiting each other really really fast,
about the same frequency as the blade on a blender.
Imagine that, such massive objects rotating so quickly.
These orbiting masses created ripples in the fabric of spacetime called gravitational waves,
and the closer they orbited the bigger these waves got,
until the black holes collided at half of the speed of light.
And when they merged they formed a new black hole that rang kind of like a bell,
throwing out colossal amounts of energy as gravitational waves
until it settled into a perfect sphere.
And all of this happened in 0.2 seconds.
And in the collision, they turned a huge amount of mass into gravitational wave energy.
They lost a mass equal to three times the mass of the Sun
which got turned into gravitational wave energy
by Einsteins equation E=mc^2.
This created a huge wake of gravitational waves that ripped out in all directions at the speed of light.
And, and this is the thing that gets me, over that last fifth of a second
this collision released more than ten times more energy than
total output of all of the stars in the entire rest of the Universe!
It just completely boggles the mind!
Meanwhile on Earth At this exact time
our planet was looking very different to what we see now.
It was a barren wasteland
没有草没有树 其实 动植物都没有
there was no grass or trees, in fact no plants or animals at all.
Life at this stage had only come as far as
microscopic multicellular creatures that lived in the sea.
And while the gravitational waves tore through space towards us
all of the complex life on earth evolved and grew:
plants and animals developed, amphibians crawled on land,
很多生物都灭绝了 爬行动物 恐龙和哺乳动物
there was extinctions, reptiles and dinosaurs and mammals, more extinctions.
Primates evolved into all of human civilization
right up until Saturday 12th November 2015
when the scientists at LIGO turned it on to begin their initial tests.
仅仅2天 嘣 引力波飞速而来
A mere two days later and … boop … the gravitational waves flew past us
and the first direct detection on Earth was made.
而且 引力波发出的声音就是 嘣
And that sound … boop … is actually what these waves sounded like.
Even though gravitational-waves are ripples in spacetime and not ripples in the air
they vibrate at similar frequencies,
so we can actually turn them into sound waves and listen to them
boop … It might not sound very impressive,
but detection of gravitational waves means a huge amount for science.
The results of this detection have already been profound.
This is the very first time that black holes have been directly detected,
in fact gravitational waves are the only way you can directly detect them!
Consider this: all of our knowledge about the Universe to this date
has come to us through telescopes that measure light
从电磁波谱的光 像无线电波 可见光
from the electromagnetic spectrum like radio waves, visible light,
x-rays, or through detectors that measure subatomic particles.
But all of these existing telescopes are completely blind to gravitational waves.
This means that gravitational wave astronomy is an entirely NEW THING,
a whole new way to observe the Universe.
And it has already solved some important science questions:
gravitational waves exist,
blackholes exist, and gravitational waves travel at the speed of light.
Here’s how LIGO detectors work.
There are two L shaped buildings,
one in Hanford Washington,
and another in Livingstone Louisiana make up LIGO
and also there’s another detector called VIRGO
which is in Italy near Pisa.
Having several detectors on Earth
means that scientists can verify a signal from space
by seeing if it appears at all of the different detectors,
and they are far apart because it means they can see roughly
in what direction the wave is coming from by triangulating the signal
Even with these waves travelling at light speed
there is a delay of a few milliseconds between each of the detectors.
Each of these L shaped buildings houses a thing called a laser interferometer
where a laser is produced,
then split into two beams which each fly down a 4km long arm,
and then they are reflected off a 40kg pure silicon
test mass which is hung from the ceiling.
It is hung from the ceiling on a series of pendulums
to insulate it from any outside disturbances
like trucks moving past, or people sneezing.
When you are trying to detect distances with a sensitivity less than the nucleus of an atom,
getting rid of external noise is very important.
Each arm bounces the laser back and forth 400 times
to make the arm effectively 1600km long,
and each time around the laser is boosted to make it stronger.
Then after its journey the light is collected and recombined in such a way that
if the arms are the same length
the light from each arm cancels out producing no signal
But if a gravitational wave ripples past,
it will stretch or contract each of the arms
making the light from each arm arrive at the detector at slightly different times.
That means that when they are recombined, they will no longer fully cancel out,
and a signal will appear.
And this is exactly what happened on the 14th November.
It’s crazy to me to imagine that if they had turned on this detector just two days later,
they would have completely missed the signal
that had been travelling towards us for 1.3 billion years.
This makes me wonder
what other signals from deep space have we already missed,
and what is still out there screaming towards us waiting to be discovered
that we just don’t know about yet.
In fact at the time that I made this video
LIGO has detected another even stronger black hole collision,
and we can expect a lot more in the coming months and years.
It has already achieved many of its science goals,
there are many more it is aiming for.
For example it might be able to give us a more accurate measure
of how fast the universe is expanding,
and so how much dark energy there is.
It will hopefully be able to look at what makes stars go supernova,
and might be able to probe the very nature of spacetime
and see if it is made of things called cosmic strings.
But the most exciting thing is that we don’t know what it will find.
This is one of the best parts of science, when you’ve got a new tool
to peer into a realm of reality that you’ve never been able to access before.
Who knows what you’ll find?
Maybe you’ll discover things that help explain some of the great mysteries of the Universe,
maybe we’ll find things that we can’t explain at all,
and then we have to come up with new physics.
In any case I find it super exciting
and I can’t wait to see more results.
So there you go, those are the basics of gravitational wave astronomy.
Hey I just want to say thanks for all the comments on my last video,
I wasn’t able to reply to them all,
but I did read them all and the amount of positive feedback was amazing.
So it makes me want to carry on and make more and more of these videos so thanks for that.
As for this video, I didn’t get to cover everything I wanted to.
This filed of gravitational wave astronomy is amazing.
In eight minutes you can only cover so much, so if you are left with any questions
please put them in the comments and I’ll try to reply to them
or I’ll collect together the most popular ones
and do something later, a Q&A or something like that.
Also I have set up a Patreon account
and so if you want to help support me make these videos
就去看看吧 另外 感谢收看 我们下期再见
go take a look at that, otherwise, thanks for watching and I’ll see you on the next video.