Time moves slower on Mercury than it
does on Earth.
A day on Mercury is 58 days, 15 hours,
and 30 minutes on Earth. We can tell
this by observing Mercury through
telescopes and [music] timing how long
it would take for Mercury to complete
one full orbit around the Sun.
However, if we travel to Mercury and
started our timer there, we would not
[music] get the same figure.
This is because time does not move as
constantly as you might think.
It can be affected by speed. The faster
you go, the slower time goes for you.
But, it can also be affected by gravity.
And Mercury is much closer [music] to a
massive source of gravity in our solar
system, the Sun.
But, how much does gravity affect time?
Could we live to incredible ages if we
all went to live on Mercury?
I'm Alex McColluch, and you're watching
Astrium. Join with me today as we answer
this question and explore the [music]
strange and warping world of general
relativity and gravitational time
dilation.
>> [music]
>> I hope by the end of this video to have
earned your like and subscription.
To begin with, what exactly is time?
Mark Twain once said that time is what
keeps everything from happening all at
once. [music]
It separates yesterday's events from
today's and today's from tomorrow's.
As you sit here and watch this video,
you are not doing what you were doing
yesterday or tomorrow. You are watching
this video.
You are constantly traveling through
time in the direction of the future at
the rate [music] of 1 second per second.
This is the normal speed for traveling
through time and is shared by you and
everyone else on the planet.
But, this speed doesn't have to be the
case.
As discovered by Albert Einstein, the
faster we travel through space, the
slower our time becomes.
Up until the point where if we somehow
could travel at the speed of light, time
would stop moving for us.
If we were to travel to an object four
light-years away from us, to people here
on Earth, it would look like it took us
four years.
To us, it would look like we arrived at
our destination instantaneously.
Although this sounds incredible,
scientists have proven that this
actually happens.
They took two clocks. One they put on a
plane and flew around the world with it
on board, increasing its speed for an
extended amount of time, while leaving
the other at a stationary location.
When they eventually returned and
compared the clocks again, they found
that the clock that had traveled faster
had actually recorded less time, in line
with what general relativity had
predicted.
General relativity is a real thing.
But Einstein also predicted that this
would happen with gravity.
The closer you are to a large source of
gravity, and the more mass that source
of gravity has, the slower time will go
for you.
This was proven to be correct in 1959
with the Pound-Rebka experiment, where
they shot gamma rays from the top of a
tall building to the bottom and recorded
whether anything had happened to the
light waves.
They found that the frequency of the
waves increased the closer to the Earth
they were. Kind of like cars bunching up
in congested traffic, the [music] peaks
of radiation moved closer together.
Time slowed down ever so slightly for
the gamma radiation, the closer it got
to the Earth.
This effect is significant enough that
satellites have to be programmed to
account for it, because when they are
further away from the Earth's gravity,
time moves more quickly for them.
Without this correction, their internal
clocks would go out of sync with our
clocks on Earth, which would cause
problems.
And yes, that's right. The Earth's
gravity is enough to cause time to slow
down for us.
So, you're probably now thinking, "How
much faster would time be going for me
if I wasn't on Earth or if Earth had no
gravity?"
Well, it's a matter of perspective. The
procession of time will always appear
the same from your perspective. You
won't feel like you're going slower,
[music] although you may notice some
weird stuff going on around you.
But, it's the outside observer [music]
that has to deal with you slowing down.
To find out an exact answer to the
question, Einstein devised the following
[music] equation.
We will be using this equation a lot in
the rest of the video, so let's try to
understand it.
It looks a little scary, but it's not as
bad as it looks.
T0 is the answer we're looking for, how
much time would pass on the object we're
interested in, in this case, the Earth.
TF is how much time would pass in a
place completely unaffected by gravity.
Such a place doesn't [music] exist, by
the way, but it is our hypothetical
stationary clock.
G is the gravitational constant, a
number they added to basically make the
sums work out. We're not going to worry
about where it came from. All we're
interested in is the fact it's always
this.
And don't worry too much about that
metric at the end.
M is mass, specifically the mass of the
object creating the gravity.
R is the distance from the center of the
object, and C is the speed of light, a
figure we know to be roughly 300 million
m/s. [music]
So, to find out how much slower time
goes on Earth compared to somewhere
where there was no gravity, all we have
to know is how much mass the Earth has,
how far we are from its center,
and then we can decide how much time we
want to compare.
Just to ease ourselves in, I decided to
try this formula out with 60 seconds.
If 60 seconds passes in our place
unaffected by gravity, how much time
passes on Earth?
I plugged in the information and got our
result.
For every 60 seconds that passes, people
on Earth experience only 59.9999996
seconds.
In other words, 6 billion seconds have
to go by before we notice a difference
of 4 seconds.
This happens roughly once every 190
years.
So, realistically, you'll only gain
about 2 seconds over the course of your
entire lifetime because of the Earth's
gravity compared to a place with no
gravity.
This is a little underwhelming.
However, we here on Earth are not just
affected by our planet's gravity. We are
also affected by the gravity of the Sun,
which in terms of our equation is much
more significant. So, let's try this
again by inputting the mass of the Sun
and our distance from it into the
equation.
This gives us the new figure. How much
time goes by for us on Earth for every
60 seconds of actual time that goes by?
59.9999994
seconds.
While this is a more significant number,
it does mean that you are only seeing a
difference of a second once every 3
years or so.
Or in other words, about 24 seconds over
the course of an average human lifetime.
But, Mercury is much closer to the Sun.
Surely, time is significantly altered
here compared to Earth with a massive
object like the Sun so close by.
How much more does time slow down for us
on Mercury?
The equation is essentially the same as
before. The only new number we need is
the distance between the Sun and
Mercury.
The answer to our final question of how
much time passes on Mercury every 60
seconds is
59.999999
seconds.
60 million seconds need to go by before
there's a single seconds difference.
This happens roughly once every 1.9
years, which scaled up across a lifetime
equates to about 38 seconds difference.
Only 14 more seconds than Earth.
But from your point of view, you're not
really gaining those seconds because
from your perspective time would be
moving normally.
It would be the Earth that is moving
ever so slightly faster from your point
of view.
So, if you pointed a telescope towards
Earth, you'd see 14 more seconds of
Earth's history than you would if you
were living there.
As Mercury is fairly inhospitable, this
probably isn't worth it.
So, does Mercury's proximity to the Sun
allow time to move slower for you
compared to Earth? Yes, but only by a
difference of about 14 seconds over your
entire lifetime.
In reality, time dilation due to gravity
is not actually that significant, at
least not in our own solar system.
The mass of the Sun is not enough to
really stretch time that much for us,
even if we were as close to it as
Mercury.
The Sun would need to be millions of
times more massive before we would begin
to experience time dilation to any
significant degree.
However, it's still a fascinating
insight into the way that time flows in
our universe and reveals that time is
not as constant as we might think.
And of course, it begs one final
question.
Are there any objects in the universe
that are massive enough that we'd
experience significant time dilation
around them?
There are.
And this is where things start to get
very noticeable to an outside observer.
At the center of galaxies exist [music]
objects known as super massive black
holes.
These objects have masses that are
millions to billions of times the mass
of our sun.
In other words, if you were to fall
towards them, time could slow down for
you so much that it would approach zero.
If you were to get close enough to one
of these giants and later escape it, you
could spend half an hour around there
and emerge to find that [music] hundreds
of years had passed to the people back
on Earth.
People on Earth with a powerful enough
telescope would see you barely move.
Even your breaths [music]
would take days to complete.
In fact, there would be a point right at
the edge of such a body that as you
approached it, your time would hit zero.
What would happen if you passed that
point? Our equations break down and so
cannot explain what lies beyond that
horizon. Science does not yet have the
answer, although there are certainly
many theories.
But that might be better explored in
another video.
Movement through space is inevitable.
You might not feel it, but right now
you're sitting on a planet that is
spinning on its axis, moving you at up
to 1,600
km/h depending on your latitude.
The Earth circles the sun.
The sun moves through the Milky Way.
The Milky Way moves through the local
group of galaxies.
And the local group moves through our
local supercluster.
And even that supercluster of galaxies
is in motion, heading towards the
Shapley Supercluster, where scientists
believe that there is a particularly
large concentration of galaxies.
I hope all this motion isn't making you
too dizzy.
But on the grandest scales of our
universe, something strange is
happening.
Scientists have begun to realize that
there is not enough mass in the Shapley
Supercluster to pull us towards it at
the rate that we see.
About 50%
of the cause of that motion is
unaccounted for.
In 2017, researchers discovered a
possible source for the rest of that
motion.
But it's not something pulling us.
It's something pushing.
I'm Alex McConaughey and you're watching
Astrium. Join with me today as we
explore the evidence for the region of
space known as the Dipole Repeller and
try to understand how in a universe
filled with gravity something can push
us instead of pull.
Let's start with a little context.
It's taken a long time for astronomers
to recognize that this motion was taking
place.
To understand that we were moving,
scientists first had to make a map of
the regions of space around us.
And although we as a species have been
mapping the stars since the beginnings
of civilization, it was only thanks to
Mr. Hubble in 1924 that we proved there
are actually stars in other galaxies
outside the Milky Way.
Before then, although astronomers had
seen [music] fuzzy little clusters of
light in the night sky that we now know
to be galaxies, the common wisdom was
that these were gas cloud spiral nebula
within the Milky Way, not something that
existed beyond it.
Hubble noticed several Cepheid stars, a
type of variable star with a cycle of
variation closely linked to their
luminosity, within one such [music]
spiral nebula,
and used the fact that this luminosity
was very predictable based on distance
to calculate very precisely how far away
they were.
Surprising many, he proved that they do
exist beyond the Milky Way.
Since then, astronomers have been
scrambling to catch up with the new
reality of galaxies and superclusters
and set about mapping the locations and
distances to every galaxy they could
find.
Hubble had been studying Andromeda, our
closest neighboring major galaxy, but
there were others.
By the 1970s, Stephen Gregory, Laird
Thompson, and William Tifft definitively
proved that galaxies didn't just float
randomly in space, but converged into
superclusters, large filamentary or
sheet-like structures that exist between
large bubbles of void. They mapped the
first supercluster, the Coma
Supercluster, which covered a region of
space
>> [music]
>> 100 million light-years across.
A whopping 95%
of all galaxies are found in these
superstructures.
It wasn't until 2014
that our own supercluster, the Laniakea
Supercluster, was fully mapped.
You might be surprised to hear it
happened in that order. After all,
wouldn't common sense say it's easier to
map regions of space closer to us rather
than ones further away?
Why was it only nearly a century after
the discovery of other galaxies existing
>> [music]
>> and 40 years after mapping the first
supercluster that we finally turned to
our own?
The answer is that mapping the local
regions of space around us is
surprisingly [music] challenging.
And it's all because of the Milky Way
getting in the Milky Way.
The Milky Way is full of gas and dust,
and these regions are so difficult to
penetrate [music]
that we can't really see beyond them.
Towards the center of the Milky Way,
this effect gets so bad that scientists
[music] have dubbed the whole area the
zone of avoidance.
Very little light gets through it.
Not being able to see in this area is
particularly relevant to our current
discussion for reasons we'll get to
later.
In contrast to our local area, distant
galaxies are easier to study as all you
have to do is point your telescope at a
single point in the night sky and let it
drink in the light. Mapping the area
around us requires looking at every
point in the night sky.
It's proved quite difficult, although
astronomers are now getting a better
handle on it. Thanks to their efforts in
looking at thousands of galaxies in our
galactic neighborhood and by studying
galaxies' peculiar motion or their
motion relative to the cosmic background
radiation that ignores the expansion of
the universe,
scientists have been able to group
gravitationally bound galaxies into
superclusters.
At the center of our Laniakea
Supercluster, lies a point known as the
Great Attractor, an area 150 to 250
million light-years away from us that
we, in the Milky Way, are slowly
drifting towards.
Sadly, the Great Attractor lies within
the zone of avoidance, so it's tricky to
see clearly what lies there.
To confound things further, the Laniakea
Supercluster was also found to be
moving. It travels towards another
supercluster known as the Shapley
Supercluster, taking us along with it.
At the center of this supercluster lies
the Shapley Attractor, which also is
believed to be incredibly dense in terms
of mass.
But, frustratingly, this second
attractor also lies in the zone of
avoidance, making it difficult to see
exactly what's going on over there, too.
Thankfully, as X-ray and infrared
telescopes improved, it became possible
to take a better look at these two
attractors.
But, this is where the mystery begins.
What mass is there is not quite massive
enough to account for our apparent
motion in that direction,
which is why a team of researchers from
the University of Hawaii published an
article in Nature to attempt to explain
this discrepancy.
The team was made up of Yehuda Hoffman,
Daniel Pomarede,
R. Brent Tully, and Helene M. Courtois.
[music]
Together, they created a new kind of map
of the local universe around us.
>> [music]
>> This time, instead of simply noting
galaxies' positions, they highlighted
their motions instead.
This allowed them to create a map of
flow lines, which, after mathematical
analysis, allowed them to make
assumptions about the locations of
masses in our nearby superclusters.
Their results were surprising.
A lot of our local mass is moving
towards the Shapley Supercluster, which
is what you would expect.
But, they also found a lot of mass is
moving away from another specific region
in space.
They call this mystery patch of space
the Dipole Repeller.
Together, the Dipole Repeller and the
Shapley Attractor both contribute about
50% to our motion, working together in
tandem, hence the name dipole.
But, how?
Gravity is a force that,
>> [music]
>> so far as we have observed, only pulls.
What lies in this region [music] of
space that allows a push to happen?
Could it be a strange collection of
white holes, the theoretical opposite of
black holes that we've talked about in
videos before?
Or perhaps some [music] sort of
anti-gravity.
Not quite.
The answer is
nothing.
The Dipole Repeller is a void, a bubble
about 100 million light-years across
that does not contain many, if any,
major galaxies.
It's one of the bubbles that exist
between the filaments of the universe's
structures.
But, interestingly, our galaxy's current
motion lines up much more satisfyingly
with the push from this region than it
does from the pull of the Shapley
Attractor,
which is a compelling argument in this
theory's favor.
So, how does nothing push?
The answer is actually that this could
be a pseudo force more than it is a real
one.
Imagine a universe where all the
galaxies were spaced equally.
While the gravity of galaxies above
would pull you, the gravity of galaxies
below you would do the same. So would
the pull of galaxies to your left and to
your right. In this way, the varying
forces of all these gravities would
balance each other, leaving you to not
really travel in any direction. You'd be
in a perfect equilibrium.
But, what would happen if we removed the
galaxies from one of these directions?
What would [music] happen if we added a
void?
Well, then the scales would tip.
There would be one direction that no
longer pulls [music] you, leaving the
opposite direction's pull to act on you
unimpeded. As a result, you would move
away from the void as [music] if it were
pushing you.
Everything near a void will thus move
away from it, making it seem to have a
repelling gravitational force.
>> [music]
>> It is something of an optical illusion.
There is another [music] explanation for
why a void space might push you, which
might work [music] in tandem with the
pseudo force.
The universe is expanding, and that
expansion shows no sign of slowing down.
In fact, it seems to be speeding up.
However, this expansion is counteracted
by gravity, and in areas where there is
more gravity, less expansion seems to
take place.
Stars aren't just being pushed away from
each other, they're also being pulled
together.
Conversely, in a void, there is nothing
to contain this rampant expansion.
[music]
As a result, void spaces literally will
swell compared to their supercluster
counterparts.
It might be done by warping the fabric
of reality, but this also achieves a
sort of push.
Since 2017, not much more has come to
light about the existence [music] of the
Dipole Repeller. As this is still an
expanding field of research, there's
some debate about whether it's really
there or how influential it might be on
our galaxy's current motion.
It is difficult to observe due to the
Milky [music] Way's obscuring effect,
and there is always a challenge inherent
in trying [music] to spot the absence of
a thing rather than its presence.
However, Tully and his fellow
researchers hope that with the aid of
future ultra-sensitive surveys in
multiple spectrums of light, it will be
possible to map out the [music] few
galaxies that lie in this void and
generally confirm its existence in the
region they hypothesize.
We are constantly moving through space,
but even the science of space [music] is
always in motion.
Each new galaxy, along with data
regarding its redshift and travel speed,
helps us develop a broader understanding
of the movement of the galaxy.
It's fascinating to think that not all
of that motion is caused by the pull of
gravity.
It might turn out that sometimes the
universe just needs a little push to get
going.
Tune into the universe with the right
equipment and you will hear it humming
no matter where you look in the sky.
Of course, it's not possible for sound
to travel in the vacuum of space.
Instead, we are talking about the hum of
gravitational waves.
You might already be familiar with a
similar signal discovered decades ago.
It's called the cosmic microwave
background and it is the afterglow of
the Big Bang.
This glow of microwave light was a
groundbreaking discovery. It was the
smoking gun that solidified the Big
Bang's place as the leading theory of
our universe's beginning.
Yet, this light signal had a glaring
weakness.
Despite it being the earliest light of
our universe, it was emitted almost
400,000
years after the Big Bang.
If we want to go back earlier, we will
need a background signal made not of
light, but of the ripples of the fabric
of space-time.
In 2023, astronomers at the NANOGrav
collaboration have announced that by
using a clever trick, they may have
detected just that.
Using exotic stars as a galactic size
gravitational wave detector, they
measured the faint hum of space-time
itself.
This hum of gravitational waves might
just be the reverberations of the very
creation of our universe.
I'm Alex Mackolgan and you're watching
Astrium. Join me today as we learn about
the secrets of gargantuan gravitational
waves and the ingenious new method
astronomers have used to detect them.
In a previous episode, we covered the
LIGO detector and how it uses lasers to
precisely measure the minuscule changes
in the length of a tunnel caused by
gravitational waves passing through.
These gravitational waves stretch and
squish space-time causing changes in the
time it takes for light to travel from
one end of the tunnel to the other.
But detectors like LIGO are only
sensitive to a specific kind of
gravitational waves.
Like light waves and sound waves,
gravitational waves can come in
different frequencies and LIGO is only
able to listen in to one part of the
spectrum, the high frequencies.
That's when these binaries are spiraling
in on each other rapidly, completing
hundreds of rotations per second before
colliding in a great bang.
But that is also why LIGO cannot listen
in on the much earlier stages of these
binaries when the objects are orbiting
each other at a much greater distance.
Because these orbits are much slower,
the gravitational waves they produce are
of a lower frequency and energy.
So you could say, rather than a great
bang, they give off a faint hum
combining into one collective signal.
Bigger black holes give greater
contributions to this signal.
The loudest contributors would consist
of pairs of supermassive black holes,
the kind you'd find at the center of
galaxies.
These binaries, which consist of black
holes billions of times the mass of our
sun, are expected to be rare cuz they
only form when something truly
spectacular happens, the merger of two
galaxies.
When galaxies merge, because of the vast
amounts of empty interstellar space in
them, they mostly phase through each
other.
Very few, if any, actual collisions of
stars or planets happen.
But, the stars gravitationally attract
one another and merge into one
collective galaxy.
And the nuclei of these galaxies, the
supermassive black holes, form binaries
that continue orbiting one another long
after the galaxy merger appears to be
complete.
It's these remnants of galaxy mergers
that we would hear most strongly in the
ultra-low frequency range, specifically
around the nanohertz range.
Yet, just like how the human ear cannot
hear sounds with ultra-low frequencies,
LIGO is deaf to this cosmic cacophony.
If we could listen in, we would gain new
insight into galaxy mergers and black
holes.
But, that's not all.
One of the most exciting things about
the gravitational wave background is
that it is a window to the very first
moments of our universe's existence.
Let's quickly recap what we know about
the cosmic microwave background.
This microwave light is the earliest
light that we can detect because for the
first 400,000 years, the universe was
opaque to light.
What does it mean for the universe to be
opaque?
Initially, the universe was far too hot
for neutral matter to exist.
Electrons and protons were unable to
bind together and form neutral atoms
simply because they had too much energy.
This state of matter is called plasma,
the same stuff that stars are made of,
and the universe was filled with it.
Light gets trapped in plasma, unable to
move through space without bumping into
the charged electrons and protons, being
continuously absorbed and emitted.
But, once the universe expanded and
cooled down enough for the protons and
electrons to coalesce into neutral
hydrogen and helium atoms, it suddenly
became transparent. The light was
finally freed.
This light was then stretched by the
expansion of the universe until it
redshifted into microwave light.
This is why the cosmic microwave
background is the earliest light of the
universe.
But gravitational waves are not made of
light.
They are made of space-time itself.
Plasma is no obstacle to them.
To gravitational waves, the universe has
been transparent from almost the very
instant of the Big Bang itself.
They can penetrate past this plasma
barrier and offer us a glimpse of the
very first moments of the universe's
creation.
Because these primordial gravitational
waves have been traveling for so long,
even if they were originally produced
with a high frequency, they would be
greatly stretched by the expansion of
space-time.
They would be redshifted into ultra-low
frequency gravitational waves, the same
kind that would come from black hole
binaries.
Together, these multiple sources will
give us a so-called stochastic
gravitational wave background existing
over a range of low frequencies.
How can astronomers retrieve this
treasure trove of information about
black holes, the Big Bang, and more?
Can we build a better LIGO and detect
how the Earth is bobbing up and down in
these gargantuan gravitational waves?
Well,
LIGO works by sending two beams of light
being sent along perpendicular tunnel
arms.
The beams are reflected by mirrors at
the ends of each of those tunnels and
they return to the starting point and
are compared with each other.
This is so that if space-time gets
distorted along one direction, the
information from the other direction can
be used to get a sense of how much it
has changed.
Unfortunately, if we were to design a
new LIGO to detect these gravitational
waves, we would need to have tunnel arms
that are much larger than anything we
could build on Earth.
This is because the frequency of a wave
is inversely proportional to its
wavelength.
Ultra-low frequency waves have an
ultra-long wavelength and would require
us to look at changes over a much
greater distance to notice the effects
of these gravitational waves.
For nanohertz waves, that is one full
fluctuation taking a billion seconds, a
quick calculation
wavelength equals speed of wave over
frequency
tells us that the wavelength would be
around 10 to the power 17 m.
For a sense of scale, the distance
between the Earth and the Sun, or an
astronomical unit, is 10 to the power 11
m.
A light-year is around 10 to the power
16 m.
The wavelengths we are discussing are
tens of light-years long.
Clearly, we cannot build anything
suitable for this task on Earth or even
in the solar system.
We need to go beyond our stellar
neighborhood.
But rather than wait for humanity to
ascend beyond the confines of our solar
system and become a space-faring
civilization, astronomers at NANOGrav
have used a clever trick.
You may recall our recent episode on
pulsars and how we discussed that these
ultra-dense neutron stars are able to
spin with remarkable stability.
They emit light along their poles and we
can detect the pulses that we receive as
those of vast beams of light swoop
around and hit the Earth again and
again.
These pulses come with such precise
certainty that they are used as clocks
of the cosmos, able to keep time across
vast distances.
Therefore, we can use already identified
pulsars to supply us with reliable
signals from thousands of light-years
away.
The pulsars are like the mirrors at the
end of the LIGO arms, but rather than
sending us back a signal we sent across,
they are generating their own.
NANOGrav have cleverly concocted an
imaginary interferometer that stretches
for thousands of light-years, and all we
need to use it are the good old radio
telescopes that we've been using for
decades.
But, our calculations will only be as
good as our observations.
When choosing pulsars to observe, we
need to make sure that they are suitable
for the task.
The best candidates are hand-picked and
added to a group called a pulsar timing
array.
For NANOGrav, they have made use of 68
pulsars, which were chosen for the
following special properties.
They had to be spread all across the
sky, so we can figure out how these
massive gravitational waves are
affecting space-time in all directions.
Also, they had to be millisecond
pulsars, which means they complete
millions of rotations per second.
In our recent pulsar video, we talked
about how these fast-spinning pulsars
are expected to be the most stable and
dependable when it comes to the
regularity of their pulses.
In fact, their stability even rivals
that of some atomic clocks. We could not
ask for better timekeepers to be dotted
across space and time.
This regularity means that we can take
measurements of the time their pulses
arrive, and continue taking these
measurements over multiple years to see
if there are any changes happening.
Of course, the times of arrival can vary
due to all sorts of factors, such as the
change in distance between the Earth and
the pulsar as both objects continue
moving around space, but we can take
this into account quite easily.
However, there are less predictable
factors, such as random fluctuations in
the interstellar gas the light is
traveling through, causing delays that
aren't coming from the stretching of
space-time.
How do we get rid of the random noise in
our data from pulsars located thousands
of light-years away?
With statistics, we can isolate the
gravitational wave background from the
random noise by considering correlations
between pairs of pulsars in the pulsar
timing array.
Just like LIGO, one tunnel isn't
sufficient. We need two pulsars on each
end of our imaginary tunnels.
If similar fluctuations show up with two
pulsars, it is unlikely that the random
noise aligned in such a way to make it
appear by chance that they are acting in
unison.
It is far more likely that there is a
common cause underlying both those
fluctuations, such as the possibility
that the photons from the two pulsars
are riding the same gravitational wave
that are washing by them both.
We can then relate these correlations of
pulsar pairs with their angular distance
in the sky.
If there is no gravitational wave
background, we expect there to be no
correlation, and the graph to show a
straight line at zero.
However, if the data shows a special
curve called a Hellings and Downs curve,
that would indicate that a gravitational
wave background is responsible for the
variances.
That's what NANOGrav have succeeded in
doing.
On the 28th of June, 2023,
based on over 15 years of continuous
observations of their 68 pulsars, they
announced strong evidence for the
gravitational wave background.
This is the graph we've been waiting
for.
This is a hallmark of gravitational
waves, which stretch along one direction
and squish in the perpendicular
direction.
This is exactly what we are seeing with
this graph.
When pulsars are separated at right
angles, their photons are experiencing
an opposing effect as they travel to
Earth.
The error bars in the data appear quite
large, but the researchers have
calculated that the data shows that the
existence of a gravitational wave
background is statistically significant
at the three-sigma level.
This means that there's only a one in
1,000 chance that this is a false alarm
and that this data was the result of
mere chance.
In physics, the five-sigma level is
considered the gold standard for
discovery, but three-sigma is still very
strong evidence.
As they collect and analyze more data,
astronomers will be able to reduce those
error bars and finally announce an
official discovery.
So, what secrets does this gravitational
wave background reveal?
While we cannot pick apart the sources
of the collective noise just yet, we
hope that these techniques will
eventually allow us to listen in to
specific supermassive black hole
binaries and figure out what the signal
can tell us about the creation of the
universe.
Until then, astronomers will continue to
use their galaxy-sized detector to
calculate how we are bobbing up and down
in the turbulent sea of space-time.
Have you heard about NANOGrav before?
What secrets do you think gravitational
waves could reveal? Let us know in the
comments below.
On the 14th of September, 2015,
scientists at the Laser Interferometer
Gravitational Wave Observatory detected
gravitational waves directly for the
first time. A stunning achievement that
led to the 2017 Nobel Prize in Physics.
Why was this significant?
Well, here's an analogy.
Let's imagine that human beings evolved
without the ability to see light. For
thousands of years, we'd fumble in the
dark, relying on our other senses until
one day someone invented a machine that
could perceive light for us.
In time, we'd see everything from the
tips of our noses to the farthest flung
galaxies.
This analogy captures the magnificence
of LIGO.
It's about much more than proving a
scientific prediction. LIGO enables us
to perceive the physical universe and
understand reality on a new level.
Like photons, gravitational waves travel
at the speed of light as they ripple
across space-time.
Their signals are all around us.
By listening for gravitational waves
with some of the most sensitive
instruments ever built, scientists are
recording tremors of distant, violent
events,
the formation of black holes, supernova
explosions, and potentially exotic
phenomena we haven't discovered yet.
So, what are gravitational waves? What
causes them? And why is LIGO's ability
to detect them already transforming our
understanding of the universe?
I'm Alex McCollgan, and you're watching
Astrium. Join me today as we learn about
gravitational waves, unpack the
groundbreaking technology behind LIGO,
and anticipate some of the stunning
developments that lie around the corner.
Gravitational waves are one of the
stranger implications of Albert
Einstein's general theory of relativity.
As we've covered previously, space-time
is a model that combines the three
dimensions of space and the fourth
dimension of time into a single
manifold.
All objects with mass create curvature
in space-time, and objects with a lot of
mass create a lot of curvature, which we
experience as gravity.
A simple way to visualize this is to
think of a pool ball resting on an
elastic surface and a bowling ball
resting on that same surface.
The more massive bowling ball will
create more curvature.
As objects move across space-time, that
curvature changes position with them.
One of the amazing consequences is that
when objects of a certain mass
accelerate, they can send ripples across
space-time as gravitational energy.
While this requires a special set of
conditions, namely a very massive object
undergoing acceleration,
such a cataclysmic event would send
ripples or gravitational waves outward
at the speed of light.
Think of them like ripples on a pond,
but instead of water, they travel
through the fabric of space-time in all
directions.
As in the pond analogy, these
disturbances become weaker as they
radiate outward.
To an observer, the distance between
objects would appear to expand and
shrink as the gravitational wave passes.
Mind-boggling to imagine.
Yet, although Einstein predicted the
existence of gravitational waves, he was
pessimistic about our chances of ever
detecting them.
He thought that these disturbances would
be so small as to escape our ability to
measure them.
And who could blame him? Many of the
changes in distance that LIGO seeks to
measure are 1/10,000
the length of a proton.
Yes, you heard that correctly. 10,000
times smaller than a single proton.
And yet, these signals would come
encoded with all kinds of information
about their origins.
When they originated, how far they
traveled, and what kind of event
produced them.
This is where LIGO comes in.
It consists of two observatories funded
by the United States National Science
Foundation and operated by MIT and
Caltech.
Among its driving forces are renowned
physicists Kip Thorne, Rainer Weiss, and
Barry Barish,
all of whom shared the 2017 Nobel Prize
for their decisive contributions to the
detection of gravitational waves.
LIGO is essentially a large-scale and
very sensitive interferometer,
an invention that's been around since
the 1880s.
An interferometer essentially measures
what happens when light waves are
combined from two or more sources.
For example, you could use an
interferometer to test whether light
travels at different speeds through
different substances, such as through
air or water.
Even a subtle difference in speed will
produce an interference pattern when the
light waves combine, much like what
happens when two ripples on a pond
intersect.
If the peak of one ripple hits the
valley of a second ripple, they will
subtract from each other, producing a
flat surface.
However, if the peaks line up exactly,
it means that the waves are in phase and
add to each other.
This is essentially what the
interferometer measures with light.
By seeing how in or out of phase two
light waves are, an observer can infer
the relative speed of the waves.
And the larger and more powerful the
interferometer, the more sensitive it
is.
Here's how it works.
LIGO has two observatories located in
Hanford, Washington, and Livingston,
Louisiana.
Why two?
Well, you need at least two detection
sites [music] to triangulate where the
signals are coming from.
Each observatory continuously fires a
powerful laser at a beam splitter
positioned at a 45° angle.
The laser beam has to operate at around
750
kW.
Powerful enough to vaporize you
completely if you got in its path.
The splitter then splits the laser beam
perpendicularly.
The light in each arm travels down a 4
km vacuum cavity with a mirror at the
end of it.
The beams then bounce between this
mirror and a recycling mirror at the
other end nearly 300 times, increasing
the distance from 4 to 1,200
km.
Remember what we said? With
interferometers, bigger is better.
After completing nearly 300 trips, the
laser beams combine at the beam splitter
and head to a photodiode, which is a
light-sensitive semiconductor.
If undisturbed, the beams will be in
phase, meaning their frequencies will
subtract each other and no light will
arrive at the photodiode.
But if there's a gravitational wave, the
distance each beam travels will be
slightly different and they'll be out of
phase.
The photodiode will pick up a signal
indicating the presence of a
gravitational wave.
Now, this is how it works in a perfect
world. But in reality, the
interferometer is constantly picking up
noise.
To minimize this, LIGO uses incredibly
smooth 40 kg mirrors suspended by silica
threads.
Any particles in the interferometers
arms are also a problem, which is why
LIGO pumps the air from its vacuum
chambers to 1 trillionth of atmospheric
pressure.
But there's another problem.
At these minuscule levels, even quantum
mechanics are a nuisance because they
introduce randomness into photon
behavior.
LIGO mitigates this with an optical
cavity, which squeezes the light.
This squeezing minimizes the light
phases noise and squeezes it into
amplitude noise, which the
interferometer doesn't measure.
In other words, the quantum randomness
will show up more in the height of the
waves.
Quantum randomness is a fact of life. It
can't be eliminated, but it can be
shifted. Much as you might move clutter
from your bedroom floor to your closet.
The chaos isn't gone, just out of sight
for the moment.
Plus, the goal isn't to eliminate noise
completely, but to get the best signal
to noise ratio possible.
That's a pretty good overview of how
LIGO works.
So, what has it discovered?
As I mentioned earlier, LIGO detected
its first signal in 2015.
Named GW150914,
scientists studied the data and learned
that it was caused by the merger of two
black holes about 1.6 billion
light-years away.
These black holes, which were 29 and 36
solar masses, became a binary and
spiraled around each other until they
merged and released a blast in the final
20 milliseconds that was so powerful.
Now, get ready for this number because
this is what the scientists actually
think.
It contained 50 times the combined light
power of every star in the observable
universe.
At the risk of sounding crude, that is
nuts.
I've read this fact many times over and
I still cannot comprehend what it means.
Yet, after traveling for 1.6 billion
years and finally reaching LIGO, the
disturbance was so faint, it moved
LIGO's 4-km arm 1/1000 of the width of a
proton.
To visualize this,
imagine the distance between us and
Proxima Centauri
and changing it the width of a human
hair.
That is the level of precision LIGO was
able to detect.
If that's not one of the most
astonishing feats in human history, I
don't know what is.
And this was just the first
gravitational wave LIGO detected.
The second detection occurred 3 months
later in December 2015.
That signal also came from a black hole
merger, which took place 1.4 billion
light-years away.
Over its initial three runs, LIGO
recorded more than 80 black hole
mergers, and in August 2017, it detected
the merger of two neutron stars.
Named GW170817,
this signal was notable for being the
first gravitational wave to be
corroborated by electromagnetic
observations from 70 observatories
across [music] the planet.
This was a breakthrough, not only in
gravitational wave detection, but in
multi-messenger astronomy.
It turns out LIGO was just warming up
during these three runs.
As of May 2023,
LIGO has begun its fourth run with
better sensitivity than ever.
After its latest round of upgrades,
which kept LIGO offline for 3 years, the
observatories now have more reflective
mirrors, better mirror suspension, and
improved light squeezing with lower
quantum uncertainty.
And this time, LIGO also has the support
of KAGRA,
a new interferometer observatory in
Hida, Japan.
KAGRA is located underground, making it
the world's first subterranean
gravitational wave observatory and also
the first to use cryogenic mirrors.
During an engineering run on the 18th of
May, LIGO scientists say they already
received a signal that was possibly
caused by a neutron star being swallowed
by a black hole.
We'll have to wait a while for
confirmation, but if these early results
are any indication, LIGO is about to
blow the doors off our understanding of
gravitational wave generating phenomena.
So, what other developments lie ahead?
India is preparing a collaborative
project called LIGO-India or INDIGO,
which will help LIGO triangulate better
location data.
In 2027 to 2028, LIGO will implement its
LIGO-Voyager upgrade, which will achieve
higher sensitivity with four times
heavier mirrors and higher frequency
lasers.
And in the more distant future, a
third-generation facility has been
proposed called Cosmic Explorer.
This facility would feature two new
observatories with arms spanning 40 km
and 20 km respectively.
Remember, with interferometers, bigger
is better.
But the proposal that really excites me
is the Laser Interferometer Space
Antenna or LISA.
This would be the first space-based
gravitational wave observatory, which
would utilize three spacecraft in a 2.5
million-kilometer-long configuration.
This interferometer would be so big and
so precise, scientists hope it would be
adept at uncovering exotic and
theoretical sources of gravitational
waves, such as cosmic strings and other
speculative phenomena.
In theory, it could help us stare
directly into the fabric of reality.
With a planned launch date of 2037,
we're still over a decade away, but it's
never too early to start counting the
years.
So, there you have it.
An overview of LIGO and how scientists
are using gravitational waves to better
understand the universe.
They give us evidence of extremely
remote and ancient phenomena that cannot
be measured by other means. And they
could be a secondary way to measure
observations made by other instruments
like the Webb Telescope or Hubble.
In [music] time, this revolutionary
field should allow us to understand the
nature of our universe, its history, and
even its future.
I hope you found this episode as
fascinating as I have.
>> This video by Dan Burns explains gravity
in a visual way that's really elegant.
What do you think is going to happen to
the balls when the teacher drops them?
As soon as he lets them go, they start
orbiting around the heavy object that is
pressing down on the 2D plane.
This 2D plane represents space-time, and
the heavy object symbolizes a massive
object warping space-time. Or in basic
terms, it creates gravity. This mimics
gravity in our solar system really well.
You can imagine these balls as objects
or planets orbiting our sun. Although,
the difference in the solar system is
that there is no friction in space. They
don't lose energy, so they keep
orbiting.
While this is a simulation of 2D space
with objects circling at the edges
moving in towards the center, in 3D
space, objects move in towards the
center from all directions, above and
below, as well as the sides. But why do
objects in our solar system only orbit
in one direction? Check out the pinned
comment for more. Gravity can pack a bit
of a punch, but did you know gravity is
not tied directly to the size of the
planet causing it? Take a look at this
simulation of a weight falling on a car
on different planets and moons to see
what I mean. Smaller celestial bodies
like the moon or Pluto have less gravity
than Earth, causing the same mass to
bounce off the simulation car with much
less damage. Gravity on the moon is 1/6
as powerful as on Earth, and gravity on
Pluto is only 1/15 as powerful. However,
it's not a planet's size that really
matters, but it's density.
Although Uranus has a radius four times
larger than Earth, its gravity is only
90% of ours. This is because the gas
that makes it up is not very dense,
making this impact comparable.
The same is even more true of Saturn
with nine times our radius, but 1.08
times our gravity.
With enough mass, even low density is
overcome. Jupiter's gravity is 2.4 times
that of Earth's, and as for the gravity
of the Sun at 27.9 times, it's no joke.
The results of this simple experiment
may surprise you.
We have here two different ramps that
the metal ball can roll down. The one on
the right is smooth and uniform, while
the one on the left is bumpy with dips
and rises.
In a moment, Professor Kelly will roll
two balls down each ramp at the same
time.
What do you think will happen?
Both balls are the same weight, and
there are no tricks involved such as
magnets. [music]
This is pure physics in action.
If you're like me, you might expect both
balls to reach the bottom at the same
time, but this doesn't occur.
The professor uses a pencil to drop both
balls at once.
And there you have it. Although both
balls went the same distance down, the
dips provided the left ball with a
greater chance to accelerate without
friction putting the brakes on. This
gave the ball enough momentum to speed
ahead of the ball on its right, even
when it hit the bumps on its journey.
When it comes to speed, momentum is a
powerful thing.
You may have noticed this video didn't
have any sponsors and that's because it
was brought to you by our astronauts on
patreon.
Consider joining our patreon to keep
these videos thriving even when they're
sponsor free. It's the reason we can
research deep into the topics we love
without cutting corners or chasing
clicks.
Every new astronaut allows us to explore
bigger ideas and make every upload even
better than the last. So if you've ever
thought about being a bigger part of
this channel, join the crew to power
Astrum and keep space curiosity alive.