[MUSIC PLAYING]
The curvature of spacetime
plays tricks on our eyes.
Much of the deep universe
is shifted and magnified
by the warping effect of
gravitational lensing.
[THEME MUSIC]
Our brains evolved
in a Euclidean world,
or pretty close to it.
We have hardware to
build internal models
of our environment in which
space is a simple 3D grid,
static with time.
In the world of our
mind's eye, light
travels in straight lines.
This lets us map the real
world into the imaginary.
We just catch
photons with our eyes
and trace their paths backwards.
This works beautifully, as
long as those light paths
are truly straight.
But add a pool of
water or a glass
lens or a funhouse mirror,
and our internal model fails.
We perceive illusory and
distorted images as our mind's
eye tries to enforce
over-simplistic physics
on a complex reality.
Well, it turns out
that the whole universe
is a giant funhouse
mirror, a rippling pond,
and many things are not
where or what they seem.
In the real universe, both
space and time can be curved.
And the path traveled by
light follows the curve.
Here's our playlist on
curved spacetime, time,
if you want to go
deep into this idea.
Einstein's general
theory of relativity
describes the real universe as
a flexible, dynamic dimensional
grid that only resembles
our mind's eye Euclidean
lattice in the absence
of mass and energy.
The curvature produced
by mass gives gravity.
Light follows this
curvature, and so gravity
bends the path of light.
The prediction of general
relativity that gravity
deflects the path of light
rays was one of the first
to be directly verified.
In 1919, British astrophysicist,
Sir Arthur Eddington,
loaded a ship full of
astronomers and set sail
for the island of Principe
off the West coast of Africa
and sent a second
ship to Brazil.
The mission?
To catch an eclipse of the sun
and to measure the tiny change
in the position of nearby
stars due to the deflection
of their light by the
sun's gravitational field.
And here's the photo he
took during the eclipse.
The stars had shifted.
Their light paths were
slightly deflected,
making them appear a bit
further from the sun.
The deflection angle
was exactly what
Einstein's theory predicted.
This particular confirmation
of general relativity
was the one that shot
Einstein to his great fame.
"The New York Times"
article is wonderful.
I find it especially
touching that it assures us
that we need not worry
about the dangers
of gravitational lensing,
which is, of course, what we've
come to call this phenomena.
The gravitational field
of any massive object
converges passing light rays,
like a badly designed lens.
For stars, this effect
is typically small.
And Einstein
originally felt that it
would be acute but subtle,
and not particularly relevant
phenomena.
That's not the case.
When we look out
there at the universe,
we see gravitational
lensing everywhere.
It's become a very powerful
tool for studying the universe.
At its most spectacular,
we see extreme warping
of the shapes of
distant galaxies.
As their light travels through
the deep gravitational wells
of intervening galaxies
and galaxy clusters,
they are greatly
magnified in brightness
and stretched into
arcs and rings.
These are beautiful,
but they're also useful.
The illusion results
from our mind's eye
projecting straight lines
onto a curved spacetime.
When brains don't
suffice, we instead
build model universes
in our computers.
Their spacetimes can
curve any way we choose.
Within these simplified
virtual universes,
we can hunt through
vast possibility space.
Somewhere in that
parameter space
is a configuration of
lens and light source
that will collapse
those distorted images
into the true galaxies
that created them.
Find that
configuration, and we've
mapped the gravitational
field, the distribution
of mass of the lens.
We've weighed many galaxies
and galaxy clusters this way.
And we've confirmed
that the vast majority
of mass in this universe is
in the form of dark matter.
When the distortion of the
light source is this obvious,
we call it strong lensing.
But the phenomenon doesn't
just distort and magnify.
Sometimes we see the same
object through multiple paths
through space.
This is the Einstein Cross,
an extremely luminous
distant quasar powered by
a supermassive black hole
feeding on its surroundings.
In fact, each of these
spots is that one quasar
viewed via four different
paths through the universe.
You can see the
nearby spiral galaxy,
whose gravitational
field bends spacetime
to create these paths.
To see multiple
quasar images, you
need a near-perfect
alignment between the lensing
galaxy and the quasar.
While we know of
a 200,000 quasars,
we only know of about 100
that are lensed this way.
The flickering of the
lensed quasar images
carries with it many secrets.
The quasar is a vortex of
superheated matter falling
into a black hole.
This is a violent
fluctuating process.
By measuring the time
delay between fluctuations
for those different
paths, we can actually
measure the path lengths.
Distances are one of the hardest
things to measure in astronomy
but are essential for cosmology.
Lensing distance
measurements have allowed us
to measure the Hubble
Constant, which tells us
the rate of expansion
of the universe,
independently confirming the
results from other methods.
Also coded in this flickering
is information about the heart
of the quasar itself.
Light passing through
the starry lens galaxy
brightens and dims due to
the gravitational fields
of individual stars in that
lens in a process called
microlensing.
As those stars sweep in front
of the quasars in a vortex,
its different parts
change in magnification
to different degrees
and at different times.
We can read this flickering to
map regions near the black hole
many orders of magnitude
smaller than any telescope could
resolve.
Although this sort of obvious
strong lensing is rare,
the effects of gravitational
lensing are everywhere.
Weak gravitational
lensing slightly
warps the shapes of essentially
all galaxies in the universe.
When we look at hundreds
or thousands of galaxies,
we can spot
correlations in the way
their elongations are aligned.
We see that they encircle
the vast strands and nexuses
of dark matter that form
the cosmic web, allowing
us to understand its structure.
Even within the Milky Way,
we see the effect of lensing.
Compact stellar
bodies-- black holes,
neutron stars, and
brown dwarves--
occasionally pass in
front of other starts
and lens them into brief
flashes of increased brightness.
This is how we've been able
to count these otherwise
near-invisible stellar bodies.
Despite Einstein's pessimism,
gravitational lensing
has become an important staple
in the astronomer's toolkit.
It's allowed us to
decode the universe
in ways that would've
been impossible in boring
Euclidean space.
Of course, the most extreme
gravitational bending
of light results
in the most awesome
of all astrophysical objects,
the black hole itself.
The lightspeed flow of
spacetime at the event horizon
results in old light
paths pointing inwards.
This gives us the
black in black hole.
Light falling below the event
horizon is lost forever,
so we don't describe
it as being lensed.
But just outside
the event horizon,
we find the most extreme
gravitational lensing
in the universe.
The photon sphere
hovers at about half,
again, the height of
the event horizon.
This is a region where
light paths are so strongly
curved that photons can actually
orbit the black hole, forming
a shell of light.
There are no stable orbits
this close to a black hole.
So photons will inevitably
spiral inwards or outwards.
As outspiraling light
escapes the photon sphere,
it joins with
severely lensed light
from any surrounding
whirlpool of hot plasma
to form a bright ring
around the black hole.
This ultimate
gravitational lensing
has not yet been observed.
But real lensing simulations
show us what black holes
should look like up close.
Both the black hole
and the wormhole
from the movie "Interstellar"
are amazing examples.
Next time you look up
at the sky, remember,
your eye is following
strange curved paths.
The stars are mostly where
you see them-- mostly.
But look through a telescope
at very distant galaxies,
and all are brightened, shifted
and warped by the weird lens
of a curved spacetime.
I guess gravitational
lensing is pretty cool.
Who am I kidding?
It is awesome.
In fact, it's one of my
main areas of research
as an astrophysicist.
If you want to learn more
about what I'm up to with that,
check out the mini
documentary I made
with the American Museum
of Natural History-- link
in the description.
OK.
We recently started talking
about quantum physics
by looking at the bizarre
phenomena of quantum tunneling.
Let's see what you
guys had to say.
Flynn Kruchell would like
to know whether he really
can teleport to the moon.
Actually, no.
You can't quantum
tunnel to the moon,
because to properly
tunnel, you need
to spontaneously find
yourself at a point
in your wave function that
has an equal or lower energy
state than your starting point.
Think about that alpha
particle trying to tunnel
through the
potential energy wall
of the strong nuclear force.
An exponentially decaying
part of its wave function
is actually inside that wall.
The particle could find itself
located anywhere that its wave
function is non-zero.
That means it can spontaneously
be in the region of the wall
where the strong nuclear
force is pulling it back
towards the center.
To escape, it needs
to tunnel all the way
to the other side, where it
finds itself in a lower energy
state.
Same with trying to tunnel
out of a gravitational field.
You'd be tunneling
to a higher energy
state, which is impossible.
[INAUDIBLE] Mayo asks
whether my interpretation
of the de Broglie wavelength as
a range of possible locations
is only true for the Copenhagen
interpretation of quantum
mechanics.
OK.
So some of the language
I used to describe
the collapse of
the wave function
and possible positions did echo
the Copenhagen interpretation.
But I don't
necessarily endorse it.
For those of you
who are unfamiliar,
the Copenhagen
interpretation was
one of the early interpretations
of some of the weird quantum
effects developed in the 1920s.
The idea is that
a physical system
doesn't have the familiar
classical properties
like position,
momentum, spin, et
cetera, until it is observed.
Prior to that, it exists
only as its wave function,
which is a distribution
of probabilities
of these properties.
This description of
things as probabilities
works, but it's also not
deterministic in that
in order for the wave
function to become
a set of physical
properties, there
needs to be a completely random
sampling of its probability
distribution.
Alternatives exist.
The de Broglie-Bohm
pilot wave theory,
the many-worlds
interpretation, and others,
allow a deterministic
interpretation
of the so-called collapse
of the wave function.
But from the standpoint
of us, the observer,
the effect is the same.
The wave function
that we calculate
defines the probability that we
will observe a particular set
of physical properties.
This is something we'll come
back to in a lot of detail,
so forgive me for my
rather crude answer.
Mystyc Cheez and others
ask, what is really
meant by an observation
in quantum mechanics?
Well, that's a question
that physicists have
argued about since the 1920s.
Observing a quantum system
means doing something to it
that collapses its
wave function into
the classical physical
properties like position
and momentum.
Some interpretations
suggest that an observation
means any interaction that is
thermodynamically irreversible.
Which is another way of
saying that a particle's wave
function gets so
hopelessly mixed
with those of other particles
that its observable quantum
behavior can't be tracked.
Again, more on this later.
However, one view that's
not really favored
is the idea that a
conscious observer is needed
to collapse a wave function.
This is absolutely not the
standard interpretation
of an observation these days.
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