[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.
[THEME MUSIC]