Is Gravity An Illusion? | Space Time | PBS Digital Studios


[MUSIC PLAYING] Isaac Newton said
that an apple falls because a gravitational
force accelerates it toward the ground, but
what if it’s really the ground accelerating
up to meet the apple? [THEME MUSIC] Suppose I drop an apple. According to Isaac
Newton, the ground can be considered at rest, Earth
applies a gravitational force to the apple, and that
force causes the apple to accelerate downward. But according to
Einstein, there’s no such thing as a
gravitational force. Instead, it’s more
appropriate to think of the apple as stationary
and the ground– along with everything on the
ground– as accelerating upward into the apple. Now what I just said sounds
preposterous and maybe even moronic, but
it’s not sophistry. There’s something
substantive here, and today I’m going to clarify
what exactly this point of view means, why Einstein
came to adopt it, and how it planted the seeds
for what would eventually become general relativity. You ready? OK, bear with me for
a minute because we need to begin with some
Physics 101 and Newton’s laws of motion. To analyze motion,
you need what’s called a frame of reference. That’s just some X-Y-Z axes
to label points in space and a clock to track time. The reason you need
a frame of reference is that you can
only measure motion relative to other things. If that concept is
not familiar to you, you need to pause
me right now and go watch this super awesome 1960s
black and white video from MIT all about frames of reference. It’s amazing and I promise
you won’t be disappointed. Welcome back. Now, Newton’s laws
can’t tell you whether a frame of
reference is really at rest or really moving at
constant velocity because that distinction
is meaningless and simply a matter
point of view. However, interestingly,
Newton’s laws can tell you whether
your frame of reference is really accelerating or not. Here’s how that works– take
an object with no forces on it and let go of it. If it stays right where it is,
then your frame of reference is not accelerating and we
call it an inertial frame. Now in Newtonian
physics, inertial frames are special because Newton’s
second law, F equals ma, is only valid in
inertial frames. In other words, the
net force on an object will equal that object’s
mass times its acceleration only if you’re measuring
that acceleration using an inertial frame. For example, suppose
that you’re in a train car that starts
accelerating uniformly forward along a flat track. Relative to the
car’s interior, you will accelerate backward,
even though you can’t identify any horizontal forces on you. So inside the train car, F
decidedly does not equal ma and the train car’s frame of
reference is not inertial. In contrast, a frame
attached to the tracks pretty much is
inertial– at least if you disregard
Earth’s rotation, because relative to that frame,
you don’t accelerate at all. Instead, the train car
accelerates forward underneath you. Now more generally,
any frame that accelerates relative
to an inertial frame will not be inertial. You got that? Inertial frame and
non-accelerating frame are synonyms in
Newtonian physics. In fact, you can think
of inertial frames as the standard against which
you measure true acceleration. And from the perspective
of inertial frames, motion obeys a simple
rule– F equals ma. All right, let’s look at
things from the train car’s frame of reference though
a little more carefully. Inside that
accelerating train car, not only does everything
accelerate backward for no apparent
reason, everything accelerates backward together. You, a book, and an
elephant will all lurch toward the back of the
car with the same acceleration. Remember, from the
preferred point of view of the inertial frame that’s
attached to the tracks, you, the book, and the
elephant are all stationary and it’s only the train car that
actually accelerates forward to intercept you. So of course you
move in lockstep as viewed in the
train car’s frame. But hold on a second. There’s something
else familiar that makes people, books,
and elephants accelerate in lockstep– the
Newtonian force of gravity. In fact, in the absence
of air resistance, that’s the defining
feature of gravity. So in the train car’s frame,
which is accelerating forward, it’s as if there’s an additional
gravitational field that points backward. So accelerated
frames of reference mimic a gravitational field
in the opposite direction of the frames acceleration. That’s interesting. If you combine that extra
fake gravitational field with the actual gravitational
field of the Earth, which points down, it
looks like there’s a net gravitational field
inside the car that points at some angle down and back. Destin at “Smarter Every Day”
has a pretty famous video of a helium balloon
in an accelerating car that happens to illustrate
this point really well. Destin generously gave
us permission to show it, but you should check
out the full video by clicking over here
or following the link we have down in the description. Now as you can see, when
Destin hits the accelerator, a pendulum hanging
from the ceiling tilts back while a balloon
that’s tied to the floor tilts forward. Destin explains that
air is piling up in the rear of the car and
getting slightly denser there, so the balloon is just
trying to go toward the less dense air near the front. All of that is true. But there’s another way to
think about this situation. You can also think that the
car’s forward acceleration is mimicking some extra
gravity pointing backward. Combine that with Earth’s
real gravitational field and it’s as though the
total gravity inside the car points down and back at
around a 30-degree angle. That is the new vertical
and the pendulum string and the balloon string
are just aligning with the vertical the
way they always do. The pendulum hangs
down and the balloon aims up because air is
denser on the ground and less dense at
higher altitudes. In fact, the accelerated frame
of reference of Destin’s car is completely indistinguishable
from having that car stationary on the surface of
some other planet with slightly bigger
gravity than Earth and tilted upward
by about 30 degrees. You see what I mean? If you blacked out the windows
and put perfect shock absorbers in the minivan, then for all
Destin and his kids know, they’re completely at
rest, tilted upward on another planet in a
perfectly inertial frame. Huh. Now in Newtonian
physics, this is just an accounting trick that
has no broader significance. Really, Destin’s
car is accelerating and this extra backwards
gravity is fake. But Einstein asked,
hold on, what if the so-called “real”
downward gravity from Earth is also fake, a side
effect generated because Earth’s surface is
really accelerating upward? Now, you know what
Newton would say. He’d say, that’s crazy. He would remind us that
inertial frames are the standard for measuring
true acceleration, so you can only say Earth is
really accelerating upward if you can identify an inertial
frame relative to which Earth’s surface
accelerates upward, and there’s obviously no
inertial frame like that, right? Well, not so fast,
says Einstein. Maybe there is. What about a frame
that’s in freefall? Think about it. If I put you in a box
and drop you off a cliff, then in the frame of
the box, everything just floats, weightless. The falling frame of
the box behaves just like a stationary
inertial frame that’s way out in intergalactic space
where there’s no gravity. So why can’t the box’s
frame be inertial? Well because, Newton says,
that frame can’t be inertial. It’s really
accelerating downward at 9.8 meters per
second squared. The interior just
seems like zero G because the downward
acceleration acts like a fake extra upward
gravitational field that, from the perspective of the
box, just happens to exactly cancel the real downward
gravitational field of Earth by coincidence. Really, Newton? Really? Einstein says, look buddy,
I’m just following your rules. You established
the test for what an inertial frame is–
release a force-free object and it stays put. Stationary frames in
intergalactic space pass that test. But freely-falling
frames here on Earth also pass that test if
your so-called gravity is fictitious. More to the point, Newton,
if you’re inside the box, there’s no way for you
to know that you’re not in intergalactic space. This inability to distinguish
freefall from lack of gravity has a name, by the way. Einstein called it the
equivalence principle, and if you buy it, then maybe
the falling frames really are inertial. If so, then it’s the
falling frames that establish the standard
of non-acceleration, in which case, it’s
really the ground that’s accelerating upward
and what we’ve always been calling
a gravitational force is an artifact of being in an
accelerated frame of reference. It’s not different from
the weird, backward jolt that you experience on the train
that you know perfectly well isn’t being caused by anything. So why are you insisting
that the downward jolt we experience every day on
Earth has a physical origin? Maybe gravity, just like that
backward jolt on the train, is an illusion. Doesn’t that point
of view seem simpler? Now Newton says,
nice try, Einstein, but you forgot something–
Earth is round. Down isn’t really down,
it’s radially inward, and this creates two
problems with thinking about freely-falling
frames as inertial or thinking about
gravity as an illusion. First, two objects
in a falling box are falling toward Earth on
not-quite-parallel radial spokes. So from the perspective
inside the box, they won’t actually
remain stationary. They accelerate
toward each other slightly, even though there
are no forces on them, in seeming violation
of F equals ma. Second, by your criterion,
Einstein, orbiting frames of reference– like
on the space station– should also be
considered inertial. But those frames accelerate
relative to frames that are just falling straight down. And if you recall the
beginning of the episode, inertial frames aren’t
supposed to accelerate relative to each other. Huh, that’s a good point. So it looks like game
over for Einstein, right? Well, not quite. It turns out that there’s a
loophole that makes Einstein’s viewpoint self-consistent. The rule that inertial frames
can’t accelerate relative to each other turns
out only to be true if the world has what’s
called a flat geometry. If instead the world is a
non-Euclidean and curved spacetime, then straight
line at constant speed doesn’t mean what
you think it means and it turns out that inertial
frames in a curved spacetime can do almost
anything they want. It took Einstein about
seven years to realize that. But once he did, a
beautiful model of the world emerged called
general relativity. It makes several predictions
that Newton’s theory of gravity does not, and so
far, it has passed all its experimental tests. And one of the central
precepts of general relativity is that we inhabit
the curved spacetime. And in that curved spacetime,
the orbit of the ISS is a constant-speed
straight line. The arc of a basketball
during a three-point shot? Constant-speed straight line. But you, sitting perfectly
still in this chair watching this video? You, my friend, are
accelerating, giving you the impression that there’s
a force of gravity when, in fact, no such thing exists. Wait a minute– how can geometry
and straight lines possibly work the way I just said? Patience, grasshopper. We’ll tackle that another time. For now, just reflect on
Einstein’s inspired thinking and how he got there,
maybe next time you get in a car or a train. We’ll reconvene next time
our accelerated paths cross in curved spacetime. Last week, we debunked media
coverage of so-called habitable exoplanets like Kepler-186f. Let’s dive right into your,
as usual, great comments and questions. Many of you asked about the
upcoming James Webb Space Telescope or JWST. When it launches
in 2018, will it be able to characterize
exoplanetary atmospheres? Yes and no. Primarily, JWST is
an infrared telescope that will see
exoplanets because, contrary to
Earthenfist’s comment, planets do glow,
in the infrared. But the caveat that
I gave in the episode still applies– JWST will
see super-Earths maybe that are very
close to red dwarfs because only those planets
will heat up enough to be bright in the infrared. Earth analogs in Earth-like
orbits around Sun-like stars are not going to be visible. Now, JWST could
see dimmer planets if it had enough
continuous observation, but that probably won’t
happen because JWST, just like the Hubble
telescope, has to be shared with lots
of other astronomers who aren’t looking
at exoplanets. BukueOner and dulez
ninjaman asked, so why the focus on habitable
exoplanets when we’re never going to go there
and we still haven’t explored our own solar system? But remember, astronomers
have other reasons for studying exoplanets–
just basic science. They want better understandings
of how planetary systems form, of how proximity to
different kinds of stars affects the
atmospheres of planets, and so forth– the prospect
for life, whatever. We can’t reposition
the planets here, so other star systems are the
laboratories for these kinds of investigations. Lutranereis points out that one
problem with media reporting might be that the reporters lack
adequate science background, and that’s a good point. I’m a trained
astrophysicist and I have a pretty tough time just
getting some facts straight for the show. I try not to make mistakes,
but sometimes I do. Reporters also have deadlines,
which doesn’t always help. Lukos0036 suggested
that maybe interest in space without
sensationalist headlines won’t happen until
space travel becomes more accessible and
immediate in people’s lives. It’s an interesting idea. You might be right. And finally, Pikminiman give
us some really nice feedback about the show which I and the
rest of the team at Kornhaber Brown really appreciate. For those who are
curious, I write the scripts, which then
go through revisions and great group editing with
Andrew Kornhaber, who produces, and Kyle Kukshtel,
who also directs. Some topics come
from me, some are brainstormed with Andrew, Kyle,
and the other producer, Eric Brown. BJ Klophaus does the film and
sound editing and sound effects and Michael Leng does the
animation and graphics. It’s a big effort
by a team of people every week to
bring what we think is clear thinking to
interesting science topics and it means a lot to us and me
that you guys find it valuable. And Pikminiman is right– I
think our comments section is among the best on
YouTube, so you guys also make this channel great and
I really want to thank you. [THEME MUSIC]

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