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]

100 thoughts on “Is Gravity An Illusion? | Space Time | PBS Digital Studios

  1. Absolute bullshit that proves Einstein was a quack, and all the scientists worshipping are part of a cult of quacks. If the earth keeps expanding every time we dropped something, we would notice. The earth has remained a constant size. And the Earth travels around the sun because of the FUCKING FORCE OF GRAVITY! The sun is fucking growing, because the Earth would be burned to ash.

  2. Seems there is no such thing as inertial frame since everything in space, including space itself, is moving. You'd have to have a fixed point by which to measure from. Right or wrong?

  3. Why do massive objects bend space-time and what do they bend it into? (Related: We always see this sphere on a 2-D rubber sheet analogy for how massive objects fold space time but space is 3 dimensional right? And so massive objects should fold space time in all three spatial dimensions right, or we wouldn't be able to put a satellite into polar orbit which clearly makes no sense. Is my thinking all wrong and if so, what I'm I getting wrong?).

  4. This guy is just going to make people think theyre too stupid to ever understand relativity…This is the worst explaination of General Relativity I have ever seen .
    I am familair with space time, relativity and quantum mechanics and i am struggling to understand the examples you chose to use and the way you explain them .

    This video is NOT for a newcomer, this video is NOT for a beginner.
    There are far better explainations out there guys….crash course does a good job.
    But just remember this, it's hard for anyone to understand, even Einstein. Humans can't comprehend Dimensions we don't live in so don't get frustrated and give up. Keep watching different explanations and slowly but surely it'll click, in a way. I don't think any of us will ever truly be able to understand it

  5. What if we drop two apples, at the same time, one in Cairo (Egipt) and one in Newcastle (Australia)? By your description of "what Einstein said", that would mean that the Earth is accelerating in two (almost) opposite directions, at the same time. Is that possible? You may want to find a better way to explain this; one that does not go that obviously against common sense because, in essence, the idea of bringing sophisticated concepts down to Earth is very good.

  6. Jungle conclusions…. https://chrome.google.com/webstore/detail/threelly-ai-for-youtube/dfohlnjmjiipcppekkbhbabjbnikkibo

  7. Everytime I watch one of your videos, I actually feel like I'm being untaught. You guys are horrible teachers, stick to your lane and SLOW DOWN. After awhile it seems like you forget what you were even talking about. Absolutely terrible.

  8. WHAT TOTAL BULLSHITE !! like Rogue CIA DISINFO = FAKE NEWS START WARS BLACK =WHITE ISLAM=PEACE !! Why the fark can you NOT THINK ??? TALK LOUD FAST PASSIONATELY DOES NOT MAKE ANYTHING MORE OR LESS TRUE…the END of STORY KNOCK OUT to this INsanity is 3 Points APPLE TREE EARTH NEWTON is more correct BECAUSE THE APPLE FALLS ie MOVES AWAY and is FURTHER AWAY from the TREE than the EARTH after the FALL than BEFORE…so for you low IQ NEWSCASTERS BEFORE FALL at Time START APPLE AT tree FAR from Earth VERSUS AFTER FALL at Time END APPLE AT EARTH FAR FROM TREE !! GET IT ??!! THAT IS WHY WE SAY THE APPLE FELL TO TH EARTH FROM THE TREE it is the COMPARATIVE SPATIAL DISPLACEMENT !!!! You farkin morons BUT as You clearly Understand the General Theory ?!! oh you do not !! but you clearly understand the TENSOR MATH RIGHT ???!! Oh NOT AT ALL !!! But You understand this Nonsensical BS Yes use einsteins Theory for Calculations BUT AS IF IS NOT THE SAME AS IS !! Apparent STAR LIGHT POSITION on EARTH NOW (ie BILLIONS YEARS OLD LIGHT) IS NOT THE SAME AS ACTUAL STAR LOCATION in SPACE!!!

  9. Yah space isn't flat either its curved as well and gravational waves make up the curvature it's weird but it cant be any other way

  10. now I finally understand…flat-earthers are really just supporters of Einstein's view of inertial frames, I'm going to join them

  11. Time dilation in a gravitational field is also proof that we are in an accelerated point of reference. GPS satellites have to account for time dilation in both special and general relativity.

  12. If we feel gravity here on Earth, because we are accelerating, then it should depend on the direction of motion whether the gravity is positive or negative. That means on opposite side of the planet people should be floating in air or at least weight of the same object would differ on the opposite side? Is it really happening?

  13. Sitting smock under his nose is the simplicity of downward falling motion across the globe, it's happening in the now as a physical act, unlike the mythical Gravity.
    Sunlight energy splits the water molecules freeing the oxygen hydrogen gases, which by it's molecules motion rise upwards into the atmosphere to again continue it gas bonding cycle, and it's by taking on a state of solid mass new water is created, which as water is motionless and can not sustain it's rise in the atmosphere, so it falls downward as RAINDROPS ! Zero Gravity.

  14. Why people just assume the Earth is round? The speaker in the video gives evidence for everything else they speak about, then just assumes the Earth is round, and tries to model geometry to meet this assumption. Why can't the world just be Euclidean, the Earth flat, and then it fits Einstein's observation perfectly, without resorting to this weird non-Euclidean geometry. Roundness of the Earth has never been proven!

  15. Ah… so you're one of those teachers that like to take something people already understand and then explains it in a way that confuses the shiznit out of them lol. Fortunately I am impervious to your evil sorcery and already know of Einstein's Elevator thought experiment. Now away with you Bamboozlor!

  16. If a ball that repelled water was submerged, it couldn't repel it as there is nothing to take its place, therefore it would create a force pushing back, while warping the space around the ball, if a planet in space is taking up the area in space the same forces are created, as the area the planet is taking doesnt just disappear the space it takes has to go somewhere, the planets size and density would determine how much force is pushed back into the planet creating gravity. But the opposite also happens, if something is so dense it doesnt take up enough space which causes it to pull everything towards it like a black hole.

  17. Well, gravity still makes sense. It's not a "coincidence". I know that you're trying to dumb it down for masses but the book remains stationary because there's no force on it.
    Just a simple question, if gravity doesn't exist, why does the elevator or whichever frame that contains book only fall down?
    Einstein theorized it based on a jumping man falling to *down*. Why exactly is the object going downward? You'll say that it's due to the Earth coming up.
    So, if I'm a massive guy with my mass being comparable to that of Earth, will I have my own gravitational force? Will I be able to jump away from Earth?

  18. In a way, gravity is an illusion. It is relative acceleration driven by expansion of the earth which is driven by a "matter/empty space" density gradient. Density gradients are real. General relativity and bending of space-time is just mystical theory invented 100 years ago at a time when we had only just invented the automobile.

  19. The gravitational field magnetism is always present weather the apple is air born or not its just free to accelerate to centre of mass when dropped, you could say matter just likes company. Unless your a flat earth nut ball throw back that thinks we are rising threw space with the sun as our mobile above the craddle of earth in some sort of warped snow globe dome? I personally think the shape of our space bubble cosmos? resembles closely to diagrams of magnetic fields like a donut spiraling through space at the + pole or a smoke ring black hole at centre constantly recycling mass throught the centre faster than any know speed with a gravity so great the speed defeated friction some how? Like a giant matter grav cannon sling shoting matter it constantly re churns itself over time un countable? Then circles back in the shape of the field Making the life sustaining goldylocks zone of planets harder to find or longer odds. Its all shaped dependent on your point of view entering a black spiral, exiting a big bang? half way round a flat plain? Its a vision in progress based on observed natural patterns micro scopic & teloscopic, micro & macroscopic universes drawing parralels DNA strands waves planetry spirals threw space atoms, Gravity maybe the be all & end all strongest & weakest force dependent on proximity to mass? Outstrips light speed at greatest concentrations folds space back in on itself? Im not going to try to prove it though lol.

  20. The same is true with time. So as the hands of a clock may appear to rotate about the clock, the clock, and world around it, in fact rotates counter-clockwise about the hands and mechanism.

  21. If gravity is just acceleration, then the question isn't 'where does gravity come from?' The real mystery is 'where is the invisible acceleration?' So where is it? The surface of the earth isn't expanding radially outward to hold us onto it. The only thing I can think of is maybe space time is flowing like a river towards matter density. A gravity well is just space time streaming into a chunk of matter's schwartzfield radius and space time continues on through even when the matter collides. We're being pinned to the earth the way a fire-hose would pin you to a wall.

    Clearly I'm guessing wildly, but this is bothering me. Where is the acceleration we experience as gravity?

  22. I mean the earth IS traveling at so so speed in space so for the earth to move extremely fast towards anything thats "falling" would make sense

  23. What if I jump at the same time as a guy in China? Does the Earth move in opposite directions at the same time?

    Space pushes you down because Earth bends it.

  24. How tf orbit circle can be a straight line? Ok while writing this realised that orbits are not circles but spirals..

  25. Hold on hold on hold on. If the parabola of basketball is a "constant speed straight line" and an orbiting satellite is a constant speed straight line then space is so curved that triangles ought to have very different sum of angles than 180 degrees. How come we don't say that? Or is it that space-time is curve more than space? Explain.

  26. Curving space is naked empire.

    Nucleus of atoms expanding and recycling expanding pushing force which have example nature of expanding light. Expanding light waves is dark pushing force for us, but because we can register photons, we know there is also waves.

    Expanding lights interactive with eachother and thats why old light is redshifted light cosmologys way.

    Also expanding light moving faster and faster same way what matter and light expanding.

    That explain
    1. Bending light near star and galaxy
    2. Cosmologys way redshifted light
    3. "Gravity" redshifted light

    🤔

  27. Love how he says the "Frames of Reference" video is from MIT, not U of T.
    It clearly says University of Toronto.

  28. Physics is amazing. How we figured this out is equally more insane and marvelous. Math is a human construct. We did this.

    Dolphins are alien technology. Math is not.

  29. Why in the twin paradox, the twin in the ship is always younger even if I change the frame of reference between them? How does nature know who is traveling fast?. Scenary: You can remove acceleration from the experiment assuming the measure between two points (A and B) in outer space (not on earth) one twin traveling at constant speed near the speed of light between time T0 and T1 and the other twin stays stationary at point A. How can I change the frame of reference between twins and obtain the same result( twin in the ship younger than the other twin at time T1 when the ship cross point B)?. TY

  30. explain to me im heavier than an apple right, are you saying the earth will move faster toward me ? you have half of the answer right here, but no…I disagee, but you are right in thinking something is wrong.

  31. Im not going to explain how it all works study electromagnetism vs gravity. Youre right gravity isnt gravity, gravity is a mistake in identity thats all I have to say

  32. So if some drops an apple on the other side of the planet at the same time I drop an apple, does the Earth split in half?

  33. This guy does'nt make a good teacher. He's talking very fast. Can't keep up and is hard for non native english speakers.

  34. Isaiah 24:20
    The earth reels like a drunkard, it sways like a hut in the wind; so heavy upon it is the guilt of its rebellion that it falls–never to rise again.

    In the natural they can not understand the meaning of earth 🌍 falling.

    But here what means spiritually. Human genome 🧬is traced from first Adam so for example the flesh is produced by sun and the earth like wife and husband. Moon is like their son.

    That is why moon is an example of DAVID. Humans especially Muslim think moon is a women that means they are not correct but incomplete because they are in their slavery in ISHMAEL and HAGAR 2nd heaven Queen if demonic forces. Jeremiah 44

  35. 6:16 It doesn't make sense to me, can someone help?
    I can't imagine that me or a book would just float in the box. Depending on the box's mass, I would be either pressing on one of the sides (the "floor") with my body if the box had less mass than me, or being stuck to the opposite side (the "ceiling") if the box had more mass than me.
    Where's my mistake and what do I need to understand to get what the speaker's talking about at 6:16?

  36. The funny thing about it is, how so many people dislike his speed in explaining such complexities ~ truthfully, most can’t begin to comprehend such profoundly difficult concepts regardless of teacher or how fast or slow they speak. Both Newton & Einstein we’re geniuses, they focused their entire mind on whatever they were puzzled by and made decisive conclusion, conclusions still being tweaked and modified now.

  37. Curved spacetime? Okay. If we just have the Earth accelerating in three dimensions, then a load of people are going to fall off because it can only accelerate in one direction so only people on a portion of one side will be pressed against the Earth by its acceleration.

    If everyone on every segment of the globe experiences being pressed against the earth by its acceleration then it must be expanding because that's the only way every part of the sphere can be accelerating. Expanding or exploding in fact.

    So is TIME the dimension in which it is accelerating if this theory is correct or am I being far too three-dimensional in my thinking?

  38. Crazy idea what if objects fall not because of this mystical force known as gravity but because of magnetic attraction!

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