What Physics Teachers Get Wrong About Tides! | Space Time | PBS Digital Studios
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What Physics Teachers Get Wrong About Tides! | Space Time | PBS Digital Studios

[MUSIC PLAYING] As we’ve seen before
on “Space Time,” gravity affects the motion
of all objects identically. So if gravity from
the Moon and the Sun is really responsible for
tides in the ocean and water is water, then why don’t
we see tides in lakes? Guess what? Whatever you believe about
why ocean tides exist is probably wrong, even
at the most basic level. In fact, every YouTube video
I’ve ever seen about tides, including ones made
by smart people, explains the tides incorrectly. Typically, they show
this diagram, along with an explanation that
goes something like this. The moon’s gravity is
stronger at Point A and weaker at Point B than
it is at Earth’s center. The net effect of
this differential of the Moon’s gravity
across the Earth is to stretch the
oceans out like taffy, ergo why the oceans bulge
out at opposite points along the Earth/Moon line. Now, that explanation
sounds plausible and a lot of well known
scientists give it, but as we’ll see,
it’s not correct. Don’t get me wrong. The facts are correct. There really is a
gravity differential from the Moon at points A and B.
And at least in this simplified model, there would be two
tidal bulges at opposite ends of the Earth/Moon line. Plus, if Earth could rotate
underneath those bulges with no friction between
the ocean and Earth’s crust, then at a given
location on the globe, you would experience
two high tides per day as you pass through each bulge
and two low tides per day as you pass through the spots
at 90 degrees to the bulge. All of that is true. What’s wrong is the
explanation for the bulges. They aren’t actually
being lifted or stretched by that gravitational
differential. Something much more
subtle is happening that even many professional
astronomers and physicists misunderstand– including
me, for many years. So don’t feel bad. Tides are tricky. But today, we’re going to
set the record straight and, in the process, understand
how it could be that the ocean has tides, but lakes, bathtubs,
and cups of coffee don’t. So right up front, I want
to make some assumptions to simplify the analysis
and to remove unimportant factors from the picture. That way, we can better
isolate qualitatively what’s really causing the tides. Here we go. Assumption one– we’re going
to use Newtonian gravity. It’s not that Einstein
can’t explain tides. He can. But curved spacetime
will only add complexity without actually
making things clearer. Assumption two–
let’s ignore the Sun. For simplicity, we’ll focus only
on the influence of the Moon. The Sun’s effects are going
to work analogously, anyway. Assumption three– we’re
going to pretend the Earth is uniformly covered
with one humongous ocean and no continents. And finally,
assumption four– we’re going to pretend that we can
switch Earth’s gravity on and off whenever we want to. You got it? OK. Let’s take a closer look at
the Moon’s gravity differential and how it manifests
itself from the perspective of a frame of reference
attached to Earth’s center. Forget about the
oceans for a minute and just imagine two
small blocks, A and B, at opposite ends of the Earth’s
surface along the Earth/Moon line. Turn off Earth’s gravity. What happens to the blocks? Well, relative to a frame
out in the ambient space, Block A accelerates
toward the Moon more than Earth’s center
and Earth’s center accelerates toward the
Moon more than Block B. So from the perspective
of Earth’s frame, both blocks will separate from
the surface as if acted on by some
invisible outward force. That weird invisible
outward force is called the tidal force. And in Newtonian
physics, it’s fake. It’s an artifact of
Earth’s frame of reference. Remember, Earth itself is
accelerating towards the Moon, so according to Newton,
Earth’s frame is non-inertial. Now, if you need a refresher
on non-inertial frames, you can check out our
earlier video on the topic. But the bottom line is
that in Earth’s frame, the tidal force looks like
anti-gravity, at least along the Earth/Moon line. And here’s the thing. Just like the fake forces that
you perceive in an accelerating train car, tidal forces should
make all objects accelerate off the surface identically,
regardless of their mass. An object’s resulting
acceleration relative to Earth’s surface is
called the total acceleration of that object, and it should
be identical for a pebble and a pony. But if that’s true, the tidal
force along the Earth/Moon line can’t be raising or
stretching the two bulges that lie along that line. Because if that were the
case, then water in lakes should also be lifted
and, for that matter, so should sand
and rocks and you. Remember, the object’s
mass doesn’t matter. Well, we don’t see things
levitate during high tide. More important, the math
of assuming the bulges are being lifted doesn’t work out. The tidal acceleration
on objects due to the Moon’s differential
gravity along the Earth/Moon line works out to only
1/10,000,000th of an Earth g, and you can’t lift something by
pulling up on it with a force that’s 10 million times
smaller than its Earth weight. Plus, even if you turned
Earth’s gravity off, you would never notice
an outward acceleration of one micron per
second per second. Nevertheless, those bulges
in the ocean are real. So if the ocean isn’t
being stretched, then how do they get there? The key is to look at the tidal
acceleration of objects that are not on the Earth/Moon line. For instance, a block
at this location is going to be pulled
this way by the moon. But of course, the whole Earth
is pulled that way by the Moon, chasing after the block. So relative to the
Earth’s surface, the block’s tidal acceleration
is almost radially inward– in other words, down. In fact, if we map out the
tidal acceleration vectors that you’d see at different
points on Earth’s surface, they look like this. As you can see,
tidal forces only act like anti-gravity if you’re
right on the Earth/Moon line. At most places,
those vectors are largely tangent to
Earth’s surface, which would push water sideways. Now, we’ve drawn these
vectors kind of big to help you visualize them. But in reality,
they’re microscopic. Remember, the radially
inward acceleration caused by Earth’s own
gravity on objects is 10 million times bigger. However, the surface
area of the ocean is also enormous, so those
tiny tangential sideways pushes on all the chunks
of water added up over half the
surface of the planet can produce a pretty decent
increase in water pressure. So I think you can start to
see what’s happening here. The ocean isn’t being
lifted or stretched. Instead, thanks to the
cumulative sideways traction everywhere else, it’s being
squeezed toward the Earth/Moon line and piling up there. Basically, the moon is
turning the entire ocean into a planet sized
hydraulic pump and the ocean is bulging
along the Earth/Moon line in the same way that
a blister or a pimple will bulge up in the
center if you start to squeeze it from the side. So why don’t lakes have tides? Well, largely for
the same reason that it’s very hard to pop small
pimples– less traction and bad hydraulics. See, unlike the
oceans, a single lake is not a contiguous,
planet sized body of water. Lakes just don’t have enough
area for the tiny pushes on it to build up enough pressure
to change the water level. Now technically,
really big lakes, like Lake Michigan
in North America, can generate enough pressure
to produce mini tides, maybe with a couple of
centimeters difference between low and high tide. But since winds and boats
and aquatic sloshing will all create ripples that
are way bigger than that, those mini tides just
aren’t noticeable. The same is true for any
enclosed body of liquid– a swimming pool, a bathtub, a
human body– which is basically a big sack of water– and
a cup of coffee technically all experience tides. They’re just microscopic. Also, remember that Earth
itself isn’t perfectly rigid. So when water in a swimming
pool rises a tiny amount, Earth’s surface is also
rising by tiny amount, making the change in water
level relative to the surface of the planet even
less noticeable. Now, everything that I’ve
just said is oversimplified, but I think it gets
the main point across. Namely, tides have a lot
more in common with pimples than they do with taffy. OK. We’ve got a few
loose ends to tie up. First, the Sun–
its effects on tides are analogous to
those of the Moon, but they’re only
about a third as big. The Sun is more massive,
yes, but it’s also much further away. Now, when Earth, the Moon, and
the Sun all line up in space, the effects are additive and you
get extra-large spring tides. When, instead, they make a
90 degree angle in space, there’s partial cancellation and
you get extra small neap tides. Second, in the simple model
of a water world Earth, the math says that the water
level should vary by about 3/4 of a meter between
high and low tide. But some places see
smaller tides than this while other places, like
the Bay of Fundy in Canada, have tidal swings of
over 10 meters each day. So why the variations
from place to place? Well, location relative to
the plane of the Moon’s orbit is certainly part of it. But mostly, it’s that nooks
and crannies in the continents affect the details
of how pressure gets distributed through the
ocean in non-uniform ways. For example, some
lakes and rivers that have direct ocean
inlets do have tides. But instead of rising
gradually, high tide can come in through the inlet
like a moving wall of water called the tidal bore,
which is pretty cool. Also, some variation
in water level is just sloshing that’s
not directly related to tides at all. The bottom line
is that the finer details of tides
in the real world are just kind of complicated. So are other things that
I haven’t mentioned, like how the rate of Earth’s
rotation and the ocean tides affect each other. For discussions of
all of that, I humbly direct you to other videos and
articles that are linked down in the description. Finally, Porscher911,
one of our viewers, once asked how Miller’s planet
in the movie “Interstellar” could’ve had such a huge
waves without the astronauts themselves being
stretched or levitated. I think that other stuff was
supposed to be at play there. But nevertheless, when it
comes to liquid on a planet, the squeezing aspect
of tidal forces will almost always be more
important than the stretching, even in the craziest
regions of spacetime. Last week, we
finished our series on general relativity
and curved spacetime. You guys had a lot
of great questions that I’ll address in a second. But first, a comment about
one of our earlier episodes. A while back, we did an episode
clarifying misconceptions about so-called
“habitable” exoplanets. Now, prior to the
airing of that episode, at the 2015 NASA
Space Apps Challenge, a colleague of mine–
Professor Emily Rice from the College
of Staten Island– had done a very similar talk
about exoplanet atmospheres. She and I had been talking
about exoplanets back and forth for a while, including in
the context of this show, and there was a lot of
overlap between our two talks that I wasn’t aware of at the
time that we aired our episode. If I had been, I would have
cited her talk explicitly, and I’d like to rectify
the situation now. So down in the description,
you’ll see a link to her talk about alien atmospheres– it’s
very good– at the NASA Space Apps Challenge, along with
a link to other information from her website. Now to your comments. A lot of you ended up asking
very similar questions. Rather than address your
comments individually, I’m going to address
the most frequently asked questions in bulk. A lot of you asked
about gravitons in the following context. You may have heard that
in quantum field theory, forces are described
as being mediated by some kind of particle
like electromagnetism by the photons, strong
nuclear forces by the gluon, and so forth. So if gravity is
not a force and it’s considered spacetime
curvature, then why do people talk about gravitons? That’s an excellent question
and it’s hard to answer, but here’s the
rough bottom line. Thinking about things
in terms of gravitons and thinking about
spacetime curvature are not necessarily
mutually exclusive. It’s just instead of quantizing
a sort of standard field, you’re– that you think
of as a force field, you’re quantization something
different when you talk about the quantum version
of general relativity. And it turns out that you
can do it self-consistently. You can quantize any
classical theory, as long as you are
restricting yourself to looking at
macroscopic regimes– large scale things– and you
get pretty good agreement with general relativity there. It’s when you start looking at
very small scales, like what the quantum version
of gravity tells you on very small scales
or very high energies, that a lot of infinities
start popping up in the theory that you can’t get
rid of, which is what we don’t have a fully
self-consistent, quantized version of gravity yet. But from the
philosophical perspective of quantum field
theory, you should be able to quantize anything. And figuring out how to
do that is just still an unsolved problem in physics. Related to this question,
people ask the converse of it. Is it possible, then,
to geometrize or treat other forces like
electromagnetism in geometric terms? Well, that’s actually been
worked out in some contexts. You can look up something called
Kaluza-Klein theory, which is a way of trying to
get a geometric version of electromagnetism
plus general relativity. But in general, these are two
sort of parallel paradigms. One is trying to
geometrize a lot of stuff, which was a big thing in the
sort of early 20th century. And the other is to try to take
field– classical field theory versions of something
and then add to them the machinery of
quantum mechanics to sort of get a quantized
version of the world. And these two things proceed
on sort of parallel tracks. It’s hard to answer these
things well in 30 seconds without just throwing
out a lot of buzzwords and giving you what’s not
really an explanation. But I don’t think there’s
really any alternative that wouldn’t mislead you. You’re asking very heavy stuff. A lot of you wanted
clarification on how curvature of time
is what’s responsible for, say, circular orbits around
the Earth being geodesics. First, remember that I was
speaking very, very loosely. There’s really only
spacetime curvature. You can’t really break it up
into separate space and time curvatures. That doesn’t mean anything. But here’s what I did mean. Suppose that you take a frame
attached to Earth’s center, with clocks– that frame’s
not going to be inertial– and then you work out what the
equations of general relativity say the geodesics should
be around the Earth and you down project
them into that frame. So you down project
everything into 3D spatial and temporal
terms with respect to that not inertial frame. It turns out that the pieces
of the geodesic equation– the things the tell you
what the geodesics should be– that resemble–
that basically give you Newton’s laws of
motion as if there were a gravitational force–
are just the pieces of the geodesic equation that
involve “time” components when projected into that frame. That’s really all I meant. There’s actually some good
lecture notes by Sean Carroll that I’ve linked in
the description that give you a slightly more
mathematical treatment of this. Go up to equation 4.22 in
the link that you see below. A lot of you were
expressing confusion about how two points
on Earth’s surface can both be “really”
accelerating if Earth’s surface itself is not expanding. Be careful. We’re using the word
acceleration here in two different and
mutually exclusive senses. In the Newtonian
sense, acceleration would mean put a frame of
reference at Earth’s center. Use a clock as if
it runs– pretending that it runs the same
rate everywhere and ask, relative to that clock, are
the coordinate positions of two points on Earth’s
surface changing? And the answer is, of
course they’re not. So in that sense, those two
points are not accelerating. But Einstein says, hold on. In the four-dimensional
curved spacetime sense, you need to give something some
kind of inherent, absolute 4D geometric meaning. And acceleration in
that context means, is your tangent vector
being parallel transported along your worldline? For points on Earth’s
surface, that’s not true. They’re not geodesics. So the way that we have
to reconcile things is, is there a
problem with saying that falling frames, like
attached to an apple falling at both those places, are the
ones that we should really use as the standard, even in
the three-dimensional sense, of non-acceleration. Einstein says, you don’t
really have a choice. You have to do that because it’s
only the falling frames that are “purely” non-accelerating
in the four-dimensional sense. The only reason why
that seems to conflict with the idea of using a
single frame at Earth’s center and saying that those two
points are not accelerating is that that frame
at Earth’s center that you’re using the
gauge that is not inertial and so that shouldn’t be the
standard for saying that things are not accelerating. In other words, Einstein
says the standard of non-acceleration can
only be defined locally in small spacetime patches. If you try to talk about
it in any other context, you’re assuming something
geometric about the world– mainly that it is a 3D,
Euclidean world that works the way you
think– you learn it works in 10th grade
that simply isn’t true. Finally, I do want to reference
one individual comment. Iwon’t tellmyname
was asking what courses he would need to take to
learn about general relativity. He got some answers
from WingedSoda. And then he replied, OK. Great. And I guess I got lots
of time because I’m only going into eighth grade. That’s right. You got lots of time,
Iwon’t tellmyname and you’re asking all
the right questions. Keep learning and listening
to as much as you can, even if it doesn’t
make sense at first. This stuff takes years
to grasp and you’re getting a head start Glad
you’re watching the show. [MUSIC PLAYING]

About James Carlton

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100 thoughts on “What Physics Teachers Get Wrong About Tides! | Space Time | PBS Digital Studios

  1. Good video. Question: Given that two bodies orbiting are mutually bound by a centripetal force, does not the circular (or more correctly the elliptical) motion not also generate a centrifugal force? Apart from rotation on its axis, the earth also rotates around the earth-moon barycentre which is located offset from its centre. Is it possible that this centrifugal force accounts for liquid motion on the side facing away from the moon ?

  2. Before I watched this video I did not have a clue regarding tides, moon earth, and now, I am none the wiser (my lack of understanding has nothing to do with the guys explanation it is just the way my mind works which a mystery in itself)

  3. Why do some of these YouTubers speak soooo fast? Slow down. I like to absorb and process things as I listen actively, otherwise you are just noise.

  4. At the 4:10 mark up to the 6:10 mark the presenter in the video describes the physical conditions that result in the 'tidal bulge'. If one removes the moon from the picture but keeps the rotation speed of the earth there would still be a bulging of the ocean waters although symmetrically distributed and less conspicuous. This oblateness, the flattening of a spherical body at its poles with a simultaneous bulging towards its equatorial plane, relatively small but quite real, is due to the dynamics of its spinning about its polar axis, something seen in other astronomical objects including stars. This can only occur with bodies composed of condensed matter, that is, non gaseous matter. While that last phrase is not echoed in the hallowed halls of orthodoxy, it is within principles of actual science.

  5. Hold on there you young whipper snapper. I'm a real scientist, and I will explain to you how the tides work, and it's rather simple…Magic, evil spirits, and Ozzy Osborn control the tides. Global warming? Blame that one on Keith Richards.

  6. Still utterly misses the main point about tides. You all go on about some big bulge and never look at an actual map of tides. There is no bulge that sweeps around the planet. And 3/4 of a metre? Wow. Stop the bulge nonsense! The kids end up thinking the bulge is real instead of an abstract calculation of gravity forces. Gravity is the engine that drives the system and the moon is in time with them. That is all. Don't use the bulge picture and if you do, don't colour it blue.

  7. Before you get to 2 minutes you mess up. Quite a large number of lakes do have tides… they are just small. Stopped watching and disliked… try again.

  8. actuuuuuuually the Force of gravity due to the Earth isn't 10 million times larger than the tidal force due to the moon, its 295467.39 =>
    Dont take my word for it, heres the math: Fearth/Fmoon = (d^2 * m * g) / (G * M * m) = (d^2 * g) / (G*M)
    However, the tidal force IS 3.38 million times SMALLER than the gravitational force.. but that's completely different

  9. After all the time it is still fascinating that it actually makes sense to switch the speed of his videos to 0,75 and get a reasonable pace without even noticing a slo-mo until the music begins. Actually you get a quarter of a video „for free“ compared to new videos.

  10. ENERGY affect's the motion of all object's, and not Gravity !!!!!

    Gravity is a mythical concept that's mentally applied by Newton, solely to guarantee needed results to it's point of origin, the BRAIN !

  11. Lakes ponds caves swimming pool bath tub's and whatever is directly connected to the oceans has tides .

  12. Here is a simpler explanation of tides. Just as the planets around distant stars produce a wobble of the star, the moon produces a wobble of Earth. The water covering the Earth's surface is thrown in the opposite side of the moon by inertia. As a result the water level rises opposite the moon and lunar gravity increases the level of the water by direct action of gravitational forces

  13. Hi, thanks for the video. As we know continental plate float on magma. They move and get pushed against each other and the pressure builds up to a point that one day it gives way and causes earth quake. I once thought that if the Moon causes tides in the way you just described then surely it can also trigger earthquake are areas of the plates that are about to snap. Then I realised one scientist has already guessed that and predicted a minor earthquake and guessed the time exactly right. So I was wondering if you could investigate that and make a video on it as no one seems to be aware of that.

  14. Yes. If it was merely the pull of gravity on the water then the tide would be more of a teardrop shape with a single high tide every day. The pull on the side of the earth opposite to the moon would be the lowest tide and the highest nearest the moon. This would result in, roughly, one high and low a day.

  15. Flat earth confirmed.
    Just kidding love rewatching these older episodes and I love the direction this channel has gone. This honestly is my favorite channel


  17. I enjoy watching these PBS videos, but I can't cope with these (surely) accelerated videos from this guy. If I wanted to listen to a person talking like this I'd find somebody who recently snorted something illegal.

  18. It was depressing to watch this video and to see how many positive comments it got. It is so excessively complicated that it is difficult to say how wrong it is. The only thing one can conclude is that this video did not help anyone's understanding, and that those who claim to understand it, have not understood anything.

  19. Your first assumption is wrong. To assume something means you really dont know. Bad Experiment using faulty methods

  20. Do not forget the effects of all the fish in the oceans. Fish do things in unison as they swim along in fish schools. The tides really fall when all the fish in the oceans drink water and the tides rise when they go pee.

  21. Pro tip: Tackle fewer topics & treat them more thoroughly…bring us from basic understanding to nuances.
    At this speed, you're talking to a tiny tiny fraction of 1 percent of people…and that tiny fraction already knows most of what youre saying, so they aren't learning much of anything either.

  22. Tides go up from where the moon is facing. How is it that on the opposite side of the earth where wherever the moon is facing, there is also a high tide when the moon isn’t facing there meanwhile on the left and right side of it, its low tides?


  23. There are also two more forces at play that could be mentioned. The gravitational force of the moon is tiny but the whole system "resonates" as it's continually pushing and pulling the water up and down every 6 hours in various places forcing the water tidal effect to amplify in some places causing big tides. If we had no moon and no tides and suddenly introduced the moon back in the system, the tides would take many cycles to grow to normal levels. The second one not mentioned is the fact that the earth also rotates slightly around the centre of mass of both the earth and moon. This point is slightly off centre of the centre of the earth towards the moon. This is the force that helps force the water out at the opposite side of the earth from the moon.

  24. Some say the moon is just too large to be too close to the earth and why doesn't it get drawn into earth? They conclude it must be hollow…why does it proceed different to earth spin?

  25. It makes since that it's a compressive force, not a stretching/pulling force. The pulling notion always bothered me because if it was solely a pulling by gravity along the Earth-moon line, the FAR side of the Earth should have an even lower tide than the "sides" of the Earth-moon line (because the water is being pulled in the direction of the moon), NOT a high tide, like the side nearest the moon.

  26. I was always confused, every time I saw an explanation.
    The whole time I'm thinking… I get why it rises on the moon side, but WHY THE OTHER?! (not plausible)
    Low key I led my whole life just ignoring that other tide (and bathtubs).
    This immediately made much more sense.
    It's where you said "here's that refresher course" that I realized why they don't teach this in high-school.
    Almost like in order to answer one answer accurately, you have to answer all of them first.

  27. the lack of detailed explanation to back up the overall claims means you could just as easily have said tides are due to the spin effect of the quantum universe. Whats annoying is that you have taken basic physics and made it sound like only you have the insight to bring it all together. Why didn't you just draw the vector diagrams for for the gravitational gradient around the whole earth. At 7:37 you show the sun in proportion to the earth and it is obvious that the direction of the gravitational pull at the earth poles will be horizontal , not at an angle as you showed for the moon . We know the sun produces a tidal effect but how can that be using your argument , is there something magical about the sun ?

  28. so your telling me 8000 miles makes so much a difference that tides exist. this is coming from the guy who thinks everything in space is cgi
    what a douche

  29. Space time lol space isn't a principle you can't do anything with it , space is like a shadow shadow isn't something it's the absence of light cold isn't anything and can't be measured because it's the absence of heat or energy space isn't a principle you can't do anything with it like bend it your shadow won't do anything

  30. For me you don't explain this very well, but you have a lot of happy followers so I don't need to go into it, assuming you don't claim to be the author of this knowledge is there source that you got it from I could refer to? You've suggested most, even physics profs don't know it, I'd like to narrow my search.

  31. Sorry, nothing new here. Plus you are wrong about being unable to detect the tides in lakes. Even a damped tube in the water will show the change in height. Sub-hunter aircraft have long had the capability to time-average wave heights which (if you have ever seen it) completely flattens out the wave patterns leaving only the underlying average water height. Useful to detect the bulge caused by a sub moving at depth but sensitive enough to reveal changes in a lake.

  32. What should the tides have looked like in interstellar?
    And why wasn't that planet tidally locked?
    And wouldn't that much tidal friction have overheated the planet?

  33. He is actually incorrect to say that the Sun’s influence is less than the Moon’s because it is so far away. As he explained the Moon’s influence is ‘squeezing’ not ‘pulling’. The Sun is so far away that the influence is almost parallel so the squeezing is not focussed as is the Moon’s. Also the North Atlantic acts as a large bowl of almost perfect size and depth to cause the tide to oscillate and reinforce the height of tide. To see how it works, take your coffee cup and swirl the liquid about. On the edges the liquid rises and falls regularly in rotation. Just like the North Atlantic.

  34. The scientific method would allow you to focus on the independent variable which in this case you’re claiming that the independent variable is gravity how do you very gravity on the waves or on the tides to prove that it’s a gravity affecting the tides it’s another ad Ad hoc explanation..

  35. So is there a reason why we don't have atmospheric Tides – or why these forces don't affect the air in the sky?

  36. First lie right off the bat ! gravity does not affect all things equally !, like gas pressure ! gravity does not pull gas pressure down gas pressure moves in all directions why is gravity selective? Mass attracting mass is a complete failure and pseudo science… nowhere can you see this demonstrated in molecular form or in mass form…

  37. There's a lot of lying going on. SATAN is doing his best to manipulate people into believing things that are not true. And there's so many people believing the lies that it makes it seem like the people that know the Truth are the liars. It says in the Bible that a whole lot of people will be deceived and will not make it into HEAVEN. I believe those people are the ones that fell for SATAN'S lies.

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