

Gravity visualized (2012) [video] - shbhrsaha
https://www.youtube.com/watch?v=MTY1Kje0yLg

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mhartl
I've never really understood this visualization. It's a cheat in so many ways
(wrong dimensionality, reliance on actual gravity to work, etc.). While such
"gee-whiz" demonstrations are great at getting people excited about science, I
worry that they create false expectations. They often produce the _feeling_ of
understanding without conveying _actual_ understanding (and the limitations
thereof).

The unfortunate truth is that many subjects in physics, including general
relativity, are impossible to visualize in the conventional sense. I learned
GR from Kip Thorne at Caltech and wrote my Ph.D. dissertation on the dynamics
of rotating (Kerr) black holes, and my intuition for the subject is still
fuzzy and inchoate. The best you can do is have a cobbled-together half-
picture that kind-of, sort-of works, some of the time. Human brains simply are
not designed to understand, at a deep intuitive level, things like the
geometry of four-dimensional pseudo-Riemannian manifolds.

~~~
ksk
Wow, you sound very confused to me. Your "false expectations" bit is nonsense.
Using your logic, kids shouldn't be told to wash their hands before they eat
because it creates the "false expectation" that by washing their hands they're
getting rid of all harmful germs. (re: They often produce the feeling of
understanding without conveying actual understanding (and the limitations
thereof)

Reality is complicated and kids already know that. Its just that they're way
better than adults at being comfortable with not knowing everything.

> Human brains simply are not designed to understand, at a deep intuitive
> level, things like the geometry of four-dimensional pseudo-Riemannian
> manifolds.

"Hey kids ! Lets get real.. you won't understand this stuff at a "deep
intuitive level" but please choose physics as a career." \- Doesn't sound
motivational to me.

~~~
mturmon
You're not giving the above comment the respect it deserves.

Whenever you offer up an analogy, you want to choose it carefully, and be sure
to convey its limitations. The video didn't discuss its limitations (which is
OK, it's just a video showing off a rig). So we're left to do it here. And
trust me, learning GR with Kip Thorne
([http://en.wikipedia.org/wiki/Kip_Thorne](http://en.wikipedia.org/wiki/Kip_Thorne))
makes the above commenter qualified to offer an opinion.

~~~
ksk
>Whenever you offer up an analogy, you want to choose it carefully, and be
sure to convey its limitations.

Yes, maybe if you're presenting at a conference or a lecture or something like
that. Not if you want to get a bunch of kids motivated/interested in a topic.
In the case of this video, the limitations can only be understood by people in
that field. Sure, You can hand-wave a general clarification like "this isn't
actually how it is, because <insert boring text that nobody will remember>" \-
which would be pointless (IMO) and convey no real information. Because the
"real" information takes several years of academic training to understand and
comprehend.

>And trust me, learning GR with Kip Thorne
([http://en.wikipedia.org/wiki/Kip_Thorne](http://en.wikipedia.org/wiki/Kip_Thorne))
makes the above commenter qualified to offer an opinion.

Sorry but it changes nothing in my mind. If the OP had said "Hey I would have
said it this way and guess what I've successfully used it to motivate X, Y and
Z into taking up science" \- I'd have been way more impressed and given the
"above comment the respect it deserves".

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taf2
Mr. Burns the best science teacher ever. I remember he introduced the concept
of Hertz to a half a sleep class by introducing the Avis and proceeding to
casually talk about frequencies of Avis. After about 30 seconds he could tell
which of us had read the chapter assigned the night before and which hadn't
based on the laughter... His classes were the best.

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danellis
This visualization always bothered me. The marbles 'stick' to the surface, but
only because of the gravity pulling them from underneath. In the absence of
gravity, marbles coming in from the outside could just continue in a straight
line parallel to the ground, unaffected by the mass in the middle.

The only way I've been able to reconcile this in my mind is to think that this
is a visualization of gravity warping spacetime in a _2D_ universe, and that
is why the masses are stuck to the surface, but I've never seen that said in
any of the explanations.

Am I right in thinking about it that way?

~~~
Arnavion
>The only way I've been able to reconcile this in my mind is to think that
this is a visualization of gravity warping spacetime in a 2D universe, and
that is why the masses are stuck to the surface

Yes, that is correct. The sheet models a 2D universe, so objects are not
allowed to "leave" the sheet, just like the Earth can't leave the three
spatial dimensions it inhabits in the real universe.

For the same reason, an object on the sheet universe can't see (or even
imagine) the concept that its sheet is bending. After all, the sheet is
bending in a dimension that it cannot know about.

The analogy breaks down when you try to think where the force pulling the
object down the bent spandex comes from. The objects on the sheet get pulled
down a bend because of the Earth's gravity, but in the actual universe there's
no 4D Earth that's pulling objects down a 3D slope.

~~~
jonsen
_... there 's no 4D Earth that's pulling objects down a 3D slope._

What _is_ pulling it then?

~~~
Arnavion
There is no answer to this question. Relativity simply allows one to calculate
how much curvature is created in a particular mass configuration, and what the
resulting gravitational forces are. It doesn't try to explain why the
curvature occurs.

This link explains it better than I did:
[http://www.thenakedscientists.com/forum/index.php?topic=3887...](http://www.thenakedscientists.com/forum/index.php?topic=38879.0)

~~~
amelius
Just wondering, if acceleration == gravity (equivalence principle), how would
this be visualized in this model?

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computerjunkie
What an amazing way to explain this concept. I'm a visual thinker, and I
always preferred these kind of explanations of concepts. I just wish I had
teachers who taught more like this in my science subjects during Secondary
School.

~~~
plusfour
[https://www.youtube.com/watch?v=jlTVIMOix3I](https://www.youtube.com/watch?v=jlTVIMOix3I)

~~~
computerjunkie
Another great way to make a graph more interesting.

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jwuphysics
This is definitely a nice, intuitive way to introduce people to the theory of
gravity. However, I was a little confused about how he tried to model dark
energy.

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ww520
I think his dark energy illustration doesn't work. Dark energy is the
inflation of spacetime, not mass warping spacetime (well at least we don't
know what is causing the inflation). To make the illustration work, he needs
to pull the whole sheet out along the rim, making the whole sheet bigger.

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zxc1234
This gravity visualization has been allways very problematic for me. It is
down right confusing.

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simar
Bravo, what an amazing way to explain the concept.

~~~
fizwhiz
Indeed. It was particularly interesting how he described the "emergent" nature
of the direction in which the celestial bodies finally conform to
(clockwise/anticlockwise if applied to a 2d space). Wouldn't it be possible
for a few planets to escape a collision and orbit in an opposite direction? Is
collision necessary imminent from a stochastic perspective?

