5 ms·

Gravity visualized (2012) [video]

97 points by shbhrsaha 12y ago

simar 12y ago
Bravo, what an amazing way to explain the concept.
- fizwhiz 12y ago
  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?
jwuphysics 12y ago
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.
- ww520 12y ago
  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.
danellis 12y ago
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?
- scobar 12y ago
  I'm also curious to learn the answer to your question. The way I've always seen space-time modeled in illustrations, and now this video, is like a single plane. I've always imagined that plane as one of infinite planes that form a sphere surrounding the mass whose gravity is warping the space-time. If I'm wrong, I'd appreciate being corrected.
- stormbrew 12y ago
  I don't have a lot of confidence in my own understanding of relativity, gravity, etc., but I think you're taking the analogy too literally. In spacetime the object isn't warping the 'surface' because of something pulling on it, and the demonstration is not about explaining why matter warps spacetime, only demonstrating the effects of that warping.
- Arnavion 12y ago
  >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 12y ago
    ... there's no 4D Earth that's pulling objects down a 3D slope. What is pulling it then?
    - Rapzid 12y ago
      I suppose it's possible that mass interacting with the higgs field could give rise to motion in the way an object will be pushed from a high density area of liquid to a low density area as mentioned in another comment here..
zxc1234 12y ago
This gravity visualization has been allways very problematic for me. It is down right confusing.
computerjunkie 12y ago
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 12y ago
  https://www.youtube.com/watch?v=jlTVIMOix3I https://www.youtube.com/watch?v=jlTVIMOix3I
  - computerjunkie 12y ago
    Another great way to make a graph more interesting.
taf2 12y ago
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.
mhartl 12y ago
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 12y ago
  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 12y ago
    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.

97 points by shbhrsaha 12y ago

simar 12y ago

Bravo, what an amazing way to explain the concept.

fizwhiz 12y ago
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?

jwuphysics 12y ago

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.

ww520 12y ago
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.

danellis 12y ago

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?

scobar 12y ago
I'm also curious to learn the answer to your question. The way I've always seen space-time modeled in illustrations, and now this video, is like a single plane. I've always imagined that plane as one of infinite planes that form a sphere surrounding the mass whose gravity is warping the space-time. If I'm wrong, I'd appreciate being corrected.
stormbrew 12y ago
I don't have a lot of confidence in my own understanding of relativity, gravity, etc., but I think you're taking the analogy too literally. In spacetime the object isn't warping the 'surface' because of something pulling on it, and the demonstration is not about explaining why matter warps spacetime, only demonstrating the effects of that warping.
Arnavion 12y ago
>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 12y ago
  ... there's no 4D Earth that's pulling objects down a 3D slope. What is pulling it then?
  - Rapzid 12y ago
    I suppose it's possible that mass interacting with the higgs field could give rise to motion in the way an object will be pushed from a high density area of liquid to a low density area as mentioned in another comment here..

zxc1234 12y ago

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

computerjunkie 12y ago

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 12y ago
https://www.youtube.com/watch?v=jlTVIMOix3I https://www.youtube.com/watch?v=jlTVIMOix3I
- computerjunkie 12y ago
  Another great way to make a graph more interesting.

taf2 12y ago

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.

mhartl 12y ago

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 12y ago
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 12y ago
  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.