Advanced Concepts of Gravity

[AlgeKalipso]

This is so slow and trivial. I go to Stanford and classes are way faster, at least five times.

[mattyisagod]

"We live in an infinite universe"

o rly?

[mrrn100]

The 1st particle imbalance of the infinite model is sparked because this Universe is finite (in the sense that it's size is measurable). Either we can measure it's size or it's mass, but not both at the same time. That's all.

Good modelling though on the origin of Black Energy.

[Zurround100]

this boring guy has nowhere near the charisma of the late Carl Segan or Michio Kaku hes a BORE

[Oxydox]

more like elementary concepts in gravity lawl

[sabaths1fan]

more of these UCTV plz

[pirskwayrd]

finite AND uniform? mkay...

[mindgrapes]

wtf is the university of california doing giving this clown a stage? are they really letting people who never read steven hawking speak to students about cosmology?

[ricomajestic]

@mindgrapes steven hawking is so overrated. Most physcists don't even put him in the top ten list of physicists. He hasn't even won the Nobel Prize. He is really famous for writing a very popular book.

[mindgrapes]

@ricomajestic granted, but I was invoking Hawking merely for the history of physics he gives, not any of his own work:

The idea that an infinite distribution should be gravitationally static was, in fact, put forward by Newton, but (according to Hawking in BHOT) this idea is simply wrong. Even with an infinite uniform distribution, matter would still collapse. (I guess this is due to the fact that gravity is bound by the speed of light, but don't really know) . Anyways, this guy gets it wrong.

[CACBCCCU]

@mindgrapes Seems it couldn't be lightspeed limit if nothing is moving in the first place. I'd say it has to do with Einsteinian gravity's "curving" of spacetime, which I'd call a "positive" or "negative" curvature depending on perspective. In any event it makes hypersurfaces sperical (recall the balloon with dots painted on it model of the universe), not ultimately flat. Einstein's GR universe was "unbounded, but finite" not infinite.

[mindgrapes]

@CACBCCCU Hmm, ok counterfactual. Suppose an infinite universe of evenly distributed point masses were poofed into existence. My thinking was that because gravity takes time to act over distances, the masses will behave effectively as if they were in a finite distribution, and collapse into each other. Writ large, i suppose you could say the universe remains infinite, but that infinity gets smaller as everything contracts...

Are we saying the same thing in different ways?

[CACBCCCU]

@mindgrapes It's a strange problem, I'm not sure what the point of it is. Making all the masses appear simultaneously out of nothing doesn't make it any stranger though, I think the result would still be the same. Seems there's still nothing to break the symmetry. FWIW, I'm told Einstein initially thought the universe was infinite, so when he first came up with the cosmological constant it's not clear to me whether it was to keep the universe from collapsing or to give it large-scale flatness.

[mindgrapes]

@CACBCCCU Taking your points in reverse order: I *think* the cc does not in itself imply any particular geometry of the universe, as its value would have to be derived observationally. But this is a bit off the original topic...

I'm not sure i followed you with "there's still nothing to break the symmetry" - could you elaborate plz?

My point with the thought experiment was just to avoid any potential confusions introduced by actual observations of the universe

[CACBCCCU]

@mindgrapes

What I mean by "nothing to break the symmetry" is that since all the particles appear simultaneously, each of their gravity fields begins expanding and overlapping identically and simultaneously. Because the entire particle distribution is supposed to be symmetrical around every particle it seems evident that all the expanding fields must also be symmetrical around every particle. I guess that's the simplest way I can describe it.

[mindgrapes]

@CACBCCCU OK, well I totally agree with that. But I think that would hold true regardless of whether you took a Newtonian or Einsteinian view of gravity. My original posting was intended to criticize the Newtonian idea (articulated by this lecturer) that if there is an infinite symmetry of gravitational forces the universe wouldn't collapse. I'm arguing, and I think this is implied by GR, that this is not the case (absent a cosmological constant, of course).

[CACBCCCU]

@mindgrapes

I understand. Ignoring anything hypothetically antigravitational about so-called empty space, such as the CC, if all the distances between the mases shrink together, no mass is favoring one side over the other, for a closed ring or an infinite line. Ignoring edge effects, a sufficiently long line would suffice to show the same thing somewhat, but basically the idea seems like a dead end of positively metaphysical proportions and, if so, I'd prefer a +/- mass checkerboard array.

[TaterGumfries]

@mindgrapes You comment don't make sense. UC hired the boy because he's a good researcher and gots lots of good papers published. How you think they ought to decide who to hire?

[trogsanonymous]

This is advanced?

[daenumen]

@trogsanonymous very very advanced.

[fuckingghey]

@trogsanonymous So advanced that its come full circle and were at the basics again...

[tepanec]

wtf is talking about?

[BaronVonLichtenstein]

I don't know. If there is really such a thing as gravity, how come fat chicks aren't surrounded by thin guys?

[JohnnieNaked]

@BaronVonLichtenstein Fatness is repellant!

[Oxyster7]

@BaronVonLichtenstein fat white chicks are surrounded by skinny black dudes. Perhaps they have different gravitational properties?

[ashoka22]

at 1:22 arent there two more point masses also at 1:53 doesnt it make sense that the 4 masses cancel yes... with there being 9 masses in total 3rd from the right being the example two masses on either side cancelling would there not be 4 masses applying the force of gravity

[mrqsilveira]

(3:40) ... cause we live in an infinite universe.

I HAVE NEVER REALLY THOUGHT OF THAT!!

At first glance the universe does not seem to be infinite. So, I imagine this depends on the perspective one takes, in this case a infinite uniform distribution of objects.

SO, WHAT ARE THE INTERESTING PERSPECTIVES UNDER WHICH WE APPREHEND THE WORLD AS EITHER INFINITE OR NOT?

[Jacnas]

Wait a minute, infinity - infinity is not zero, it's undefined. Besides, gravity propagates at the speed of light, not infinitely fast as Newton believed. Since the universe has finite age, we're only gravitationally influenced by a finite amount of mass even if the universe contains infinite amount of mass.

[Dearolor]

that's a good point. I wondered if gravity travelled.

[JackSarfatti]

several idiotic comments above.

[1acount]

advanced my ass,this is the same brainless big bang "rationalization" bulshit,thier own theories dont make sense and are contradictory,if all of time and space were made in thier hypothetical "big bang,than before the big bang there would not be anything to undergoe and mysterious"fluctuation"to somehow magicly creat the universe out of nothing,and no time or space for it to happen in,thier always talking about the "fabric of spacetime" bieng bent by mass(somehow)yet they say space is a "vacume"

[1acount]

how can nothing(vacume) be "bent"?aparently from thier models,they think "space time" is made iether of math or gridlines,this is the kind of bulcrap that bowadays is accapted as "advanced science"in reality its a total mathematical abstraction that has no meaning and no relation to reality,its idiots like this that distract people from the fact that antigravity technology has been devoloped wich proves the "law" of "universal" gravitation wrong(just like the "law" of conservation of mass)

[nanotech71]

make a vid, show us what you mean...

[stonerj0e]

what is a vacume ?

[ashoka22]

you use it to clean floors.... LOL

[robheus123]

The main question though which is not touched upon in this video is wether this equilibrium state is a stable or unstable one: would this configuration of masses in infinite space under newtonian laws of gravity contract globally, or would there just be local clustering of matter due to small inhomogeneities?

Note that the situation is not the same when General Relativity is considered, since we know that under GR there is either contraction or expansion, no stable equilibrium.

[1acount]

newtons law of universal gravitation is wrong,gravity is not deppendant on mass,this is proven by simple observation,for instance,under the universal gravitation claptrap,pens and dust and things of lesser mass should have stuck to the astronauts in zero g,they didnt,and,ignoring air friction,things of vastly different masses fall at the same rate,same acceleration,a thing verified to a high degree of precision,

[Holidats]

Are you getting confused with an object moving at a velocity relative to another object in a vacuum or objects moving at different velocities in a vacuum?

[lcabosa]

advanced? startrek was a better entrepetation for gravity, go farm some pink rabbitts for a living :o )

[Icix1]

when you think about it the universe wouldn't be infinite.

[05swanbe]

pretty straight forward. He could have explained what happened to the objects further down the line... particles pair up, right? Is this principal the same with a line?

[Naraphim]

Nothing about this explanation is "advanced."

[Zeldovich]

What makes him think the universe is infinite? It's nto even a scientific concept, as it's not testable.

[mergeform]

I agree it's not infinite but why do they all wish for an infinite universe ?

Microwave background is at 3 degrees Kelvin temperature, if it was infinite it would be at absolute 0 degrees Kelvin.

[pyrrho314]

is this leading to something about how quantum gravity might work or what? darkenergy's pressure? what what what

[Tetrodioxide]

Yes, it has to do with quantum gravity, but not directly. This is a large-scale consideration, and what it shows is how there would be local clustering of matter initiated by simply displacement. This explains how we then get the localized structures of galaxies, stars, and so on from an initially homogenious universe from displacemnts. What caused these initial local concentrations though?: Quantum flucations in the early universe, which influenced local space as the universe expanded.