The Mysteries of Gravity: A Must-See Video Guide

[Forestman]

 

[tiny-tim]

That video (7.5 minutes) is highly misleading.

The first 5 minutes describe the problem with Newtonian gravity being that it is instantaneous, whereas in fact it travels at the speed of light, so that if the Sun disappeared, there would be no effect on the Earth until 7 minutes later.

The last 2 minutes correctly describe Einstein's theory (of matter "warping" space-time), but relate it only to the speed of gravity.

There is no mention, for example, of light rays being bent near a massive object.

This is highly misleading, since it explains almost nothing …

one could perfectly adjust Newtonian gravity to move at the speed of light without general relativity ("warping") …

the whole video concentrates on a feature (the speed) which is not an essential departure from Newtonian gravity, and fails to explain the thought experiments which led Einstein to see (for example) that light is bent by gravity. ​

 

[Forestman]

You might not have learned anything new, but I am sure that many other people did.

 

[tiny-tim]

You might not have learned anything new, but I am sure that many other people did.

Agreed, but what's the point of learning something that's highly misleading?

(and of recommending it, when there's plenty of good material elsewhere?)

 

[Forestman]

There wasn't anything misleading. At the time of Newton many people did believe that gravity had an infinite speed.

 

[tiny-tim]

There wasn't anything misleading. At the time of Newton many people did believe that gravity had an infinite speed.

Yes, of course they did.

But that's not an essential feature of Newtonian gravity, and as I said, one could perfectly adjust Newtonian gravity to move at the speed of light (in the same way as an electric field does).

And the speed of gravity is not an essential feature of Einstein's gravity.

Einstein's special relativity (nothing to do with gravity!) showed that nothing travels faster than light … which presumably meant gravity also …

the problem (which took Einstein 10 years) was to explain the bending of light, not the speed of gravity.

The whole video treats the speed of gravity as being fundamental to GR. ​

 

[Forestman]

The video clip is just one segment of the Elegant Universe movie. You need to watch the whole thing and give it a chance. The book is even better, and goes into a lot of detail on GR. It is true that it does not deal with the mathematics of GR, but before one can learn the math one first must have a conceptual understanding of the subject. The language comes first, and the math comes second. Otherwise a person just gets turned into a bean counter. I really want to understand the mathematics of GR one day, that is why I am currently in the process of teaching myself advanced calculus.

 

[tiny-tim]

The video clip is just one segment of the Elegant Universe movie. You need to watch the whole thing and give it a chance. The book is even better …

Are you saying that this video is part of the 2003 PBS Nova series film based on Brian Greene's book "http://en.wikipedia.org/wiki/Elegant_Universe" "?

 

[Forestman]

Yes, absolutely.

 

[zincshow]

Agreed, but what's the point of learning something that's highly misleading?

(and of recommending it, when there's plenty of good material elsewhere?)

I liked it.

Not sure what is there that is "highly misleading"? Appreciate if you would post links to the "good material elsewhere"

Just to clarify: you said "speed of gravity is not an essential feature of Einstein's gravity". I think that statement is "highly misleading". If it was infinite, would you still have "Einstein's gravity"?

 

[tiny-tim]

If it was infinite, would you still have "Einstein's gravity"?

yes, why not?

(it would breach special relativity, and causation, but the speed of gravity is not an essential feature either of general relativity or of Newtonian gravity)

 

[zincshow]

yes, why not?

(it would breach special relativity, and causation, but the speed of gravity is not an essential feature either of general relativity or of Newtonian gravity)

I suspect you know more then I do about the subject, so I will treat this as a learning moment for myself.

What determines the amount of curvature of space close to a large object. I have always assumed that a higher speed of light would mean less curvature and a lower speed would mean more. Ie. infinite speed of light implies no gravity?

 

[tiny-tim]

IWhat determines the amount of curvature of space close to a large object. I have always assumed that a higher speed of light would mean less curvature and a lower speed would mean more. Ie. infinite speed of light implies no gravity?

I can't offhand remember the formula for bending of light , but the https://www.physicsforums.com/library.php?do=view_item&itemid=166" factor is √g₀₀ (where g₀₀ is the coefficient of dt² in the metric) … it has nothing to do with c.