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I was reading A Briefer History of Time by Stephen Hawking and Mlodinow. I found something silly. On page 36 at the bottom, it says the following :

If, say, the sun suddenly disappeared, Maxwell's theory tells us that the earth wouldn't get dark for about another eight minutes (since that is how long it takes light to reach us from the sun) but, according to Newtonian gravity, the earth would immediately cease to feel the sun's attraction and fly out of orbit. The gravitational effect of the disappearance of the sun would thus have reached us with infinite speed, instead of at or below the speed of light, as the special theory of relativity required.

Doesn't Maxwell's theory then also predict that the sun can not suddenly disappear? I mean, it would be like travelling with infinite speed as well. So how is this example falling in place?

Also, at page 58, it says:

Newton, and others, should have realized that a static universe would be unstable, for there is no comparable repulsive force to balance the gravitational pull that all stars and galaxies exert upon each other.

And in his previous book A Brief History of time he had written that Newton realized this flaw and argued that the universe will be stable if there are an infinite amount of stars.

Why this difference (or I should say lie)?

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closed as primarily opinion-based by Valter Moretti, Nathaniel, John Rennie, Kyle Kanos, Brandon Enright Mar 6 '14 at 16:19

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

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    $\begingroup$ Why do you think it is opinion based? It has proper answers given below (except for the second part)... $\endgroup$ – user3459110 Apr 11 '14 at 9:03
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I don't see your problem. We are dealing in hypothetical situations that lead to paradoxes and inconsistencies, so there is no problem with postulating what would happen if...?, even if the "if..." is impossible.

He could have as easily said what would happen if the sun moved suddenly, we would see it move after 8 minutes, but gravitationally feel it move (according to Newton) instantly. But you could continue to argue over the logistics of moving a sun sized object which is irrelevant to the central point of the argument (that Maxwell's light travels at a finite speed and Newtonian gravity does not).

Doesn't Maxwell's theory then also predict that the sun can not suddenly disappear?

Not really, you need Einstein to say that it cannot travel faster than light, and even then simply disappearing would "only" violate conservation of mass-energy, so neither Maxwell nor Newton.

And in his previous book A Brief History of time he had written that Newton realized this flaw and argued that the universe will be stable if there are an infinite amount of stars.

previous book? I thought you were only talking about Brief History of time. In any case Having an infinite number of stars does not make the system stable or solve the lack of force to counteract gravity.

Why this difference (or I should say lie)?

I think lie is a very harsh for what is ultimately a gedanken experiment and at worst a minor slip.

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  • $\begingroup$ Thanks for the thought experiment link. As the title says, I am talking about Briefer history of time and then comparing it to its predecessor Brief History of Time. Even if the sudden disappearing of sun is violating the conservation of mass-energy, I can't imagine why that should be used as an example. Even I am not saying that infinite stars would solve the problem. But the fact that both the books showing contrasting scenes, being written by the same author, is really noticeable. That's all. $\endgroup$ – user3459110 Mar 6 '14 at 12:44
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    $\begingroup$ If you don't use impossible things in thought experiments, you'll have very little to think of. The thought experiment described was to address a problem of speed of gravity, not to show the whole picture of the Universe at once. $\endgroup$ – Ruslan Mar 6 '14 at 13:11
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As I understand Hawking, the point that he is trying to make is that Newton's theory of gravity is actually incomplete and needs to be improved. So Hawking finishes on page 36, saying that Einstein tried to find that better theory of gravity for 6 years and finally in 1915 published his general theory of relativity. In this theory it takes now also 8 minuets for the earth to cease to feel the sun's attraction.

The fact that the sun suddenly disappears is only a shorter and more intuitve version of saying "some changes in both the gravitational field and electromagnetic radiation created by the sun". If the sun would suddenly disappear, both would change, that is the entire point, nothing more.

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  • $\begingroup$ That's quiet obvious. I am just trying to highlight that the given example is not suitable in my view. But as @user288447 pointed out, that's just a thought experiment, so that sorts up everything. $\endgroup$ – user3459110 Mar 6 '14 at 12:47

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