Physics Question

joe-90 said:

Did you watch Horizon the other night? A leading scientist said that - not me. He reckons we know absolutely nothing about it.

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Well I never watched the program but if he did say that then he knows as much about gravity as you do.

More likely he never actually said that and you are making it up.

We know several things about gravity (and when I say 'we' I exclude you from the group)

1) The attractive force between two masses is proportional to the product of the masses
2) The attractive force between two masses is inversely proportional to the distance squared between the two masses

(In newtonian physics, in fact the law is named after him)

From these simple rules, we (and when I say 'we' I exclude you from this group) can derive using mathematics lots of interesting other equations.

e.g.
The escape velocity of an object
The radius of a geosynchronous orbit

Which can be tested experimentally to demonstrate the original premise holds.

So we (and when I say 'we' I exclude you from this group) know rather a lot about gravity and we are learning more every day (and again, Joe, when I say 'we' are learning more every day I unfortunately have to exclude you from this group)

Like I said, we know fook all about gravity and I include you in that group. All you have done is mention the observable effects of gravity. That doesn't mean you know what it is.

Now people like me paid for your education so stop being so sanctimonious and at least add to the sum total of human knowledge without being a fookin passenger.

You've got the education - so use it and stop being a follower. I don't have your education - I just paid for it, but at least I'm capable of original thought, and that is something that you seem incapable of.

You are educated - whilst I am clever.

I know which I prefer.

chapeau said:

We know several things about gravity

1) The attractive force between two masses is proportional to the product of the masses
2) The attractive force between two masses is inversely proportional to the distance squared between the two masses

(In newtonian physics, in fact the law is named after him)

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We don't actually "know"these things, and nor did Newton. Using these propositions made for a far more accurate model than the previous ones and as yet seem to be a pretty good approximation (although the advent of relatvity does show that some of Newton's models are fine up to a limit but aren't entirely accurate for higher speeds and huge distances).

Thus we may observe certain things associated with the properties of gravity. But we don't actually know eff all about what it is.

Now people like me paid for your education so stop being so sanctimonious....

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I suspect the taxpayer paid for both our educations. Happy to go down that path but I'll start to talk about value for money.

chapeau said:

Now people like me paid for your education so stop being so sanctimonious....

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I suspect the taxpayer paid for both our educations. Happy to go down that path but I'll start to talk about value for money.

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I suspect they paid a lot more for yours though.

Ego is of little worth in a debate about hypotheticals

joe-90 said:

There is a link between space, mass and gravity that no-one understands.

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You forgot to include a massive factor......























































TIME

I was short of it.

calorific said:

My own suspicions are already stated - instantaneous disappearance of the sun would lead to an instantaneous non-gravity effect. I.e. it is as though it were never there.

There is no sudden "implosion" of a collapsing space, nor a wobbling rubber sheet IMO. For anyone who thinks that there would be a "sucking-in" effect - it could be argued that this would counteract the gravity that might be "en route" - thus the effect would be the same; i.e an instantaneous zer-gravity effect and not an 8 or so minute delay.

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That's an interesting viewpoint. Nicely reasoned and well argued but I suspect that you're wrong.

You're right about gravity, of course. We know very little about it as a force other than it's attractive and obeys the inverse square law at a local scale. It's astonishingly weak as a fundamental force and there's never really been a successful theory to unify gravity with the other fundamental forces. My personal favourite theory (M theory) offers one idea that gravity isn't actually a part of our own universe but rather 'leaks' from a nearby brane. Quantum loop gravity may eventually provide some answers if it pans out. It's fascinating stuff though.

Back to your proposition though. We know that space-time distorts around a mass - Einstein's predictions on that have been proved to be true by observation many times (the rubber sheet analogy works well in two dimensions although the real effect is far more difficult to visualise). Hypothetical instantaneous removal of a mass that's responsible for the largest distortion in out locality is going to leave space-time with a distortion and nothing left to maintain it. Whichever way you look at it, that distortion has to change and that's not going to be instantaneous - it's going to propagate out from the centre at a particular velocity. Now, the universe itself can exceed the speed of light - we know this but that's in its own expansion - a gross effect if you like. Localised areas appear to conform to standard physical laws (by observational data) and so the propagation of the effect is more likely to be limited to the speed of light than be instantaneous.

Yes but the problem with that is that you would have gravity in the universe that didn't relate to any mass , and that can't be.

Some of the stars we see in our galaxy have long since exploded and ceased to be. Yet we still see their light and will only find out about their fate when the lightwaves from the explosion reach us.
Why wouldn't the same effect happen with gravity?

sooey said:

Some of the stars we see in our galaxy have long since exploded and ceased to be. Yet we still see their light and will only find out about their fate when the lightwaves from the explosion reach us.
Why wouldn't the same effect happen with gravity?

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This is a different question to that of the op. The exploded stars' matter are still in existence and so the cumulative gravitational effects are still in situ, albeit spread across a larger volume. The op questions the sudden non-existence of a body, which creates a different conjectural soluton.

sooey said:

Some of the stars we see in our galaxy have long since exploded and ceased to be. Yet we still see their light and will only find out about their fate when the lightwaves from the explosion reach us.
Why wouldn't the same effect happen with gravity?

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Because light is a package of energy called a photon whereas no-one knows what gravity is. Gravity distorts space and bends light - but no-one has the slightest idea how that happens. All we do know (by observation) is that it is ALWAYS associated with mass. No mass = no gravity.

This is a different question to that of the op.

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Well obviously it's a different question but it's a very pertinent question which takes the discussion further and I suppose it's too much to expect everybody on here to see that.

The information that the star went bang does not reach Earth instantaneously. The information that arrives via what we call light, the fastest thing possible, takes a finite time. Information also arrives via neutrinos again takes a finite time. Why should the information that is gravity mediated be instantaneous?

Theoretically they don't and there are graviational waves detectors in place and in development which are designed to show this is the case.

It's only pertinent if you want to compare apples with oranges.

A photon is an energy particle that can happily exist after the source that produced it has gone. It's like a bullet from a gun.

Gravity, however, is a fundamental property of space that cannot exist without the space/mass that produced it.