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Gravity, what's the attraction...


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Hi all,

I came up with this question a while ago and have never found a definative answer. So I pose this question here.

It's a thought experiment of sorts. I'll set out the apperatus for you.

Imagine a single star orbited by a single body. Let's call it a planet.

The planet orbits around the sun in an unremarkable fashion and is quite happy.

Now, suppose we suddenly remove the star from the system. Does the planet feel the effect of this immediately and start heading of in a straight line?

This would mean that information can travel faster than light.

Or does the effect take time to reach the planet, in effect giving gravity a definable speed. i.e. under the speed of light. Personaly, I don't see why gravity would have to be constrained to the speed of light and favour my instant effect theory. Imagine the possibilities in communication, if we could transmit data faster than light.

I'd be interested in everyones thoughts on this.

Best Regards

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Personaly, I don't see why gravity would have to be constrained to the speed of light

Very good observation. It turns out that the "speed of light" is not just the speed of light, it's a fundamental feature of nature that expresses the fact that time and space are really one entity with a certain ratio expressing the "exchange rate".

Any thing at all that has no mass is constrained to travel at a speed equal to that ratio, call it the speed of spacetime.

Light has no mass and so travels at the speed of spacetime. Gravity has no mass (according to Einstein theory) and so travels at the speed of spacetime.

Light signals are far easier to detect than gravity signals (which haven't been spotted yet) and so we've called the speed of spacetime, the "speed of light".

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Interesting.

I was just sat thinking we shouldn't be calling it the speed of light and you explain perfectly, that's it's really space time that sets the limit. Marvellous.

So, if my planet was the same distance from the star as our own from Sol, then it would take about 8 minutes for the gravity to cease acting upon it?

One thing I find quite frustrating about the universe, is how unknowable it is. All we can ever see of it, is how bits of it were at different points in time. Take M81 for example. We see that how it was 12 million years ago. For all we know, it's been eaten by a massive space worm since then.

Every part of the sky we look at is from a different point in time. There is no "Universe Today" just the universe of yesterday and the day before that 20 million years before that.

I wonder if anyone has ever plotted, say, just the Messiers to plot where they might be right now.

Just a thought.

Best Regards

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bearing in mind that we know very little about anything outside our own little galaxy and that the laws of physics have been developed to fit what we think we know over only the last 2,000 years or so and the universe, as we understand it, is at least 13 billion light years old

It is possible, or even probable, that our knowledge of time and space are in reality the equivalent of a caveman knowing how to start a fire.

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Yes, I think that's right, gravity effects would be "delayed" by 8 minutes. Of course, things can't just disappear because energy must be conserved and energy produces a gravitational field.

One thing I find quite frustrating about the universe, is how unknowable it is.

Quite the opposite: the amazing thing about the universe is that it's transparent. You can look up on a clear night and see forever. It only takes a bit of cloud (in the first few kilometers) to stop that from happening but otherwise it's clear for billions of light years...

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Yes, I think that's right, gravity effects would be "delayed" by 8 minutes. Of course, things can't just disappear because energy must be conserved and energy produces a gravitational field.

laws of energy conservation are only valid in a closed system because we can't make them work without setting artifical boundaries. If the star choses not to exist and the planet felt like going around and around in circles because it enjoyed doing it who are we to tell them otherwise? :)

Edited by Photosbykev
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If we were to lose our mind and enjoy it, who's to tell us otherwise.
I have reported the loss to the forum moderators but no one has found it yet :)

But I was being quite serious. about the amount of knowledge we think we know, rather than the sentient star and planet

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I reckon the planet would feel the effect immediately and start heading off in a straight line.

I don't believe gravity attracts anything. Gravity instead is just a bend in space-time. The more 'mass'ive an object the more bend in spacetime or the more gravity it has, and instead of attracting objects towards it, it is really just bending spacetime more and objects are falling towards it.

If you took out the object or the star then the bend in spacetime would no longer be there so the planet would just head off in a straight line immediately.

If you instantly took out an object that massive though it would probably create a gravity wave, so I don't know what the planet would do in this situation.

Edited by Kitch87
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The force of attraction acting on the Bodies is calculated using

G m1 m2 = F (the force acting between the objects)

r^2

If m1 is the mass of the star and m2 is the mass of the planet

Now consider the Star were to disappear, then m1 --> 0 instantly,

Hence the Force retaining the Planet in orbit ----> 0 instantly and the Hence the Planet will immediately move off in the direction it was travelling at that instance.

Consider a child spinning a ball, tied to a piece of string, around his/her head

If you now sever the string the ball will immediately move off in a straight line.

hope this helps

Edited by Nillchill
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If you instantly took out an object that massive though it would probably create a gravity wave, so I don't know what the planet would do in this situation.

I am partial to this theory and if this wave does occur then I would imagine it to force the planet further away from the point where the star originally was then slow it down and begin to pull it back towrds the centre, however I do think that the panet would end up further away from that centre than when it first started.

Jonathon.

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In the case of an object bending space time and causing other less massive objects to fall towards it, if the object is removed immediately, does that mean that space time `flattens` itself at the speed of light?

If a Black Hole can be described as `puncturing` space time, if it was removed immediately would space time be able to repair itself?

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The force of attraction acting on the Bodies is calculated using

G m1 m2 = F (the force acting between the objects)

r^2

If m1 is the mass of the star and m2 is the mass of the planet

Newtonian physics doesn't work when r is a large value (and by large, I mean any distance where the time taken for light to travel that distance can't be ignored.)

You have to start looking at general relativity, which limits the transfer of any information (gravitational or otherwise) to the speed of light. The speed of gravity could of course, be less than the speed of light, but it explicitly predicts gravitational radiation, and predicts it to propagate as a wave at the speed of light.

Carl.

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I see it is discussed on pages 73/4 of Brian Greene's "The Elegant Universe".

If a large mass comes on the scene, space will warp... the distortion will not be instantaneous. Rather it will spread outwards from the massive body... Einstein was able to calculate how fast disturbances to the fabric of the universe travel... precisely the speed of light... we on earth would visually learn of the sun's destruction at the same moment we would feel the gravitational consequences - about eight minutes... [] ... gravitational disturbances keep pace with, but do not outrun, photons.
Somewhat abbreviated from the original. :)
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The problem is that the question posits the instantaneous removal of the planet and then science disallows the instantaneous for any theory of what follows...

I like to think of Newton saying to Halley that he did not want to be considered the author of a theory requiring instantaneous action at a distance. He was surely right to have this doubt. In GR the doubt is assuaged because the field into which a body strays is already in place before it gets there so there is nothing instaneous about it any more.

Olly

Edited by ollypenrice
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Special relativity states that NOTHING can go faster than the speed of light, (apart from the expansion of space-time). This means that any natural forces occur after the light. This may sound strange because its a force. But a force isnt just some invisible thing a force is carried by a particle, these are called Bosons, the Gravitational force is carried by the Graviton, electronmagnetic by the Virtual photon, the strong nucleur by the Gluon and the weak nucleur by the W Boson. Using this and the particle theory of light we can see that the photons (light) reach us before the other particles such as the Gravitons. So if the star disappeared, the orbiting body wouldnt feel any effects until the light had reached the body. An example could be the andromeda galaxy. It is heading towards us but seeing as it is about 2.9 million light years away it could have hit us already. So it takes 2.9 million years for its light to reach us. If it hit us tomorow, we wouldnt actually know about it for another 2.9 million years.

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Really? Surely that would mean that Andromeda itself is travelling faster than the light it's emitting?

Thats a good question. The answer is no. Let me use a different example, if the sun randomly disappeared we wouldnt know about it for 8 minutes because thats the time it takes to reach us. As for andromeda because we are only just recieving the light from 2.2 m years ago the galaxy may have become closer to us already, it may have even hit us! Does that help? So basically because we are still seeing it way back in the past because its original light is reaching us, its new light from its current state is on its way.

Its awkward to describe but i see why you are confused but this is basically what will happen.

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If the Andromeda Galaxy was about to hit our's, then it would (obviously) be much closer than the 2.2 or 2.9 million light years stated by Sammy, and the light emitted by it would reach us much quicker. We would therefore be able to watch it as it approaches.

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If the Andromeda Galaxy was about to hit our's, then it would (obviously) be much closer than the 2.2 or 2.9 million light years stated by Sammy, and the light emitted by it would reach us much quicker. We would therefore be able to watch it as it approaches.

No we wont... as it approaches us yes it will be closer but we will still be recieving its light from when it was 2.1, 2.0, 1.9 and so on. We wont be able to see it.

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