Gravity

CuLT

Ok, so i'm not a physicist, but reading up on gravity has left me somewhat perplexed, to my mind, the examples of gravity contradict themselves.

Example One: Gravity is what holds the planets in orbit, correct? Now, it is also said that although this powerful force affects the planets it does not affect sub-atomic particles to any great degree.

I have no problem with this arguement, it seems valid to me, the whole "graviton particle" theory seems to stick.

Its when Black Holes are introduced that the (to my eyes) contractiction comes in. Black Holes suck everything into them according to current theory, comets, light, sub-atomic particles, etc. However, this would indicate that gravity under these circumstances prevails over everything.

Considering the theory from the first example claims that gravity has little to no effect on sub-atomic particles I thought the 2nd quite odd.

So, tell me, do I just not know enough? I think I'm missing out on some knowledge here.

Btw, I may have explained this poorly, I hope you get the gist of what im saying.

Redshift

It was thought that black holes don’t give out any energy at all (except when engulfing stars). However, in 1971 a Russian physicist, Yokov Zel’devich thought that black holes may emit protons and other particles. It was proven by Stephen Hawking three years later that black holes emit subatomic particles. These are now called Hawking radiation. From the previous proof, Hawking also concluded that black holes can evaporate.

I found an interesting article that may go so way to answering your question here http://star.arm.ac.uk/~csj/essays/bh.html

It is generally accepted that once inside a black hole that laws of physics no longer apply so don't let them wreck your head too much:)
Maybe someone else can shed a little more light on this.

Ryan

albertw

Originally posted by Redshift
It is generally accepted that once inside a black hole that laws of physics no longer apply

The laws of physics apply alright, for a given value of `the laws of physics`

it does not affect sub-atomic particles to any great degree.

Not really true. Classic laws break down in this realm and need tweaking (Einstien, Friedmann, Hawking etc have done a fair bit of tweaking!).

The force of gravity on one ubject from another is proportional to the product of their masses, divided by the square of the distance between them. This more or less holds for sub atomic particles also. However down there you have to contend with the strong and weak neuclear forces which are much stronger then gravity, and make gravity appear to have no affect. In a black hole scenario, gravity is overwhelming the stronger force.

Its when Black Holes are introduced that the (to my eyes) contractiction comes in. Black Holes suck everything into them according to current theory, comets, light, sub-atomic particles, etc. However, this would indicate that gravity under these circumstances prevails over everything.

Well zoom out a bit. Somewhere you have an `object` which is sucking in lots of mass. Its mass build up and so the gravitation force objects feel from it is increased. That makes sence under classical mechanics.

Whats actually happening to the particles as they go into the black hole is another matter, and the top of Many books. One that is not bad is Kip Thornes, Black Holes and time warps though its very much a `poplar science` title. Stephen Hawkings books are worth looking at, though read something else first, as his books are tedious and not that well written.

Hawking radiation does allow radiation from a black hole, but its not because gravity fouls up and some particles can escape. Basically (I think, someone correct me if Im wrong) particle and antiparticle paris can be created outside the event horizon. if only one particles goes into the black hole then it must have negative mass and energy and so the black hole will shrink. hmmm.. that makes a lot more sence in my head than it does written down. Get a book on it, Ill just end up confusing myself, and i never did really understand it in the first place! :-)

If you want visible tangible proof that gravitation affects sub atomic particles, have a look for information on gravitational lensing, and any hubble telescope images of it or of the Einstien ring phenomena. basically large galaxies can act as a lens, so light (photons) from even further distant objects gets distorted by them.

Cheers,
~Al
www.irishastronomy.org

davej

Considering the theory from the first example claims that gravity has little to no effect on sub-atomic particles I thought the 2nd quite odd.
So, tell me, do I just not know enough? I think I'm missing out on some knowledge here.
Btw, I may have explained this poorly, I hope you get the gist of what im saying.

Gravity is the weakest force by a long ways:

It is 10000000000000000000000000000000000000000 times weaker than the electromagentic force for example. But, and this is a big but, gravity is cumulative while the other forces obviously are not (they can cancel each other out for example).
This is the knowledge you are probably "missing" above.
Also Gravity has an effect over large distances and can't be "blocked". So if you take the case of a blackhole where there are trillions of trillions of particles compressed into such a tiny space, the cumulative force of gravity is overwhelming.

davej

CuLT

I didn't have anything to back up my reasons last night so I apologise ,

I have included snippets of text from the source of my information. my scanner
is acting up so I'll just include quotes instead.

"Fundamental Forces of Nature

The gravatational force:
This is always an attractive force and is hardly noriceable unless the masses in question are quite large e.g. planets.
While this force keeps planets in orbit around the sun and would appear to e a powerful force it has a negligible effect
on the elementary particles that make up the nucleus"

Although I'd prefer to agree with davej, according to the notes the effect is negligible. Then again, seeing as my knowlege of such
matters is Leaving Cert Honours level, so a qualifying end to that particular quote might be "-negligible unless black holes are introduced"
at college level.

Negligible is the key word for me in this.

Capt'n Midnight

Two points about gravity

a) Not all forces obey an inverse square law - depending on the distances involved different forces predominate.

b) unlike most other forces gravity is always attractive - ie. more matter always means more gravity - with other forces more matter means the attraction and repulsive forces tend to cancel.

====================================

OK then - How fast is gravity ?

When you look at the sun - you are looking at where it was 8 minutes ago, not where it is now. If gravity travelled at the speed of light then the motion of the earth would be directed to where the sun is seen to be. But the motion of the earth is directed to where the sun actually is.

So how fast is gravity ?

albertw

Originally posted by Capt'n Midnight OK then - How fast is gravity ?

When you look at the sun - you are looking at where it was 8 minutes ago, not where it is now. If gravity travelled at the speed of light then the motion of the earth would be directed to where the sun is seen to be. But the motion of the earth is directed to where the sun actually is.[/B]

Is it? What would happen if the sun disappeared? would we continue to orbit for 8 minutes?

Newton though gravity was intantaneous, Einstein disagreed.

The answer apparently is that gravity does move at the speed of light, there was a recent (well this year anyway) paper from NASA claiming to prove it, it involved observations of jupiter occulting a quasar I think. General reltivity assumes this to be the case anyway, and the reason this made the news was that it was the first time this bit of general relativity has been experimentally verified.

So strange as it may sound, if the Sun suddenly vanished, the Earth would continue in orbit for nearly 9 minutes before litterally flying off on a tangent. Thats my understanding anyway.

Cheers,
~Al
www.irishastronomy.org

Sev

Originally posted by CuLT
Considering the theory from the first example claims that gravity has little to no effect on sub-atomic particles I thought the 2nd quite odd.

Gravity effects everything. You've picked up the wrong idea from a quote in the leaving cert physics book which mentions that gravity can be considered to have a negligible effect when dealing with particles on a subatomic level, such that calculations can be made without even considering gravity, because the relative strength of the strong force, weak force and electromagnetic forces are so much greater.

Think about it, two dust particles might bond together relatively strongly by electrostatic attraction (electromagnetic force), but the gravitational attractive force between those two dust particles is incredibly small, it is dwarfed by the forces of the electric charges. Because the force of gravity is so weak it is only seen to have any noticeable effect when the mass producing it gets very very large, but it always exists.

deimos

hmmmm, it is nice to know that my foot is creating a gravitional force, even though its small....... its still dragging a galaxy a couple of billion light years away towards my foot, or my foot towards it, or them both towards each other?

"The existence of negative energy near black holes was guessed by Stephen Hawking in 1974. Hawking predicted that a black hole should faintly glow with heat radiation. The radiation has to come from somewhere, and since nothing (even engery) can get out of a black hole, it seemed that the only explanation was that negative energy flowed into it" -
Paul Davis : "How to build a time machine"

very good book, readable in about 1 hour, comphrending it could take a lifetime though......

CuLT

I think Sev hit it there, even though it doesn't mention that it's for the purposes of calculation maybe its just assumed .

He explained it better to me outside the examination hall anyway...

As far as the effect of gravity goes I would presume it to be ridiculous that the effect of gravity would instantaniously vanish if the source disappeared.
It would imply the existance of some force which was exerted faster than the speed of light which is supposed to be impossible afaik...
I'm basing this on the theoretical existance of gravitons btw.

Capt'n Midnight

...The shape of the quasar's motion gave a speed of gravity nearly identical to that of light, with an error margin of plus or minus 20%. "You can rule out a speed of gravity greater than twice the speed of light with a high degree of confidence," Fomalont told the American Astronomical Society...

Capt'n Midnight

the propagation speed indicated by observational evidence and experiments: not less than 2 x 10^10 c.
http://www.ldolphin.org/vanFlandern/gravityspeed.html

either way it's bloody quicky stuff

Now suppose the speed of gravity was actually 20% less than the speed of light - you'd still end up falling into a black hole...

Nietzschean

Originally posted by Capt'n Midnight
the propagation speed indicated by observational evidence and experiments: not less than 2 x 10^10 c.
http://www.ldolphin.org/vanFlandern/gravityspeed.html

either way it's bloody quicky stuff

Now suppose the speed of gravity was actually 20% less than the speed of light - you'd still end up falling into a black hole...

your original post about the quasar is the currently accepted view on the topic, it was done using widely accepted theory and methodoligy which is quite simple in principle and does give a value about that of c, it completely ruled out any value in or around 2x10^10c.

And according to newtonian mechanics gravity act's instantiously, and an effect equivilent of this could not be ruled out until the recent experiment using the radio telescopes with the quasar as the theory of brane worlds gave a posibility for gravity to take 'short cuts' through space and thus in theory be effectively instantiously.

Capt'n Midnight

Vg=c +/- 20% ?
But isn't this the same sort of expirement where you can prove that a photon can travel two paths at the same time - by getting inteference patterns from either side of a galaxy used as a gravity lens - ie. the rate of photons is so low that two can't arrive at the same time.. - and why didn't anyone notice then...

Still does not explain some other observed effects of the speed of gravitational effect...

Syth

I heard that gravity was the effect of spacetime being warped and things that seem to mave in circles (eg earth around sun) are actually travelling in a straight line in 4D spacetime. It looks like they're going in helixes, but because spacetime is bent, distances aren't what they seem and so a helix is the shortest distance between some 2 points. So I heard this explains why gravity is instantaneous, cause it's not like a real force, it's spacetime being bent...