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Author Topic: why can nothing go faster than light  (Read 13775 times)

Offline lightarrow

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Re: why can nothing go faster than light
« Reply #25 on: 20/10/2006 16:26:53 »
Exactly.
 

Offline science_guy

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Re: why can nothing go faster than light
« Reply #26 on: 20/10/2006 16:29:36 »
I've always liked Italian Stuff. :D:D

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Offline lightarrow

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Re: why can nothing go faster than light
« Reply #27 on: 20/10/2006 16:32:23 »
quote:
Originally posted by science_guy

I've always liked Italian Stuff.
Thank you.
 

Offline dgdavisjr

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Re: why can nothing go faster than light
« Reply #28 on: 22/10/2006 02:20:24 »
I'm curious as to the explanation for why gravitational force can move not only faster than light, but instantly. If the sun stopped emitting light, we would still see light for about eight minutes as the last photons traveled from it to us, but if it's gravity suddenly disappeared, we would not continue in our orbit for eight minutes. We would immediately commence a fairly straight line divergence effected only by the gravity fields of the moon and the other planets and debris of our rapidly scattering planetary system. [?]

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Offline science_guy

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Re: why can nothing go faster than light
« Reply #29 on: 22/10/2006 07:46:12 »
I believe that gravity is not really a force, but a bend in space.  This can be illustrated by looking at an eclipse of the sun.  Becuase the sun's gravity bends space, what we would interperate as a curved line becomes a straight line, and therefore light must follow that path, and in doing so, you can see stars behind the sun during an eclipse.  Becuase gravity is a bend in space, we also must follow this curve in space, and settle in it, which pulls our body towards the center of the "hole" in space.  Because of this, the force does not need time to reach us, because we are already in it, and it would affect us immediatly after appearance/disappearance.

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Offline lightarrow

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Re: why can nothing go faster than light
« Reply #30 on: 22/10/2006 10:09:29 »
Sorry, but where did you read that gravitational force moves instantly?

http://en.wikipedia.org/wiki/Speed_of_gravity

quote:
General relativity predicts that gravitational radiation should exist and propagate as a wave at the speed of light.
« Last Edit: 22/10/2006 10:17:30 by lightarrow »
 

Offline Soul Surfer

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Re: why can nothing go faster than light
« Reply #31 on: 22/10/2006 11:17:18 »
There IS one question in this sequence that I have not yet found a clear answer to and that is does the gravitational attractive force force between two bodies only depend on the rest mass of the bodies or does it increase with the increasing velocity?  I think that it may only apply to the rest mass.  In which case materials orbiting black holes will actually find it very difficult to get into the hole because one definition of a black hole event horizon is that it is the point where the escape velocity reaches the speed of light.  The orbital velocity is a little bit lower than this but is still quite close to the velocity of light and so the relativistic mass ingrease is very noticable requiring a lot of extra enegy input to get an orbit that low.

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Offline lightarrow

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Re: why can nothing go faster than light
« Reply #32 on: 23/10/2006 07:52:56 »
quote:
Originally posted by Soul Surfer

There IS one question in this sequence that I have not yet found a clear answer to and that is does the gravitational attractive force force between two bodies only depend on the rest mass of the bodies or does it increase with the increasing velocity?  I think that it may only apply to the rest mass.  In which case materials orbiting black holes will actually find it very difficult to get into the hole because one definition of a black hole event horizon is that it is the point where the escape velocity reaches the speed of light.  The orbital velocity is a little bit lower than this but is still quite close to the velocity of light and so the relativistic mass ingrease is very noticable requiring a lot of extra enegy input to get an orbit that low.
1.The gravitational force depends on the space-time curvature, wich depends on the stress-energy tensor (G_mu_nu = 8(pi)T_mu_nu) which depends on every form of energy, not rest mass only; so it depends on relativistic mass, not simply rest mass.
2.Orbiting near a black hole, a body probably loose energy as gravitational waves, so loosing velocity and hence going in an inner orbit.
 

Offline jysk

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Re: why can nothing go faster than light
« Reply #33 on: 23/10/2006 10:55:50 »
I causiously suggested that the influence of gravity was instantanious in a post earlier in this thread. I carefully choose the word "influence" rather then the word "speed". I might be the only guy here who thinks there is a difference. Syhprum hunted down and shared an artical that addressed the speed of gravity. Thank you for that Syhprum.

That paper concluded that the speed of gravity = C(g)

The mathmatic support they offered is a bit over my head so I won't even try to misinterprate it (lol), but the logic by which they built the model seemed damned to result in a number which is about equal to the speed of light.

Mike
 

Offline lightarrow

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Re: why can nothing go faster than light
« Reply #34 on: 23/10/2006 11:52:23 »
quote:
Originally posted by jysk

I causiously suggested that the influence of gravity was instantanious in a post earlier in this thread. I carefully choose the word "influence" rather then the word "speed". I might be the only guy here who thinks there is a difference.
When you will study physics more deeper at school, you will understand that there is no difference between "influence" and "speed", in the sense that the "influence" must have a "speed of influence". You will learn that physics treats very well defined things, otherwise it's not physics, but literature, instead.
Anyway, I hope you don't take these as critics on you; your interest in physics is a positive thing in itself, ok?
« Last Edit: 23/10/2006 11:57:21 by lightarrow »
 

Offline thebrain13

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Re: why can nothing go faster than light
« Reply #35 on: 23/10/2006 23:42:36 »
so light arrow, does that mean that if  I were traveling very fast, all the time in circles on earth, that I would measure the strength of gravity as greater than 9.81 ms^2?
 

Offline lightarrow

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Re: why can nothing go faster than light
« Reply #36 on: 24/10/2006 07:46:05 »
quote:
Originally posted by thebrain13

so light arrow, does that mean that if  I were traveling very fast, all the time in circles on earth, that I would measure the strength of gravity as greater than 9.81 ms^2?

Soul Surfer talked about "gravitational attractive force force between two bodies" and not about "gravitational acceleration". Since the acceleration, in the first approximation, is given by force/mass, if force and mass increases of the same amount, the acceleration doesn't varies.
I said "in the first approximation" because, relativistically, F = m*a it's not true anylonger.
 

Offline Soul Surfer

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Re: why can nothing go faster than light
« Reply #37 on: 24/10/2006 09:09:50 »
The plot thickens!  sorry I probably was a bit loose in my expression a was really talking about the way a body accelerates under gravity as its velocity beomes relatavistic

I have been trying to understand how strongly relatavistic motion affects the orbits of objects close to (or even inside) a black hole.

So you are saying that the acceleration of a body under gravity towards the centre of gravity as it approaches the event horizon of a black hole does not change as its mass increases as a result of its velocity increase.  How then does angular momentum fit into this?

so you are saying that the acceleration on a body as its mass increases in a relatavistic orbit around a black hole

This is the best information I have found so far
http://www.fourmilab.ch/gravitation/orbits/
as you will see the orbits are extremely peculiar

This of course only applies to non rotating black holes  the important thing is what happens when the hole is rotating?

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« Last Edit: 24/10/2006 09:42:22 by Soul Surfer »
 

Offline lightarrow

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Re: why can nothing go faster than light
« Reply #38 on: 24/10/2006 12:34:00 »
quote:
Originally posted by Soul Surfer

I have been trying to understand how strongly relativistic motion affects the orbits of objects close to (or even inside) a black hole.
It's a good question! By the way, the document you pointed to is very nice!
quote:
So you are saying that the acceleration of a body under gravity towards the centre of gravity as it approaches the event horizon of a black hole does not change as its mass increases as a result of its velocity increase.
I was talking about gravitational acceleration, not about the total resultant acceleration.
« Last Edit: 24/10/2006 12:35:39 by lightarrow »
 

Offline dgdavisjr

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Re: why can nothing go faster than light
« Reply #39 on: 25/10/2006 06:03:59 »
I believe I made myself unclear through the usage of the word "moves". My point, however, remains. The influence of the sun's gravitation (or sudden loss thereof) would be instantly apparent to us even if we were on Pluto, while it would be some time before we realised that it had been removed due to light speed lag.

D.G.Davis,Jr.
 

Offline lightarrow

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Re: why can nothing go faster than light
« Reply #40 on: 25/10/2006 15:22:23 »
Quote
The influence of the sun's gravitation (or sudden loss thereof) would be instantly apparent to us even if we were on Pluto, while it would be some time before we realised that it had been removed due to light speed lag.
What does, physically mean "influence"?
What does, physically mean "realise"?
If you don't give a precise physical meaning to these words, you are not talking about physics, but about something else.
 

Offline naturalperson

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Re: why can nothing go faster than light
« Reply #41 on: 27/10/2006 06:34:58 »
"thats not entirely true.  you are always where you are, and when you are.  The only thing that arrives before, is your image.  If light was not a barriar, than the next milestone would be instantanious.  Even if you take no time at all to move from point a to point b, the image would appear at point b slightly after the image at point a dissappears.  if you were to travel faster than instantanous, than you would meet up with yourself at the point, and there cannot be two of you in existance."

But the questions is why can't "anything" travel faster than light, and so, it seems, that conceiving of the question in terms of you or me, or man, misses the point of the question. Nature changes when observed at different scales. The "facts of nature" described by classical physics and "the facts of nature" described at the quantum scale of accuracy are radically different.

The theoretical physicist, David Bohm, speculated that there is another domain that is existent at a scale smaller than Planck's Constant - Man just has not developed the technology to explore that domain. ( more accurately Bohm speculated infinite domains). The speed of light is a limitation that defines the domain of the human experience. If something travelled faster than light, that something would have a radically different expereience of "reality" than our own.

"Never express yourself more clearly than you think." (Niels Bohr)

 

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Re: why can nothing go faster than light
« Reply #41 on: 27/10/2006 06:34:58 »

 

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