Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: thebrain13 on 16/12/2006 23:37:35
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How will a stationary object view the strength of gravity of a non stationary object.
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The original question has been changed radically and my original reply does not now apply.
I will put my reply to the revised question in the correct place in the conversation
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Does anyone know the answer?
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As far as I can see, your question could be rephrased as :
Does an object moving relativistically increase in mass to such an extent that it can gravitationally affect another body external to iself ?
And as far as I'm aware yes, the relativistic body does gravitationally affect the other body due to it's increased mass as it approaches the speed of light.
I may be confusing the objects own mass with the mass as perceived by others outside and how that actually affects things but I think that's the case.
Basically if you're going fast enough you turn yourself into a mini black hole.
Someone please correct me if I'm wrong.
If I am then I'd like to ask some questions of my own to clear up a few things.
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Does anyone know the answer?
You never ask simple questions! I would like to know GR enough to answer! However, I suspect there aren't many people around, able to answer it. Consider that it's not even completely understood what happens to a (single) relativistic rotating ring of matter! (See Ehrenfest Paradox).
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I get the feeling nobody knows how to answer this. I think gravity has problems when you introduce motion. Anyways these are my thoughts. Now keep the principle of relativity at the forefront of your thinking.
Changes in anything, due to relative motion are always in the other guys frame of reference. So you could not conclude that you would become a mini black hole if you were traveling fast enough.
Now lets consider two earth sized planets. In special relativity saying earth is traveling thousands of miles an hour has no meaning. We always consider our frame of reference at rest. So from both earths frame of reference the strength of gravity would always be 9.81 ms^2 even if one of these powerfull alien ships changed our perception of time relative to the stationary planet next to us. However that doesnt mean we cant view the other objects strength of gravity as changed. And I believe in this case it would have to. The stationary planet will view its own gravity as not changing, it doesnt matter that the other planet is moving around and ageing slower, the other planet will always fall the same. So since the moving planets frame of reference ages slower, its strength of gravity as viewed from the stationary planet would have to be lower, otherwise the moving planet would view a change in its own strength of gravity. Remember the moving planet is aging slower, if its strength of gravity stayed the same from the stationary planet the strength of gravity from its own frame would be bigger, violating the principle of relativity.
So I believe that in order for the principle of relativity to be true, the strength of gravity would have to depend on the relative time of the gravity-emitting object. Also I think using this same path of thought you can prove that blackholes are impossible, because if time stops flowing(like in blackholes) gravity does too. So you can stop thinking about all those blackhole related paradoxes.
If anyone followed my logic, I cant wait to here your response.
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I now understand the question but it was originally posed in an incorrect way. It is a question that I have already asked on this and other boards and have not yet received a clear answer.
The question is, we know that the inertial mass of objects increases as they approach the speed of light. Does this inertial mass have any effect on the gravitational effects of the rest mass.
Remember the name of Einstein's theory "relativity" AS I currently understand it it depends on the relative velocities between the two objects.
If the two objects are travelling next to each other at the same speed out in space far from anything else it mustn't make any difference how fast they are travelling with respect to the rest of the universe the gravitational attraction must just be the rest mass
however if they are moving with respect to one another things may be different.
I do think that thinking too much about time as it might be perceived by the relavant objects is a bad approach and more likely to cause confusion. Time dilation does seem to be one of the things that the general public latch on to and in general it is not important in the initial stages of understanding (although it may be important later in some of my more abstruse investigations)
For moving objects the situation only arises to any significant extent if the objects have very high gravitational fields like things that orbit close to black holes and neutron stars (which are only slightly larger that a black hole of the same mass).
Anything going at around the speed of light passes straight through the whole solar system in an hour or so so it would need to be continually pushed in order to hang around for any time and that's what makes the original question inviable. OK relatavistic effects slightly change the orbit of the planet mercury by causing its perihelion to move by a tiny amount (a few seconds of arc)but that's not what we're thinking about. the object would also be slightly deflected by the sun's gravitational field and the inertial mass element would reduce this deflection
I know the orbits of objects around black holes get very distorted and have received and quoted this illustration on these pages before.
http://www.fourmilab.ch/gravitation/orbits/ This applet allows you to play with the orbits of objects close to the event horizon of a black hole.
I think that the only way to get a real grip on it is probably to bite the bullet and work through the equations properly.
The way I visualise it is that in a normal elliptical orbit an object passes through its closest approach quite quickly so in the relativistic case as the oject gets close it goes faster and becomes heavier and more difficult to accelerate and deflect in its orbit so it goes a bit slower than the non relatavistic case this causes it to "hang around" near its closest approach point.
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"Does an object moving relativistically increase in mass to such an extent that it can gravitationally affect another body external to itself ?"
It is well known that subatomic particles moving at relatavistic speeds have a longer life than would normally be expected due to time dilation effects and a greater mass.
I see no reason why this additional mass should be exempt from Newtons law
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I can measure the gravitational attraction between two large metal balls on the earth's surface using a torsion balance it appears to be the same in all directions and depends only on the mass of the balls. This experiment has been done many times to an extreme degree of precision. These balls are moving in the same direction at a few thousand miles an hour as the earth rotates a different and chaging direction at eighteen miles a second as the earth goes round the sun at two hundred miles a second as the sun goes round the galaxy and about 500 miles a second with the galaxy towards the great attractor.
It is clear that the grvitational force between two objects cannot depend on their absolute velocity as a fundamental tenet of relativity theory itelf.
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What is 'absolute velocity' I thought after Michelson and Morley there was no such thing any more we have no æther to zip through.
The question was when the mass of a body moving in our frame of reference at a relatavistic speed is increased is there a corresponding increase in its gravitational attraction , I believe there is.
I can put it in an even simpler manner, do electrons travelling in a linear accelerator at such a speed that they become more massive weigh more?
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I believe soul surfer answered that allready, gravity is a function of rest mass. There is no increase in gravity from the perspective of a moving observer.
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I find the phrasing of the original question rather ambiguous, I believe Heliotrope agrees with my interpretation.
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There, I changed my original question, now none of you dumbies can misunderstand it, or claim that you cant carry out the experiment. [;)]
And soul surfer you could keep an object relatively in the same place with a high relative velocity, it would just have to accelerate alot. And thats irrelavant anyways, you can still measure a change of gravity even if the object is not close to you.
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The instantaneous value of the gravitational attraction between two bodies does not depend on the relative velocity of the two bodies but the resultant acceleration can this is because if the bodies are moving very fast (a significant fraction of the velocity of light)with respect to each other there is an inrease in the force that is needed to accelerate them.
This is possibly easier to unnderstand in terms of the design of an electron accelerator called a Microtron.
The rest mass of an electron is about 500,000 electron volts (a measure of energy) so if I accelerate an electron with an electric field of 500,000 volts it will gain one extra rest mass of energy and weigh twice as much as a stationary electron that is it will be twice as hard to deflect with a magnetic field it is possible to design acavity that will accelerate an electron up too several times its rest mass like this but every time it gets accelertated its orbit in the magnnetic firld gets bigger because the additional minertiaral mass makes it more difficult to deflect but the charge stays the same.
This is the best description i have been able to find with a quick google on
+microtron +electron +accelerator +"rest mass"
http://nobelprize.org/nobel_prizes/physics/articles/kullander/index.html
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Are you saying that an object with more relativistic mass responds less to gravity from a stationary viewers reference point?
So if an apple passed right above earth traveling at .99c it would not fall at 9.81ms^2 anymore from the earths point of view?
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or does that only apply to relative radial velocity?
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How will a stationary object view the strength of gravity of a non stationary object.
I know everyone is offering answers here, I would like to indulge your patience for a moment as this is something I know nothing about!! Could you please explain what is meant by Time dialation? I have never heard the term in my life!! Then I will go back and read these posts with a better understanding.. Thank you and sorry for the interuption of your flow here!
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Hi karen
Time dialation is basically the real reletavistic effect which occurs when something is moving at speeds close to the speed of light. In other words if you jumped into a space ship and flew at 99.99999 percent the speed of light 1 second for you could be like 1 year for everyone left back on planet earth. fly at near the speed of light for a year and then return home and everyone may have died centuries ago ,basically the faster you move the slower your time ticks away compared to everyone you left back on earth.
ItS a real way to time travel,but only into the future.
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Thank you.. That will help me understand the rest! Nice explanation Michael.. easy to understand..
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Did I lose you soul surfer?
My conclusion for why black holes dont exist seems simple enough and consistant with relativity. I know einstein wrote two papers on why black holes dont exist, but I havent heard his argument for why not. Does anyone know his argument, or a good link explaining it?
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No you did not lose me. I too have my suspicions about extreme gravitiational objects and expect that as usual the reality is much more complex than our ideas. however it does seem that there is some observaional evidence that it is possible for event horizons to exist and for things to fall through them.
If you want to find out more about serious alternative theories look up gravastars
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How will a stationary object view the strength of gravity of a non stationary object.
In outer space in vacuum your inertia does not change whatever moves you make.
I mean, if you dance rumba or stay still your inertia is the same.
Do we agree?
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This is a convenient topic that someone has exhumed from the distant past for no clear reason but it gives me the opportunity to say that I have now found the book that explains gravity and relativity in the clearest ways using basic high school maths.
this is "Gravity From The Ground Up" by Bernard Shutz
In the process he works his way through Astronomy Physics and Cosmology
This book clearly answers the questions That I have posed here explaining how gravity and motion can itself create gravity.
This book is totally brilliant and takes a totally innovative approach to teaching this important and poorly understood area of science.