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Skyfire
2004-Nov-29, 12:23 AM
There is something that has been going round in my head for a while now and I haven't found an answer anywhere yet (probably not using google search propely!!). Apologies if anyone has already done this:

A hypothetical situation - imagine a body, perhaps a large star or even a black hole suddenly appears at some point in the universe (I said it was hypothetical!). Every body interacts gravitationally with every other body in the universe.

HOWEVER, does this gravitational effect on ALL bodies happen instantaneously, or does the "gravity effect" of this sudden new body that has now appeared travel outwards at the speed of light (or maybe at the speed of gravity - WOW, getting heavy here!) to influence all other bodies? Is this in the realm of the "gravity waves" that seem to be under research at the moment? If it were to appear somewhere in our galaxy, say close by at maybe 100 light years, would it be 100 years before the solar system (and earth) are affected by it? Or would the effect be instant??

In other words, what is the "speed of gravity"? Is it light speed? Or something else??? Anyone have any ideas, or any theories or science you can point me at????

AGN Fuel
2004-Nov-29, 01:05 AM
Relativity tells us that there can be no 'instantaneous' interactions - that the maximum speed of any interaction is the speed of light.

If, for example, the sun were to magically disappear, it would still take ~8.5 minutes before the sun's gravitational influence on the Earth was lost.

Squink
2004-Nov-29, 02:05 AM
First speed of gravity measurement revealed (http://www.newscientist.com/news/news.jsp?id=ns99993232)
(Jan 2003)
The speed of gravity has been measured for the first time. The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.
Of course, it was only a few months until this popped up: Berkeley Lab Physicist Challenges Speed of Gravity Claim (http://www.lbl.gov/Science-Articles/Archive/Phys-speed-of-gravity.html)
(Jun 2003)
Albert Einstein may have been right that gravity travels at the same speed as light but, contrary to a claim made earlier this year, the theory has not yet been proven. A scientist at Lawrence Berkeley National Laboratory (Berkeley Lab) says the announcement by two scientists, widely reported this past January, about the speed of gravity was wrong.

swansont
2004-Nov-29, 02:18 AM
There have also been measurements of binary pulsars, whose orbital decay is consistent with the speed of gravity being c, with very good agreement.

lti
2004-Nov-29, 06:03 AM
i myself have often pondered the speed of gravity question. Wasnt there an article published in New Scientist last year about it being proven to have speed c? or is that the berkely guys u speak of?

On another similar note (if u dont mind the implications of relating gravity to the force on a string), imagine a ball traveling in a circle due to the centripetal force provided by the string. imagine the string was 2 ly long. if i was to cut the string, would the ball instantaneously fly off on a tangent, or would it continue to accelerate towards the centre of the circle despite there being no unbalanced forces now acting on it.

Skyfire
2004-Nov-29, 09:56 AM
...<SNIP>...

imagine the string was 2 ly long. if i was to cut the string, would the ball instantaneously fly off on a tangent, or would it continue to accelerate towards the centre of the circle despite there being no unbalanced forces now acting on it.

Yes, that's the kind of thing that got me thinking about this in the first place.

Thanks for the responses so far, I have tried following the link given but the system here at work seems to have a problem getting to it. I'll have to wait till later, or when I'm home to read it. I thought that this must have been looked into but I hadn't found anything on it. Althoug that's probably to do with me being useless at putting in the right search information.

Normandy6644
2004-Nov-29, 08:29 PM
i myself have often pondered the speed of gravity question. Wasnt there an article published in New Scientist last year about it being proven to have speed c? or is that the berkely guys u speak of?

On another similar note (if u dont mind the implications of relating gravity to the force on a string), imagine a ball traveling in a circle due to the centripetal force provided by the string. imagine the string was 2 ly long. if i was to cut the string, would the ball instantaneously fly off on a tangent, or would it continue to accelerate towards the centre of the circle despite there being no unbalanced forces now acting on it.

I'm pretty sure it would keep moving in a circle until the gravity wave reached it, at which point it would move on the tangent line. This is the same idea as orbiting planets. If the Sun suddenly disappeared, our orbit wouldn't change for 8.3 minutes, after which point things would be very different. :D

AGN Fuel
2004-Nov-29, 11:09 PM
On another similar note (if u dont mind the implications of relating gravity to the force on a string), imagine a ball traveling in a circle due to the centripetal force provided by the string. imagine the string was 2 ly long. if i was to cut the string, would the ball instantaneously fly off on a tangent, or would it continue to accelerate towards the centre of the circle despite there being no unbalanced forces now acting on it.

Without working through the numbers, it will take time for the loss of tension to propagate through the string. The ball would not instantaneously fly off at a tangent.

Skyfire
2004-Nov-30, 12:04 AM
On another similar note (if u dont mind the implications of relating gravity to the force on a string), imagine a ball traveling in a circle due to the centripetal force provided by the string. imagine the string was 2 ly long. if i was to cut the string, would the ball instantaneously fly off on a tangent, or would it continue to accelerate towards the centre of the circle despite there being no unbalanced forces now acting on it.

Without working through the numbers, it will take time for the loss of tension to propagate through the string. The ball would not instantaneously fly off at a tangent.

Yes, but aren't you then assuming some "mass" within the string? Surely for this hypothetical experiment we can assume a special string with no mass? I know!! "Photonic string"!!!! Hey, that could be just what we need for these mind experiments....

Evan
2004-Nov-30, 04:20 AM
The string experiment has nothing to do with gravity at all. We know from conclusive experimental evidence that ordinary forces such as electromagnetism, the weak force and the strong force propagate no faster than the speed of light. Those are the forces in the "string" and the object will feel the tension in the string until the change in force in the string propagates at the speed of light or slower. It doesn't matter if we postulate a massless string with no elasticity either. Changing the position of the string relative to the point it swings about is conveying information about the state of the weight at the end. It is a cardinal tenet of relativity theory that information cannot be conveyed faster then light.

Squink
2004-Nov-30, 05:38 AM
It is a cardinal tenet of relativity theory that information cannot be conveyed faster then light. And recently proven again:
Physicists in Switzerland have confirmed that information cannot be transmitted faster than the speed of light. Nicolas Gisin and colleagues at the University of Geneva have shown that the "group velocity" of a laser pulse in an optical fibre can travel faster than the speed of light but that the "signal velocity" - the speed at which information travels - cannot Good News for Causality (physicsweb) (http://www.physicsweb.org/articles/news/8/11/10/1)

Skyfire
2004-Nov-30, 10:32 AM
It is a cardinal tenet of relativity theory that information cannot be conveyed faster then light.

I thought the "speed of gravity" experiments and gravity waves research has not yet proved (or disproved) this yet. From what I understand the debate is still ongoing and until some method is found to measure gravity waves (or whatever properties of gravity are measurable to determine speed) properly we still won't be sure. Or have I missed something?

Actually, there's another thought. Light is shown to consist of the wave/particles we know as photons, massless "light packets" is the best way I can think of describing them. What form is it thought that gravity waves take? I know that gravitons have been mentioned, but I don't think they have been proved yet. Or have they??

I know that relativity suggests light speed to be the maximum speed for, in effect, everything. Or again, am I wrong? I know tachyons are a theory, but don't know whether they are proven???

Evan
2004-Nov-30, 02:35 PM
Relativity does not predict gravitons. Relativity considers gravity to be a consequence of the "shape" of space as affected by matter. The jury is still out on the speed of gravity. Some measurments indicate that it is limited to c. It may not be affected by the information rule though. How does one transmit information with gravity? Information about the state of a collapsing star could be inferred from the gravity signature so perhaps that is a clue.

As for tachyons, they are strictly hypothetical. The math of relativity is symmetrical around the speed of light and the equations can be performed by assuming a particle that travels faster than light. This is a mathematical curiosity only.

A Thousand Pardons
2004-Nov-30, 02:44 PM
This is a mathematical curiosity only.
What do you mean by that? Are you saying that we have proof that they don't exist? Or proof we can't detect them?

Kaptain K
2004-Nov-30, 02:54 PM
Either way, you can't build a ship out of tachyons, since (if tachyons exist) relativity says that they must move at a speed in escess of "c" in all frames of reference - including other tacyons! :o

Skyfire
2004-Nov-30, 03:12 PM
...... relativity says that they must move at a speed in escess of "c" in all frames of reference ....

C's in all the frames..... is that NASA hoaxing again?! :lol:

(sorry, it was too obvious, I couldn't resist it!! 8) )

Skyfire
2004-Nov-30, 03:24 PM
Now here's a thought that has just struck me.

<WARNING NOTE: this is NOT A THEORY, just my speculation! This could come under the "bad science" heading if anyone takes it as "something they read somewhere">

Why are black holes....err... well..... black? Because the light can't escape due to the amount of gravity .... that's what is generally well known...

However, surely it has more to do with the fact that the speed of light is still C, but at the event horizon the time dilation effect has "stretched" all light wavelengths to an infinite length? Therefore we in effect see the light rays emitted with infinite wavelength, and thus we see "black" light..... ?!! (or an absence of light...)

Someone is now going to show me the "bad science" here.... please do. I am just rambling really, but it makes a kind of sense in my mind. I would much rather put my mind right than go on thinking along the wrong lines!

Skyfire
2004-Nov-30, 03:30 PM
But then, light is affected by gravity, especially in not being able to escape black holes, so why is gravity seemingly not afected similarly, if it is also affected and controlled by the speed of light?

It has been said that a black hole formed out of a neutron star would show the same gravity field as the neutron star, as the mass creating the black hole was the same. Is that correct?

A Thousand Pardons
2004-Nov-30, 03:34 PM
But then, light is affected by gravity, especially in not being able to escape black holes, so why is gravity seemingly not afected similarly, if it is also affected and controlled by the speed of light?
I like the way you think! :)

Kaptain K
2004-Nov-30, 04:47 PM
A black hole can have a magnetic field. So, a black hole can have a magnetic field and a gravitational field, even though the "carriers" of both (photons and gravitons) cannot escape from a black hole!

Evan
2004-Nov-30, 04:49 PM
According to relativity theory gravity is a distortion in space-time. The possibility that such distortions can only propagate at the speed of light does not imply that something matter or energy is actually moving, therefor the fact that the escape velocity at the Schwartzchild radius equals c has no effect.

Tachyons are not thought to exist. It is simply a consequence of Einstein's mathematics. The equations can be satisfied by postulating a particle that moves faster than light. It is a result of how Einstein deals with infinities in his math. The tachyon has the property of requiring ever more energy to slow it to light speed. At light speed the tachyon would require infinite energy. It is the mirror image of a regular particle accelerated to light speed. A conseqence of this is that a non decelerated tachyon must travel at infinite velocity, an absurd idea that falls out of the math.

Evan
2004-Nov-30, 04:52 PM
Kaptain K,

No magnetic field for a black hole. Black holes have only three properties. Mass, charge and angular momentum. This is called the "No Hair" principle, as in "Black holes have no hair". Gravitons are a QM construct not verified by experiment.

Kaptain K
2004-Nov-30, 05:11 PM
Kaptain K,

No magnetic field for a black hole. Black holes have only three properties. Mass, charge and angular momentum. This is called the "No Hair" principle, as in "Black holes have no hair". Gravitons are a QM construct not verified by experiment.
If black holes do not have magnetic fields, then, what exactly is accelerating the particles in the "jets" to near light speed?

Evan
2004-Nov-30, 05:14 PM
That is a property of the infalling matter in the accretion disk, not the black hole. That matter is outside the Schwartzchild radius so no paradox occurs.

Skyfire
2004-Dec-01, 10:06 AM
It has been said that a black hole formed out of a neutron star would show the same gravity field as the neutron star, as the mass creating the black hole was the same. Is that correct?

Does anyone know the answer to this? Is it the same, or greater/lesser? It still puzzles me.

Ari Jokimaki
2004-Dec-01, 10:19 AM
A hypothetical situation - imagine a body, perhaps a large star or even a black hole suddenly appears at some point in the universe (I said it was hypothetical!). Every body interacts gravitationally with every other body in the universe.

Lately I have been wondering how mass behaves in this same hypothetical situation.

The setting for my thoughts is this (and assuming gravity travels at speed of light):



3ly 2ly
S1 SX S2

1ly

O (observer)


At t = 0 star SX appears out of nowhere.

After this the observer, who is in 1ly distance from SX, estimates mass of SX by measuring the gravitational effect SX has on the observer. Result is 0kg, because gravitational effect of SX hasn't reached the observer yet.

At t = 1 year gravitational effect of SX reaches the observer, and after that the observer gets meaningful reading for the mass of SX. Observer also tries to define SX's mass from star S1's and S2's reaction to mass of SX. This fails because gravitational effect of SX hasn't reached the stars S1 and S2 yet.

At t = 2 years gravitational effect of SX reaches star S2, and after that the observer can use S2's reaction to define the mass of SX.

At t = 3 years gravitational effect of SX reaches star S1, and after that the observer can use S1's reaction to define the mass of SX.

What I don't know is how SX reacts to masses of S1 and S2. Does SX feel the gravitational effect of S1 and S2 already at t = 0, or does SX feel them at t = 2 years and t = 3 years?

The question that follows from this: Is mass relative to the observer?

scourge
2004-Dec-01, 10:37 AM
It has been said that a black hole formed out of a neutron star would show the same gravity field as the neutron star, as the mass creating the black hole was the same. Is that correct?

Does anyone know the answer to this? Is it the same, or greater/lesser? It still puzzles me.

I'm not sure what you mean by "a black hole formed out of a neutron star," but I'll guess you mean a neutron star that acquires enough mass to collapse into a black hole...

In any case, gravity is gravity--whether it takes the form of a black hole or a neutron star, the field is still gravitational--that is, it acts according to its mass, nothing more.

The Bad Astronomer posted this helpful link with respect to black holes: Black Hole Facts (http://cosmology.berkeley.edu/Education/BHfaq.html)

Regarding the 'speed of gravity,' I find it helpful to consider the speed of light 'the speed of reality.' If the Sun were to wink out of existence, then that reality is still eight minutes away for us--put simply, it hasn't happened for us yet. I think GR tossed the idea of 'absolute time' along with 'absolute motion.'

AGN Fuel
2004-Dec-01, 11:51 AM
It has been said that a black hole formed out of a neutron star would show the same gravity field as the neutron star, as the mass creating the black hole was the same. Is that correct?

Does anyone know the answer to this? Is it the same, or greater/lesser? It still puzzles me.

The mass is the same, so the gravitational influence on a distant object is unchanged. Replace our sun with a solar mass black hole and the Earth will continue to orbit about it (albeit much darker & colder than before!)

Where your source of confusion might arise is that a solar mass black hole has a very small radius. This permits the foolish adventurer to approach much closer to the centre of the body than if it were a normal star. Gravitational force is inversely proportional to the square of the distance; tidal effects are inversely proportional to the cube of the distance. The nasty effects such as spaghettification occur at very close distance to the black hole.

Evan
2004-Dec-01, 04:37 PM
At t = 0 star SX appears out of nowhere.

After this the observer, who is in 1ly distance from SX, estimates mass of SX by measuring the gravitational effect SX has on the observer. Result is 0kg, because gravitational effect of SX hasn't reached the observer yet.


The observer cannot know of the existence of this suddenly appearing star until the light as well as the gravity reaches him. No two objects in the universe can occupy the same place at the same time. The corollary is that no objects in the universe occupy the same time in space. My time is not the same as your time. This has very significant effects on how things are percieved when relativistic effects are involved. It falls in the area of relativity theory that deals with Simultaneity (http://www.cs.sbcc.cc.ca.us/~physics/flash/relativity/Simultaniety.html)

Zjm7891
2004-Dec-01, 09:00 PM
What I don't know is how SX reacts to masses of S1 and S2. Does SX feel the gravitational effect of S1 and S2 already at t = 0, or does SX feel them at t = 2 years and t = 3 years?


From what I gander, SX will feel their effects the moment it is created as spacetime is already bent from the other planets. However, as I said, that is from what I gander. So, supposing you only feel the effects of S1, and SX your mass might be one thing, but the moment the gravitational field of S2 "hits" you your mass would change.

EDIT: Ooops!! not mass, apparent weight. sorry! I always mix those two up in my moments..

AGN Fuel
2004-Dec-01, 10:34 PM
At t = 0 star SX appears out of nowhere.

After this the observer, who is in 1ly distance from SX, estimates mass of SX by measuring the gravitational effect SX has on the observer. Result is 0kg, because gravitational effect of SX hasn't reached the observer yet.


The observer cannot know of the existence of this suddenly appearing star until the light as well as the gravity reaches him. No two objects in the universe can occupy the same place at the same time. The corollary is that no objects in the universe occupy the same time in space. My time is not the same as your time. This has very significant effects on how things are percieved when relativistic effects are involved. It falls in the area of relativity theory that deals with Simultaneity (http://www.cs.sbcc.cc.ca.us/~physics/flash/relativity/Simultaniety.html)

The interesting part of this question is the last - if an object appears ex nihilo into a gravity field of another object is it immediately affected by that object, hence apparently countering the claim of 'no instantaneous interaction'.

I think Zjm7891 is on the right track though with the solution - the spacetime at the point of the sudden appearance of SX is already shaped by the gravity of S1 & S2 - as such, SX will immediately act in response to that shape at that location. However, it will take 2 & 3 light years respectively for S1 & S2 to act in response to SX.

Evan
2004-Dec-02, 12:04 AM
but the moment the gravitational field of S2 "hits" you your mass would change.

Mass would change? How so? Mass is a property of matter. Gravity is a property of the mass of matter. Mass is not a property of gravity.


The interesting part of this question is the last - if an object appears ex nihilo into a gravity field of another object is it immediately affected by that object, hence apparently countering the claim of 'no instantaneous interaction'.

There is no instantaneous interaction there. When the mass appears it does instantly warp spacetime where it exists but that warp spreads at the speed of light (given that is the speed of gravity). Until that warp reaches another mass the space may as well be "flat".

Any two masses attracted to each other by gravity are both contributing to the shape of spacetime. The dimple in the rubber sheet analogy can only be stretched so far and then it fails.

AGN Fuel
2004-Dec-02, 04:02 AM
The interesting part of this question is the last - if an object appears ex nihilo into a gravity field of another object is it immediately affected by that object, hence apparently countering the claim of 'no instantaneous interaction'.

There is no instantaneous interaction there. When the mass appears it does instantly warp spacetime where it exists but that warp spreads at the speed of light (given that is the speed of gravity). Until that warp reaches another mass the space may as well be "flat".

Any two masses attracted to each other by gravity are both contributing to the shape of spacetime. The dimple in the rubber sheet analogy can only be stretched so far and then it fails.

I'm not disagreeing with you. :D The nuance of Ari's question though was not what effect the ex-nihilo appearance of SX will have on S1 & S2, but rather what effect S1 & S2 will have on SX and when...


What I don't know is how SX reacts to masses of S1 and S2. Does SX feel the gravitational effect of S1 and S2 already at t = 0, or does SX feel them at t = 2 years and t = 3 years?

(my emphasis)

...to which I suggested that the effect would be immediate because spacetime at (x.y,z=0) where SX 'appears' has already been shaped by S1 & S2. I am not implying instantaneous interaction with S1/S2, but rather with the local spacetime shaped (in part) by their remote influence.

Evan
2004-Dec-02, 04:28 AM
I have a feeling that this isn't a valid thought experiment as a test of facets of relativity since it postulates something that is prima facie impossible, not just an idealization of conditions.

AGN Fuel
2004-Dec-02, 05:09 AM
I have a feeling that this isn't a valid thought experiment as a test of facets of relativity since it postulates something that is prima facie impossible, not just an idealization of conditions.

Until we find the first White Hole! :lol: :lol: :lol:

Ari Jokimaki
2004-Dec-02, 07:47 AM
The observer cannot know of the existence of this suddenly appearing star until the light as well as the gravity reaches him.

#-o Yes, that was wrong.


From what I gander, SX will feel their effects the moment it is created as spacetime is already bent from the other planets.

That makes sense. I'm just wondering that if we look at this from particle physics point of view, would the answer be different since an exchange of gravitons is needed for gravitational force. But I quess that all objects constantly send gravitons all over, so gravitons from S1 and S2 would also be already present when SX appears.


Mass would change? How so? Mass is a property of matter. Gravity is a property of the mass of matter. Mass is not a property of gravity.

Perhaps we are talking about weight rather than mass (?).


I have a feeling that this isn't a valid thought experiment as a test of facets of relativity since it postulates something that is prima facie impossible, not just an idealization of conditions.

Yes, probably impossible at least in the scale I presented it, but there is some matter creation, virtual particles for example, so I think that this could be valid on a smaller scale. And while this looks bad from current theories, this thought experiment arises from Arp & Narlikar variable mass theory, where matter is created and its mass starts from zero. So I'm just probing to see if there could be a natural reason for the mass of newly created matter to start from zero and grow when it "communicates" with other masses by gravity.

Skyfire
2004-Dec-02, 10:47 AM
Yes, probably impossible at least in the scale I presented it, but there is some matter creation, virtual particles for example, so I think that this could be valid on a smaller scale. And while this looks bad from current theories, this thought experiment arises from Arp & Narlikar variable mass theory, where matter is created and its mass starts from zero. So I'm just probing to see if there could be a natural reason for the mass of newly created matter to start from zero and grow when it "communicates" with other masses by gravity.

So apparently we already know the CONCEPT is NOT an impossibility, although not on the scale we are discussing.

I am always very wary of using the word "impossible" in most circumstances, particularly in the realms of science. I would much rather use "it is not possible with the current known set of knowledge and theories". I know the concept of "white holes" don't seem to fit either, but who knows??

If we were to take our modern computers back say 100 years and show even scientific people of the time something like an immersive virtual world flight combat simulation game (EDIT: or of course the Lunar Lander simulation!!) I'm sure they would say "but that's impossible!!" However, hopefully the more open minded of 100 years ago would say, "ok, that's impossible with todays knowledge and technology, but can you try to explain to me some of the basics of how it works etc.."

If we took it back about 300 to 400 years the cry would most likely be "It's magic!" and we might get burned at the stake as warlocks and witches!

I would like to think that we are now at a science knowledge level that, should the same happen to us (or say a much more advance alien ship lands) we would look at the items and not declare them "magic", but say, "ok, we don't understand how or why this works and maybe contradicts some of our scientific theories, but we want to find out, and learn about it."

/ramble off

I must say I am surprised and pleased that so many people have responded and kept this thread going. A lot of fascinating discussion! Thanks. Keep it up!

Edited to add the comment about the Lunar Lander simulation

eburacum45
2004-Dec-02, 11:05 AM
Mass travelling through wormholes (if they existed)also would suddenly appear, as would energy emitted by white holes (if they existed);

as both are supposed to be at least theoretically possible the scenario of new mass/energy suddenly appearing is not completely inconceivable.

Evan
2004-Dec-02, 02:51 PM
as both are supposed to be at least theoretically possible the scenario of new mass/energy suddenly appearing is not completely inconceivable.


Just because we can conceive of something does NOT make it possible. Even if something actually fits within the framework of tested theory that still does not make it possible, tachyons for instance. The only thing we know for sure is that we don't know everything.

John Dlugosz
2004-Dec-02, 09:32 PM
Mass travelling through wormholes (if they existed)also would suddenly appear,

No, I don't think so. A wormhole is just a topology of space. The two objects will "feel" each other through the shorter path between them (the wormhole) before one of them goes "through" the wormhole.

ljbrs
2004-Dec-03, 02:12 AM
It has been said that a black hole formed out of a neutron star would show the same gravity field as the neutron star, as the mass creating the black hole was the same. Is that correct?

A neutron star does not have enough mass to become a black hole. If it were massive enough to be unseen there would be an event horizon beyond which it could not be viewed. We would be able to view the stuff falling toward the event horizon, but we would not be able to view the black hole. We would be unable to view the infalling "stuff" once that "stuff" were within the event horizon of the black hole. Neutron stars can be observed. However, neutron stars would most likely have an accretion disk which might obscure some of the view. Black holes cannot be seen because they would be within the "Schwarzchild Radius." We can observe the effects of black holes by measuring the speeds of objects falling toward the unseen black holes.

I do not plan to get anywhere near such astronomical/astrophysical beasts.

The Sun cannot become a black hold or even a neutron star (unless other stars were to merge with the Sun to give it enough mass to do so. The Sun is quite a distance from other stars. Perhaps it will get its chance when the Andromeda galaxy merges with the Milky Way in the distant future.

Cheers!

ljbrs :wink:

ljbrs
2004-Dec-03, 02:25 AM
I should have written "Chandrasekhar Limit" because it is the limit (an imaginary boundary which increases with increasing mass) beyond which the black hole cannot be observed. Actually, neutron stars can gain mass through stellar collisions or accretion of other matter and eventually become (unseen) black holes.

ljbrs :oops:

Andromeda321
2004-Dec-03, 03:02 AM
The Sun is quite a distance from other stars. Perhaps it will get its chance when the Andromeda galaxy merges with the Milky Way in the distant future.
The fact that two galaxies collide does not really mean the stars themselves will likely collide. It's much more likely that the sun (if it's still around then, I don't remember how many billions of years it is until the collision) will just be flung out into intergalactic space then get hit by another star.
However that is billions of years into the future. Every 200 million years the sun completes one rotation about the center of the Milky Way which means it passes through spiral arms (where there are more stars) on a fairly regular basis. Although we are in such a gap now we'll be surrounded by close(r) stars in a few million down the line. Even then I'm skeptical another star will hit the sun due to the immense distance involved. So we'll probably just have the standard white dwarf tombstone.

Evan
2004-Dec-03, 03:25 AM
ljbrs,

The Schwarzschild radius is the point where the escape velocity of a black hole just exceeds the speed of light. This is variable depending on the mass and therefore the size of the black hole. The Chandrasekhar Limit is the maximum mass that a white dwarf can attain without becoming a black hole. This is not variable and is about 1.44 solar masses.

eburacum45
2004-Dec-03, 08:23 AM
Mass travelling through wormholes (if they existed)also would suddenly appear,

No, I don't think so. A wormhole is just a topology of space. The two objects will "feel" each other through the shorter path between them (the wormhole) before one of them goes "through" the wormhole.

Yes, there will be some gravitational effects felt through the throat of the hole;
but the full effect of the mass would not be felt until the mass came out of the mouth of the hole. It would appear from a distance as if the mass of the object decreased in one location and increased in another, those locations being widely separated.
I think.

Kaptain K
2004-Dec-03, 02:09 PM
ljbrs,

The Schwarzschild radius is the point where the escape velocity of a black hole just exceeds the speed of light. This is variable depending on the mass and therefore the size of the black hole. The Chandrasekhar Limit is the maximum mass that a white dwarf can attain without becoming a black hole. This is not variable and is about 1.44 solar masses.
Emphasis added.

Shouldn't that be "...a white dwarf can attain without becoming a neutron star"?

Evan
2004-Dec-03, 02:35 PM
Kaptain K,

Your'e right. I should have said "a neutron star or a black hole."

Kaptain K
2004-Dec-03, 06:53 PM
[-X A white dwarf that edges over the Chandrasekhar limit will collapse into a neutron star only. The minimum mass to collapse into a black hole is approximately twice Chandrasekhar's limit!

pghnative
2004-Dec-03, 07:01 PM
Evan's revision is correct --- look again at his original context.

Evan
2004-Dec-03, 07:41 PM
Or a quark star, if such exists.

Skyfire
2004-Dec-05, 12:51 PM
Or a quark star, if such exists.

What's the definition of a Quark star? In fact, what's a quark? I have often heard it mentioned, but never been really sure of the definition.

Evan
2004-Dec-05, 01:04 PM
First, what is a quark? (http://www2.slac.stanford.edu/vvc/theory/quarks.html)

Second, quark stars (http://www.space.com/scienceastronomy/astronomy/new_matter_020410.html).