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Would space-time get disturbed like the wake and bow wave from a boat, if something massive like a planet or star was suddenly accelerated (instant velocity even?) or oscillate like the air around an implosion if said planet/star was made to disappear?
Order of Kilopi
It would cause gravity waves.
Member
I believe the current understanding is that black holes might spin or twist space time as well as curve it. If space-time was also twisting, would it continue to twist and gradually stop twisting in the event that the gravity source suddenly disappeared?
Order of Kilopi
Some how I suspect the "snap" from something like a BH disappearing would have to do something interesting to the surrounding space time other then just have it start mimicking flat space/time. What I do not know.
Order of Kilopi
I think the problem with questions like that is that it would violate the conservation of matter, so you are asking a hypothetical question about something that cannot happen in the first place.Originally Posted by Grahamshortuk
As above, so below
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Obviously, but then again many thought experiments postulate impossible scenarios such as riding on a photon. Its just an attempt to explore and understand the nature of something. In this case; The Space-Time / Gravity / Mass relationship. I dont believe questions like this actually HAVE a problem. On the contrary, they are useful.
Order of Kilopi
Well, one problem is the second of your questions, in the OP ("if said planet/star was made to disappear?")
Disappearance can't happen in GR, so your question is meaningless.
Alternatively, you need a quite different theory (of space, time, gravity, etc) in which that can happen, in order to get a sensible answer. I, myself, do not know of any such theory.
Has the first of your questions been answered, to your satisfaction? If not, I'll have a go at providing an answer.
Banned
A long long time ago a far greater mind than mine said something that still rings as true...
'You can not creat or destroy matter. You can only change its form.' Energy can become matter and the reverse may also apply.
Now in your question we want to know if a object of mass could move away at greater than c. Or just ceas to exist.. and NO.
If you make some thing vanish... where is it ? If its still within this universe then its not vanished. There is no provision for leaving.
So the answer must be NO. The matter of the object might change. Become heat and energy. Whatever, it just can not vanish.
Established Member
Honestly, do folks not recognise a thought experiment when they see one? It doesn't matter if something being asked about can or can't really happen, the point of framing the question is to try to figure out what would happen if it was possible. It involves things like extrapolation from known facts/theories, educated guesswork, and imagination, which are all very useful for figuring out science.
I recall reading a book about Gravity Waves years ago from the university library (I forget the name of the book unfortunately. I think it was "Understanding Gravity Waves"?) and remember it saying that if the sun was to suddenly vanish then the consequences would propagate out at the speed of light. So we'd carry on orbiting around the Sun as if it was there for about 8 minutes (the speed of light delay between Earth and Sun) after it disappeared, and then the earth would fly off at a tangent from our orbit since we'd "know" that the central mass wasn't there anymore.
Order of Kilopi
i love the internet sometimes.. someone asks a question about something that is impossible- and sometimes states that they know it to be impossible.
most of the responses are usually along the lines of "that's impossible so i won't answer it"..
whatever happened to "there are no stupid questions"?
Established Member
" What would happen to space-time if something huge suddenly accelerated/disappeared?
Would space-time get disturbed like the wake and bow wave from a boat, if something massive like a planet or star was suddenly accelerated (instant velocity even?) or oscillate like the air around an implosion if said planet/star was made to disappear?"
EDG gave the best answer. Since we have no idea what could cause a disappearance, the Earth (and other planets etc) going off at a tangent would occur. I suppose oscillate and gravity waves would also result, near where the disappearance occurred, but I would expect the disturbance to decrease as the square of the distance at locals outside the present boundary of our solar system. An Earth size planet disappearing would have about 330,000 times less impact than one solar mass disappearing. Neil
Order of Kilopi
This is all well and good.
However, when it comes to "if said planet/star was made to disappear?", how would you go about answering it?
You could, perhaps, ask whether such a thing is possible, within GR; specifically, how would you model it in the EFE? If you, or anyone, can find a way to answer this, go ahead, please do so. If you can't, then say so.
Or you could ask the question within the framework of a different theory of gravity, perhaps Newtonian.
But here's the key thing: you cannot ask such a question in a theory-free way! Why not?
If this is merely an illustration of the fact that 'gravity' propagates at c (in GR), then fine.I recall reading a book about Gravity Waves years ago from the university library (I forget the name of the book unfortunately. I think it was "Understanding Gravity Waves"?) and remember it saying that if the sun was to suddenly vanish then the consequences would propagate out at the speed of light. So we'd carry on orbiting around the Sun as if it was there for about 8 minutes (the speed of light delay between Earth and Sun) after it disappeared, and then the earth would fly off at a tangent from our orbit since we'd "know" that the central mass wasn't there anymore.
You could, of course, make the Sun 'disappear' by somehow converting it to photons, which would stream away at c. But the photons are mass-energy, just like the Sun is (or was)! So their gravitational effect would remain.
Order of Kilopi
(bold added)
But why do you say this? What is the basis for your - what seems to me to be - pure speculation?
Order of Kilopi
I think you have it wrong; "that's impossible so i can't answer it" is more appropriate, don't you think?
It's not a stupid question; merely one that cannot be answered (within GR; it's meaningless), or can only be answered within a different theory of gravity.whatever happened to "there are no stupid questions"?
There are plenty of unanswerable questions; think of the famous "Do colourless green dreams sleep furiously?" or "Why is a raven like a writing desk?"
Established Member
That's not the point of the question - the point is "if it disappears, what would happen", not "how could we make a planet/star disappear instantaneously"? The hows and whys of it are not the issue. Again, that's why it's a thought experiment.
Do you ask how you're supposed to make a device consisting of a single radioactive atom with a 50% chance of decaying that triggers the cyanide capsule in the box containing Schrodinger's Cat? If everyone did that then we'd be hung up on the mechanics of the box instead of the actual issue that thought experiment raises. It spectacularly misses the point of the exercise.
Established Member
"Impossibility" has nothing whatsoever to do with anyone's ability to answer a question. More to the point, thinking about this specific question raises issues such as the "speed of gravity", which otherwise one may not think about if one only allowed oneself to consider questions about things that were physically possible or realistic (in our current understanding).
Order of Kilopi
I agree it's thought, but it most definitely is not a thought experiment.
The point of a thought experiment (Gedankenexperiment) is to test an idea, within the framework of some model, or theory.
In this case, if you can find a way, using the EFE (or any other GR equations) to make "a planet/star disappear instantaneously", you're doing a Gedankenexperiment. If you can't, then it's not.
Quite the opposite.Do you ask how you're supposed to make a device consisting of a single radioactive atom with a 50% chance of decaying that triggers the cyanide capsule in the box containing Schrodinger's Cat? If everyone did that then we'd be hung up on the mechanics of the box instead of the actual issue that thought experiment raises. It spectacularly misses the point of the exercise.
This particular thought experiment is based on, and depends totally on, the relevant quantum mechanics (theory, or model). Remove quantum mechanics, and you don't have a thought experiment.
The analogy you are making is quite misleading - you can, in principle, build a box, and a trigger mechanism; you cannot, even in principle, make "a planet/star disappear instantaneously".
Order of Kilopi
If [impossible hypothetical] then [unpredictable result].
Plug in your favorites. It matters not.
Order of Kilopi
Still missing the point.
Perhaps an analogy might help.
Here in this section of BAUT you'll find a great many questions about things like FTL, and what the universe might look like if we could 'see' everything instantly. There are also quite a few on 'gravitons'.
FTL questions can be easily discussed, because the framework within which these make sense - SR - is well-defined.
However, questions about any 'wizard's eye' perspectives on the universe are really only answerable if you use something like an absolute, Newtonian, model*.
And graviton questions can only be answered by entering huge caveats - things like 'what if the gravitons emitted by the Earth were destroyed before they could get to the Sun' are unanswerable (in any meaningful, scientific sense).
Now of course one can speculate wildly ... but such speculation cannot, by any stretch of the imagination, be called scientific.
* excepting, of course, drawing charts with 'world lines', etc.
Order of Kilopi
I'm sort of going over some ground that nereid has covered a lot, but I was the one who brought it up, so I should answer. I'm not trying to say that the question is stupid. I'm just saying that you have to be careful when making thought experiments that are not possible in principle. Because you can sort of get into this mindset of, OK, if this happens, then this weirdness happens, and therefore there is something wrong with GR or quantum mechanics or what not. But the "problem" can be based on something that can't happen anyway. The idea that EDG brought up with the cat is different, because as Nereid said, you can in principle make such a device. If you can make a planet disappear magically, then all bets are off and we no longer really understand anything about the universe.
As above, so below
Established Member
Hi Nereid: Perhaps it can't be answered, but I perhaps foolishly answered the questions. Oscillations and shock waves are the usual result of sudden changes. Can you think of a sudden change that does not produce shock waves and oscillations? Do sudden mass movements produce gravity waves? Neil
Order of Kilopi
Sure, the annihilation of an electron-positron pair produces pair radiation - are two gamma ray photons equivalent to "shock waves and oscillations"?
When you cause a fly to die, by spraying it with some insecticide, does the 'sudden change' - the fly's biological demise - "produce shock waves and oscillations"?
Established Member
What would happen if frogs had wings ? They wouldn't bump their butts.
Banned
The "thought experiment" proposed is not so much "thought" as it is an imagination experiment. It imagines that the "pull" gravity has on space/objects can be cut like the tension on a rubber band to snap the band to a lower tension with comensurate waves. The problem with calling this a "thought experiment" is that we don't have any reason to think, that gravity has an existance seperate from mass. The model that we think about has gravity intrinsic to mass.
The only answer I can think of that even comes close to answering the OP would be to say that to "disappear" the mass you must disappear it for all of its existance disappearing all its gravity for all of its existance too. The result being that the universe evolves differently, the trajectories of all the masses in it move to places absent the mass other then they did with the mass.
Established Member
The way I see it, you can try to answer the question and actually be helpful to the OP, or you can threadcrap and be unhelpful to the OP - Neil and I are at least attempting to answer the question based on what is known about physics and gravity, which is most likely what the OP is after. Nitpick over semantics all you like, but you're not helping the OP understand anything relevant. I'm sure the OP realises that mass doesn't just appear or disappear like that as it is, but it doesn't mean the question isn't an interesting one to consider. If the question offends you that much, why be all snooty about it and be all sarcastic at the OP about it? Nobody's forcing you to answer it, after all. Sometimes I wonder if people are just being deliberately obtuse, obstructive and unconstructive here.
Just to satisfy those other people though, one could say "Ok, imagine the sun suddenly got pulled through a wormhole that spontaneously appeared, sent it many lightyears away, and then disappeared". Bam, the Sun is gone somewhere else, see? Does that really add anything to the exercise? No, it doesnt. Though I'm sure they'll just nitpick about how wormholes aren't possible, or don't work like that, or something else, and again you won't be helping the OP understand anything relevant to what he's asking. Again, missing the point. Think of it as "suspending your disbelief" if it makes you feel any better.
Last edited by EDG; 2010-Jun-11 at 04:58 AM.
Established Member
Nonsense. Just because you make a planet disappear, that doesn't have to change anything else about how we understand the universe for the purpose of the exercise. Teleporting a planet away for these purposes doesn't suddenly make every other law of physics bunk or broken - claiming so is just intellectual laziness.
Established Member
Wrong.
There is indeed such a thing as a nonsensical question.
Learning to ask sensible questions is valuable.
A question involving gross failure to conserve mass-energy is nonsensical. This violation of basic physics is not very subtle.
Established Member
yes , based on the size of the object and within the parameters of direction
Established Member
I tend to agree that there are question that are meaningless in a given context. It is not necessarily limited to questions about breaking laws of physics. One commonly mentioned is "What do you find when you go north of the North pole?". The previous poster's nic is incidental
The dog, the dog, he's at it again!
Established Member
look the reason that both space and time become space-time is because first , time is based on the speed of light
the cleaner the space ( devoid of mass ) , light speed holds
the more mass the more light bends or slows down
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