Quizzes & Puzzles2 mins ago
Gravitational Waves, Einstein’S Ripples In Spacetime, Spotted For First Time
Not a question, but something for anyone who's interested.
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Tremendous achievement, on a par with the discovery and subsequent use of radio waves. Einstein himself paid homage to the great Michael Faraday and James Clerk Maxwell, both brilliant British scientists, when he formulated his theory of relativity. This gives the astronomers the ability to observe previously undetected galaxies in the previously unreachable reaches of the universe.
I have a niggling doubt in my head that this is going to turn out in the same way as the last time scientists announced they had discovered gravitational waves, a couple of years ago. That turned out to be a false alarm -- this time I think I'm just being paranoid, but here's hoping the announcement stands up to further scrutiny this time. Anyway, an exciting discovery and a century-old prediction turns out to be true.
Clover, I'll try to provide a more detailed explanation later today.
Clover, I'll try to provide a more detailed explanation later today.
//In four ground-breaking lectures in November 1915, Einstein explained that gravity wasn't an instantaneous interaction between matter across infinite distance, but that gravitational interaction was mediated through the fabric of space-time, limited by the speed of light. Matter and time were no longer separate but fundamentally intertwined. Matter warps the fabric of space-time and "bends" the energy travelling through it.//
//An asymmetric gravitational system - such as the Earth and moon orbiting each other - should produce gravitational waves, but they will be so small as to be immeasurable.
So it takes a truly cataclysmic event to produce gravitational waves that could come close to being measured, such as the collision of massive black holes or neutron stars, or the formation of the universe itself.//
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There are some graphics at the bottom of this link that help to imagine gravity waves. I have mentally imagined them as precious radio waves, I am fortunate in having studied radio and light waves during many years involved in the repair of electronic appliances including microwaves.
//An asymmetric gravitational system - such as the Earth and moon orbiting each other - should produce gravitational waves, but they will be so small as to be immeasurable.
So it takes a truly cataclysmic event to produce gravitational waves that could come close to being measured, such as the collision of massive black holes or neutron stars, or the formation of the universe itself.//
http://
There are some graphics at the bottom of this link that help to imagine gravity waves. I have mentally imagined them as precious radio waves, I am fortunate in having studied radio and light waves during many years involved in the repair of electronic appliances including microwaves.
I'll answer jomifl's post first and then come back to yours clover.
"I can't quite see how they can be detected locally since if space and time are distorted simultaneously the effects would cancel."
This is where the precise experimental set-up matters. The key idea behind the experiment is that two mirrors are set up at right angles to each other. Except, very possibly, with some stupidly bad luck (ie a wavefront oriented in exactly the right way), it is almost certain that a passing wavefront will compress space in very slightly different amounts for each mirror. Coupled with the stupidly high sensitivity of the apparatus (roughly speaking the disturbances caused by most gravity waves are in the order of one part in a million million million, or less), this should allow gravity waves from most directions to be detected from all but (possibly) two directions.
"I can't quite see how they can be detected locally since if space and time are distorted simultaneously the effects would cancel."
This is where the precise experimental set-up matters. The key idea behind the experiment is that two mirrors are set up at right angles to each other. Except, very possibly, with some stupidly bad luck (ie a wavefront oriented in exactly the right way), it is almost certain that a passing wavefront will compress space in very slightly different amounts for each mirror. Coupled with the stupidly high sensitivity of the apparatus (roughly speaking the disturbances caused by most gravity waves are in the order of one part in a million million million, or less), this should allow gravity waves from most directions to be detected from all but (possibly) two directions.
I suppose that, like almost every other explanation out there, this is going to come up a little short because it's trying to explain some seriously complex mathematics without using any of that maths whatsoever. Try explaining a musical piece without being able to listen to it or using any musical terms at all. Anyway, here's my attempt to explain what's going on.
The first key point is that gravity is actually a very weak force indeed. Stupidly weak. It's only when you have a whopping great amount of matter in one place that you start to notice it at all. This makes it anyway an incredibly hard force to study and understand, because its true nature is only really understandable in the most extreme conditions in the universe. The event that's been reported as observed in this experiment was two black holes, each with a mass about 30 times that of the sun, colliding and tearing each other apart, spinning really fast and finally merging in around a fifth of a second, before spitting out an energy equivalent to the mass of three Suns that sent ripples propagating through the universe at the speed of light, over a billion years ago. And the effect measured was something like a hair's width over the orbit of Neptune. Even if you don't quite understand what the effect is itself, the idea of being able to detect something that precise is in itself stupidly impressive. But anyway.
The picture to have in mind with gravity is the usual one thrown in at this point: space and time are a rubber sheet, and gravity is the effect caused when a mass rests on that sheet and distorts it. Far away from the mass spacetime is flat -- nearby, things are curved. The heavier the mass, the greater the curvature, etc etc. This analogy is really quite useful, particularly if you get a chance to set it up (eg use lots of fish-netting stretched out over some metal frame, put a basketball in the middle, and start trying to send ping-pong balls in straight lines; the paths these balls take are not going to be that far off from the paths a comet moving near to the sun would take).
The really interesting stuff happens when you start playing around with the large mass in the middle. Get it to move, rotate, orbit another large object really fast, spiral inwards. The shape of spacetime in this case starts to be affected in much more dramatic ways, essentially because it's no longer a static shape but one that will change over time. This seems intuitive enough, I think -- the object is moving, and if it's curving spacetime then the curves will move along with it in some sense. If in addition the motion is dramatic enough and rhythmic enough, then the resulting changes in shape will pulse and take on the form of waves, rippling through spacetime in all directions. As any such wave passes a given point, the shape of space (and the passage of time) would, briefly, be distorted. Space would stretch or squash, time would speed up a fraction and then slow down a fraction -- and then it would be back to normal an instant later (unless the source of the gravitational waves is long-lasting, in which case this would happen several times in a row).
Again, though, all of these effects are only really noticeable at the most extreme scales. I think it's probably true that, in principle, you can make gravitational waves yourself (by, eg, spinning around on a chair), but the characteristic size of the resulting waves would be probably equivalent not to a hair in the solar system, but to (at most) an atomic nucleus in the visible universe. These really are tiny effects. It is little wonder, then, that Einstein, when he predicted the existence of waves, noted despondently that he would not expect to see them in any experiments. I'm sure he would be thrilled to have been wrong about that.
I've not read this explanation back yet -- I hope it's fairly clear, but if not or if you have any other questions let me know.
The first key point is that gravity is actually a very weak force indeed. Stupidly weak. It's only when you have a whopping great amount of matter in one place that you start to notice it at all. This makes it anyway an incredibly hard force to study and understand, because its true nature is only really understandable in the most extreme conditions in the universe. The event that's been reported as observed in this experiment was two black holes, each with a mass about 30 times that of the sun, colliding and tearing each other apart, spinning really fast and finally merging in around a fifth of a second, before spitting out an energy equivalent to the mass of three Suns that sent ripples propagating through the universe at the speed of light, over a billion years ago. And the effect measured was something like a hair's width over the orbit of Neptune. Even if you don't quite understand what the effect is itself, the idea of being able to detect something that precise is in itself stupidly impressive. But anyway.
The picture to have in mind with gravity is the usual one thrown in at this point: space and time are a rubber sheet, and gravity is the effect caused when a mass rests on that sheet and distorts it. Far away from the mass spacetime is flat -- nearby, things are curved. The heavier the mass, the greater the curvature, etc etc. This analogy is really quite useful, particularly if you get a chance to set it up (eg use lots of fish-netting stretched out over some metal frame, put a basketball in the middle, and start trying to send ping-pong balls in straight lines; the paths these balls take are not going to be that far off from the paths a comet moving near to the sun would take).
The really interesting stuff happens when you start playing around with the large mass in the middle. Get it to move, rotate, orbit another large object really fast, spiral inwards. The shape of spacetime in this case starts to be affected in much more dramatic ways, essentially because it's no longer a static shape but one that will change over time. This seems intuitive enough, I think -- the object is moving, and if it's curving spacetime then the curves will move along with it in some sense. If in addition the motion is dramatic enough and rhythmic enough, then the resulting changes in shape will pulse and take on the form of waves, rippling through spacetime in all directions. As any such wave passes a given point, the shape of space (and the passage of time) would, briefly, be distorted. Space would stretch or squash, time would speed up a fraction and then slow down a fraction -- and then it would be back to normal an instant later (unless the source of the gravitational waves is long-lasting, in which case this would happen several times in a row).
Again, though, all of these effects are only really noticeable at the most extreme scales. I think it's probably true that, in principle, you can make gravitational waves yourself (by, eg, spinning around on a chair), but the characteristic size of the resulting waves would be probably equivalent not to a hair in the solar system, but to (at most) an atomic nucleus in the visible universe. These really are tiny effects. It is little wonder, then, that Einstein, when he predicted the existence of waves, noted despondently that he would not expect to see them in any experiments. I'm sure he would be thrilled to have been wrong about that.
I've not read this explanation back yet -- I hope it's fairly clear, but if not or if you have any other questions let me know.
Just a daft thought jim before I go out for a while. We have now been able to detect 'gravity waves'. Now radio waves were there before we 'discovered'' them. As were light waves etc. etc. We have learned to generate, or capture radio waves, and subsequently amplify and re transmit them. Likewise light waves. I wonder if we will ever have the ability to generate, amplify, or emit gravity waves for the benefit of mankind. The alternative of course to harnessing such a force for aggressive means is truly scary. (I love my sci/fi stories too)
I suppose it's not totally impossible but again the point about the scale difference of these things rules it out pretty much definitively, certainly within our lifetimes. Radio waves, etc, emerge from electric circuits and the characteristic scale of electromagnetic forces is many billions of billions of times stronger than gravity. Like I tried to hint at in my post, the amount of energy dumped into this particular gravity wave was equivalent to completely destroying our Sun three times over. Even if you gathered all the matter in our Solar system and the one next to it I still don't think you'd have enough matter (that you'd then also have to blow up perfectly) in order to disturb spacetime enough to emit a wave.
So basically no, to all intents and purposes harnessing gravity waves ourselves is completely impossible and will remain so for long into the future.
So basically no, to all intents and purposes harnessing gravity waves ourselves is completely impossible and will remain so for long into the future.
Jim, //harnessing gravity waves ourselves is completely impossible and will remain so for long into the future.//
I ‘m not sure we should assume that. Science does tend to move in astonishing leaps and bounds. The past hundred or so years proves that. I hijacked this from agchristie on another thread, I hope she doesn’t mind.
//Years ago, I can imagine Einstein was asked what planet he was on!//
I think there's something in that.
I ‘m not sure we should assume that. Science does tend to move in astonishing leaps and bounds. The past hundred or so years proves that. I hijacked this from agchristie on another thread, I hope she doesn’t mind.
//Years ago, I can imagine Einstein was asked what planet he was on!//
I think there's something in that.
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