Brace yourselves for gravitational waves

[Excited Penguin]

I just couldn't stop smiling while watching this
https://www.youtube.com/watch?v=DoOPEPVYAnU


Here's also an article I just found that I thought put things in perspective really well.
http://www.forbes.com/sites/startswithab...490e954b15

And what we’ve seen, for the first time, is not just one of the greatest predictions of Einstein’s General Relativity, although we did just verify that. And it isn’t just that we took our first step into the world of gravitational wave astronomy, although LIGO will doubtlessly start seeing more of these signals over the coming years; this is as exciting for astronomy as Galileo’s invention of the telescope, as we’re seeing the Universe in a new way for the first time. But the biggest news of all is that we’ve just detected two merging black holes for the first time, tested their physics, found a tremendous agreement with Einstein, and seen evidence that this happens over a billion light years away across the Universe.

[Alex K]

Yes, that is what I alluded to earlier, it is an impressive confirmation of Einstein's field equations under extreme conditions.

To put the discovery into context one has to note that we already had very strong indirect evidence for many years that gravitational waves roughly worked the way Einstein predicted, namely from the Hulse-Taylor Pulsar measurements. This is a binary system which speeds up because it loses energy via gravitational waves and the two neutron stars approach each other. The change in frequency fits exactly the prediction from general relativity for how much energy should be radiated by a pair of neutron stars.

[vorlon13]

So, to clarify, gravitational radiation is conveyed and is also entirely "composed' of distortions in space-time ?

We don't need or expect an actual particle for this ?

And a follow up, this gravitational energy is radiating away from the merger site, and is being subsumed into the existing universal flux of gravitational energy, and that is mostly a relic of the formation of the universe when literally the entire mass of the universe was rending space-time in the same way, just at a larger and more violent scale.

And, the LIGO devices only see (to borrow a term from electricity) the AC component of the flux, the 'DC' component of the universal background radiation is unknown in magnitude, and it's 'density' (terminology?) is steadily decreasing in proportion to expanding volume of the universe ? And do we know the average 'density' of that DC flux, and if not, could it be large enough to be (pick one) dark matter, dark energy, 'sumtin else', or E. all of the above ?

[Brian37]

I just couldn't stop smiling while watching this
https://www.youtube.com/watch?v=DoOPEPVYAnU


Here's also an article I just found that I thought put things in perspective really well.
http://www.forbes.com/sites/startswithab...490e954b15

And what we’ve seen, for the first time, is not just one of the greatest predictions of Einstein’s General Relativity, although we did just verify that. And it isn’t just that we took our first step into the world of gravitational wave astronomy, although LIGO will doubtlessly start seeing more of these signals over the coming years; this is as exciting for astronomy as Galileo’s invention of the telescope, as we’re seeing the Universe in a new way for the first time. But the biggest news of all is that we’ve just detected two merging black holes for the first time, tested their physics, found a tremendous agreement with Einstein, and seen evidence that this happens over a billion light years away across the Universe.

I love Neil, he really makes science fun to listen to.

[vorlon13]

Easy on the eyes too . . .

[Alex K]

Easy on the eyes too . . .

And real tall, I heard

[Alex K]

@Vorlon, let me get a bit technical to answer your question, if it is unintelligible I wil elaborate.

Yes, so in general relativity, the thing that carries the dynamics and contains the degrees of freedom of spacetime, is the so called metric tensor - a matrix of 10 numbers which tell us how far away two neighboring points in coordinate space are from each other physically. In approximately flat space and using standard cartesian coordinated, this metric tensor has just four components which boil down to telling you how to measure time and space distances, and all the other numbers are zero in flat space

It is these entries of the metric which get modified by masses and energy and thus mediate gravitation and also determine the movement of objects, and they are also the thing which the Einstein field equations determine. If you start with the metric for flat space, and then consider small deviations from that and plug that into the Einstein field equations, you notice that these small deviations adhere to something like a complicated version of Maxwell's equation, and hence have wave solutions , sourced by changing mass and energy distributions.

[Excited Penguin]

@Vorlon, let me get a bit technical to answer your question, if it is unintelligible I wil elaborate.

Yes, so in general relativity, the thing that carries the dynamics and contains the degrees of freedom of spacetime, is the so called metric tensor - a matrix of 10 numbers which tell us how far away two neighboring points in coordinate space are from each other physically. In approximately flat space and using standard cartesian coordinated, this metric tensor has just four components which boil down to telling you how to measure time and space distances.

It is these entries of the metric which get modified by masses and energy and thus mediate gravitation and also determine the movement of objects, and they are also the thing which the Einstein field equations determine. If you start with the metric for flat space,  and then consider small deviations from that and plug that into the Einstein field equations, you notice that these small deviations adhere to something like a complicated version of Maxwell's equation, and hence have wave solutions , sourced by changing mass and energy distributions.

I think I became a little smarter just reading this.

[Alex K]

So if you now take the *deviations* from the flat space metric tensor, and treat these numbers as a field akin to the electromagnetic field, the rules of quantum theory suggest that for any given frequency, there is a smallest deviation from flat space which corresponds to a massless bosonic particle -the graviton -with spin 2, just as from the EM field we get photons. If you plug this into the usual formalism to obtain a kind of more complicated QED, the leading resulting Feynman graphs give you the usual gravitational attraction via exchange of gravitons, in analogy to the em force from feyman graphs with photons. The trouble only starts when one has to consider how the gravitons interact with each other at very high energies...

In this case one naively gets results like the probability of radiating off one more graviton always being higher , which would lead to probabilities larger than 1, and so the notion of individual particles with well-defined interactions breaks down. Suddenly, an infinite number of free parameters would be needed to specify all the possible ways in which 2,3,4.... gravitons interact, and this kills predictivity

[Excited Penguin]

Just one thing I couldn't wrap my head around at all. What are these deviations you keep referring to, exactly?