Here's an update on the Black Hole Merger modelling:

www.space.com/scienceastronomy/060418_bhole_merger.html

By the way I don't agree with the idea of gravitational waves or particles, they have not been found because they don't exist. Gravity is merely a distortion of spacetime caused by matter, it is not a force in the true sense. I may have to eat these words, but I think I'll be waiting some time.

Here's an update on the Black Hole Merger modelling:

www.space.com/scienceastronomy/060418_bhole_merger.html

By the way I don't agree with the idea of gravitational waves or particles, they have not been found because they don't exist. Gravity is merely a distortion of spacetime caused by matter, it is not a force in the true sense. I may have to eat these words, but I think I'll be waiting some time.

I'm afraid you'll have to eat those words. Although there are no DIRECT observations of gravitationsal waves there is direct evidence of gravitational energy being radiated away. That radiation is by definition a gravitational wave and as we all know from QM waves can be considered particles and vica-versa.

You said that Gravity is a distortion of spacetime, that is according to Einstein, hence you are saying you believe in relativity. Are you aware that according to relativiy gravitational waves HAVE to exist? Your view is not consistent as far as I can see. If we stick with Einstein's view of the universe then gravity waves are ripples in spacetime eminating from moving spacetime distortions caused by massive objects. The classic example is blackholes orbiting each other, they are very ,assive so they make a huge distortion in spacetime and as they move they send ripples out through spacetime, those are Gravitational waves.

Basiclly, if you believe Einstein you have to belive in Gravity waves and if you look at the results from current research the evidence is very strongly in favour of Einstein and gravity waves.

Bart.

Here's an update on the Black Hole Merger modelling:

www.space.com/scienceastronomy/060418_bhole_merger.html

By the way I don't agree with the idea of gravitational waves or particles, they have not been found because they don't exist. Gravity is merely a distortion of spacetime caused by matter, it is not a force in the true sense. I may have to eat these words, but I think I'll be waiting some time.

I'm afraid you'll have to eat those words. Although there are no DIRECT observations of gravitationsal waves there is direct evidence of gravitational energy being radiated away. That radiation is by definition a gravitational wave and as we all know from QM waves can be considered particles and vica-versa.

You said that Gravity is a distortion of spacetime, that is according to Einstein, hence you are saying you believe in relativity. Are you aware that according to relativiy gravitational waves HAVE to exist? Your view is not consistent as far as I can see. If we stick with Einstein's view of the universe then gravity waves are ripples in spacetime eminating from moving spacetime distortions caused by massive objects. The classic example is blackholes orbiting each other, they are very ,assive so they make a huge distortion in spacetime and as they move they send ripples out through spacetime, those are Gravitational waves.

Basiclly, if you believe Einstein you have to belive in Gravity waves and if you look at the results from current research the evidence is very strongly in favour of Einstein and gravity waves.

Bart.

what no crystaline spheres? .
stephen.

By the way I don't agree with the idea of gravitational waves or particles, they have not been found because they don't exist. Gravity is merely a distortion of spacetime caused by matter, it is not a force in the true sense. I may have to eat these words, but I think I'll be waiting some time.

I'll agree with you about it not being a force.

I guess you mean gravity waves dont exist in the same way that say electromagnetic waves do. But I think what people are trying to detect as gravity waves are ripples in spacetime. Consider a binary star. As these stars rotate around each other their distortion on spacetime moves, and in effect creates a little ripple. Such a ripple should be detectable. It may be helpful to think of this as two posts rotating around each other on a pond; or the more conventional spacetime example of two balls rotating around each other on a rubber sheet.

As for the graviton, I dont think it exists either. The only people who seem to be convinced that it does exist are the string theorists. In fact one of the reasons that there is a graviton hunt at all is that the standard model doesnt need it but string theory predicts it. Which is rare case of string theory actually predicting something different to other theories that is testable. If you dont use string theory you don't need the graviton. The standard model is happy to see gravity as a feature of spacetime so no particle required.

Cheers,
~Al

Albert,

Couldn't have put it better myself. It's all in the mind, for too long classical physics mentality has ruled leading to misconceptions like these. I hate it when the term gravity is used. There is no such thing as gravity.
'Spacetime waves' or ripples would be a more correct term to use instead of gravity waves. Even then, I'm not sure how wavelike, these 'waves' would be, or if they would even propogate. Certainly two objects rotating about each other will cause variable distortions in spacetime in their vicinity, but does the distortion propogate like a wave? I don't think so. I believe that if spacetime waves are eventually monitored we will see that they are localised, and do not propogate beyond a sphere of influence - a kind of standing wave, for want of a better term.
Anyway it's time that physics classes in school started teaching basic relativity. Once the classical idea of gravity has set in it can be too late to really understand what Einstein was on about. I'm sure he'd turn in his grave if he knew that gravity myths were still being perpetrated by leading physicists.

Bart, sorry I didn't explain myself better first time - chew chew chew - nope still don't like it.

OK, I'm still confused here. Gravity IS a force. It can do work, hence it is, by definition, a force. The fact that this force is a result of space-time curvature does not chance the fact that it is a force!

There is nothing wrong with the classical representation of gravity. Clasical physics says nothing about the cause of gravity, just about the effect of gravity. Namely that the force of gravity follows and inverse square law and that is correct.

Classical physics describes what gravity does, Einstein gave us an insight into how it does it, there is no conflict here.

As for whetehr or not gravity waves propagate, I'm not qualified to give a proffesional opinion on the matter but I have to wonder why they would not. The effects of gravity are not local, they are massively non-local working on the giga-parsec scale and above so why should the ripples be local? I may be wrong but I'm pretty sure Einstein believed in gravity waves because they do seem to be a direct prediction of his theories. Those theories have stood up well to verification so far so why assume it will be different for gravitational waves?

Anyhow, we are a lucky generation, there is every chance that we will find out one way or another within the next two decates whether or not there are space-time ripples passing through us on a regular basis. There are very sensative instruments in the making now and if, 20 years from now, those instruments have not found the gravity waves theorists are expecting then science will have a major question to ask if itself, if they are detected then we will know they are real. Either way, should be fun!

Bart.

Bart, I think we are in compete agreement on gravity. You are just looking more from a newtonian perspective, well with newtonian language anyway, while I'm looking more from GR. Trying to mix both leads to headaches as I've found out many times! (eg last week I convinced myself that black holes have NO gravitational effect, and I still havent sorted it out fully in my head!)

OK, I'm still confused here. Gravity IS a force. It can do work, hence it is, by definition, a force.

Not according to GR. A planet orbiting a star is taking the straight path as far as it is concerned. There is no force acting on it, its going on a straight line through warped space. If there was a force it would either act to push the planet out of orbit or pull it towards the star.

The fact that this force is a result of space-time curvature does not chance the fact that it is a force!

Oh yes it does! (cue panto!). Its not a result of space time curvature, it is space time curvature. The object always moves in a straight line unless a force acts on it. Its a subtle but important bit of Einsteins argument.

There is nothing wrong with the classical representation of gravity. Clasical physics says nothing about the cause of gravity, just about the effect of gravity. Namely that the force of gravity follows and inverse square law and that is correct.

Perfectly true.

Classical physics describes what gravity does, Einstein gave us an insight into how it does it, there is no conflict here.

GR does not fit into newtonian physics that way though. Einstein gave us GR with its spacetime ideas which have gravity built in, no force needed. Thats why he was the first to say that gravity is not a force.

As for whetehr or not gravity waves propagate, I'm not qualified to give a proffesional opinion on the matter but I have to wonder why they would not. The effects of gravity are not local, they are massively non-local working on the giga-parsec scale and above so why should the ripples be local? I may be wrong but I'm pretty sure Einstein believed in gravity waves because they do seem to be a direct prediction of his theories. Those theories have stood up well to verification so far so why assume it will be different for gravitational waves?

The gravity ripples of binary stars or black holes are indeed predicted by GR. Though detecting them will be tricky. Gravity moves at the speed of light[1] so the fluctuations will be quick. Secondly the ripple looses intensity in the order of the square of the distance so by the time it gets to us it will be very hard to detect because the effect will be small. To put some numbers on this the LIGO observatory which is designed to detect these gravity waves has two 4km tunnels which lightbeames get fired down and reflected back to an interferometer. They are expecting to detect gravity waves that distort the lengths of a path by 10^-18 m. Yep, 10 to the minus eighteen of a meter over 4km. And they are looking for big events, like the merger of black holes. IIRC they estimated a 20% chance of detecting a single event by 2010, and since they havent found anything yet want to build a more sensitive detector.

Cheers,
~Al

[1] Trust me ok! You know the rubber mat model of the solar system with the sun being a heavy ball in the middle warping the space around it? Well if the sun disppeared the rubber mat would flatten out from the center at the speed of light. In fact as the space flattened around the plaets they would go off in tangents according to newton; or in GR terms they would continue to go in a straight line as they had been doing anyway!

We're not going to agree on 'Gravity' here and we're certainly not going to solve the 'gravity problem'. Suffice to say that it still has science baffled - why should matter influence spacetime in the first place? The classical model has already been shown to be not quite the whole picture. Einsteins model will not be the whole picture either, I've read much on this subject, including Loop Quantum Gravity, all the flavours of String Theory, M Theory and Supersymmetry and none have yet been shown to be fully correct. One of the latest theories which also addresses dark matter claims that gravity leaks into our universe from other "parallel" universes, and this does away with the need for dark matter - personally I really think this one is a load of B"£$%^.

You can think of gravity as a force if that makes you feel more comfortable, to all intents and purposes it acts like a force, and we'll all still use the inverse square law because it works, but it isn't really a force so there is no point in looking for Gravitons, and gravity waves when you need to be looking for fluctuations in the curvature of spacetime. It's a subtle difference but once you get used to not thinking of gravity in the classical sense it opens up whole new ways of thinking.

When you say it does work what do you mean? You have an object moving through spacetime, it encounters a highly curved region of spacetime, it carries on moving in a straight line, however the line is now curved, so it has apparently moved in a different direction. No force has been applied.

You learn something new everyday, the planets are going in straight lines.
Its a perspective I hadnt thought of.

You learn something new everyday, the planets are going in straight lines.
Its a perspective I hadnt thought of.

HaHa!
Their going in straight lines through curved spacetime.

exactly

This has been an interesting discussion.
Albert - the force is strong in you, and yet we still have not met.
Sorry did I really just type that?
See you at the next SDAS and hopefully we can finally meet or if you're at the IAS on Monday...

This is certainly one of those threads that really stretches the mind .... imagining 4 dimensional straight-lines that are curved makes one's head a little sore!

Looking at orbits as being planets moving is straight lines is cool and a nice way to look at the universe. However, that does not work for all gravitational effects. A swininging pendulum swings because gravity exerts a force on the weight at the end of the pendulum pulling it down. How can you re-explain Galileo's simple observations without reffering to Gravity as a force?

Bart.

Came across this on a google search.

That General Relativity predicted gravitational waves wasn't seen at first.
In fact it wasn't proven to physicist level rigour until the 1940s.
(and it wasn't until the 60s that the proof was mathematically consistent)

If you keep within the context of Newtonian Physics then gravity is a force as is Electromagnetism.
However when you move into GR it is not.

As has been said, objects follow straight lines in spacetime. The reason this leads to orbits is to do with GR itself.

The straight line objects follow isn't a path through space, rather it is the object's timeline. When in the presence of a large mass near by time lines will tend to wrap around the massive object's timeline.

In a sense, the Earth "exists" around the Sun. What is a tick of the clock on Earth corresponds to moving around the Sun to an outside observer.

All currently observed effects of gravity can be explained by General Relativity. Even mundane stuff such as swinging water in a bucket wasn't fully explained until GR.
A pendulum swings the way it does because its timeline is directed toward the Earth and away from the Earth, over and over again. From an observer stationary with respect to the Earth, who cannot envision time as an extended dimension, this looks like the pendulum swings up and down in 3-space.

In relation to this discussion, there are actually two types of spacetime curvature. The first kind gives rise to gravity, the second is what causes gravitational waves.

The standard model is happy to see gravity as a feature of spacetime so no particle required.

Which Standard Model are you refering to?
The Standard model of Particle Physics or Cosmology.

The standard model is happy to see gravity as a feature of spacetime so no particle required.

Which Standard Model are you refering to?
The Standard model of Particle Physics or Cosmology.

Sorry, Particle Physics.

In relation to this discussion, there are actually two types of spacetime curvature. The first kind gives rise to gravity, the second is what causes gravitational waves.

Can you elaborate on that a bit?

Can you elaborate on that a bit?

Gravitational waves are caused by Weyl Curvature and Gravitation is caused by Ricci Curvature.

Einstein's General Theory shows how matter can generate Ricci Curvature, but not Weyl Curvature.
However under the right conditions Ricci Curvature can generate Weyl, which is how Black Holes create gravitational waves.

The interesting thing is that in the Universe's past the curvature was completely Weyl and as time progressed Ricci Curvature increased.
The progression from Weyl to Ricci is basically an entropic process.

One of the problems with Quantum Gravity is that you can quantise Weyl Curvature, but you can't quantise Ricci.

Sorry, Particle Physics.

The relationship between the Standard Model and GR is an interesting one. Despite what you always hear, Quantum Theory and General Relativity do work together and don't contradict each other.

The only problem is that the Standard Model is a Quantum Field Theory, which were always formulated in flat spacetime until the 80s.
(Which isn't a problem because you rarely need to understand particle physics in a Strong Gravitational Field)

It seems that you can put Quantum Field Theories in a curved spacetime, but it has only been done for a select few spacetimes, because in most spacetimes Feynman diagrams don't work as well as usual.

Son Goku,

The only problem is that the Standard Model is a Quantum Field Theory, which were always formulated in flat spacetime until the 80s.
(Which isn't a problem because you rarely need to understand particle physics in a Strong Gravitational Field)

And the event horizon censures the proceedings anyway.
Can you recommend any literature on Weyl or Ricci Curvature ? I'd like to follow this up a bit more.

I still think nature has something strange to throw at us regarding 'gravitational waves'. I think there is still something that we're missing. "Science without observation" and all that - why have gravitational waves not been observed yet ? - and yes I know it's not a simple matter to observe.
This months Astronomy magazine has a section on how merging black holes can be thrown free of their host Galaxy by a gravitational wave re-bound, however, many Galaxies are the result of mergers, why don't we see any evidence for this eviction of merging black holes? presumably you would need to look for broken galaxies, or does the evicted black hole re-capture it's host galaxy after a period ?

I'll start chewing on the first few syllables and see how they taste.

Can you recommend any literature on Weyl or Ricci Curvature ? I'd like to follow this up a bit more.

If you have Roger Penrose's "Road to Reality" then there is an excellent discussion on it in Chapter 17 or 18, can't remember which.
He then goes on to talk about Weyl and Ricci curvature in his chapter on entropy.
Even his other book "The Emporer's New Mind", talks about it a bit.

(Believe it or not 90% of all entropy in the universe came from Weyl curavture being replaced by Ricci)

On a more advanced level Wald's "General Relativity" is pretty much the bible on the differences and relationships between Ricci and Weyl curvature.

One of the biggest areas of research in GR today is what spacetime does when there is no matter.
Since matter only causes Ricci Curvature, Weyl Curvature can do what it wants independant of matter.
When you get rid of all matter space is Ricci-flat and then you have to talk about just Weyl.
What Weyl did on its own was one of Einstein's favourite questions, as seen in this famous picture:

The equation reads "Ricci Curvature = 0?".
It then went on to discuss Weyl Curvature.

A final down to Earth example of both, if I'm not making this post too long, is the Moon and the Earth's tide.
Astronauts on the Moon's surface jump up and fall down because of Ricci Curvature. However this Ricci curvature causes Weyl Curvature and the Weyl Curavture causes the Earth's tides.

I still think nature has something strange to throw at us regarding 'gravitational waves'. I think there is still something that we're missing. "Science without observation" and all that - why have gravitational waves not been observed yet ? - and yes I know it's not a simple matter to observe.

We still have a while to go in understanding Weyl Curvature's role in the universe, so I wouldn't be surprised if there were surprises.

Son Goku,

I do have Roger Penrose's "Road to Reality", haven't read it all yet - it's a selective reading book really, that you dip in and out of for relevant info, a bit tough to read all in one go. I'll check out the chapters you mention - thanks for that. I'll also see if I can pick up a copy of Wald's "General Relativity" at Amazon.

While were at it, what are your impressions on particles of 'space'?, i.e. that space itself must have a quantum existence somewhere near the planck scale. At this scale there would be more space particles on the head of a pin (paraphrasing) than there are all other particles in the entire universe. But maybe that's for another day and another post.

I do have Roger Penrose's "Road to Reality", haven't read it all yet - it's a selective reading book really, that you dip in and out of for relevant info, a bit tough to read all in one go.

I know what you mean, very mathematical. Although it's a fantastic guide to physics. The only downside is that he spends three chapters on Quantum Mechanics, but only one chapter on Quantum Field Theory, which I find odd because most of modern theoretical physics is Quantum Field Theory.

While were at it, what are your impressions on particles of 'space'?, i.e. that space itself must have a quantum existence somewhere near the planck scale. At this scale there would be more space particles on the head of a pin (paraphrasing) than there are all other particles in the entire universe. But maybe that's for another day and another post.

It's better a better idea than the graviton I think and has managed to give predictions. However I'm still firmly part of what is called the "Third Road", advocated by Roger Penrose and a few other British Physicists.
I could explain this if you wish, it usually makes for a good discussion on Theoretical Physics.

However I'm still firmly part of what is called the "Third Road", advocated by Roger Penrose and a few other British Physicists.

Is this third road anything to do with Twistors ?
If you can elaborate more or point me to a descriptive link of "Third Road" then I'd appreciate it.

Basically a greater understanding of decoherence, wavefunction "jumping" and QFT and GR's relationship is needed before we can continue.

Penrose's Twistors were part of this, as he rewrote GR in the mathematics of QFT to make comparison easier.

The litrature is few and far between, because not many people are in this area. (Simply because it doesn't grab most people's attention.)

I've a bit more reading to do then.

Thanks for the pointers.