PDA

View Full Version : Is gravity curved space?



ccart3r
2009-Dec-26, 01:25 PM
Ok, I'm sorry if this is a dumb question. I just want to make sure I have the correct understanding.

Growing up like many others, we were told that gravity was/is pulling us down. Yet as I grew up, learnt that it's actually space that's pushing us down.

So my question is, if it's space that's pushing us, what is gravity doing? Is gravity just holding all the mass from flying out, but not actually pushing or pulling?

Or is space/gravity working as one in the same? Is the "curved" space actually gravity and I'm just thinking of it as two separate things? Again, one of these questions were I believe I understand yet I question myself so much that I start believing the wrong idea.

Growing up, I always thought gravity is what's pulling us to the earth, since it's actually space, what is the gravity doing? Is gravity the result of space curving? I understand their are many things we still don't understand about gravity, yet I'm trying to understand the relationship between gravity and space, or at least curved/warped space.

Am I wrong in thinking of Space and Gravity as two separate things. Or is it that the curved space "pushing" is the gravity.

Thank You.

antoniseb
2009-Dec-26, 02:02 PM
Hi ccart3r

Welcome to the BAUT forum.
Your 'dumb' question is more misinformed than dumb. It is very useful for calculation purposes to imagine that gravity is an attraction between masses... i.e. gravity is pulling us down. If you want to try and think about it in terms of General Relativity the idea that space is pushing us down doesn't quite capture it.

Going a little further, in the future, we may have some model of how things work that is like GR in the big world, and explains why Quantum Mechanics works in the small scale. Who knows if the ideas of pushing or pulling, or going with the flow will have any meaning in that sense.

I guess my main point is that all explanations are models, and the terminology you use depends on which model of explanation you are using.

hhEb09'1
2009-Dec-26, 04:43 PM
I've changed the thread title from "dd" to something more descriptive. It may attract our more knowledgeable users to answer here too.
Or is it that the curved space "pushing" is the gravity. That is almost right. But it is not so much that space is curved, as is spacetime.

cosmocrazy
2009-Dec-26, 04:53 PM
Welcome to BAUT ccart3r,

No its not a dumb question. Yes gravity is modelled to be curved/warped space-time under GR (general relativity) which is currently our best model of how the universe works on the macroscopic level. As antoniseb mentioned for calculation purposes its easier to consider gravity an attraction between masses.

A very crude analogy of how space-time is warped is whats called the "rubber sheet analogy" - http://theory.uwinnipeg.ca/users/gabor/black_holes/slide5.html

Don't take this too literally though as the sheet does not represent actual space-time very well. But it can give you some idea of the very basics of how general relativity models gravity.

What gravity actually is is still not quite fully understood. It appears that at quantum levels GR doesn't model gravity very well and so a quantum theory of gravity is used - http://en.wikipedia.org/wiki/Quantum_gravity#Quantum_mechanics_and_general_rela tivity Unifying quantum mechanics and GR is one of the problems many scientist are working on and they are hoping to get some answers from the LHC to help find the link for harmonising the two.

ccart3r
2009-Dec-26, 05:52 PM
Hello, Thank you for fixing the title, I had changed it but it never switched. Strange. Ok I believe I'm understanding now. I wasn't sure what actually was "curving" spacetime.

Reading that quote actually helped "Mass tells Spacetime how to bend, Spacetime tells Mass how to moveď

Just so I make sure I fully understand the general idea. I'll use the Earth and Moon for this. The Moon is "falling" towards Earth's center gravity, because of A. Gravity pulling masses together AND? B. Curved Spacetime is pushing as well. Yet, The Earth's mass warping spacetime added with the speed at which the moon is traveling, creates it's orbital rotation.

Now If I have that correct, My final two questions would be. What keeps the Sun from say collapsing in on it's own gravity? My guess would be a release of some energy within the sun?

And final question, Are galaxies bound to a gravitation force? And does the theory of super massive black holes at the center of every galaxy have to do with that?

cosmocrazy
2009-Dec-26, 07:42 PM
Hello, Thank you for fixing the title, I had changed it but it never switched. Strange. Ok I believe I'm understanding now. I wasn't sure what actually was "curving" spacetime

Reading that quote actually helped "Mass tells Spacetime how to bend, Spacetime tells Mass how to move“.

In simple terms yes.



Just so I make sure I fully understand the general idea. I'll use the Earth and Moon for this. The Moon is "falling" towards Earth's centre gravity, because of A. Gravity pulling masses together AND? B. Curved Spacetime is pushing as well. Yet, The Earth's mass warping spacetime added with the speed at which the moon is travelling, creates it's orbital rotation.

Again basically yes.



Now If I have that correct, My final two questions would be. What keeps the Sun from say collapsing in on it's own gravity? My guess would be a release of some energy within the sun?

Yes the release of energy by the process of fusion is what keeps the internal pressure high enough to stop the collapse.



And final question, Are galaxies bound to a gravitation force? And does the theory of super massive black holes at the centre of every galaxy have to do with that?

Yes galaxies are kept together by gravity (and also galaxy clusters) and it appears that there maybe a very massive black hole at the centre of most if not all known galaxies.
In our Milky Way galaxy there is believed to be a super massive BH which all the stars rotate about mostly within the spiral arms. Finding that these black holes appear to exist in each galaxy has given some scientists a reason to believe that they may have actually played a part in the formation of the galaxies rather than the other way round, but I don't have the details on such.

Jeff Root
2009-Dec-26, 08:42 PM
Growing up like many others, we were told that gravity was/is
pulling us down. Yet as I grew up, learnt that it's actually space
that's pushing us down.
I have never heard of space "pushing". There is an attempt at a
theory of gravity called "pushing gravity", but in that speculation,
it is not space that does the pushing, but some kind of particles.
The general theory of relativity, which is by far the best theory
of gravity that we have so far, doesn't say anything about space
pushing in any way.

The curvature of spacetime as described by general relativity is a
four-dimensional curvature. The presence of mass or energy causes
the three-dimensional space to curve into the time dimension. The
larger the mass or energy that is causing the curvature, the greater
the curvature is. Greater curvature is the same thing as stronger
gravity. You can tell exactly how much curvature is present from
the strength of the gravity, and you can tell exactly how strong the
gravity is from the degree of curvature.

Since the gravity of the Earth or the Sun is caused by only a very
small amount of curvature, it is very difficult to measure it directly
using geometric means. As you probably know, it was first done by
comparing the positions of stars in the sky which appeared close to
the Sun during an eclipse, to their positions earlier and later when
the Sun was not near them. The Sun's gravity warped spacetime
around it enough to measure the change in the paths taken by the
starlight as it went past the Sun. That isn't the most direct way to
measure spacetime curvature, but it is as close to direct as we can
get with the weak gravity fields nearby and the large distances over
which we have to measure. If we could get to a black hole, the
intense curvature would allow more dramatic demonstrations.

The curvature of space dimensions into the time dimension means
that the amount of space is reduced and the amount of time is
increased. The increase in time can more easily be measured than
the curvature of space because we can easily make incredibly
precise clocks. Global positioning system satellites use such clocks
to function, and the difference between the "rate" of time where
the satellites orbit compared to the "rate" of time at Earth's surface
is easily measured. Clocks in orbit and at the surface get out of
synchronization exactly as predicted by general relativity, unless
they are adjusted to compensate for the gravity difference.

The rubber sheet analogy is very good in several ways, and very
misleading in other ways. It gives a good visual impression of where
the curvature is located and how strong the curvature is. However,
it isn't actually the curvature of the sheet that shows the curvature
of spacetime, but the angle of the sheet's surface. The steeper the
side of a dent in a rubber sheet, the greater the spacetime curvature
it represents. The up-down direction in the rubber sheet does not
not represent a direction in space, but time. It is ironic that gravity
is used in the rubber sheet analogy to show how gravity works.

-- Jeff. in Minneapolis

north
2009-Dec-26, 09:27 PM
ccart3r

by the way the moon is moving AWAY from the Earth

( I noticed that nobody addresses this perspective of his )

Jeff Root
2009-Dec-26, 09:43 PM
I wasn't sure what actually was "curving" spacetime.

Reading that quote actually helped "Mass tells Spacetime how to bend,
Spacetime tells Mass how to move“
That is a really great characterization by John Wheeler of the relation
between matter and spacetime. It says a lot, very succinctly, in a
completely nonmathematical way.



The Moon is "falling" towards Earth's center of gravity, because of
A. Gravity pulling masses together AND? B. Curved Spacetime is
pushing as well. Yet, The Earth's mass warping spacetime added
with the speed at which the moon is traveling, creates it's orbital
rotation.
Gravity is a force of attraction between all bits of matter.

A completely different way of saying exactly the same thing is to
say that matter curves spacetime, and curved spacetime causes
matter to accelerate in the direction of the curvature.

The Newtonian description of gravity is very, very good, and can
be used for almost all purposes in which the behavior of gravity
needs to be described. But sometimes the Newtonian description
isn't good enough, in which case the general relativity description
is required.

The Newtonian description is very simple, but it doesn't work well
in some extreme situations, such as in very strong gravity. It is
an approximation to general relativity, even though it is constructed
in a completely different way from how general relativity is built.
Nature has made gravity such that it can be described either as a
force of attraction or as a curvature of spacetime. That is quite
an astonishing fact. But what are we to make of it? Something to
ponder for a lifetime!



What keeps the Sun from say collapsing in on it's own gravity?
My guess would be a release of some energy within the sun?
The Sun consists of hot gas, so yes, energy provided by fusion
in the Sun's core currently provides the heat which keeps the
Sun's matter in gaseous form. Earth's atmosphere is also gas,
much cooler than the Sun. It is also heated by the energy of the
Sun's fusion. Both the Sun and the Earth's atmosphere would
collapse to solid matter if the heat energy were removed.

The Earth's lithosphere is already solid. It is prevented from
collapsing further by electric repulsion between atoms. The Sun's
mass is so great that the weight of all that matter, if cooled to the
temperature of Earth's lithosphere, would collapse past the point
of being a solid. The electric force which prevents atoms from
collapsing would not be enough, and they would collapse further
to a state called "degenerate matter". That is the condition of a
white dwarf star. In a star more massive than the Sun, the force
keeping degenerate matter from collapsing further would not be
enough, and the matter would collapse all the way to neutrons.
That is a neutron star, made mostly of neutronium. And finally, in
a slightly more massive star yet, even the nuclear force which
prevents collapse of the neutrons would not be strong enough to
resist the pressure, and the matter would collapse completely,
forming a black hole.

So, given enough mass, gravity wins.



Are galaxies bound to a gravitation force? And does the theory of
super massive black holes at the center of every galaxy have to do
with that?
Galaxies near each other are gravitationally bound together.
Galaxies in a cluster are gravitationally bound together. Interaction
between galaxies could throw a galaxy out of a cluster with enough
speed that it would no longer be bound. That happens to individual
stars, which can be thrown out of galaxies so that they are no
longer bound to the galaxy they originated in.

Supermassive black holes at the centers of galaxies appear to be
very influential in forming those galaxies. It isn't clear that they are
crucial to that formation. It is the total gravity of all the matter in
a galaxy that holds it together. Exactly how a galaxy forms in the
first place is a very longstanding question that is very slowly being
answered.

-- Jeff, in Minneapolis

DrRocket
2009-Dec-26, 09:46 PM
Ok, I'm sorry if this is a dumb question. I just want to make sure I have the correct understanding.

Growing up like many others, we were told that gravity was/is pulling us down. Yet as I grew up, learnt that it's actually space that's pushing us down.

So my question is, if it's space that's pushing us, what is gravity doing? Is gravity just holding all the mass from flying out, but not actually pushing or pulling?

Or is space/gravity working as one in the same? Is the "curved" space actually gravity and I'm just thinking of it as two separate things? Again, one of these questions were I believe I understand yet I question myself so much that I start believing the wrong idea.

Growing up, I always thought gravity is what's pulling us to the earth, since it's actually space, what is the gravity doing? Is gravity the result of space curving? I understand their are many things we still don't understand about gravity, yet I'm trying to understand the relationship between gravity and space, or at least curved/warped space.

Am I wrong in thinking of Space and Gravity as two separate things. Or is it that the curved space "pushing" is the gravity.

Thank You.

For most purposes, there is nothing wrong with thinking of gravity in terms of the Newtonian model. In that model gravity is an attractive force that acts between masses and drops of as the square of the distance separating them. That is the model that is used in virtually all orbital calculations, ballistics, engineering and in much of astronomy oas well.

The more precise representation of gravity in general relativity, does not represent gravity as the curvature of space, but rather as curvature of spacetime. Orbits are not geodesics in space but rather geodesics in spacetime. The use of curvature in general relativity is a fairly subtle mathematical issue, and is rather poorly represented by the usual rubber sheet or trampoline with a bowling ball that one finds in the popular literature.

The curvature of spacetime is defined in terms of the curvature tensor, which has no simple classical analog. It is determined by the distributin of matter/energy in the universe, and matter is the domiinant factor. So evern in general relativity, in which the curvature of spacetime is the manifestation of gravity, it is matter that is primarily responsible for the curvature and the intuition that you have from ordinary Newtonian physics is stll a good guide except in extreme situations, as with black holes.

undidly
2009-Dec-27, 12:59 AM
I have never heard of space "pushing". There is an attempt at a
theory of gravity called "pushing gravity", but in that speculation,
it is not space that does the pushing, but some kind of particles.
The general theory of relativity, which is by far the best theory
of gravity that we have so far, doesn't say anything about space
pushing in any way.

The curvature of spacetime as described by general relativity is a
four-dimensional curvature. The presence of mass or energy causes
the three-dimensional space to curve into the time dimension. The
larger the mass or energy that is causing the curvature, the greater
the curvature is. Greater curvature is the same thing as stronger
gravity. You can tell exactly how much curvature is present from
the strength of the gravity, and you can tell exactly how strong the
gravity is from the degree of curvature.

Since the gravity of the Earth or the Sun is caused by only a very
small amount of curvature, it is very difficult to measure it directly
using geometric means. As you probably know, it was first done by
comparing the positions of stars in the sky which appeared close to
the Sun during an eclipse, to their positions earlier and later when
the Sun was not near them. The Sun's gravity warped spacetime
around it enough to measure the change in the paths taken by the
starlight as it went past the Sun. That isn't the most direct way to
measure spacetime curvature, but it is as close to direct as we can
get with the weak gravity fields nearby and the large distances over
which we have to measure. If we could get to a black hole, the
intense curvature would allow more dramatic demonstrations.

The curvature of space dimensions into the time dimension means
that the amount of space is reduced and the amount of time is
increased. The increase in time can more easily be measured than
the curvature of space because we can easily make incredibly
precise clocks. Global positioning system satellites use such clocks
to function, and the difference between the "rate" of time where
the satellites orbit compared to the "rate" of time at Earth's surface
is easily measured. Clocks in orbit and at the surface get out of
synchronization exactly as predicted by general relativity, unless
they are adjusted to compensate for the gravity difference.

The rubber sheet analogy is very good in several ways, and very
misleading in other ways. It gives a good visual impression of where
the curvature is located and how strong the curvature is. However,
it isn't actually the curvature of the sheet that shows the curvature
of spacetime, but the angle of the sheet's surface. The steeper the
side of a dent in a rubber sheet, the greater the spacetime curvature
it represents. The up-down direction in the rubber sheet does not
not represent a direction in space, but time. It is ironic that gravity
is used in the rubber sheet analogy to show how gravity works.

-- Jeff. in Minneapolis

""It is ironic that gravity
is used in the rubber sheet analogy to show how gravity works.""

The analogy works in an accelerating space ship just as well.
GR says gravity is equivalent to acceleration.

Works in a centrifuge also because of the acceleration towards the center.



So the curve (slope) is not in a physical direction but is a change in the rate of the passage of time?.
Agrees with the slowing of time in a G well which is measurable even here(on Earth) but not why mass accelerates towards a place with slower time.

WHY does mass accelerate to places where time is slower?.

north
2009-Dec-27, 01:04 AM
""It is ironic that gravity
is used in the rubber sheet analogy to show how gravity works.""

The analogy works in an accelerating space ship just as well.
GR says gravity is equivalent to acceleration.

Works in a centrifuge also because of the acceleration towards the center.



So the curve (slope) is not in a physical direction but is a change in the rate of the passage of time?.
Agrees with the slowing of time in a G well which is measurable even here(on Earth) but not why mass accelerates towards a place with slower time.

Why does mass accelerate to places where time is slower?.

just a little but important defintion , centrifuge is the moving of things OUTWARD from the center

DrRocket
2009-Dec-27, 01:09 AM
just a little but important defintion , centrifuge is the moving of things OUTWARD from the center

Wrong, as usual.

The acceleration in a centrifuge is radially inward towards the center of rotation. This is elementary physics, which should be known to every freshman who has ever taken a physics class.

hhEb09'1
2009-Dec-27, 01:32 AM
Wrong, as usual.

The acceleration in a centrifuge is radially inward towards the center of rotation. This ie elementary physics, which should be known to every freshman who has ever taken a physics class.Technically right, though that's a different concept. "Centrifuge" literally does mean "center fleeing", even dictionary.com (http://dictionary.reference.com/browse/centrifugal) has it as "moving or directed outward from the center" so it's easy to see where north is comng from

DrRocket
2009-Dec-27, 02:19 AM
Technically right, though that's a different concept. "Centrifuge" literally does mean "center fleeing", even dictionary.com (http://dictionary.reference.com/browse/centrifugal)has it as "moving or directed outward from the center" so it's easy to see where north is comng from

The issue was originally clearly stated by Undidly here (http://www.bautforum.com/space-astronomy-questions-answers/98606-gravity-curved-space.html#post1650067) as relating to the direction of the acceleration vector.

A semantic argument is irrelevant.

ccart3r
2009-Dec-27, 03:11 AM
Thank you everyone, for helping me understand or I should say visualize better.

Andrew D
2009-Dec-27, 03:03 PM
And final question, Are galaxies bound to a gravitation force? And does the theory of super massive black holes at the center of every galaxy have to do with that?

Here's the kicker: Objects on cosmological scales of distance, like distant galaxies, are not observably bound to eachother by gravity. In fact, they're moving apart at ever increasing velocities.

Truth of the matter is (get it?), we don't really know what gravity is. We know how it behaves and can predict its effect on objects with great precision, but we don't understand it like we understand the nuclear forces or electromagnitism.

Leonard Susskind has an incredible video lecture series for people with only a little background in physics and math. I've been watching one lecture every day or so for a while now, and it's totally changed the way I see the universe. Here's a post with links (http://www.bautforum.com/space-astronomy-questions-answers/98450-physics-resources-online.html).

ccart3r
2009-Dec-27, 03:10 PM
Here's the kicker: Objects on cosmological scales of distance, like distant galaxies, are not observably bound to eachother by gravity. In fact, they're moving apart at ever increasing velocities.

Truth of the matter is (get it?), we don't really know what gravity is. We know how it behaves and can predict its effect on objects with great precision, but we don't understand it like we understand the nuclear forces or electromagnitism.

Leonard Susskind has an incredible video lecture series for people with only a little background in physics and math. I've been watching one lecture every day or so for a while now, and it's totally changed the way I see the universe. Here's a post with links (http://www.bautforum.com/space-astronomy-questions-answers/98450-physics-resources-online.html).

Thank You for the link. I'll be sure to watch.

Question for anyone, I love watching/listening to lectures. Is there a site that maybe lists lectures/speakers to cities and what not? I can't seem to find a way to look up lectures for say astronomy/physics/ or just science in general. I'd love to attend lectures/conferences etc.

Cougar
2009-Dec-27, 06:18 PM
Here's the kicker: Objects on cosmological scales of distance, like distant galaxies, are not observably bound to eachother by gravity. In fact, they're moving apart at ever increasing velocities.

Right, except we don't want to imply "moving", like bits in an explosion. "They appear to be moving apart at ever increasing velocities" might be a little better. Since we don't yet really have any well-tempered theory on the vacuum energy of spacetime and its effect on the apparent overall expansion (Frank Wilczek (http://en.wikipedia.org/wiki/Frank_Wilczek) notwithstanding), we apparently can't yet say what's really going on out there in this spacetime 'medium' we find ourselves in. It seems to be expanding. How does it do that? What we can say is, this object we see whose light has been traveling, say, 10 billion years to reach us, it is in a very different frame of reference than our frame. Einstein, bless his heart, spent about 10 years figuring out how we can quantitatively reconcile any such frames within an overall variable background. "General Relativity" is still about the only thing we can say with some confidence about the overall dynamics of the universe. At least, that was my 'takeaway' from that oft linked article, Expanding Confusion. Please correct if wrong.


Truth of the matter is (get it?), we don't really know what gravity is. We know how it behaves and can predict its effect on objects with great precision, but we don't understand it like we understand the nuclear forces or electromagnitism.

Right. We don't really know how it works. This freaked out Newton, as one might imagine. Clearly the gravity was "coming from" the large mass, but how the heck could it relay this 'information' through empty space without ever touching the object it is gravitationally affecting? This is still a question, although a more developed question since Newton's time...

Now if you'll excuse me, I have a proposition to put forward over in the ATM forum.... :eek:

DrRocket
2009-Dec-27, 09:31 PM
Right, except we don't want to imply "moving", like bits in an explosion. "They appear to be moving apart at ever increasing velocities" might be a little better. Since we don't yet really have any well-tempered theory on the vacuum energy of spacetime and its effect on the apparent overall expansion (Frank Wilczek (http://en.wikipedia.org/wiki/Frank_Wilczek) notwithstanding), we apparently can't yet say what's really going on out there in this spacetime 'medium' we find ourselves in. It seems to be expanding. How does it do that? What we can say is, this object we see whose light has been traveling, say, 10 billion years to reach us, it is in a very different frame of reference than our frame. Einstein, bless his heart, spent about 10 years figuring out how we can quantitatively reconcile any such frames within an overall variable background. "General Relativity" is still about the only thing we can say with some confidence about the overall dynamics of the universe. At least, that was my 'takeaway' from that oft linked article, Expanding Confusion. Please correct if wrong.



I don't quite understand your reference to Wilczek. The only connection of which I am aware is his statement (no ready reference) that the misprediction of the cosmological constant in terms of vacuum energy (it is off by about 120 orders of magnitude) is the most important outstanding problem in quantum field theory.

tommac
2009-Dec-27, 11:12 PM
Is it true that the curving of time actually plays a greater role as the attractor than the curvature of space?

Jeff Root
2009-Dec-28, 12:32 AM
They are the two aspects of one thing. Like the two ends of a baseball
bat: One which you hold in your hands; the other which hits the ball. You
can't say that one plays a greater role than the other. You either have
both, or neither.

-- Jeff, in Minneapolis

cosmocrazy
2009-Dec-28, 01:14 AM
They are the two aspects of one thing. Like the two ends of a baseball
bat: One which you hold in your hands; the other which hits the ball. You
can't say that one plays a greater role than the other. You either have
both, or neither.

-- Jeff, in Minneapolis

Jeff,
I like this analogy very well put!

grant hutchison
2009-Dec-28, 01:15 AM
Is it true that the curving of time actually plays a greater role as the attractor than the curvature of space?The time curvature is predominantly responsible for the orbit of objects that are moving slowly in comparison to light-speed; space curvature has minimal effects under those circumstances. But the planet Mercury is moving fast enough to sample space curvature around the Sun to an extent that has been measurable (as a deviation from a purely Newtonian orbit) for a century or so.
At velocities close to light-speed the two curvatures become equally effective. Hence Einstein's result that light is deflected twice as much by gravity as would be predicted by Newton.

Grant Hutchison

Ken G
2009-Dec-28, 04:32 AM
And although that is both a useful and insightful way to think about it, I feel I should point out the nitpick that dissecting space and time curvature implies a natural yet arbitrary choice of coordinates. Time would be characterized by synchronized clocks by observers with no relative motion, and space would be characterized by a globally constant speed of light c. Those are natural choices, but strictly speaking, there is no such thing as "time curvature" and "space curvature"-- reality itself does not appear to distinguish them, that's all happening in our own arbitrary coordinate systems. Nevertheless, we do use these coordinate systems, and we think about reality that way, so the dissection that you refer to is indeed useful and valid. I'm only mentioning it because there is a tendency for people to start imagining "curved space" as a physically real thing, like a highway entrance ramp made from concrete or some such thing. That does not appear to be the case, space appears to be whatever we make it. The "real" curvature appears to be in spacetime, as has been mentioned.

Ken G
2009-Dec-28, 05:08 AM
So the curve (slope) is not in a physical direction but is a change in the rate of the passage of time?.That is a natural way to think about it, in the standard way of generalizing the way our own clocks work into a global coordinatization of time.
WHY does mass accelerate to places where time is slower?.There are several ways to think of this, but one way is a kind of distortion in spacetime. Imagine a one dimensional space along the y axis (up and down), and put the time dimension along the x axis. If there's no gravity, and you give an object an upward speed, it follows a straight diagonal line up and to the right in this x-y plane (where x is really t). But if you put in gravity, the "grid" we are drawing in this x-y plane is not a bunch of squares any more, because time is "running faster" at higher y values. That means that each tickmark in the x direction gets wider as you go up to higher y values-- a change in x of 1 at y=0 would be a little bit more than 1 at higher y. That distorts the grid, such that the gridlines themselves bend slightly. The line taken by the object is relative to those distorted gridlines, so as the lines of fixed t bend toward the right, the diagonal line of the x(t) trajectory also bends to the right. Eventually it bends completely to the right, at which point the object is moving entirely in the time direction-- it is instantaneously motionless. After that, the grid distortion will turn its x(t) trajectory downward, and it starts moving down and to the right in the x-y plane. It has stopped going up and is now "falling" back down, all "because of" what is "happening" to time at higher y.

undidly
2009-Dec-28, 09:00 AM
That is a natural way to think about it, in the standard way of generalizing the way our own clocks work into a global coordinatization of time.There are several ways to think of this, but one way is a kind of distortion in spacetime. Imagine a one dimensional space along the y axis (up and down), and put the time dimension along the x axis. If there's no gravity, and you give an object an upward speed, it follows a straight diagonal line up and to the right in this x-y plane (where x is really t). But if you put in gravity, the "grid" we are drawing in this x-y plane is not a bunch of squares any more, because time is "running faster" at higher y values. That means that each tickmark in the x direction gets wider as you go up to higher y values-- a change in x of 1 at y=0 would be a little bit more than 1 at higher y. That distorts the grid, such that the gridlines themselves bend slightly. The line taken by the object is relative to those distorted gridlines, so as the lines of fixed t bend toward the right, the diagonal line of the x(t) trajectory also bends to the right. Eventually it bends completely to the right, at which point the object is moving entirely in the time direction-- it is instantaneously motionless. After that, the grid distortion will turn its x(t) trajectory downward, and it starts moving down and to the right in the x-y plane. It has stopped going up and is now "falling" back down, all "because of" what is "happening" to time at higher y.

The x(t) ,y graph with the distortion as t changes cannot be drawn on a flat
surface.It needs a an extra dimension at right angles to x(t) and y.
This is analogous to the 4 th physical direction (if there is such a thing).

""After that, the grid distortion will turn its x(t) trajectory downward, and it starts moving down""
Explains why the trajectory is curved.
And if it has no trajectory,if it is not moving?.
Why does it start moving (accelerating ) towards slow time.

EDG
2009-Dec-28, 09:17 AM
The thing I don't get about gravity is this - if it's essentially a manifestation of the curvature of space-time, then why do we expect it to have a particle (the graviton) associated with it? (at least, I assume that people are still looking for the graviton or some kind of "gravity transmitting particle"?).

RussT
2009-Dec-28, 12:06 PM
I don't quite understand your reference to Wilczek. The only connection of which I am aware is his statement (no ready reference) that the misprediction of the cosmological constant in terms of vacuum energy (it is off by about 120 orders of magnitude) is the most important outstanding problem in quantum field theory.

EDG asked...
The thing I don't get about gravity is this - if it's essentially a manifestation of the curvature of space-time, then why do we expect it to have a particle (the graviton) associated with it? (at least, I assume that people are still looking for the graviton or some kind of "gravity transmitting particle"?).

Here is a question I have never seen addressed before...

IF, the Higgs Boson or Graviton IS found, how will that solve the 10^120 OOM problem?

Ken G
2009-Dec-28, 01:20 PM
The x(t) ,y graph with the distortion as t changes cannot be drawn on a flat
surface.It needs a an extra dimension at right angles to x(t) and y.Actually, I believe that is only true if you use a locally "square" gridding on the surface. The purpose of the "distorted" grid I described is to give the same effect as curvature but on a flat piece of paper. It might be a kind of local approximation of the full curvature, useful only for the x(t) trajectory in weak gravity, I don't know. I first saw it described by Feynman.

And if it has no trajectory,if it is not moving?.It is always moving in an t-y plot, it will just start out moving sideways.

DrRocket
2009-Dec-28, 01:30 PM
Here is a question I have never seen addressed before...

IF, the Higgs Boson or Graviton IS found, how will that solve the 10^120 OOM problem?

It won't. Why would you expect that it would ?

grant hutchison
2009-Dec-28, 01:33 PM
And although that is both a useful and insightful way to think about it, I feel I should point out the nitpick that dissecting space and time curvature implies a natural yet arbitrary choice of coordinates. Time would be characterized by synchronized clocks by observers with no relative motion, and space would be characterized by a globally constant speed of light c. Those are natural choices, but strictly speaking, there is no such thing as "time curvature" and "space curvature"-- reality itself does not appear to distinguish them, that's all happening in our own arbitrary coordinate systems. Nevertheless, we do use these coordinate systems, and we think about reality that way, so the dissection that you refer to is indeed useful and valid. I'm only mentioning it because there is a tendency for people to start imagining "curved space" as a physically real thing, like a highway entrance ramp made from concrete or some such thing. That does not appear to be the case, space appears to be whatever we make it. The "real" curvature appears to be in spacetime, as has been mentioned.Yes, thanks. :) I woke up this morning realizing that I hadn't said which observers measured the space/time distinction I had described.

Grant Hutchison

DrRocket
2009-Dec-28, 01:37 PM
The thing I don't get about gravity is this - if it's essentially a manifestation of the curvature of space-time, then why do we expect it to have a particle (the graviton) associated with it? (at least, I assume that people are still looking for the graviton or some kind of "gravity transmitting particle"?).

General reltivity and quantum theory are fundamentally imcompatible. GR is a deterministic theory, and QM predicts on probabilities.

In order to develop a unified consistent theory there is much ongoiing research into a quantum theory of gravity, which would presumably be such a theory. The graviton, if it exists, is the name chosen for the particle that would carry the gravitational force.

No one is yet actually looking for a graviton. What they are looking for is a consistent theory that would include the graviton. Once there is a theory that would describe the properties of that particle then one might be able to recognize it if one saw it. It may be so weakly interacting that detection would be difficult.

Ken G
2009-Dec-28, 01:38 PM
The thing I don't get about gravity is this - if it's essentially a manifestation of the curvature of space-time, then why do we expect it to have a particle (the graviton) associated with it? (at least, I assume that people are still looking for the graviton or some kind of "gravity transmitting particle"?).
I don't get this either. I think the idea is that if gravity is "really" something happening to spacetime, then it is not unifiable with the other forces. I don't see what would be so terrible about that, but certainly the goal of science is to unify as much as possible, so it is natural to look for unification. If there can be unification, then gravity is either mediated by a particle that requires certain properties (like spin 2), or else the other forces aren't really mediated by particles they are spacetime effects of a more complicated nature (I don't know about any suggestions of that nature).

DrRocket
2009-Dec-28, 01:43 PM
... or else the other forces aren't really mediated by particles they are spacetime effects of a more complicated nature (I don't know about any suggestions of that nature).

I have seen suggestions (without specifics) that it is GR that will ultimately stand and QM that will have to change, but I don't see how the force particles would go away either. It seems to me that the photon, for instance, is on pretty solid empirical ground and making it disappear would be quite a trick.

Ken G
2009-Dec-28, 02:07 PM
I have seen suggestions (without specifics) that it is GR that will ultimately stand and QM that will have to change, but I don't see how the force particles would go away either. It seems to me that the photon, for instance, is on pretty solid empirical ground and making it disappear would be quite a trick.Yes, I don't get how all that would work. It is rumored that Penrose pursues that approach, perhaps because he fears the current particle picture will never be able to accomodate intelligent thought whereas giving space and time more complex fractal-like properties might somehow do that, I don't follow his argument there. I believe that many mathematical physicists already look on particles as having a somewhat dubious ontology, preferring to see them in terms of symmetries in various mathematical structures that are more fundamental than the particles themselves. Heisenberg once said words to the effect that we need to think in terms of elementary symmetries rather than elementary particles, and I knew one particle physicist who would flat out state that "particles are a hoax." But the rest of physics can't even turn around without referring to them, so I don't know quite what these people are really saying there other than my usual caution not to ever take physics ontologies too seriously.

ccart3r
2009-Dec-28, 02:32 PM
How does Loop Quantum Gravity Theory work with curved spacetime and gravity?

DrRocket
2009-Dec-28, 02:43 PM
... and I knew one particle physicist who would flat out state that "particles are a hoax."

I would be interested in understanding what he means. I am sure that he has a point, but I don't understand what it is.

I am pretty certain that elementary particles are not little marbles.

But I am also certain that there is quite a bit of substance to the particle model of quantum field theory. If not, the boys at CERN have pulled off a very elaborate and expensive practical joke.

cosmocrazy
2009-Dec-28, 02:52 PM
Yes, I don't get how all that would work. It is rumoured that Penrose pursues that approach, perhaps because he fears the current particle picture will never be able to accommodate intelligent thought whereas giving space and time more complex fractal-like properties might somehow do that, I don't follow his argument there. I believe that many mathematical physicists already look on particles as having a somewhat dubious ontology, preferring to see them in terms of symmetries in various mathematical structures that are more fundamental than the particles themselves. Heisenberg once said words to the effect that we need to think in terms of elementary symmetries rather than elementary particles, and I knew one particle physicist who would flat out state that "particles are a hoax." But the rest of physics can't even turn around without referring to them, so I don't know quite what these people are really saying there other than my usual caution not to ever take physics ontologies too seriously.

I'm with you on this one Ken, I just can't see how one can describe gravity as spacetime curvature and as a particle carrying force as well. The one thing that intrigues me though is that light itself is considered to be a quanta of energy with the characteristics of a "wave particle duality", being neither in classical terms but displaying either and/or both attributes dependent on the circumstances. Maybe if the "graviton" is discovered then it may well act in a similar manner. Somehow interacting with spacetime as a wave form but then displaying the characteristics of a particle when interacting with matter. Still... how it would actually warp spacetime still baffles me. Its rather mind blowing stuff. :)

cosmocrazy
2009-Dec-28, 02:55 PM
I would be interested in understanding what he means. I am sure that he has a point, but I don't understand what it is.

I am pretty certain that elementary particles are not little marbles.

But I am also certain that there is quite a bit of substance to the particle model of quantum field theory. If not, the boys at CERN have pulled off a very elaborate and expensive practical joke.

Could it be possible that like the photon, fundamental particles are indeed neither a particle or a wave form but display either or both attributes dependent on the system they are engaged and how they interact within that system at that time?

Ken G
2009-Dec-28, 04:47 PM
I would be interested in understanding what he means. I am sure that he has a point, but I don't understand what it is.

I am pretty certain that elementary particles are not little marbles.
I think what he meant was something along the lines that particles are a way of organizing the outcome of observations, but they don't really exist, perhaps in the "marble" sense. I don't think he felt that the spot that appears on a detector corresponds to a particle that was emitted by the source and followed some kind of trajectory to the detector, I think he just felt that there were fields and symmetries and selection rules and it all comes out that outcomes are quantized but there's no such thing as particles, at least not in the way the language around them gets used. In my experience, if you ask ten particle theorists how they like to think about what particles are, you get ten different answers, and none of them sound anything like the usual picture, and would probably elicit a pretty puzzled expression from their counterpart observers who actually detect these things. Perhaps it is high time that particle theorists got together and achieved some kind of consensus about just what is the "particle" that appears in their job titles!

Ken G
2009-Dec-28, 04:52 PM
Maybe if the "graviton" is discovered then it may well act in a similar manner. Somehow interacting with spacetime as a wave form but then displaying the characteristics of a particle when interacting with matter. Still... how it would actually warp spacetime still baffles me. That's an interesting possibility, that the graviton wave function might itself have some effect on spacetime. That's not standard quantum mechanics, where the wave function and space and time are three different things. It's not clear that if the graviton is already affecting spacetime, that you need it to mediate interactions with matter that is already experiencing that spacetime. I just don't know how this unification business is supposed to work. String theory is supposed to allow gravity to act like a normal particle, but I don't know much string theory.

EDG
2009-Dec-28, 05:14 PM
No one is yet actually looking for a graviton. What they are looking for is a consistent theory that would include the graviton. Once there is a theory that would describe the properties of that particle then one might be able to recognize it if one saw it. It may be so weakly interacting that detection would be difficult.

Isn't that a bit backwards though? It sounds like we're assuming that a graviton should exist, and are looking for a theory that explains it - but we have no evidence to suggest that it does actually exist in the first place.

As far as I can see, if gravity is a function of the curvature of spacetime then it's a fundamental property of spacetime itself isn't it? And if that's the case, why would it need any particles at all?

It seems to me that we're looking for for something that has no reason to exist.

EDG
2009-Dec-28, 05:16 PM
I don't get this either. I think the idea is that if gravity is "really" something happening to spacetime, then it is not unifiable with the other forces. I don't see what would be so terrible about that, but certainly the goal of science is to unify as much as possible, so it is natural to look for unification. If there can be unification, then gravity is either mediated by a particle that requires certain properties (like spin 2), or else the other forces aren't really mediated by particles they are spacetime effects of a more complicated nature (I don't know about any suggestions of that nature).

Yeah... the latter would possibly make more sense to me - that early on in the universe all forces were manifestations of fundamental properties of space time, but as things cooled off and they separated out, they became particle-based. I guess that's ATM, but I'm surprised that nobody seems to be taking that approach to the problem.

cosmocrazy
2009-Dec-28, 05:26 PM
That's an interesting possibility, that the graviton wave function might itself have some effect on spacetime. That's not standard quantum mechanics, where the wave function and space and time are three different things. It's not clear that if the graviton is already affecting spacetime, that you need it to mediate interactions with matter that is already experiencing that spacetime. I just don't know how this unification business is supposed to work. String theory is supposed to allow gravity to act like a normal particle, but I don't know much string theory.

I have just found a simple article about super-string theory and it basically explains how theorists expect gravitons to fit in with GR. here is the link let me know what you think. http://www.superstringtheory.com/blackh/blackh4.html
thanks

gzhpcu
2009-Dec-28, 06:24 PM
As far as I can see, if gravity is a function of the curvature of spacetime then it's a fundamental property of spacetime itself isn't it? And if that's the case, why would it need any particles at all?


Quantum mechanics models three forces as having carrier particles. Gravity appears to be a force, hence QM would attempt to model it as having a carrier particle as well. The GR approach, that it is just a warping of spacetime, is hard to envision, I find, without envoking a higher, additional spatial dimension. It works well, but is hard to actually imagine.

EDG
2009-Dec-28, 09:43 PM
Quantum mechanics models three forces as having carrier particles. Gravity appears to be a force, hence QM would attempt to model it as having a carrier particle as well. The GR approach, that it is just a warping of spacetime, is hard to envision, I find, without envoking a higher, additional spatial dimension. It works well, but is hard to actually imagine.

Hm, when you put it that way, I see the problem! It's just interesting that all the effort seems to be in trying to merge gravity with the particle approach, rather than merging the quantum with the spacetime approach. I guess the latter is a lot harder to deal with though, as you said.

DrRocket
2009-Dec-28, 09:45 PM
Isn't that a bit backwards though? It sounds like we're assuming that a graviton should exist, and are looking for a theory that explains it - but we have no evidence to suggest that it does actually exist in the first place.

As far as I can see, if gravity is a function of the curvature of spacetime then it's a fundamental property of spacetime itself isn't it? And if that's the case, why would it need any particles at all?

It seems to me that we're looking for for something that has no reason to exist.

It is not really backward. It starts with the assumption that the universe is fundamentally quantum mechanical, and that is an assumption not a fact.
But given that assumption then forces are carried by the particles of quantum field theory, and the "graviton" is just a ready-made name for the boson that goes with gravity if and when a quantum theory of gravity is ever formulated.

You are simply taking the contrary position that gravity is just a manifestation of a spacetime continuum in the manner of Riemannian geometry, which is also an assumption. No one really knows what is going on at quantum scales and that manifold model may break down there.

The fundamental problem is that while one can formulate a quantum theory that is consistent with special relativity -- that is what quantum field theories really are -- the attempt to formulate a theory consistent with general relativity has run into fundamental problems. The basic problem is that such theories are not renormalizable and therefore make infinite predict ins that cannot be swept under the rug.

We get along because gravity is very weak compared to the other quantum mechanical forces at the usual microscopic scales, and the QM forces are negligible at the large scales at which gravitation is normally considered. But gravity is a 1/r^2 sort of force and at very very small scales should again be significant compared to the other forces, which is why you hear so much about the unknown physics "at the Planck scale". It is also why you hear so much speculation about "quantum foam" or other departures from a manifold model at that scale since that would cause general relativity and the spacetime picture to break down.

It is also why you hear so much about string theory. In string theories a particle is required by the mathematical consistency of the theory that is recognizable as the graviton. But string theory has its own set of problems, not the least of which is that it is not really clearly and rigorously defined. It also has failed to actually make any new testable predictions. In fact nobody really kines what string theory is, despite many statements that appear to the contrary in the popular press.

The flip side is that in extreme situations, as with black holes or the very early stages (say less than 10^-33 seconds from the big bang) of the universe, that general relativity predicts singularities that are simply indications that something is awry with the theory. One likely resolution is that for such applications quantum phenomena are important and what is needed is a quantum theory that includes gravity.

So, the bottom line is that neither GR nor quantum field theories are cast in concrete and everything is open to challenge at appropriate scales and for appropriate phenomena. Both are solid theories in the domains of validity at which they are usually applied. There is no known correct or incorrect, forward or backward, to look at the problem. There is no known answer. There is only a deep mystery to be solved through the creative process called research.

DrRocket
2009-Dec-28, 10:00 PM
The GR approach, that it is just a warping of spacetime, is hard to envision, I find, without envoking a higher, additional spatial dimension. It works well, but is hard to actually imagine.

This is a bit puzzling.

It is fairly easy to understand curvature of curves. One sees that done in an introductory calculus class.

It is more difficult to figure out what curvature means for surfaces. Gauss spent quite a bit of effort and eventually succeeded.

The extension to higher dimensions is considerably more difficult. Riemann formulated a general theory of curved manifolds, but the result is extremely abstract, and there are many different versions of "curvature" that apply. Even in GR one will find several different curvature tensors discussed.

So, at least for most people, the addition of dimensions is not a clarification.

Moreover, I am not sure why adding a spatial dimension would help. It is very important to recognize that in GR there is no such thing as either a spatial or a time dimension, there is just a Lorentzian manifold. The curvature is not a curvature of time or of space but of the whole manifold. Neither time nor space are unambiguously identified, except for a specific local observer, and curvature is not a local concept. Curvature is an invariant of the theory.

Think about special relativity. Any time-like vector can be the time coordinate for some reference frame, that is for some observer. So even in the special theory time and space are inextricably mixed together, and that is a flat space. Curvature just serves to further blur any imagined distinction.

I am afraid that the rubber sheet, trampoline and bowling ball, analogy that is commonly used has done as much harm as good in conveying the notion of curved spacetime and gravity. It is a very inaccurate analogy, and in fact uses gravity to somehow explain gravity. Curvature in Riemannian geometry is a very subtle thing and has no good simple analogy. It is a major thrust of any course in Riemannian geometry to figure out what "curvature" is supposed to mean. There is no simple explanation.

DrRocket
2009-Dec-28, 10:05 PM
That's an interesting possibility, that the graviton wave function might itself have some effect on spacetime.

In some sense this is the problem.

One reason that quantum field theories of gravity have failed is that the graviton interacts with itself, gravity grvitates, and that is the obstruction to renormalizability.

The photon, in contrast, does not carry charge and therefore does not feel the electromagnetic force. That makes things easier. QED works pretty well.

publius
2009-Dec-28, 11:23 PM
Actually, in QCD, the gluon carries color charge itself. QCD is far more complex than QED, but a carrier particle that interacts with the force it mediates apparently is not a show stopper. With gravity, it's more than that, and the simple explanation I've always seen given is gravity is "spin-2" a tensor, rather than vector field. Then there is the esoteric concept of "background independence", which I don't understand.

About the "graviton". I remember reading something about this by someone who understood things more than I do. This picture of a carrier particle carrying a force is just that, a picture, one way to think about it. It's a picture, or pedagogy, that easily comes of the renormalization methods of QFT.

But that's apparently not the only way. Again, the particle picture can be misleading as we tend to get this little cut and dried picture of charge particles throwing little carrier particles between each other. When you get into it, it's far from that simple.

Apparently one might come up with some quantum description of gravity that doesn't really on "gravitons" in the sense that photons carry the EM force and gluons carry the color force.

Somehow, a quantum theory of gravity should have "curvature" as being discrete, rather than continuous. Curvature would go in jumps, rather than being a smooth function.


-Richard

Ken G
2009-Dec-29, 12:05 AM
I have just found a simple article about super-string theory and it basically explains how theorists expect gravitons to fit in with GR. here is the link let me know what you think. http://www.superstringtheory.com/blackh/blackh4.html
thanks
There are certainly some hints there, but not much more. It's the kind of writing that if you could not count on the authority of the source, and if you did not know that there were actual mathematical theories underlying the claims being made, it would read almost exactly the same as flaky pseudoscience. What's more, the "great results" we find mentioned are only that a 10 dimensional string theory allows for a graviton to produce spacetime effects similar to GR (but there's no independent suggestions for why we should accept 10 dimensions as a realistic depiction of our universe, and it's hard to rule out the possibility of sheer coincidence, and the related issue of what price is too high a price for unification), and that there's a special ratio of charge to mass in a black hole (very far from real black holes) in which quantum mechanical calculations can arrive at the same entropy of a black hole as GR/thermodynamic arguments do. But again we have the problem that it only works for a special case, the "BPS" ratio of mass to charge, and so we again cannot seem to rule out sheer coincidence. So the bottom line is, although string theory is the only place we ever see any direct connections between GR and quantum mechanics, which some see as a triumph, it only happens under somewhat strained conditions (10 dimensions and strange charge-to-mass ratios), so we can still be troubled by two things:
1) it might be a coincidental connection between two unrelated theories, and
2) it might not have anything to do with our universe, extracting such a high price for unification that pre-unification is actually preferable.
I wouldn't rule it out on these grounds, but I have a hard time getting too worked up about it either.

Jeff Root
2009-Dec-29, 12:10 AM
I'm skeptical both of "unifying" forces and of quantizing everything.

That the graviton should have a spin of 2 is something I've seen for almost
five decades now. When and how was that prediction made?

-- Jeff, in Minneapolis

Ken G
2009-Dec-29, 12:17 AM
Somehow, a quantum theory of gravity should have "curvature" as being discrete, rather than continuous. Curvature would go in jumps, rather than being a smooth function.And here we have the "rub"-- the scales at which quantization occur, and gravity occur, are so vastly separated it seems hard to believe that both ends of the dynamical range would be accessible to observations. That's the same problem that dogs the interpretation of quantum mechanics to this day, we have the "correspondence principle" that says quantum mechanics cannot be wrong for macro objects, but it does not say that it is useful for macro objects. So when we lose the driving force of observations we do not yet understand, we step back from the observations and dive into theory, under the battle cry "unification at any cost," and the theorists go to town. But is it really true that a unified theoretical description is really what we need, when the result is almost totally unrecognizable as anything that relates to our universe, and does not come with any suggested tests or technologically reachable falsifiabilities that are not already built in to work? Shouldn't there be some kind of scientific rule that says we should unify all that seems to want to be unified, but not what seems like we are forcing it down its throat?

DrRocket
2009-Dec-29, 01:03 AM
Somehow, a quantum theory of gravity should have "curvature" as being discrete, rather than continuous. Curvature would go in jumps, rather than being a smooth function.


-Richard

You may well be correct, but no one has a clue what that really means. It certainly casts aside all of the tools of Riemannian geometry and even the more basic theory of differentiable manifolds, which is what is normally associated with the various notions of curvature. That leaves you in a situatiomn in which you lack not only physical insight but even the mathematical tool kit that is normally used to construct a theory. In large part that is the problem with string theory -- it lacks testable physical predictions and it lacks developed rigorous mathematics to support it.

loglo
2009-Dec-29, 01:39 AM
I'm skeptical both of "unifying" forces and of quantizing everything.

That the graviton should have a spin of 2 is something I've seen for almost
five decades now. When and how was that prediction made?

-- Jeff, in Minneapolis

Jeff,
Spin-2 comes from arguments using the Representation Theory of Groups (which I don't understand). See Keifer (http://books.google.com.au/books?id=ndBXK3yv5FoC&pg=PA28&lpg=PA28&dq=representation+theory+of+groups+graviton&source=bl&ots=V1yJNQTRxo&sig=w0iPIXGL-h4o0fUE_zgQPfFuoGw&hl=en&ei=sFY5S7CDGYygkQWIrvCACQ&sa=X&oi=book_result&ct=result&resnum=3&ved=0CBsQ6AEwAg#v=onepage&q=&f=false). I'm not sure who first put forward the argument.

publius
2009-Dec-29, 01:52 AM
I'm skeptical both of "unifying" forces and of quantizing everything.

That the graviton should have a spin of 2 is something I've seen for almost
five decades now. When and how was that prediction made?

-- Jeff, in Minneapolis

You'd have to get an expert on QFT to explain that, but basically, it comes from the math of how the various entities are represented. EM is a so-called "vector field", which means the potential function (a mathematical entity) from which the field is derived can be represented by a vector (a 4-vector in the case of the EM, the four-potential). That somehow corresponds to a spin-1 carrier particle in the quantum math.

GR gravity, however comes from a rank-2 tensor potential entity (the metric basically plays the role of potential). That requires a spin-2 quantum thingy, at least according to how such thingies are now understood.


-Richard

Tensor
2009-Dec-29, 02:10 AM
I'm skeptical both of "unifying" forces and of quantizing everything.

That the graviton should have a spin of 2 is something I've seen for almost
five decades now. When and how was that prediction made?

-- Jeff, in Minneapolis

It a result of the math used, not a prediction, per se. loglo's given you one way, within a linearized gravity theory. Hang with me while I go into some background for another reason. Tensors are used in math for a generalization or extension of scalars and vectors. A rank 0 tensor is a scalar, a rank 1 tensor is a vector. Higher order tensors would describe the relationship between two vectors or even tensors.

Jeff, I know you've heard of the stress-energy tensor in GR. That is a rank 2 tensor. The Magnetic and Electrical forces are described by a four vector. Which is a rank 1 tensor (there is the EM force tensor which is rank 2, but that describe the relationship between the Electric and Magnetic force vectors not one force). Now, in a simplified way, the rank of the tensor is kinda sorta the quantum spin (and I won't go any further as I'm pushing my limit here).

Now, odd number spins can have attractive and repulsive components (the + and - of charge). Even number spins are only attractive. So, since gravity uses a rank two tensor, it has a quantum spin of 2 (which is why speculations on anti-gravity, at least using GR, are so easy to dismiss).

Tensor
2009-Dec-29, 02:17 AM
... from which the field is derived can be represented by a vector (a 4-vector in the case of the EM, the four-potential). That somehow corresponds to a spin-1 carrier particle in the quantum math.

Richard. It corresponds because a vector is a rank 1 tensor. Rank 1, spin 1. Rank 2 spin 2. How this is represented with 1/2 or zero spin particles, well, like I said, the why (I think it may have something to do with the way forces,not particles are represented) is a bit beyond my current understanding.

OldAstrofan
2009-Dec-29, 11:03 PM
Yes galaxies are kept together by gravity (and also galaxy clusters) and it appears that there maybe a very massive black hole at the centre of most if not all known galaxies.
In our Milky Way galaxy there is believed to be a super massive BH which all the stars rotate about mostly within the spiral arms. Finding that these black holes appear to exist in each galaxy has given some scientists a reason to believe that they may have actually played a part in the formation of the galaxies rather than the other way round, but I don't have the details on such.


The super massive BH commonly called Saggitaius A* (forgive me if i get the spelling wrong) has already been proven to be a BH unless our theories of massive objects are incrorrect cuz the mass is millionsX solar mass (way beyond the critrion of BH formation)
Also check out this article if you really are interested:
"Quasar induced galaxy formation:a new paradigm ?
D. Elbaz1, K. Jahnke2, E. Pantin1, D. Le Borgne3,4, and G. Letawe5
Astronomy & Astrophysics manuscript no. elbaz˙he0450˙astroph"