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Author Topic:   General Relativity Question
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 16 of 39 (271549)
12-21-2005 6:45 PM
Reply to: Message 15 by Son Goku
12-21-2005 2:16 PM


Re: Jumpimg about
what do you think of Loop Quantum Gravity?
You keep asking me, and I keep forgetting to reply!
And now I'm going to bed after seeing Kong for the second time... even better than the first time!
So for now: Loop has some wonderful features but I still love the unification of everything (matter, space and forces) in SuperGravity, and by association String and M-theory. Loop still has a lot more work on the matter side of things.
Off to bed...

This message is a reply to:
 Message 15 by Son Goku, posted 12-21-2005 2:16 PM Son Goku has not replied

Replies to this message:
 Message 17 by Fabric, posted 12-21-2005 9:00 PM cavediver has not replied

  
Fabric
Member (Idle past 5672 days)
Posts: 41
From: London, England
Joined: 02-27-2005


Message 17 of 39 (271572)
12-21-2005 9:00 PM
Reply to: Message 16 by cavediver
12-21-2005 6:45 PM


Re: Jumpimg about
Hi peeps, theres something that has been bugging me for a while now & i was hoping you could help me out please..
ive read that gravity is not a force but the bending/wraping of space/time by heavy bodies such as the sun ect... I understand this clearly.
ive also read that physicist's who study quantum mechanics are looking for a force carryer for gravity called the" Graviton". But what i dont understand is that if gravity is the warping of space\time and not a force, why are we looking for a particle that carrys a gravitational force, it confuses me
I hope this makes sense, i think it does... Thanks 4 your time

This message is a reply to:
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Iblis
Member (Idle past 3895 days)
Posts: 663
Joined: 11-17-2005


Message 18 of 39 (271579)
12-21-2005 9:27 PM
Reply to: Message 17 by Fabric
12-21-2005 9:00 PM


off topic answer to the wave/particle/field quandary
It's because what the quantum physics guys call a particle isn't the same thing as what you are thinking of. They just call them that so you will understand that they are talking about what's going on down below the atomic level. But really, they are "entities" used to define fields, gravity is a field, the field warps spacetime, most fields do but not to such a great extent or so incredibly weakly, they hope to measure and define the properties of the entity associated with this field.

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Fabric
Member (Idle past 5672 days)
Posts: 41
From: London, England
Joined: 02-27-2005


Message 19 of 39 (271587)
12-21-2005 10:03 PM


i have tried looking on the web before about fields & the gravity field, but i hav'nt come across anything that explains it in laymens term's.
You say that the Gravity field wraps spacetime, i dont understand that , what is a gravity field, also i dont really understand what fields are in genral tbh.
should i start a new thread...
cheers

Replies to this message:
 Message 20 by Son Goku, posted 12-21-2005 10:20 PM Fabric has not replied
 Message 24 by Iblis, posted 12-22-2005 7:04 PM Fabric has replied

  
Son Goku
Inactive Member


Message 20 of 39 (271592)
12-21-2005 10:20 PM
Reply to: Message 19 by Fabric
12-21-2005 10:03 PM


ive read that gravity is not a force but the bending/wraping of space/time by heavy bodies such as the sun ect... I understand this clearly.
ive also read that physicist's who study quantum mechanics are looking for a force carryer for gravity called the" Graviton". But what i dont understand is that if gravity is the warping of space\time and not a force, why are we looking for a particle that carrys a gravitational force, it confuses me
Your basically asking a foundational question that relates to a thing called background independance. (And one that a lot of people miss)
There is quite a bit to it and several side issues that'll take more than one post.
I'd be happy to explain, but would you prefer here or in a new thread?

This message is a reply to:
 Message 19 by Fabric, posted 12-21-2005 10:03 PM Fabric has not replied

  
Fabric
Member (Idle past 5672 days)
Posts: 41
From: London, England
Joined: 02-27-2005


Message 21 of 39 (271598)
12-21-2005 10:59 PM


Thanks Son Goku for helping me out, if you could expain it to me that would be great,
I think we should start a new thread...
cheers

  
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 22 of 39 (271619)
12-22-2005 5:27 AM
Reply to: Message 18 by Iblis
12-21-2005 9:27 PM


Re: off topic answer to the wave/particle/field quandary
It's because what the quantum physics guys call a particle isn't the same thing as what you are thinking of. They just call them that so you will understand that they are talking about what's going on down below the atomic level. But really, they are "entities" used to define fields, gravity is a field
Good answer.
gravity is a field, the field warps spacetime, most fields do but not to such a great extent
Be careful. It would be better to say that gravity IS the warping of space-time. The other fields then can act as sources for this warping. There is a big difference between the grav field and the other fields.
Warning: advanced bit!!!!
Mathematically, space-time is some manifold, M. Gravity is a connection on the tangent bundle. The other fields are connections on other bundles over M.
In higher dimensional theories (Kaluza-Klein, SuperGravity, String, M-theory, etc), the idea is that you only have gravity*, but as you dimensionally reduce down to 4 dimensions, the higher dimensional gravity sectors are partitioned off and become the other fields.
*well, maybe not quite as simple as this, but it will do for now
This message has been edited by cavediver, 12-22-2005 05:30 AM

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 Message 18 by Iblis, posted 12-21-2005 9:27 PM Iblis has not replied

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 Message 23 by Fabric, posted 12-22-2005 2:15 PM cavediver has replied

  
Fabric
Member (Idle past 5672 days)
Posts: 41
From: London, England
Joined: 02-27-2005


Message 23 of 39 (271748)
12-22-2005 2:15 PM
Reply to: Message 22 by cavediver
12-22-2005 5:27 AM


Re: off topic answer to the wave/particle/field quandary
sorry i dont really understand what fields are, if you dont mind could you explain in laymans terms, what they are, Im keen to learn
cheers

This message is a reply to:
 Message 22 by cavediver, posted 12-22-2005 5:27 AM cavediver has replied

Replies to this message:
 Message 28 by cavediver, posted 12-23-2005 5:57 AM Fabric has replied

  
Iblis
Member (Idle past 3895 days)
Posts: 663
Joined: 11-17-2005


Message 24 of 39 (271816)
12-22-2005 7:04 PM
Reply to: Message 19 by Fabric
12-21-2005 10:03 PM


So long as we stick with gravity and how qm clashes melodiously with general relativity I guess we will be ok?
You said you understood the idea of a heavy object sitting in spacetime and thereby warping things toward it. I expect you have heard the analogy where you have something like a pencil or a wineglass (a rolly object) lying on a mattress (a warpable surface) and it lies still just fine, but then when you stick a more massive object in the middle of the mattress, like a typewriter or a television, all that stuff suddenly rolls right towards because it changes the shape of the mattress and thereby warps stuff toward it.
This is an excellent analogy from the general relativity viewpoint in that it causes people to nod their head about how it works even though they can't think in Einstein-level math all day. But really it doesn't explain anything, and neither does the math, it just says that gravity affects the math in a specific definable way. And that's fine for gravity, for the moment.
Meanwhile though light for example, one of our other relativity goodies in the math, has some properties that we have a lot more trouble getting people to swallow with simplistic analogies and universal math. For example light behaves like a wave, like sound for example, but has no medium to be traveling through. In other words, something is traveling, like what the atomic chemists would call a particle would, but it doesn't have any mass and moves in all directions at once, unlike a particle. The word "wavicle" was invented at this point, and some of the guys actually doing the math were able to nod their heads at that, but really it isn't a very good explanation either even for scientists.
Menwhile the guys who actually make the bombs and run the accelerators and generally investigate new ways of blowing up the earth, they were seeing something odd. Based on the math that they did, relating to very small things rather than large areas like relativity, some of the fundamental components they were breaking up particles into had equally odd impossible properties.
We ended up with a bestiary of different pseudo-particles in two families, the fermions and the bosons. The bosons, like the photon that turns out to explain light, take up space but have no mass. The fermions have mass, but don't take up any space! This, you will notice, makes it fairly hard for the mass to be the thing that is directly warping the space, it not having any of its own to work with.
Some of the bosons though, the pseudo-particles that relate to space and are expressed as fields, they do warp space, and they are attached to the fermions in normal matter. Sadly, the ones we have actually seen so far in the course of smashing things do not include the graviton, the one that would theoretically mediate the field that holds the universe together in the big vague way that general relativity requires. We have seen the ones that mediate the fields that hold atoms together though, so we certainly aren't giving up, the general idea is to find one like that only much Much weaker.
Cavediver is pointing out that some of the new attempts at unified theories are suggesting that perhaps we don't need another particle at all, perhaps the entities that hold neutrons together could also make gravity work if you just added a whole gob of extra dimensions instead, with one version of the force being diluted by going in every direction through uncounted branes and another being strengthened by having its force trapped in a very small circular dimension curved in on itself. But really, it's all gibberish until someone manages to catch signs of it in one of the big particle accelerators. (And only slightly more sensible for years afterward, I expect.)
Anyway to summarize, fields are the expression of the quantum "particles" that have no mass and mediate space, the bosons. Electromagnetism, the stuff that makes light work, is a field. Gravity, the stuff that warps smaller masses towards larger ones, is a field. Fields tend to express themselves at the maximum speed possible in our spacetime, C = 186000 miles or 300000 kilometers per second, and in all directions at once. The other set of particles, the fermions, which do have mass, are normally bound up in these fields in various ways, which is lucky because the relationship between the two seems to make our spacetime what it is.
*Oh, and the reason these little guys are allowed to break all the rules is because they are technically smaller, shorter or less whatever than one of the measurably smallest possible things that are normally possible in our curved spacetime. For example, nothing that has mass and takes up space can possibly be smaller than Planck's Constant (the distance light travels in the smallest measure of time). Yet when we smash things that are almost that small already in two, we still get stuff. But, it's stuff that does one or the other, not both.

This message is a reply to:
 Message 19 by Fabric, posted 12-21-2005 10:03 PM Fabric has replied

Replies to this message:
 Message 25 by Fabric, posted 12-23-2005 4:35 AM Iblis has replied
 Message 26 by cavediver, posted 12-23-2005 5:08 AM Iblis has replied

  
Fabric
Member (Idle past 5672 days)
Posts: 41
From: London, England
Joined: 02-27-2005


Message 25 of 39 (271924)
12-23-2005 4:35 AM
Reply to: Message 24 by Iblis
12-22-2005 7:04 PM


Thanks Iblis for your reply, i understand pretty much everything you wrote, thats what i was looking for, Im just a simple warehouse worker but find it amazine that things can be so small,
I find the quantum world amazine, it's really weird !
With your post im going to stick bits of it in google one at a time and try to learn more,
Nice 1 cheers

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 Message 24 by Iblis, posted 12-22-2005 7:04 PM Iblis has replied

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 26 of 39 (271928)
12-23-2005 5:08 AM
Reply to: Message 24 by Iblis
12-22-2005 7:04 PM


But really, it's all gibberish
Unfortunately, that pretty much sums up your post.
And you were doing so well earlier...
I expect you have heard the analogy where you have something like a pencil or a wineglass (a rolly object) lying on a mattress (a warpable surface) and it lies still just fine, but then when you stick a more massive object in the middle of the mattress, like a typewriter or a television, all that stuff suddenly rolls right towards because it changes the shape of the mattress and thereby warps stuff toward it.
This is an excellent analogy from the general relativity viewpoint
No, this is not the analogy. This is a misrepresentation of the analogy. Things do not suddenly start rolling towards the depression. This relies on "gravity" to start the rolling!
The bosons, like the photon that turns out to explain light, take up space but have no mass. The fermions have mass, but don't take up any space! This, you will notice, makes it fairly hard for the mass to be the thing that is directly warping the space, it not having any of its own to work with.
Depending on how you look at it, both bosons and fermions take up space, or neither take up space. Mass is not a function of taking up space.
Anyway to summarize, fields are the expression of the quantum "particles" that have no mass and mediate space, the bosons.
Bosons are both massless (photon, gluon) and massive (W, Z). More importantly, both bosons and fermions are described as fields.
Fields tend to express themselves at the maximum speed possible in our spacetime, C = 186000 miles or 300000 kilometers per second, and in all directions at once.
This makes no sense.
Cavediver is pointing out that some of the new attempts at unified theories
No, I am talking about our ideas that cover the last 80 years. Hardly new.
perhaps the entities that hold neutrons together could also make gravity work if you just added a whole gob of extra dimensions instead
This is totally backwards
with one version of the force being diluted by going in every direction through uncounted branes and another being strengthened by having its force trapped in a very small circular dimension curved in on itself.
I didn't even mention branes... they are irrelevant in what I am discussing.
For example, nothing that has mass and takes up space can possibly be smaller than Planck's Constant (the distance light travels in the smallest measure of time).
Planck's constant is not a distance, it is an action. There is the Planck length. I have no idea by what "takes up space" means.
quote:
Thanks Iblis for your reply, i understand pretty much everything you wrote
  —Fabric wrote
Oh dear...
This message has been edited by cavediver, 12-23-2005 05:17 AM

This message is a reply to:
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Replies to this message:
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 Message 30 by Iblis, posted 12-23-2005 2:03 PM cavediver has replied

  
Fabric
Member (Idle past 5672 days)
Posts: 41
From: London, England
Joined: 02-27-2005


Message 27 of 39 (271934)
12-23-2005 5:42 AM
Reply to: Message 26 by cavediver
12-23-2005 5:08 AM


well cavediver, why dont you explain it to me then, In Laymens terms what are fields,and how do they work, i would like to understand what fields are , also at the quantum level are there really particles, Basically i would like to know what fundamental matter is made out of. is it just energy flying around doing random stuff on a small scale but on a bigger scale it looks nice a smooth and makes up a football for instance?? cheers

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 Message 26 by cavediver, posted 12-23-2005 5:08 AM cavediver has replied

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 28 of 39 (271935)
12-23-2005 5:57 AM
Reply to: Message 23 by Fabric
12-22-2005 2:15 PM


Re: off topic answer to the wave/particle/field quandary
sorry i dont really understand what fields are, if you dont mind could you explain in laymans terms
Ok, let's try this...
I will start with a classical field. Essentially a field is a value defined at each point in space and time. This can be a simple value, like temperature. So around the surface of the earth, each point has an air temperature. This is a field. Becaue the value at each point is a simple number, we call this a scalar field.
Similarly, at each point there is a wind speed and direction. Now a speed and a direction is a vector, so wind is a vector field around the earth: at each point there are three numbers describing the vector.
Another scalar field is air pressure. Interestingly, it is the differences in air pressure that give rise to the wind, so these two fields (wind vector and pressure scalar) are intimately related. BUT, it is local differences in air pressure that give rise to local wind conditions. There is no "action at a distance", just a long chain of pressure changes causing wind causing nearby pressure changes causing neaby wind causing further away pressure changes causing further away wind causing distant pressure changes... etc. You get the idea.
Gravitation is very similar. There is a field throughout the universe, and at each point there are 10 numbers (so more complicated than a vector, this is a tensor) which describe how far away are neighbouring points. This field is called the metric, as it is how measurements are made up.
If you want to find the distance between two distant points, you add up (integrate) all of the metric distances between the infinitessimally close points that make up the jouney from your starting point to your end point. Critically, this includes distances in time as well as space.
What we think of as everyday normal space, is just space with no curvature: the metric distances are the same in all directions at all points. This is flat space(and time). When the numbers change, the distances change, and we get curvature.
Just as with the wind and air pressure, curvature at one point will cause curvature at a nearby point (to keep things consistent) and this will cause curvature at a further away point, etc, etc.
Objects travel through space-time in straight lines at constant speed. If space becomes curved, we travel along the straighest line possible... this makes us think there is a "force" called gravity pulling us in strange directions. This is just us trying to follow our straight paths.
If you think that there are times you are not moving at all, you are are simply moving purely in the time direction. If we add a nearby planet, it causes changes to metric next to it, which causes changes further out, which causes changes all the way out to where you are. Your straight line pointing straight up through time gets bent by the changed metric and now points slightly towards the planet (but still predominantly up through time), and you see this as a motion towards the planet. But there is no force. You were always moving, it is just the direction that has slightly changed.
Ok, this has gone a bit too far into GR. We are talking about fields. Remember, a field is simply a number (or set of numbers) at each point of space(and time). There are rules as to what values these numbers can take. Einstein's Equation is the set of rules for the metric field, Maxwell's Equations are the rules for the classical electromagnetic field. Wind and air pressure have their own set of meteorological rules.
Think over this, and if you are happy let me know, and I will continue into quantum fields which is where your question really lay anyway.

This message is a reply to:
 Message 23 by Fabric, posted 12-22-2005 2:15 PM Fabric has replied

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cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 29 of 39 (271936)
12-23-2005 6:00 AM
Reply to: Message 27 by Fabric
12-23-2005 5:42 AM


i would like to understand what fields are , also at the quantum level are there really particles, Basically i would like to know what fundamental matter is made out of. is it just energy flying around doing random stuff on a small scale but on a bigger scale it looks nice a smooth and makes up a football for instance??
So, just the simple stuff then?

This message is a reply to:
 Message 27 by Fabric, posted 12-23-2005 5:42 AM Fabric has not replied

  
Iblis
Member (Idle past 3895 days)
Posts: 663
Joined: 11-17-2005


Message 30 of 39 (272059)
12-23-2005 2:03 PM
Reply to: Message 26 by cavediver
12-23-2005 5:08 AM


more gibberish
Oh grand, I'm stupid after all! I was hoping that would be the case, it takes some of the responsibility off me.
While you are poking at me though, would it be possible for you (or some smart person) to help me understand the "triplets" paradox from the original post better? The way I read it, if I'm not missing something important, the third body that stays in the new inertial frame can calculate what order things took place in on the first and second body and come up with a different answer than the first body who stayed in the original inertial frame.
This isn't an issue of how long it takes light to tell them what happens, either, the paradox continues even after you convert distance (light years) into time (years). Is that right?
If so, is it only the acausality of the relationship between the bodies that makes this work? How does the universe guarantee all such relationships will be acausal?
So far the responses I've seen on this part of the question don't make enough sense to me, they include a lot of instantaneous or near-instantaneous transitions in acceleration that I don't believe occur in the real world, presumably to make things simpler, but resulting in the untrue impression that as you suddenly make a u-turn in space all that time catches up with you, kind of a thing. No one suddenly makes u-turns at near-light speeds though, so I'm trying to visualize what it would really look like as you slowly decelerate to a stop and then accelerate back whence you came.
PS: if you are going to make the "real" version of the warped mattress analogy work you are going to have to pitch pennies at it while floating in free fall yeah, but I've never yet seen anyone afflicted with it who didn't start nodding well before then, so it tends to get truncated in the classrom.
Getting people to understand the actual "twins" paradox is not a problem, they could read Heinlein in "Time for the Stars" over the weekend. But I'm still not finding any sense in this 3 bodies problem, is the assumption about the "other" inertial frame even true or is it backwards?

This message is a reply to:
 Message 26 by cavediver, posted 12-23-2005 5:08 AM cavediver has replied

Replies to this message:
 Message 31 by cavediver, posted 12-23-2005 3:52 PM Iblis has replied

  
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