Register | Sign In


Understanding through Discussion


EvC Forum active members: 65 (9162 total)
1 online now:
Newest Member: popoi
Post Volume: Total: 915,817 Year: 3,074/9,624 Month: 919/1,588 Week: 102/223 Day: 13/17 Hour: 0/0


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   What is Time and Space
cavediver
Member (Idle past 3643 days)
Posts: 4129
From: UK
Joined: 06-16-2005


Message 36 of 204 (227822)
07-30-2005 2:05 PM
Reply to: Message 30 by GDR
07-30-2005 10:23 AM


Re: Uniform Time
I don't think there is a point in asking how we measure the expansion because I don't imagine there is any hope in me understanding the answer
Hmmm, I think you seriously underestimate my explanatory powers
Remember that the light gets stretched as it crosses the expanding universe? We see this in the redshift of the observed light. If we look at the spectrum of light from a distant galaxy, we see lots of peaks and troughs in the intensity occuring at different frequencies. The peaks and troughs are emssion lines and absorption lines. We know at what frequency they should occur. As we look at further away galaxies, we see the lines occur at lower and lower frequencies: the light has been red-shifted. If we compare the red-shift between two galaxies whose distance we know, we can see how red-shift changes by distance, and we can determine the expansion of the universe.
Would an observer on Neptune have a totally different estimate of the age of the universe as us?
Different, yes. Totally different, absolutely not. The difference is totally negligible. Remember the balloon blowing up... the galaxies we draw on the balloon are not moving. In reality, they might be, slightly. And our galaxy is rotating so we are moving. And Earth is orbiting the sun, etc, etc. But all of these motions, relative to the expanding universe, are negligible. To make a difference you would have to accelerate to a good fraction of c, and then you would be moving with significant velocity with respect to the Galaxy. You would get a different answer to the age of the universe, but it would also be rather obvious that all of the galaxies around you were moving at high velocity with respect to you.

This message is a reply to:
 Message 30 by GDR, posted 07-30-2005 10:23 AM GDR has replied

Replies to this message:
 Message 38 by GDR, posted 07-30-2005 5:36 PM cavediver has not replied

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


Message 40 of 204 (227955)
07-30-2005 6:56 PM
Reply to: Message 39 by Son Goku
07-30-2005 6:26 PM


However the problem is no objects "current proper time" existed even 500,000 years after the big bang.
What about the 10^28 hydrogen atoms in my body? Are they good enough? (the 28 is a guess...) Or forget them, what about my electrons? Oh, no, wait... there is only one electron, isn't there
It was an issue in 70s cosmology and is now considered largely unimportant.
I think I can see why...
This message has been edited by cavediver, 07-30-2005 06:58 PM

This message is a reply to:
 Message 39 by Son Goku, posted 07-30-2005 6:26 PM Son Goku has not replied

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


Message 41 of 204 (228070)
07-31-2005 10:10 AM
Reply to: Message 37 by GDR
07-30-2005 5:08 PM


Re: Unobserved Moon
The infinite thing is obviously more difficult. My perception of the BB goes back to E=MC2, but I don't understand the BB as a point but as infinite energy and zero mass. If that is correct then it makes sense to me that the universe beyond the event horizon is infinite because we started off with infinite energy at t=0. If we started with infinite energy then we can't have a finite universe now.... can we?
Hmmm, as soon as I see E=Mc^2 I reach for my gun The reason for all the confusion is that we have more than one concept of mass... there is rest-mass of a particle. This is a quantum concept. There is mass in the sense of "mass" that curves space-time. This is a relativistic concept. There is "relativistic mass" of a "moving" object. This is an unhelpful concept. I could spend days talking about the differences and the confusion that reigns to this day.
For now, let's stick to relativity: the mass within a volume of space is a measure of how curved that space is within that volume. If we measure the mass of the universe today, that mass was always there, all the way back to the singulatity.
Talking about "energy" is just as bad. But, yes, an infinite universe started off with infinite energy and a finite universe started off with finite energy.
Picturing things like that kinda makes infinity seem somewhat sensible.
Absolutely. To help even more, Penrose came up with a method of drawing diagrams, which we call "Penrose Diagrams" no less. Actually, Brandon Carter came up with the idea too, so we should really call them Carter-Penrose Diagrams. We rarely did, unless Brandon was in the room and then I used to as he scared me They enable us to draw infinite spaces in finite drawings.
I got thinking about what I said about time 0 not being a point but infinite energy. According to string theory everything is little strings of energy. If that is true it would make sense that at t=0 it wasn't a point. (Wouldn't it?)
Certainly possible. That was one of the big hopes for string theory, that it would "smear" out the classical singularity. Some of my early work was modelling black holes in string theory, and seeing how the concept of singularity changed.

This message is a reply to:
 Message 37 by GDR, posted 07-30-2005 5:08 PM GDR has replied

Replies to this message:
 Message 42 by GDR, posted 07-31-2005 11:00 AM cavediver has replied

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


Message 43 of 204 (228117)
07-31-2005 2:40 PM
Reply to: Message 42 by GDR
07-31-2005 11:00 AM


Re: Unobserved Moon
I mostly see what you are saying about mass. It is really helpful as all through my reading I was wondering why I couldn't understand just what mass was. In my mind I couldn't get away from thinking of it as matter.
Yes, this is a great part of the misconception. Matter usually has mass but mass is not matter. Mass is gravitational charge, the equivalent of electrostatic charge. The difference is that gravity itself carries gravitational charge, where as photons are electrically neutral... and a good job too, because if photons were electrically charged, there would be no such thing as sight!
Couldn't the mass that has existed from the time of the BB have been in the form of energy at t=0?
Well, having just said that mass is charge, this now doesn't make a lot of sense. If you want to ask
quote:
was all the matter that we see around us in the form of energy at the BB
then I would say yes, sort of. The trouble is by talking about the matter fields at the BB we are pushing beyond the boundaries of GR and even simple quantum gravity: we're into full blown TOE.
And is the universe open or closed? Well, I would like it to be closed on aesthetic grounds. But I'm not that bothered. And don't forget that the chances that our observed 4d universe is the real "universe" are slim... it is much more likely part of a much larger multiverse/encompassing existence, which may embed our universe or more bizarrely "project" our universe.
Don't you just love the way that every one of your questions launches fifty more? That's why I love this subject... Every other science is just so obvious: Chemistry? It's just atoms isn't it. Biology/evolution... it's just chemicals isn't it

This message is a reply to:
 Message 42 by GDR, posted 07-31-2005 11:00 AM GDR has replied

Replies to this message:
 Message 44 by GDR, posted 07-31-2005 7:46 PM cavediver has not replied
 Message 97 by GDR, posted 08-10-2005 2:46 PM cavediver has replied

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


Message 47 of 204 (228346)
08-01-2005 6:10 AM
Reply to: Message 46 by GDR
07-31-2005 10:42 PM


I'm having trouble with this concept. Are you saying that all gravitons attract all gravitons? If this is the correct is it true then that the gravitational field not only holds stars, planets etc in place but also hold itself in some sort of cosmic balance?
Absolutely. Most of my favourite space-times (solutions to GR) have no matter in them at all... we call them vacuum solutions. But they are highly non-trivial. The most simple black-hole solution, the Schwarzschild metric, has no matter in it. It has a measurable mass but this simply comes from the warped space-time.
But it's not that strange. Of our four fources, only photons don't exhibit this behaviour. Gluons interact with gluons and the Weak bosons self-interact. As I alluded to earlier, it is the fact that photons don't self-interact (well, they do, but not straight-forwardly) that gives rise to the possibility of sight. Photons can travel from an object to our eyes without being disrupted by interactions with other photons crossing their path.

This message is a reply to:
 Message 46 by GDR, posted 07-31-2005 10:42 PM GDR has replied

Replies to this message:
 Message 48 by GDR, posted 08-01-2005 9:42 PM cavediver has not replied
 Message 52 by GDR, posted 08-02-2005 4:30 PM cavediver has not replied

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


Message 54 of 204 (228943)
08-02-2005 6:06 PM
Reply to: Message 49 by Son Goku
08-02-2005 1:53 PM


It certainly isn't for the lay reader
Well, I guess you're right... but Roger thinks it is which is the funniest part!
We had this great head-to-head lecture series where Hawking and Penrose gave their respective views on a number of things. Atiyah was hosting, and at the end of the first one he asks
quote:
OK, if anyone has any "relevant" questions, then fire away
The silence was defeaning

This message is a reply to:
 Message 49 by Son Goku, posted 08-02-2005 1:53 PM Son Goku has replied

Replies to this message:
 Message 55 by Son Goku, posted 08-02-2005 6:57 PM cavediver has not replied

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


Message 68 of 204 (229713)
08-04-2005 11:32 AM
Reply to: Message 67 by GDR
08-04-2005 11:03 AM


Re: interesting in this
Sorry, haven't got time to stop for a long post... but gravity does not pass information instanteously (or superluminally as we would say.) The graviton is massless and thus moves at the speed of light, as does the photon. There are cranks out there that would claim that gravity "acts" "instantaneously" or at some very high multiple of c. Neither of these concepts actually have any sensible meaning and just reveal the crank nature of their proponents

This message is a reply to:
 Message 67 by GDR, posted 08-04-2005 11:03 AM GDR has replied

Replies to this message:
 Message 69 by GDR, posted 08-04-2005 1:56 PM cavediver has replied

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


Message 77 of 204 (229863)
08-04-2005 6:40 PM
Reply to: Message 72 by Son Goku
08-04-2005 2:26 PM


Spoken like the great geometradynamicist I used to be (and still am on good days )
Sadly, too much time in quantum gravity has really persuaded me otherwise... at its most basic, given that the T in G=8piT has to be (T), it is very hard to see how G cannot be (G)... and belive me, I spent years trying to refuse this point.
But calling a graviton a particle is possibly where you think that gravitons are the domain of evil particle physicists (like Weinberg and his "no black hole" ideas). A graviton to me is simply the quantisation of the space-time... don't forget that a particle is merely a single root mode of the fourier transform of a quantum FIELD. QFT makes particle physics look like GR, not the other way round... despite what most particle physicists think and tell you!
[Edit to make the bra-kets appear... using () instead of the usual]
This message has been edited by cavediver, 08-04-2005 07:04 PM

This message is a reply to:
 Message 72 by Son Goku, posted 08-04-2005 2:26 PM Son Goku has replied

Replies to this message:
 Message 78 by GDR, posted 08-04-2005 6:46 PM cavediver has replied
 Message 81 by Son Goku, posted 08-04-2005 7:12 PM cavediver has replied

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


Message 79 of 204 (229874)
08-04-2005 7:01 PM
Reply to: Message 69 by GDR
08-04-2005 1:56 PM


Re: interesting in this
In "The Fabric of the Cosmos" Greene says that if the moon were to disappear we would see the gravitational affects on the Earth immediately whereas we would have to wait for a second and a half to see the light disappear. (Pg 63) Is this wrong or am I misinterpretting it?
Does he really say that??? Are you sure? If he does, then yes, he is wrong. But then he is a particle physicist so I shouldn't expect anything more
Actually, though, what he is talking about is not possible (as in the moon disappearing) and he reveals his "ignorance" of GR by suggesting the scenario. Only magic or an act of God could remove the moon in total. No physical mechanism exists to reproduce the effect. You could disintegrate it, but all the mass would still be there. You could annihilate it with an anti-moon, and all the energy would still be there. It could be swallowed by a passing black hole or worm-hole, but again, its mass would continue to affect things, only now as part of the hole. The moon being there and then the moon not being there is comparing two different realities, and there is no super-metric to be able to define an "immediately" between those realities. And of course, as any student of SR will immediately say... "immedaitely in whose frame?" In one frame, the earth would react before the moon disappeared and in another, it would react after the moon disapperaed.

This message is a reply to:
 Message 69 by GDR, posted 08-04-2005 1:56 PM GDR has replied

Replies to this message:
 Message 82 by GDR, posted 08-04-2005 7:42 PM cavediver has replied
 Message 112 by madeofstarstuff, posted 08-12-2005 3:14 PM cavediver has replied

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


Message 80 of 204 (229876)
08-04-2005 7:02 PM
Reply to: Message 78 by GDR
08-04-2005 6:46 PM


B*gg*r, my expectation bra-kets didn't come out properly... no wonder you were confused. I'll edit it, then see if it's any better...
No, thought not

This message is a reply to:
 Message 78 by GDR, posted 08-04-2005 6:46 PM GDR has not replied

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


Message 83 of 204 (229891)
08-04-2005 7:50 PM
Reply to: Message 81 by Son Goku
08-04-2005 7:12 PM


I'm expressing the view of some that gravity mightn't be a quantum field, or that current QFT doesn't apply to it.
I'd certainly agree with the last part of that, as has been repeatedly demonstrated. But then it's not really surprising, given its nature. One way to think of it would be that at tree level, you'd expect pair creation of black holes, but of course the black holes themselves are intrinsically not describable at tree-level or at any order of a perturbative expansion.
As for gravity not being a quantum field, I would agree that it doen't appear as the other quantum fields. But then, if the other fields are generated Kaluza-Klein style, it makes more sense. Gravity is then THE quantum field, and the other fields appear from the dimensionally reduced quantum metric. Of coures, you need fermions in there as well, so this becomes supersymmetric. And this is our classic SUperGRAvity.
As it stands the quantum aspects of gravity (or if you ask some, the general relativistic aspects of QM) haven't been worked out, so for now I'm suggesting that the lay reader stick with General Relativity.
I totally agree... just look at GDR's reply to my post
As a side note, are you from the String theory camp or the Loop Quantum Gravity camp?
Well, like you I started as an astrophysicist. Then became a dyed-in-the-wool relativist. I got dragged into string theory, but as we were a relativity group, the approach was very geometrical, with a big de-emphasis on the particle side. But I kept a very open mind, and was very much into alegbraic approaches. That's where I got my cross over into the loop community, really through the sort of stuff John Baez now pushes. But these days, I'm more into the philosophy of it all. Which is why I'm a big fan of Penrose.

This message is a reply to:
 Message 81 by Son Goku, posted 08-04-2005 7:12 PM Son Goku has not replied

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


Message 84 of 204 (229893)
08-04-2005 7:56 PM
Reply to: Message 82 by GDR
08-04-2005 7:42 PM


Re: interesting in this
Oh, I am pleased... he certainly didn't seem that screwed up 10yrs ago when I last spoke to him... but he's still a particle physicist

This message is a reply to:
 Message 82 by GDR, posted 08-04-2005 7:42 PM GDR has replied

Replies to this message:
 Message 85 by GDR, posted 08-04-2005 8:48 PM cavediver has replied
 Message 89 by GDR, posted 08-08-2005 1:57 PM cavediver has not replied

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


Message 86 of 204 (229913)
08-04-2005 9:25 PM
Reply to: Message 85 by GDR
08-04-2005 8:48 PM


Re: interesting in this
I'm trying to go to bed, but ok...
G=8piT is Einstein's Equation. It is the core of GR. G is the Einstein Tensor no less, and T is the stress-energy tensor. Essentially , G is geometry (of space-time) and T is energy and matter in space-time.
But we know that energy and matter are quantised: so if T is quantised, then so must G ( as G=T), and thus the geometry is quantised.
QFT is quantum field theory. There are two real-world quantum field theories: Electro-Weak and QCD (quantum chromodynamics) and between them they explain three of our four "forces": em, weak nuclear and strong nuclear. A quantum field is very like our rubber sheet space-time- it's a sheet extending through-out spacce-time. A particle is the smallest sized bump you can have in the sheet. Particles can be created... an up-bump and a down-bump appear and move apart. And particles can be destroyed... an an up-bump and a down-bump come together and "annihilate". This is in great contrast to quantum mechanics, where partciles cannot be created nor destroyed. There is essentially one sheet for each "force" and additional sheets for each type of matter partcile (fermions). Gravitons are just bumps in the space-time rubber sheet.
The reason that Gravity is so different is that each of the non-gravitational sheets stretches over the space-time sheet. The shape of the total space is governed by the space-time sheet... the other sheets just follow the shape.
Bed
[edit to correct thousands of typos... but I'm tired and can't be bothered]
This message has been edited by cavediver, 08-04-2005 09:27 PM

This message is a reply to:
 Message 85 by GDR, posted 08-04-2005 8:48 PM GDR has replied

Replies to this message:
 Message 88 by GDR, posted 08-05-2005 9:02 AM cavediver has not replied

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


Message 95 of 204 (231711)
08-10-2005 5:13 AM
Reply to: Message 94 by GDR
08-09-2005 6:20 PM


Re: Gravity?
I understand that under GR gravity is acceleration
No, gravity is curvature.
Remember in my initial discussion on SR, we all have a 4-velocity in space-time. The question is in what direction is this velocity pointing? What track through space-time do we take? Well, just like everyday experience we follow "straight" lines. We call these geodesics. Think of the aircraft following a great circle path around the earth... it's not straight in the usual definition, but it is the straighest path over a curved globe. It is the path something will move along if pushed and then does not subsequently accelerate.
Mass/energy curves space-time, so the geodesics are curved away from naive ideas of straight. Discovery has just returned from following a straight line which happens to be so curved it closes up into a "circular" path all the way around the earth. The orthogonal straight line (geodesic) to the orbit is a path straight towards the earth. If you "fall" along these paths, you are following the curvature of space-time. You experience no acceleration, no force. Hence being weightless both in orbit and in free-fall (ignoring air-resistance). To deviate from a geodesic, you need to exert a force.
This is the crux... it led Einstein to GR. The 1g we experience on the earth is not gravity. It is the up-push from the earth forcing us off our preferred geodesic which is to "fall" to the centre of the earth.
If we increase this force, by use of Discovery for instance, we increase this 1g force and we move further away from the geodesic. In fact, with Discovery, we exert sufficient force to move us onto a new orbital geodesic.
If you jump out of plane, nothing accelerates. You just move naturally. It is, as you say, the surface of the earth accelerating towards you.
It also is difficult to understand how the Earth is accelerating upward at both the north and south poles simultaneously
Of course it's diificult, it's GR But it is true. Acceleration does not imply "movement". It's a problem of trying to interchange 3d concepts and 4d concepts... it doesn't work.
And this is all totally local. The space-time around the earth is massively dominated by the earth, such that the sun, moon, distant stars and galaxies have no appreciable effect.

This message is a reply to:
 Message 94 by GDR, posted 08-09-2005 6:20 PM GDR has replied

Replies to this message:
 Message 96 by GDR, posted 08-10-2005 10:42 AM cavediver has not replied

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


(1)
Message 101 of 204 (232044)
08-10-2005 6:41 PM
Reply to: Message 99 by Son Goku
08-10-2005 2:54 PM


Re: Projection
This is what the whole projection thing is about
That's certainly part of it... and I claim a small credit in having a very long and detailed discussion with Lenny Susskind about black hole mappings when he was just developing his ideas. He's a particle physicist really, so needed a little coaching It all seems many lifetimes ago now
But the HP is only one aspect of what I am talking. If you scan over my posts, you should find my piece on how our space-time (target space in string speak) is just a projection from the "real" 2d string worldsheet. I love this because it reduces everything to the mathematics/geometry of Riemann surfaces. I still find this exceptionally aesthetic.

This message is a reply to:
 Message 99 by Son Goku, posted 08-10-2005 2:54 PM Son Goku has not replied

  
Newer Topic | Older Topic
Jump to:


Copyright 2001-2023 by EvC Forum, All Rights Reserved

™ Version 4.2
Innovative software from Qwixotic © 2024