Register to reply

What IS the Spacetime fabric?

by ShadowKnight
Tags: fabric, spacetime
Share this thread:
ShadowKnight
#1
Sep12-03, 12:26 PM
P: 55
I've been reading a lot about gravity lately, trying to understand it as best I can. When I think of gravity, I find it easiest to imagine the rubber sheet analogy to explain the spacetime curvature. Gravity is supposed to be caused by the curvature of spacetime. So the thing that I have no insight on is the 'fabric' of spacetime. What is it? It's talked about like it's a physical object in many texts that I've read. "A tear in the fabric of spacetime." You need something to tear or put a hole in, right? A spinning black hole will spin spacetime near the event horizon - spin what? It's also the medium that gravitational waves travel through. This again requires (in my mind) some kind of physical object, ie. sound waves travel through air - air is the physical object.

Another puzzling thing I've seen several times is the idea that the spacetime fabric travels faster than c as the universe expands and has done so since the big bang. The fact that it has always been faster than c satisfies the theory that nothing can CROSS the light barrier, but if the spacetime fabric is some sort of physical object, wouldn't it have mass? How can something with mass go faster than c - that would require infinite energy, wouldn't it? Would a (hypothetical) observer who is outside of our spacetime see the universe and everything in it moving faster than c (relative to their stationary position outside of spacetime) with the 'flow' of the spacetime fabric?

I'd appreciate any insight any of you could provide me on this. Any links would be very helpful as well.
Phys.Org News Partner Astronomy news on Phys.org
Fermi finds a 'transformer' pulsar
Astrophysicists model the formation of the oldest-known star in our galaxy
Hubble traces the halo of a galaxy more accurately than ever before
wolram
#2
Sep12-03, 12:52 PM
PF Gold
wolram's Avatar
P: 3,682
http://www.abc.net.au/science/k2/moments/s275021.htm
this link is a starting point.
but i think the stepping stones of this subject
are very insecure.
ShadowKnight
#3
Sep12-03, 01:06 PM
P: 55
Thank you Wolram - this article did provide some insight as well as a place to start. This article suggests that spacetime is made of one of three things:
1. Graviton particles (has the graviton ever been detected?)
2. Cosmic strings
3. Quantum foam
I've done reading on 2 & 3 and am fascinated by both, but it never occured to me that either of these could actually BE the spacetime fabric. Still, I'd appreciate any additional insight on this subject that any of you can provide. I'm still curious about my second question in the original post.

Eh
#4
Sep12-03, 01:20 PM
P: 683
What IS the Spacetime fabric?

Curved spacetime often gets called the aether of Einstein. But it is a little misleading, since the aether of old is out the window. To once again quote the Ask an Astronomer website:

"....First of all, space-time is not a fabric. Space and time are not tangible 'things' in the same way that water and air are. It is incorrect to think of them as a 'medium' at all. No physicist or astronomer versed in these issues considers space-time to be a truly physical medium, however, that is the way in which our minds prefer to conceptualize this concept, and has done so since the 19th century. Back then physicists talked of an ether. Today we know that ethers of the kind that behave like a physical medium are simply not present.

We really do not know what space-time is, other than two clues afforded by quantum mechanics and general relativity. General relativity as developed by Albert Einstein, says, and this is a direct quote from Einstein, that

"Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field"."


The gravitational field is spacetime. It can exist in the absence of matter and energy as a field. Lee Smolin briefly covers the concept of the field in "Three Roads to Quantum Gravity".
meteor
#5
Sep13-03, 10:24 AM
P: 915
the general theory of relativity says that nothing can travel faster than light in spacetime, but not forbids spacetime itself to surpass this limit
i agree that spacetime has to be composed of some substance, after all gravity waves are ripples of spacetime
marcus
#6
Sep13-03, 10:37 AM
Astronomy
Sci Advisor
PF Gold
marcus's Avatar
P: 23,084
Originally posted by Eh
...this is a direct quote from Einstein, that

"Space-time does not claim existence in its own right, but only as a structural quality of the [gravitational] field"."[/i]
this is a good quote
can you supply a source
I've seen Einstein quoted to this effect in other places but
have never managed to track down the book or article or interview by him where he said it
always someone else quoting einst. without a precise reference

thanks in advance if you have a lead on this!
marcus
#7
Sep13-03, 11:39 AM
Astronomy
Sci Advisor
PF Gold
marcus's Avatar
P: 23,084
Eh, thanks for the Einst. quote.
I found a reference to a piece he wrote in 1952
and appears in the umpteenth edition of one of his books
so I posted the link (to a longer passage with that quote in it)
in math forum
Eh
#8
Sep13-03, 11:51 AM
P: 683
Was it also in his rather philosophical book, "Relativity: The Special and the General Theory"?
Eh
#9
Sep13-03, 11:54 AM
P: 683
Originally posted by meteor
i agree that spacetime has to be composed of some substance, after all gravity waves are ripples of spacetime
Define substance.
marcus
#10
Sep13-03, 11:58 AM
Astronomy
Sci Advisor
PF Gold
marcus's Avatar
P: 23,084
Yeah it was something he added in 1952 to the 15th edition of that book.

Eh, the quote you raised opens up the Great Wormcan of Relativity
Rovelli spends a couple of chapters discussing it including
how Einstein got hung up on it for three years 1912-1915
(called the problem "the meaning of the coordinates")

Rovelli's draft book online at his website has the best philosoph
discussion of these questions I know, you may know others

crux is that in general relativity the field has physical meaning
but the equation relating it and matter is invariant under smooth deformations so that

space-time points have no physical meaning---do not exist

there is no Newtonian absolute space and time upon which
these fields are defined

it is not like in 9th grade where you got handed a piece of absolute graph paper and drew curves and stuff on it

the theory is background-independent

so any background you use is just a provisional convenience and replaceable at whim by a smooth deformation

points in spacetime have no meaning, only EVENTS like
the intersection of two worldlines have meaning
because a crossing of paths remains a crossing of paths even after a smooth deformation ('diffeomorphism')

the main equation of GR is "diffeomorphism-invariant" and thus the theory is background independent and cannot be based on
some arbitrarily chosen absolute space-time

this has been hard for everyone to assimilate

and has interfered with attempts to build a quantum GR
because quantum theories tend to be built on an absolute space-time (the 9-th grader's piece of graph paper, or the world as recommended to us by Messers Newton and Minkowski)

If the history of science issues interest you, do you have a link to Rovelli's book----the history of western conception of space and time is fascinating and the whole thing sort of comes to the fore
in quantizing GR.
meteor
#11
Sep13-03, 12:02 PM
P: 915
I define substance like something that exists, in contraposition of a hypothetical pure vacuum. In a pure vacuum gravity waves cannot exist (there's nothing to wave). For example in LQG, the substance can be the same loops. In string field theory that substance can be the string fields (though I'm not sure in this case)
thed
#12
Sep13-03, 12:29 PM
P: n/a
Originally posted by meteor
In a pure vacuum gravity waves cannot exist (there's nothing to wave).
One of the hardest things to accept in modern physics is that waves can propagate without a medium.

The idea first comes in with Maxwell's Equations. An EM wave is self propagating , even in a vacuum. Put another way, the speed of an EM wave is not related to the pressure of anything, just two constants, the electric permittivity and magnetic permeability of free space.

What may help in understanding spacetime is that it is 4D whereas our sense are only used to 3. When we refer to spacetime 'bending' it bends in 3D, it may be flat in 4D.

To appreciate this I like the analogy Kip Thorne uses in 'Black Holes and Worm Holes". We assume that light has constant velocity in vacuo, more precisely it is invariant between inertial frames, and follows the shortest distance between two points. We also know that gravity causes things to accelerate in it's presence. So what happens when light passes through a gravitational field?

The answer to the conundrum is that it changes direction, the speed is scalar so does not change, but change direction you change velocity and it accelerates. But that means light follows a curved path in the prescence of gravity.

What we are seeing is light following the shortest path in 4D, which to us is a curve in 3D. The path it follows is spacetime. It is purely a geometric construct.

Did that make sense?
wolram
#13
Sep13-03, 03:21 PM
PF Gold
wolram's Avatar
P: 3,682
not if you read this..............


http://www.intalek.com/Index/Project.../glossary1.pdf

"the great speed of light cover up".
Hurkyl
#14
Sep13-03, 03:43 PM
Emeritus
Sci Advisor
PF Gold
Hurkyl's Avatar
P: 16,099
I wonder how superluminal transmission of Beethoven's 40'th symphony sounded.
thed
#15
Sep13-03, 04:41 PM
P: n/a
Originally posted by wolram
not if you read this..............
http://www.intalek.com/Index/Project.../glossary1.pdf
"the great speed of light cover up".
Got as far as Tom Bearden and starting laughing.
http://www.google.com/search?q=tom+b...utf-8&oe=utf-8
ShadowKnight
#16
Sep13-03, 06:58 PM
P: 55
Originally posted by thed
What may help in understanding spacetime is that it is 4D whereas our sense are only used to 3. When we refer to spacetime 'bending' it bends in 3D, it may be flat in 4D.
My senses are certainly only used to 3D
Can anyone out there explain this analogy to me using a 2D / 3D example? I feel like I'm almost to an understanding but the light hasn't fully come on yet.
meteor
#17
Sep13-03, 08:56 PM
P: 915
I think that the comparation of light with gravity waves is erroneous
Light is not a vibration of some pre-existing field. It's a self-propagating wave with electric and magnetic fields continously inducing one another
Gravity waves are vibrations of some pre-existing field: the gravitational field (aka spacetime)
marcus
#18
Sep14-03, 10:55 AM
Astronomy
Sci Advisor
PF Gold
marcus's Avatar
P: 23,084
Originally posted by meteor

Gravity waves are vibrations of some pre-existing field: the gravitational field (aka spacetime)
what you say here about gravity waves is consistent with
what i've read

but what you say about the electric field not being "pre-existing"
worries me. I think of the electric field as existing throughout, before some particular vibration comes into the picture

maybe the fields and the vibrations in them are different in character but the difference is not exactly this "pre-existence"
condition


Rovelli has an interesting comparison of Maxwell eqn with
Einstein eqn on page 34,35 of his draft book, showing parallels and historical development

I agree with you that too simple a comparison of light with gravity is erroneous, but I may have misunderstood the precise distinction you make between them


Register to reply

Related Discussions
What is the fabric of the universe? Astronomy & Astrophysics 45
Is this the fabric of the universe? Special & General Relativity 1
From 'the fabric of the cosmos' Special & General Relativity 5
Pushing against fabric vs wall Introductory Physics Homework 4
Gravity fiels and spacetime fabric General Physics 1