Understanding the Spacetime Fabric

[Thikr]

So, I don't get it... I know Einstein said that space-time was a "fabric" but I can't visualize that, with us being in three dimensions.i Can visualize in my head (or at least understand) how space and time are really the same thing (or to my knowledge, time is just a component of space, a dimension of space more specifically). I know the "Trampoline Example", that is not the issue... I know that gravity bends the space-time fabric, but what IS the fabric? Thanks guys!

 

[bcrowell]

It's just a metaphor. It doesn't work if you try to take it literally.

 

[Thikr]

A metaphor or an example of how gravity works? I figured there was no physical fabric out in space, what is it a metaphor for?

 

[stevmg]

It's just a metaphor. It doesn't work if you try to take it literally.

Einstein wrote in flowy Germanic dialogue...

There is no fabric as bcrowell stated above. Even the trampoline analogy isn't good. But there is something to spacetime that is not nothing (double negative intended), probably some kind of potential energy that binds the universe together and has no mass itself. Otherwise, how could spacetime bend as Einstein, Hilbert and Eddington so brilliantly proved?

Steve

 

[stevmg]

Here is a "supposed" conjecture based on GR how gravity can work...

Imagine an object with mass that "distends" spacetime and thus what is "pushed out" is pushed back in. That's one example.

Another possibility is that the curvature of spacetime causes worldlines to bend and time marches on inexorably, so the curvature of worldlines means acceleration which means gravity.

I will not go further as this forum is NO PLACE for conjecture. So, guys, don't jump on me too much as I won't make up any hypotheses beyond that.

 

[Thikr]

Here is a "supposed" conjecture based on...
...any hypotheses beyond that.

Actually, that really made things simpler for me! I'm fine with knowing that the analogy i didn't understand was sort of inaccurate anyways. Anyhoo, don't jump on stevmg, I wouldn't mind hearing hypotheses, as I have yet to get any college education on physics so you can't really corrupt me yet. What I didn't get was the "distends" example, or at least I don't think I understand it fully.

EDIT: Photoshop didn't work so well, so I'll have to put this into words... ahem...

Ok, some exceedingly large object by Earth standards (like the sun) actually pushes space outwards, stretching the "insides" and pushing space out. Things in space (people, in super anti-heat suits), move to this "stretched" space, at the core of the sun, because it is "less dense", yet things like photons, which are not as affected by gravity simply go through space as normal. Since space has been "pushed" away from the sun, photons and the like are "pushed" with space. Am I sort of on the right track?

EDIT*: Oh, and I wanted to clarify that I am aware that empty space does not have any "density" per se. Also, I wanted to clarify that I know nothing about particle physics so don't flame me if my "photon" assumptions are totally wrong or just sound funny.

 

[stevmg]

The distended space is the least likely of my conjectures to be correct. The bent worldlines of GR are more likely. If time were to stand still, then there would be no gravity as the worldlines wouldn't be expanding in strainght lines or curves or whatever.

The parable of the two travelers in the first edition of Taylor/Wheeler's Spacetime Physics describes this.

Conjecture----------------------Hypothesis---------------------------Theory
Wild *** Guess-----------Scientific Wild *** Guess-----------"Proven" Wild *** Guess

*** is a word that begins with an "A" and ends in two "s" 's - a perfectly legitimate Air Force Term

To wit:

Conjecture - I was placed on aspirin to help prevent heart attacks and I bleed more easily and for a longer time than before, so aspirin is a blood thinner

Hypothesis - A whole lot of people who take aspirin to prevent heart attacks bleed more easily now, therefore aspirin probably is a blood thinner

Theory - We matched 500 random people who were placed on aspirin for cardiac prophylaxis versus 500 age-matched controls not placed on aspirin and the aspirin takers had statistically significant as well as clinically significant prolonged bleeding times compared to the non-aspirin takers. So, aspirin really is a blood thinner.

Stephen M. Garramone, MD, Col (Ret), USAF, MC

 

[Thikr]

Hehe, I forgot to mention that my formal education extends only to projectile motion and basic trigonometry... I don't want to force you to be my college professor but may I ask what world lines and all that humbug was about?

 

[stevmg]

Geez -

Can't do that in a short time and I am new at this also. General Relativity frm A to B by Robert Geroch which is cheap will explain it better than I can.

stevmg

 

[Thikr]

Cool, thanks man. I'll check it out, and thanks for all of the help!

 

[stevmg]

I'll give you a quick and dirty lesson -

Imagine a one-dimensional world - the x-axis with the origin at (0, 0)

The second component is the ict axis or time (the y-axis)

An item will exist at a posiiton in space - x and ict (time). This is a two dimensional world but only one dimension that you can see - s. Later, you generalize this to the three dimensional world + the invisible 4th dimension - ict.

A person at x= 1 and ict = 0 will not sit there. Time will march on and he will then be at 1, 1) then 1, 2) (1, 3) etc. etc. He's always moving in time but there is no perception of motion. If the straight line up is curved, then he experiences acceleration. If it is straight but at an angle, he experiences velocity (sees the x-axis moving) but no acceleration. This upward bound line is called a worldline

------------ict---worldline
------------|-------|
------------|-------|
________________________________x

The analytic geometry is different. One uses the Pythagorean Theorem in reverse

uses (ict)² - x² to get proper time

The pictures in Geroch's book explain it all and not too much extraneous detail to confuse you.

This is just a kickstart. Geroch does not go into the mathematics of it.

stevmg

 

[DrGreg]

If you consider an object moving in a straight line in space, with no gravity, you can represent its motion with a space-time graph, a graph of distance against time plotted on flat two-dimensional paper. We call the line a "worldline".

When there is gravity, you have to draw the graph on a curved surface instead. Imagine a shape like the output-end of a trumpet. You can't draw a literally straight line on a curved surface, but you can draw a line that is "as straight as possible", and that line represents the motion of an object falling freely under gravity.

There's a picture at www.relativitet.se/spacetime1.html.

 

[stevmg]

If you consider an object moving in a straight line in space, with no gravity, you can represent its motion with a space-time graph, a graph of distance against time plotted on flat two-dimensional paper. We call the line a "worldline".

When there is gravity, you have to draw the graph on a curved surface instead. Imagine a shape like the output-end of a trumpet. You can't draw a literally straight line on a curved surface, but you can draw a line that is "as straight as possible", and that line represents the motion of an object falling freely under gravity.

There's a picture at www.relativitet.se/spacetime1.html.

Better answer than mine, DrGreg...

 

[Troponin]

There are some 256,000 words in the English language. Most of those are archaic forms that have fallen out of favor, so the "actual" number is much less.

Giving worded analogies of what the mathematics is saying is always a slippery slope. Use the idea of a "fabric" as a vague basis to build upon. If you have the mathematical background...rely on that for concrete definitions.

 

[stevmg]

Einstein wrote in German... "Fabrik?"?

Was bedeutet das?