Curved spacetime?

[PeterJ]

That's an odd comment Alan. Space-time has not been shown to be a fabric or to distort. It is modelled that way. The existence of space-time is not a question.for physics but for metaphysics, same as for the existence of anything. As Ajb says, physicists can be sloppy in their language on existential questions. We should be careful when reading it. I mentioned Mohrhoff above because he has interesting things to say about this issue.

[michel123456]

 

That could sort the problem out - but that in itself doesn't suggest that a viable quantum theory of gravity could be assembled - I just don't know.

 

My moderate interest in this stems from the sort of comment I referred to earlier in which it is often claimed that an expanding universe entails the expansion of spacetime itself. This is not quite the same as saying that an expanding universe has to be described by an expanding curved coordinate system.It implies that spacetime is something which exists in itself and that could be described as expanding - in fact, the expansion of the surface of a balloon which is being blown up is often cited as an analogy - the two dimensional surface of the balloon being analogous to spacetime.

 

Take this as an example: "If we examine a piece of `empty' space we see it is not truly empty, it is filled with spacetime, "

 

from here: http://abyss.uoregon.edu/~js/ast123/lectures/lec17.html

 

and the following from Stanford : " however, the equations of general relativity are perfectly consistent with spacetimes that contain no matter at all. Flat (Minkowski) spacetime is a trivial example, but empty spacetime can also be curved, as demonstrated by Willem de Sitter in 1916."

 

from here: http://einstein.stanford.edu/SPACETIME/spacetime2.html

 

However, this quote is also given:

"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' ".

 

also from Stanford: http://einstein.stanford.edu/content/relativity/q909.html

I agree completely with you, JonG.

The "fabric of spacetime" is the worst analogy ever.

And there is no doubt in my mind that the explanations including the "curvature of spacetime" are hiding circumventing a more profound reality.

Edited January 6, 2014 by michel123456

[swansont]

The "fabric of spacetime" is a metaphor, not a definition.

 

edit: xpost with michel

[PeterJ]

Probably a silly question, but would it be possible to model gravity as a distortion of time in a flat space, or a distortion of space in a flat time, or must it be a distortion of both?

Edited January 6, 2014 by PeterJ

[Alan McDougall]

The "fabric of spacetime" is a metaphor, not a definition.

 

edit: xpost with michel

 

Thank you!

 

And it bends or distorts around a huge gravity field.

In Einstein’s theory of general relativity, he put forth the radical idea that space is not just a static, empty arena in which celestial objects go about their business. Instead, he described our universe as a sort of fabric that is curved and distorted by the objects within it. Sound crazy?

 

Maybe. But researchers at Northwestern and their colleagues within a world-wide scientific collaboration have developed ways to detect and measure these disturbances in the fabric of space.

 

Vicky Kalogera, professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences,

 

 

How can we think of space as “fabric?” And what causes these disturbances?

 

First, imagine space as an elastic membrane, spread flat. Now, set a big ball on the membrane. It will cause the membrane to sag. This is similar to how an object with mass, like a planet, affects the geometry of space. Although we cannot perceive this distortion in everyday life, aside from strong gravitational fields on Earth, it is measurable in the motion of planets due to our Sun’s gravity.

 

Now, picture another object moving in a circle—an orbit—in this distorted, sagged membrane. This creates regular disturbances that propagate through space like a wave. We call them gravitational waves, because it is gravity that keeps objects in orbit and disturb the “membrane” of space. It’s like the ripples that follow when you throw a rock in a lake.

 

In order for these waves to be large enough for us to detect them on Earth, they have to be caused by very massive objects orbiting around one another at speeds approaching the speed of light, like two black holes.

 

A black hole is an object in space with a very high mass in relation to its size, making it extremely dense. For example, a black hole with the mass of the Earth would only be an inch across—very, very dense! This makes the gravitational pull of black holes incredibly strong—so strong that even light cannot escape.

 

We know black holes exist—we’ve observed 20 of them so far, but only visually and through X-rays. Measuring the gravitational waves they produce will give us a new way to study them.

Edited January 6, 2014 by Alan McDougall

[PeterJ]

So, you don't agree that 'fabric of space-time' is a metaphor? Or can mass distort metaphors?

[Alan McDougall]

So, you don't agree that 'fabric of space-time' is a metaphor? Or can mass distort metaphors?

 

In this case yes, this metaphor that you dislike, exists as a reality, if you like it or not, and mass distorts the the thing that has been "metaphorically put" to explain a complex idea.

 

Einstein once quoted "if you can explain something to your granny", then you understand it yourself.

 

Maybe you know something Einstein did not on space-time, if you do,please enlighten us?

 

Read the paragraph below again please!

 

In Einstein’s theory of general relativity, he put forth the radical idea that space is not just a static, empty arena in which celestial objects go about their business. Instead, he described our universe as a sort of fabric that is curved and distorted by the objects within it. Sound crazy?

[MigL]

The name of the brilliant but pompous physicist/astronomer was Eddington, not Ellington, Alan.

 

Space-time and its various non-Euclidian geometries are a mathematical model which does a damn good job of describing reality.

 

It is very simple to describe a real kitchen table mathematically on a computer, and perform all sorts of operations on it such as rotations and translations, using Cartesian co-ordinates. This does not mean, in any way, that the geometric , cubic construct of X, Y and Z co-ordinates is real also.

 

This is all GR and any other theory is doing, nothing more, nothing less.

Edited January 6, 2014 by MigL

[swansont]

So Einstein used the metaphor/analogy. What exactly is that supposed to prove?

[PeterJ]

Wow. One comment and I get a patronising earful. That might be a record.

Edited January 7, 2014 by PeterJ

[Alan McDougall]

The name of the brilliant but pompous physicist/astronomer was Eddington, not Ellington, Alan.

 

Space-time and its various non-Euclidian geometries are a mathematical model which does a damn good job of describing reality.

 

It is very simple to describe a real kitchen table mathematically on a computer, and perform all sorts of operations on it such as rotations and translations, using Cartesian co-ordinates. This does not mean, in any way, that the geometric , cubic construct of X, Y and Z co-ordinates is real also.

 

This is all GR and any other theory is doing, nothing more, nothing less.

 

I stand corrected, yes "Ellington" was a pompous physicist,, my mistake!

 

I also remember something about Ellington, when someone asked him if it were true that only three people in the world really understood special relativity, he replied by asking, who is the other one? (Assuming only he and Einstein understood it) However, because this also comes from my fallible memory I will check it out to verify if it is correct!

So Einstein used the metaphor/analogy. What exactly is that supposed to prove?

 

Who said Einstein or I tried to prove anything by his quote, I just used the analogy to help explain the topic to those who are interested and don't have your level of knowledge on the subject. I did mention Ellington's and his test with the position of the planet "Mercury during an eclipse" Mercury appeared from behind the moon before it should have, and the best explanation , is that space bend to make appear earlier than precise prediction said it should.

Edited January 7, 2014 by Alan McDougall

[Strange]

 

The name of the brilliant but pompous physicist/astronomer was Eddington, not Ellington, Alan.

I stand corrected, yes "Ellington" was a pompous physicist,, my mistake!

 

My irony meter just broke.

[swansont]

Who said Einstein or I tried to prove anything by his quote, I just used the analogy to help explain the topic to those who are interested and don't have your level of knowledge on the subject.

 

It's unclear the point you were trying to make, and whether you were agreeing with those who say it was a metaphor or those who think spacetime is some sort of material.

 

 

I did mention Ellington's and his test with the position of the planet "Mercury during an eclipse" Mercury appeared from behind the moon before it should have

 

I have highlighted the part you got right.

[Strange]

The missing words include "Eddington" and "stars".

 

Here is the briefest (and reasonably accurate) description of the experiment I have found: http://www.edn.com/electronics-blogs/edn-moments/4415308/Einstein-s-theory-of-general-relativity-is-tested--May-29--1919

[MigL]

The data that Sir Arthur Eddington collected during the eclipse of 1919 ( I believe ) did not actually support the conclusions he drew from it. The data did not conclusively show that light bends the required amount around the sun; it was much too scattered.

 

Whether he chose to publish because of his strong belief in relativity or other reasons is unknown.

 

He may have been brilliant but I've disliked him since reading about his treatment of Sebrahim Chandrasekhar.

[Strange]

He may have been brilliant but I've disliked him since reading about his treatment of Sebrahim Chandrasekhar.

 

Maybe that was Ellington. Unless you mean Subrahmanyan Chandrasekhar.

[Alan McDougall]

 

Maybe that was Ellington. Unless you mean Subrahmanyan Chandrasekhar.

 

I have sinned against man and god my transgressions are unforgivably especially in light of the fact that my memory has been proved very fallible by greater minds than I

 

Finally EDDINGTON

 

It's unclear the point you were trying to make, and whether you were agreeing with those who say it was a metaphor or those who think spacetime is some sort of material.

 

 

 

I have highlighted the part you got right.

 

space-time must, therefore, be a 4-dimensional object.

 

 

http://einstein.stanford.edu/content/relativity/q411.html

 

What is a space time continuum?

In 1906, soon after Albert Einstein announced his special theory of relativity, his former college teacher in mathematics, Hermann Minkowski, developed a new scheme for thinking about space and time that emphasized its geometric qualities. In his famous quotation delivered at a public lecture on relativity, he announced that,

 

"The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality."

 

This new reality was that space and time, as physical constructs, have to be combined into a new mathematical/physical entity called 'space-time', because the equations of relativity show that both the space and time coordinates of any event must get mixed together by the mathematics, in order to accurately describe what we see. Because space consists of 3 dimensions, and time is 1-dimensional, space-time must, therefore, be a 4-dimensional object. It is believed to be a 'continuum' because so far as we know, there are no missing points in space or instants in time, and both can be subdivided without any apparent limit in size or duration. So, physicists now routinely consider our world to be embedded in this 4-dimensional Space-Time continuum, and all events, places, moments in history, actions and so on are described in terms of their location in Space-Time.

[PeterJ]

This link is to an article by John Bell discussing Hermann Weyl's views on the continuum. Weyl points out (as I read him) that a true continuum is not extended. I believe that it is mistake to think that Weyl is wrong, since when we do we have to reify space-time and create a paradoxical continuum. We have then reduced metaphysics to a muddle.

 

http://publish.uwo.co/~jbell/Hermann%20Weyl.pdf

[Alan McDougall]

This link is to an article by John Bell discussing Hermann Weyl's views on the continuum. Weyl points out (as I read him) that a true continuum is not extended. I believe that it is mistake to think that Weyl is wrong, since when we do we have to reify space-time and create a paradoxical continuum. We have then reduced metaphysics to a muddle.

 

http://publish.uwo.co/~jbell/Hermann%20Weyl.pdf

 

Is this the place to bring up metaphysics?

[PeterJ]

I didn't bring it up. Whoever brought up the reality of space-time brought it up. It is a metaphysical question.

 

Sorry. Problem with the link, Here it is again.

 

http://publish.uwo.ca/~jbell/Hermann%20Weyl.pdf

 

(Something weird is happening with my posts here, so apologies for any repetitions.)

.

Edited January 8, 2014 by PeterJ

[Alan McDougall]

I didn't bring it up. Whoever brought up the reality of space-time brought it up. It is a metaphysical question.

 

Sorry. Problem with the link, Here it is again.

 

http://publish.uwo.ca/~jbell/Hermann%20Weyl.pdf

 

(Something weird is happening with my posts here, so apologies for any repetitions.)

.

 

Who is ,"Whoever" ?

[PeterJ]

Is English not your language?

[StringJunky]

PeterJ, on 08 Jan 2014 - 2:16 PM, said:

I didn't bring it up. Whoever brought up the reality of space-time brought it up. It is a metaphysical question.

It can be a metaphysical question but because it's been asked in the physics section it will be in the context of established physics and how it is viewed or modelled from a scientist's point of view rather than a philosopher's.

[Alan McDougall]

Is English not your language?

 

Who is "Whoever"?

I didn't bring it up. Whoever brought up the reality of space-time brought it up. It is a metaphysical question.

 

.

 

Who is "Whoever" in your quote above?

 

Is English not your language?

 

Do I write so badly?

http://io9.com/how-does-spacetime-get-bent-560618783

[PeterJ]

Sorry, I just wondered why you didn't know what 'whoever' meant.

Hi Stringjunky - I take your point. Fair enough. It is just that I see no purpose in trying to decide a metaphysical question in physics. It makes for an interesting discussion but it is not going go anywhere. We have to say that 'fabric of space-time' and 'curved space-time' is a metaphor. To establish that is it is any more than this would not be possible in physics. The idea that it is any more than a metaphor is not a testable scientific theory.

 

I thought I was agreeing with Swansont but giving the reason for the necessity of his agnostic view. I didn't mean to suggest we should talk about metaphysics, just that there is a limit to the discussion within physics.

 

Also, I felt that Alan (if he is proposing the reality of the 'fabric') might not realise that he was proposing a metaphysical theory.