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geordief

An article on GR and why it is so complex

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This is the first time I have come across this explanation  set out in this way.

 

https://www.forbes.com/sites/startswithabang/2019/12/04/this-is-why-scientists-will-never-exactly-solve-general-relativity/#7b33b37734a8

 

It was not a surprise to me as the intimations have been there in the background up to now...

 

The gist seems to be that, (so far) one can learn all there is to know about the mathematics of GR  but its precise application (even a two body problem) is ,not even tantalizingly  completely out of bounds.

 

Does it seem a good article to any one else?

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I thought it was a good article. I posted a link to it in another thread because I think it makes a good counter-argument to the idea that the universe "is mathematics". Or, at least, if it is then it is made of mathematics which is unsolvable. Which is functionally much the same as it not being mathematical!

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if it is mathematical,  the reason it's seems unsolvable could be that math evolved from simpler elements, which are only as of yet unsolved. It is not unreasonable to assume math can be reverse engineered at some point. Didn't Bertrand Russel propose it sprang from logic?

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So ,instead of trying to model the Universe  as we find it you suggest we try to model it as it should be?

Sounds like a step back  unless it is one step back and two steps forward

We should be asking DTrump for advice as he is the chaos man.

You might be right about B Russel-maybe that was what turned me off him (or maybe it was his bad rep at the time)

Edited by geordief

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3 hours ago, geordief said:

This is the first time I have come across this explanation  set out in this way.

 

https://www.forbes.com/sites/startswithabang/2019/12/04/this-is-why-scientists-will-never-exactly-solve-general-relativity/#7b33b37734a8

 

It was not a surprise to me as the intimations have been there in the background up to now...

 

The gist seems to be that, (so far) one can learn all there is to know about the mathematics of GR  but its precise application (even a two body problem) is ,not even tantalizingly  completely out of bounds.

 

Does it seem a good article to any one else?

 

Yes, I agree with Ethan, but I think he is skirting round the issue and it would be very easy indeed for a lay person to get the wrong idea.

Quote

In General Relativity it isn't the net force acting on an object that determines how it moves and accelerates, but rather the curvature of space (and spacetime) itself.

 

I find this misleading because there may be other forces acting on the object that do not 'curve' spacetime. Yet these forces still influence the motion of the body.

Newton was not specific (or choosy if you like) about which forces contributed to his net force , Ethan appears to be.

 

Quote

This is exactly what General relativity is  - a series of coupled differential equations -

True enough, but other than stating that theya re difficult to impossible to 'solve' Ethan nowhere explains what he means by a solution.

Nor does he mention the uncomfortable fact that any solution to any differential equation can have arbitrary things added to it.

This is true even of the Newtonian wave equation.

Nor does he mention the other (essential) part of solving a differential equation - that of including the boundary conditions to achieve any useful solution.

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5 minutes ago, studiot said:

 

Yes, I agree with Ethan, but I think he is skirting round the issue and it would be very easy indeed for a lay person to get the wrong idea.

 

I find this misleading because there may be other forces acting on the object that do not 'curve' spacetime. Yet these forces still influence the motion of the body.

Newton was not specific (or choosy if you like) about which forces contributed to his net force , Ethan appears to be.

 

True enough, but other than stating that theya re difficult to impossible to 'solve' Ethan nowhere explains what he means by a solution.

Nor does he mention the uncomfortable fact that any solution to any differential equation can have arbitrary things added to it.

This is true even of the Newtonian wave equation.

Nor does he mention the other (essential) part of solving a differential equation - that of including the boundary conditions to achieve any useful solution.

I took the article to be aimed at the layperson and that it benefited  from keeping it simple.

I am half way through the book you recommended. (Einstein's Popular Exposition 1952 edition)

It does not disappoint and I was pleased to find that I could follow  the SR section  although I had to go over it around 3 times .

The GR  half is shaping up to be quite a bit harder.

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2 minutes ago, geordief said:

I took the article to be aimed at the layperson and that it benefited  from keeping it simple.

I am half way through the book you recommended. (Einstein's Popular Exposition 1952 edition)

It does not disappoint and I was pleased to find that I could follow  the SR section  although I had to go over it around 3 times .

The GR  half is shaping up to be quite a bit harder.

Glad you are making progress.

:-)

So long as you don't get the incorrect impression that GR completely determines motion. It doesn't

And it can be difficult to later unlearn a half truth you were told to keep things simple.

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5 minutes ago, studiot said:

Glad you are making progress.

:-)

So long as you don't get the incorrect impression that GR completely determines motion. It doesn't

And it can be difficult to later unlearn a half truth you were told to keep things simple.

Well I suppose part of my brain might have just tacitly accepted that  (but it is not a question that keeps me awake at night or hopefully a preconception I would revisit on anyone else)

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14 hours ago, studiot said:

Nor does he mention the other (essential) part of solving a differential equation - that of including the boundary conditions to achieve any useful solution.

Very true.
The boundary conditions are the 'physics' that must be applied to the mathematical model ( the diff. equations ) to obtain any kind of sensible solution.

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