I have a couple of questions about general relativity

[Uri Zlatnik]

TL;DR

Could general relativity be wrong and quantum mechanics on the right track in figuring the true cause of gravity and how it's connected to electromagnetism?

From a very young age, we are often told that space is a vacuum and a vacuum contains no matter and energy.

What we are also told from a very young age is that something cannot come from nothing.

So if a vacuum contains no matter or energy or even nothing at all then how can it curve or contain any fabric at all?

Also since we don't have a theory of quantum gravity, meaning we don't know how quantum mechanics and general relativity are connected, does it mean it could be a bit misguided to trust a theory written in 1916 which was 111 years ago?

Personally I would a bit hesitate to trust a theory written 111 years ago during War World 1 when people back then were probably ignorant of what time, energy, space, dimensions and what these things really are and what they really mean.

 

[Matterwave]

So if a vacuum contains no matter or energy or even nothing at all then how can it curve or contain any fabric at all?

I don't know about you, but if I look around I see stuff. Do you not?

Also since we don't have a theory of quantum gravity, meaning we don't know how quantum mechanics and general relativity are connected,

It is true that we don't have a working theory of quantum gravity, but we have a working theory of how the quantum field theories work inside classical background gravity. This is called quantum field theory in curved spacetimes. It is only expected to break down when, roughly, gravity is "important at Planck scales".

does it mean it could be a bit misguided to trust a theory written in 1916 which was 111 years ago?

Why?

Personally I would a bit hesitate to trust a theory written 111 years ago during War World 1 when people back then were probably ignorant of what time, energy, space, dimensions and what these things really are and what they really mean.

Quantum mechanics is maybe 10 years younger than GR if you count from Heisenberg and Schroedinger's formulations. It's older than GR if you start from the Photoelectric effect or from Planck or Bohr.

Do you let age guide all your decisions?

 

[FactChecker]

Personally I would a bit hesitate to trust a theory written 111 years ago during War World 1 when people back then were probably ignorant of what time, energy, space, dimensions and what these things really are and what they really mean.

The accuracy of the theory's predictions have been verified to astonishing accuracy up to the current day. GR adjustments are included in the GPS devices that we use every day. Without the GR adjustments, GPSs would be unusable. You may want to interpret the theory in different ways and there are still a lot of unanswered questions, but the accuracy of its calculations can not be questioned.

 

[Ibix]

it could be a bit misguided to trust a theory written in 1916 which was 111 years ago?

Imagine that tomorrow we find a complete Grand Unified Theory. There would be no new physics to discover. In a hundred years, would you mistrust it because it was old? In a thousand?

Old theories have survived all the tests that have been thrown at them - it's like the old saying about being wary of an old man in a young man's game. There's a reason that they survive in it. Similarly, GR remains a highly precise model that we've never caught making an incorrect prediction, which is why we continue to use it.

So if a vacuum contains no matter or energy or even nothing at all then how can it curve or contain any fabric at all?

On the substantive question, only popsci describes spacetime as a fabric. Physicists certainly don't. And it is described as curved because the parallel postulate (initially parallel straight lines do not cross) does not hold. It is certainly possible that this is an effect of something we don't understand, but until we work out what that is, we continue with our best model.

 

[Dale]

So if a vacuum contains no matter or energy or even nothing at all then how can it curve or contain any fabric at all?

Why would not containing energy somewhere imply that spacetime must be flat there? How can you justify this assumption?

 

[PeterDonis]

From a very young age, we are often told that space is a vacuum and a vacuum contains no matter and energy.

Yes. And then when we learn General Relativity, we learn that "no matter or energy" means that the stress-energy tensor is zero, but that in itself does not prevent spacetime from being curved, because there are solutions of the Einstein Field Equation with zero stress-energy tensor that are curved.

What we are also told from a very young age is that something cannot come from nothing

Yes. And then when we learn General Relativity, we learn that this means that the stress-energy tensor satisfies a conservation law: its covariant divergence is zero. This is enforced by the Einstein Field Equation, whose LHS is the Einstein tensor, whose covariant divergence being zero is a mathematical identity, and whose RHS is the stress-energy tensor, whose covariant divergence must then be zero because it's equal to the Einstein tensor (times a constant that depends on your choice of units).

So if a vacuum contains no matter or energy or even nothing at all then how can it curve or contain any fabric at all?

That's what we learn General Relativity to explain. See above.

since we don't have a theory of quantum gravity, meaning we don't know how quantum mechanics and general relativity are connected,

This is not quite true. We can do quantum field theory in curved spacetime, so we have some ways of connecting QM and GR.

What we don't have is a comprehensive theory that allows for quantum amplitudes for multiple different spacetime geometries.

does it mean it could be a bit misguided to trust a theory written in 1916 which was 111 years ago?

Not at all. GR has plenty of experimental confirmation within its domain of validity. All we can say because of the lack of a theory of quantum gravity is that we expect GR's domain of validity to be limited to spacetime curvatures that are well below the quantum gravity scale (which most physicists assume to be the Planck scale). That covers a lot of ground.

Personally I would a bit hesitate to trust a theory written 111 years ago during War World 1 when people back then were probably ignorant of what time, energy, space, dimensions and what these things really are and what they really mean.

Personally i would be very, very hesitant to make such pronouncements without first becoming very familiar with the extensive experimental confirmation of the theory.

Also, do you have a smartphone? Does it have GPS? Does the GPS tell you your location accurately? If so, you have strong evidence of GR's validity in a fairly wide domain, which includes the vacuum region around the Earth, where spacetime, even though it's vacuum, must be curved the way GR says in order for GPS to work.

 

[phyzguy]

Euclid's Elements were written in 300 BC, more than 2300 years ago. Should we throw out geometry because the theory is so old?

 

[Uri Zlatnik]

I want to completely apologize to Einstein and to all of you guys for being completely wrong about general relativty. I'm really sorry guys.

I was never so good at math and apparently I'm not as smart and bright as Albert Einstein and never will be.