Linking Quantum Physics and Relativity: Theory of Everything?

[darrenjb]

I am going to get laughed out of this forum but at least it will put my mind to rest when this happens. I've been thinking about this for a few years now and what better place for an answer than a physics forum?

Amongst many other things I have read A Brief History of Time seven or eight times and a takeaway from this book is that a big challenge for physics is trying to link the theories of quantum physics and relativity. As I understand it this is a challenge which is still ongoing today. A theory of everything.

I have a question which is that maybe these theories are unrelated and do not need to be linked?

To explain with my rudimentary understanding of physics..

There are four forces, the electromagnetic, the weak nuclear, the strong nuclear and the gravitational. Quantum physics is a theory for the first three of these and relativity is a theory for the last.

In quantum physics the interactions between particles can be explained by the electromagnetic, weak nuclear and strong nuclear forces and the properties of fields.

In relativity there is no force of gravity. There is no gravitational field. Objects with mass appear to be attracted to each other not through the action of any force but by the bending of space / time. For example some of the common explanations of relativity: Objects with mass travel in a straight line in space time, but it is the bending of space time which makes them appear to be affected by gravity. A planet orbiting a star is like a basketball rolling around a bowling ball on a rubber membrane - it would like to go in a straight line if not for the bowling ball which makes it look like it is orbiting.

So a question I would really like to have answered is if quantum mechanics is dealing with forces and fields, and relativity is dealing with the bending of space time, are they dealing in any way with the same thing? Should there be a theory of everything which links them together or is this unnecessary?

 

[Nugatory]

Amongst many other things I have read A Brief History of Time seven or eight times...

You should be very cautious about using this book to build your understanding. It's fine for getting a feel for some of the interesting thinking that's been going on, but Hawking didn't intend it to be an accurate description of the real physics - for that you need to be reading his peer-reviewed papers and graduate-level textbooks. It's also well to remember that "A Brief History..." was first published more than thirty years ago. Since then an entire generation has been born, grown up, gone to college, entered and completed their physics PhD programs, and discovered and published new results that won't be in Hawking's book.

A planet orbiting a star is like a basketball rolling around a bowling ball on a rubber membrane - it would like to go in a straight line if not for the bowling ball which makes it look like it is orbiting.

That explanation is repeated over and over by non-scientists trying to write about relativity, but it is seriously misleading. There's no "looks like it is orbiting" going on - the planet really is orbiting and it really is moving in a straight line. Our own member @A.T. has made a pretty good video that suggests how this can be, and it's worth searching this forum to find a link to it.

Should there be a theory of everything which links them together or is this unnecessary?

It's necessary. Quantum mechanics works really well to explain the behavior of the universe at small scales with negligible gravity, and general relativity works really well to explain the universe at large scales when gravitational effects cannot be ignored. Neither theory works well in a situation where we have strong gravitational effects at a small scale, so if we're to understand these situations we need a theory that links both gravity and QM.

 

[darrenjb]

Nugatory - thank you very much for your answer. As expected, yourself and many others have a much better grasp of this than I do.

There is still a part of this which is still unresolved for me.

Should gravity be treated as a force? If as you say above "a planet really is moving in a straight line", it seems that there is no force acting on it.

I do understand that there is a need to understand situations where there is both a small scale and strong gravity. What I don't quite understand yet is why there is a need to understand this with a unified theory that links the quantum and relativity theories together when the theories appear to be dealing with different things.

 

[Mister T]

So a question I would really like to have answered is if quantum mechanics is dealing with forces and fields, and relativity is dealing with the bending of space time, are they dealing in any way with the same thing?

You're asking the wrong question, in my opinion. There are some fundamental features of Nature that are known to us through our studies of quantum mechanics. Relativity ignores these features of Nature! There is evidently something incomplete about our knowledge.

 

[phinds]

Should gravity be treated as a force? If as you say above "a planet really is moving in a straight line", it seems that there is no force acting on it.

In Newtonian physics, which is is just an approximation (albeit a very good one in many situations) of General Relativity, gravity is a force. In General Relativity it is not a force. There is no force acting on a planet in orbit, therefore it travels in a straight line. Where you are lacking understanding is that a "straight line" in a true descriptions of large scale things (General relativity) is more properly called a "geodesic" and is described by Riemann Geometry. Should you insist on applying Euclidean Geometry to the situation, a situation to which EG does not actually apply, you would conclude that the planet was not traveling in a straight line.

 

[fresh_42]

If as you say above "a planet really is moving in a straight line", it seems that there is no force acting on it.

Yes. Planets are in free fall: they fall into the sun but are so fast that they constantly miss it. The "straight line" is a geodesic, like great circles on Earth are. It is straight within the surface it belongs to, same as airplanes fly a straight (= shortest in this context) line between two destinations, but the actual flight path on Earth is curved.

 

[Nugatory]

Should gravity be treated as a force? If as you say above "a planet really is moving in a straight line", it seems that there is no force acting on it.

That's right - in the relativistic description of gravity it is not a force and the there is no force acting on an orbiting planet.
Non-relativistic Newtonian physics does treat gravity as a force and generally that treatment works really well for problems ranging from navigating the Apollo spacecraft to the moon, to predicting the orbits of the planets, to explaining the behavior of dropped objects on the surface of the earth, to designing building and bridges that don't fall down, to calculating the trajectory of artillery shells... just about everything that humans have built, done, or observed for the past three centuries. Because it works so well we use it and teach it to this day.

However, the Newtonian "gravity is a force" model disagrees slightly with observations of objects in sufficiently strong gravitational fields. One of the first and most important examples is the planet Mercury, which because of its proximity to the sun experiences a stronger gravitational field than the other planets; its orbit deviates very slightly (google for "Mercury anomalous precession") from the Newtonian predction. This effect is explained by the general relativistic model in which gravitational effects are the result of force-free motion in a curved spacetime. And - crucially - the general relativistic model also agrees to within the limits of experimental accuracy with Newtonian gravity in all of the situations where Newtonian gravity works. If it didn't, we'd have to reject it because we'd have observations that falsified it.

I do understand that there is a need to understand situations where there is both a small scale and strong gravity. What I don't quite understand yet is why there is a need to understand this with a unified theory that links the quantum and relativity theories together when the theories appear to be dealing with different things.

We're looking for a theory that agrees with relativity everywhere that relativity works and agrees with quantum mechanics everywhere that quantum mechanics works and also works in at least some of the situations where neither of these work. The thought process is similar to the process that leads us to accept the relativistic model of gravity over the Newtonian one because the latter works everywhere that the former does, and also works for Mercury's orbit.

 

[fresh_42]

The biggest obstacle for a unified theory is the overwhelming success of what we already have!

 

[A.T.]

That explanation is repeated over and over by non-scientists trying to write about relativity, but it is seriously misleading. There's no "looks like it is orbiting" going on - the planet really is orbiting and it really is moving in a straight line. Our own member @A.T. has made a pretty good video that suggests how this can be, and it's worth searching this forum to find a link to it.

@darrenjb The video is here:



But note that it only shows radial fall. Showing orbits is not as simple, due to the 2nd spatial dimension needed.

 

[Ibix]

We know that the gravitational field depends on the mass, energy and location of matter (that's very much a B level statement, but it's good enough for this purpose). If we know that the sources of gravity can do funny quantum things like existing in superpositions of states with different energies and inexact locations, then either the gravitational field should be able to do funny quantum things in response or there has to be a reason why it doesn't. General relativity can't describe gravitational fields doing funny quantum things, but there's no way to put sources doing funny quantum things into the maths so that's not surprising.

We need a theory of gravity which can accept sources doing funny quantum things because we can see that this is reality, and our existing theories don't match it.

 

[andrew s 1905]

Is it not equally true that our current theories of funny quantum things don't consider strong gravitation? So we also need a theory of quantum funny thing compatible with gravity?

Regards Andrew

 

[cosmik debris]

So a question I would really like to have answered is if quantum mechanics is dealing with forces and fields, and relativity is dealing with the bending of space time, are they dealing in any way with the same thing? Should there be a theory of everything which links them together or is this unnecessary?

General Relativity can also be formulated as an Effective Field Theory which looks like a spin 2 field on flat spacetime. This is effective only at low energies and has found to be not renormalisable. It is however a field theory and may lead to unification if some of the obstacles can be overcome.

Cheers

 

[darrenjb]

Thank you all. I really appreciate it, as a layman, being able to ask a question here and get some really thoughtful and well written answers.

 

[Nugatory]

Is it not equally true that our current theories of funny quantum things don't consider strong gravitation? So we also need a theory of quantum funny thing compatible with gravity?

Yes. We need a theory that agrees with quantum mechanics when gravitational effects are negligible, agrees with general relativity when quantum effects are negligible, and works better than either when both quantum and gravitational effects are important.

 

[fresh_42]

Yes. We need a theory that agrees with quantum mechanics when gravitational effects are negligible, agrees with general relativity when quantum effects are negligible, and works better than either when both quantum and gravitational effects are important.

Do we have a Mercury? An observation that cannot be explained by the standard model and general relativity?

 

[Nugatory]

Do we have a Mercury? An observation that cannot be explained by the standard model and general relativity?

We do not, because we aren’t able to explore areas where both quantum mechanics and gravity must be considered: Planck-scale high energy collisions or conditions near a black hole singularity inside the event horizon. However, we do know that neither QM nor GR provide sensible answers under those conditions so even without observations we know our two best theories are incomplete.

 

[fresh_42]

However, we do know that neither QM nor GR provide sensible answers under those conditions so even without observations we know our two best theories are incomplete.

Doesn't CMB analysis involve both at the same time?