Question Regarding Gravity

[vlaiv]

13 minutes ago, andrew s said:

It is the same issue as getting a quantum theory of gravity.  Namely the treatment of time. QT requires a well defined time coordinate but you don't have one in a curved space time.

Regards Andrew 

 

Not even going to pretend that I understood what you just said - quick google search on "curved spacetime well defined time coordinate" yielded this result:

https://arxiv.org/pdf/1401.2026.pdf

Paper is titled: "Quantum fields in curved spacetime"

Might be of interest to someone

 

[andrew s]

53 minutes ago, saac said:

I agree and that is really my point Andrew.  We use the level of understanding relevant to the task; acknowledging at the same time its limitations .  It's a bit like using a general service screwdriver rather than the precision screwdriver when the basic screwdriver will suffice. We can place the satellite into orbit using Newtonian mechanics (as we have been doing since Sputnik in 1957). So that understanding whilst not complete is nonetheless effective. It ceases to be effective though for the second task concerning GPS.   I remember a similar discussion on a different forum where attempts were being made to revise the Newtonian understanding of lift as generated by an aircraft's wing.  Here a case was being advanced to supplant Bernoulli and the Navier Stokes equations with a quantum description of lift.  Ultimately a quantum description would probably be more complete but as effective?  Of course in the future who knows, maybe the quantum description may ultimately deliver  more efficient wing profiles.  Out of interest I was watching a programme yesterday by Jim Al-Kahlili  on quantum effects within biology.  He presented on some very compelling and interesting arguments that quantum effects are measurable within some very common biological processes: the Robin's navigation sense, our sense of smell, photosynthesis, and even perhaps a visible impact on evolution.  It was a fascinating programme and showed exactly what I'm trying to explain that we have levels (Russian doll) of understanding each with their own utility.  It's still available on BBC i Player. 

The Secrets Of Quantum Physics - Let There Be Life

Jim 

You use quantum tunnelling every time you turn on a switch. The inevitable oxide on the contacts is an insulator and electrons tunnel through it.

Regards Andrew 

[Tiki]

13 hours ago, andrew s said:

@Tiki all I can do is quote one of the mentors PeterDonis from Physicsforums

In response to Martin Scholtz who said: two observers, although both following geodesics, have non-zero mutual acceleration is coordinate-independent thing and it happens only in the presence of curvature.

PeterDonis replyed: This is tidal acceleration, and since both observers are in free fall, there is no force involved in the GR sense. It's simply spacetime geometry--as you say, tidal acceleration is a sign that spacetime is curved. One could talk of "tidal force", but once again, the force involved is not gravity, but internal forces inside objects that cause stresses due to parts of the object not moving on the geodesic paths that they would move on if the object had no internal forces and every part of it could move independently.

Source https://www.physicsforums.com/threads/is-gravity-a-force.975552/#post-6214766

Regards Andrew

From the same thread, another mentor 'Nugatory' says:

Gravity is indeed a force in classical physics, but to avoid the criticisms from @Dale and @PeterDonis above you will have to be a bit more precise about what that means: Newton’s first law defines an inertial frame. Newton’s second law defines force, not just as acceleration but as acceleration in an inertial frame. Thus the Newtonian definition is based on coordinate acceleration. The distinction between proper and coordinate acceleration is irrelevant to this definition; what matters is that there is coordinate acceleration in an inertial frame. Gravity as a real force (a falling object has coordinate acceleration in an inertial frame) but centrifugal force is not (produces coordinate acceleration only in the non-inertial rotating frame). General relativity (more cleanly, IMO) treats all coordinate acceleration as a mere convention and defines force in terms of proper acceleration. That definition doesn’t change the interpretation of the classical fictitious forces, but it does exclude gravity as a force.

Source https://www.physicsforums.com/threads/is-gravity-a-force.975552/#post-6214766
 

So we are both right. It depends upon how you define force. I never realized that GR used such a definition of force.

 

One thing for certain, no matter which definition of force that you use, when you fall over it will always hurt!

[andrew s]

@Tiki I agree Gravity is a force in Newtons theory but not in GR.

Regards Andrew

[saac]

19 hours ago, andrew s said:

You use quantum tunnelling every time you turn on a switch. The inevitable oxide on the contacts is an insulator and electrons tunnel through it.

Regards Andrew 

Ultimately all of our reality and all that we do stands on quantum effects. That's not what I'm contesting what I'm arguing is that it is possible to describe reality to an acceptable level without invoking the quantum.  There are perfectly acceptable models that have stood the test of time and continue to serve well - yes they may not provide the complete understanding that quantum "may" bring but nonetheless they continue to have utility.

Jim  

[andrew s]

3 hours ago, saac said:

Ultimately all of our reality and all that we do stands on quantum effects. That's not what I'm contesting what I'm arguing is that it is possible to describe reality to an acceptable level without invoking the quantum.  There are perfectly acceptable models that have stood the test of time and continue to serve well - yes they may not provide the complete understanding that quantum "may" bring but nonetheless they continue to have utility.

Jim  

I fully agree, we have very effective theories that each have a range of applicability. 

Regards Andrew 

[saac]

1 hour ago, andrew s said:

I fully agree, we have very effective theories that each have a range of applicability. 

Regards Andrew 

At the risk of going off topic but I think it is kind of related to where we took the discussion; I stumbled across this  - shadow created by entangled photons.  If I have understood it correctly the image of the shadow of the cat was created by entangled photons which did not interact with the object. Now there is something that could only be explained by quantum physics.  I don't pretend to properly understand the setup or explanation but if my description above is correct then I find that so intriguing (unbelievable really) ; I would love to see it in a lab.

Entangled Photons Shadow  

Jim

Edited November 12, 2019 by saac