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

Just caught sight of this thread: real good to see another observatory going up in the Kingdom of Fife, we can coordinate and get a cloud watch going    You are going to notice such a change when you get the observatory operational, good luck with the rest of the build. 

Jim 

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  

44 minutes ago, andrew s said:

It amuses me (easily done) that it is always mentioned that we don't  have a quantum theory of gravity but never that we don't  have a quantum theory in curved spacetime.

QFT is based on the flat spacetime of SR.

Regards Andrew 

I had to read that twice - brilliant  

Bring to mind the much loved of journalist and politicians -"we have made a quantum leap".  And yet they appear so pleased    

Jim 

39 minutes ago, andrew s said:

But, you need to make both GR and SR corrections to make GPS work as the clocks the satellites carry need correcting both for both their velocity and hight in the earths gravitational well.

Regards Andrew 

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 

2 hours ago, vlaiv said:

I think this is related to how brain works - or rather how "understanding / knowing" works. In order to know something we need to establish some sort of equivalence - mapping of the phenomena to something that ultimately boils down to our experience.

For example - it is much easier to accept that two electrons repulse one another because most people can relate that there is virtual photon exchange - momentum transfer. At some point this boils down to "hit one billiard ball with another and it will bounce off".

Most people don't have any issues with Newtonian gravity because they relate it to "fields" and them acting as some sort of elastic cord between bodies that pulls them together.

Once you start talking about space time curvature - all analogy goes down the drain. Imagine following: when most people hear about space time curvature - they expect space time to actually be "curved" so space craft orbiting a planet is following this "curve". This means that all points along that trajectory are somehow "curved straight line" or something. But at exact same time and at exact same spatial coordinate - shoot a beam of light, or put object moving at higher velocity. It will no longer follow same path - so there is no actual "curve in space time" (in the sense we started thinking about it) - confusion sets, there is nothing to relate to and we "loose knowledge" - or rather we concluded that we don't know / understand it.

 

I agree with Vlaiv but I think  we also need to acknowledge that we also have different levels of understanding. Ultimately to properly understand reality we would need to turn to a quantum description. However within reality I would say we already have very effective levels of understanding. Effective enough to deliver the technological advances we enjoy and even to deepen our scientific understanding. For example Newtonian physics presents a perfectly adequate and functioning level of understanding of much of our reality, gravity in particular -  It got us to the Moon and back after all. At the same time though we also recognise that there is another deeper level of understanding. I think it is a matter of using the appropriate level of understanding for the appropriate scenario.  So explaining the gravitational lensing we observe of galaxies we replace the Newtonian understanding and invoke GR.   However I wouldn't go worrying about bent space time to launch a satellite into low Earth Orbit, or design the Jame's Web telescope.  

Jim 

34 minutes ago, Stub Mandrel said:

What is interesting is how pathetically weak gravity is.

I can push myself away from the pull of the entire earth!

A tiny magnet can lift a big lump of steel with the whole planet pulling the other way!

Absolutely yet its reach is infinite and it is really the only fundamental force that we react with on a daily basis at an appreciable level. The em force being another but I think our recognition of that interaction is bit a more esoteric.  

Jim 

47 minutes ago, andrew s said:

I obviously agree but it is not mass that is the cause of the internal forces but the other forces principally electromagnetic (at normal scales) but also the strong and weak forces at nuclear scale. 

Simply gravity is just geometry and so impacts even zero rest mass particles.

Regards Andrew 

Absolutely, I referenced mass so that my object could be influenced by gravity and hence the internal reaction forces would appear. 

But wait, I think I may now have an epiphany due to your explanation Andrew    So if we had a sizeable object in a gravity field (deformed space time) even one with no mass would still experience tidal forces as the discrete elements within attempt to follow different geodesic paths, and the em forces between these discrete elements provide the reaction as per Newton's third law; the head would feel the feet being pulled away ! Have I got that right?  I previously thought the object would need mass but now I see that it wouldn't !

Jim 

11 minutes ago, johninderby said:

Has been interesting following this thread but just emphises that we don’t know what gravity is really. 

One of the best quotes I came across was by a physicist who said “Ultimately we don’t know what anything is“

To be fair to john I think this is a matter of language. For example how much do we need to know before we can say "yes we have a pretty good understanding of its nature". Yes there is still much to be known about gravity  - will the force exchange particle (graviton) forever remain theoretical, will we ever be able to describe gravity satisfactorily as a quantum entity.  But as Andrew said we already know way more than the tip of the iceberg and actually the extent of that knowledge and what we have done with it is pretty impressive. Look at the Moon landings, Apollo 13 (gravitational sling shots), satellites, Voyager, Cassini et al.  All  made possible by our knowledge of gravity, how it behaves and its effect on objects with mass. Then there is all the mundane stuff , the bread and butter of engineering: bridges, cars train planes and automobiles   More recently, it is surely impossible not to marvel at the team who managed to detect gravitational waves - the precision of that measurement is mind-numbing. So gravity is pretty much well understood I would say but still holds some secrets for us to tame - how exciting is that.   

Jim 

7 minutes ago, andrew s said:

Exactly, analogous in fact.

Regards Andrew 

Your right Andrew it is I hesitated lol.

Jim 

So where are we with this. Are we all in agreement that gravity is not a force but that it can "induce" or cause forces to appear in objects with mass (tidal forces or reaction forces as they may also be considered) . We need a name for this, something to mirror the grandiose of the Copenhagen Interpretation - maybe we could call it The Cloudy Weather Interpretation of Gravity  

Jim 

9 minutes ago, andrew s said:

Yes, a google of "Gravity Inside a hollow sphere " will quickly answer the question in the affirmative. In fact Newton proved the theorem himself.

Regards Andrew

Almost analogous to charge inside a hollow conductor (another inverse square law).

jim  

11 minutes ago, Thalestris24 said:

Oh ok, mine does sound like R2D2/morse code ha ha but I quite like I I just have an otc Ender3 Pro but it does what I ask of it. I've no idea what Martin 2.x is either...

Thanks for the info

Louise 

x2 I love the way it squeaks and burbles like R2D2 like it's talking some secret language; I like to think it's complaining about another complicated print.   

jim

An alternative way to dispel gravity as a force without invoking GR while at the same time remaining in Newton's classical realm is to properly refer to "gravity" as  "gravitational field strength".  Gravitational field strength describes a "potential" and is certainly in itself not a force rather it causes a force to act on mass (on Earth our weight).   It is perhaps a pedantic distinction,  that is Physics after all, but it may provide a little solace to those struggling to accept that "gravity" is not a force without having to bend space and time and think about gravity wells. 

Jim  

Could I not have a similar experience if I were not spinning but placed at the centre of a large planet sized mass at the core which is conveniently hollow to accommodate me.  Would I not experience an outward pull (gravity attraction from planet) radiating from my centre as though I were in radial motion?

Jim 

40 minutes ago, vlaiv said:

Actually that was my point - there is spin without reference point. If you were out there spinning in empty space without being able to see any reference point - you would still know that you are spinning by stretching sensation in your head and feet. If you were in elevator without windows and there was a pull towards floor - you would not be able to tell if you were suspended in gravity field or you were uniformly accelerated in space. With rotation - you would be able to tell straight away as nothing acts as "negative" gravity source centered in your belly.

Nicely demonstrating the equivalency of gravity and acceleration.

Jim 

Can we not say that without a reference point there is no spin (Newtonian not quantum) .  I'm sure we discussed this at some point in the past or something very similar. Need to search the archive when my head stops spinning  

Jim

9 minutes ago, Sunshine said:

Yeah, I stand by centrifugal, what we’re describing here is the force felt when inside a spinning “centrifuge” no?

They are in effect the same thing, centripetal being the centre seeking force resulting in circular motion , centrifugal being the reaction force to centripetal.  Two sides of the same coin  

In Vlaiv's example the force you would feel pulling on your head and feet is the reaction to the centripetal force causing the rotational motion. 

Jim 

2 minutes ago, Paul M said:

.

 we liked to get it right!

Ah ambition  

Jim 

My name is Jim , I'm an engineer and I believe in F = m^w2r  

Jim 

7 minutes ago, Sunshine said:

Would this not be centrifugal force? thanks for the great thought experiment.

An engineer would say yes and happy with that knowledge go on to successfully design high bypass turbo jet aero engines that fly thousands of passengers across the globe. A physicist would roll their  eyes at the use of the term centrifugal.   You have opened a can of worms now sunshine  

Jim 

6 hours ago, Sunshine said:

A question i pose about the nature of gravity and its effects, as i understand, gravity is not a force as is commonly believed. It seems as such but, in reality, objects are merely falling toward a larger object in a circular motion caused by the well in the fabric of space time created by that larger object, as per Einstein. Having said that, how is it possible that large bodies, say Jupiter for instance, can physically affect smaller objects such as its moon by way of stretching them and such, causing friction within, leading to geologic activity?. This is where my confusion lies, if gravity is not a force, should there not be any physical effects exerted between bodies in proximity to one another?. Shouldn't they merely orbit each other without physically affecting each other?.

 

I think what may be causing the confusion is the popular way gravity wells are represented graphically. More often or not the gravity well is represented as a sheet of material on which the planets sit within  the attendant deformation (well).  This allows us to easily visualise how the smaller mass would be attracted or fall into the larger well .  Of course space time is not a 2D sheet of material and neither is the resultant deformation forming a gravity well 2D.  It's hard to graphically represent but the gravity well around say Jupiter projects across all of its volume and not simply a plane through a particular axis.  So with this 3D gravity well we can see how every atom in a nearby moon or planet would therefore be subject to a gravity well created by Jupiter - every atom will experience an acceleration as it falls along a personal space-time geodesic as described by Andrew.  If that acceleration is resisted then the interpretation of that resistance or reaction is what we name force. Those internal reactions (resistance to the gravitational acceleration) in turn can create the heating effect from so called tidal forces.  The main point is that every atom in the moon or second body will indeed be in a gravitational well of Jupiter, the well is not 2D as often drawn. 

Jim 

The IAU has voted to rename Hubble's Law  as the Hubble and Lemaitre Law.  Georges Lemaitre was the author of what would become known as The Big Bang Theory. Unlike Hubble , Lemaitre was far from being a self publicist and in early presentation of his theory he was met with indifference and ridicule - in fact the name Big Bang was a pejorative term coined by Fred Hoy who was one of many cosmologists who favoured the steady state model.  So at last Georges Lemaitre is recognised more publicly by this renaming.  Lemaitre is a bit of a hero of mine so when I was teaching today (expansion of the universe) I had a big smile on my face when I introduced the Hubble-Lemaitre law to the class.  

Now we just need to get Henrietta Swan Leavitt recognised more widely; her work on Cepheid variables (relationship between luminosity and period) was an essential precursor for Hubble's work .

IAU Rename Hubble Law

Henrietta Swan Leavitt

Jim 

On 05/11/2019 at 20:00, Gina said:

Much better than I'm getting ATM with my printer.  Seems I have serious finger trouble or more like brain trouble in getting my settings right!  Can't blame the filament - it's a new reel freshly opened and top quality.

It's those pesky Gremlins Gina the bane of all engineers  

Jim  

What a cool prize.  I bet your tea  will taste all the better from that mug.    Well done indeed.

Jim