Question Regarding Gravity

[saac]

2 minutes ago, Paul M said:

.

 we liked to get it right!

Ah ambition  

Jim 

[Sunshine]

Physicists may have invented “centripetal” to describe “centrifugal” I shall rename this force to “spinius forcius” there, I’m sharp too!.

Edited November 9, 2019 by Sunshine

[saac]

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 

Edited November 9, 2019 by saac

[vlaiv]

I gave above example to show couple of things.

First one is very "mundane" - same way objects in gravitational field follow curved trajectories - one's legs following a curved trajectory while spinning, will experience "force". Neither of two "forces" need force carrying particles to be exchanged to give raise to force - both are consequence of "curved space-time". We might as well call them "pseudo" forces (as one often does for centrifugal force).

Other one has really baffled me for as long as I remember. Straight uniform motion is relative. If you are floating in empty space and don't feel anything, you have no way of knowing that you are "moving with respect to something". If you are accelerating along a direction - you will feel that as a force in your reference frame - but you need energy expenditure to do so. Again what is the thing that you are accelerating with respect to?

But rotation around one axis is strangest of all - if you are spun and left on your own - you will feel above mentioned forces, although same would happen in completely empty space with no reference point. You are rotating with respect to what? And you no longer even have energy expenditure unlike in accelerating case.

All of this "sounds" very counter intuitive, but I think there is very "reasonable" explanation. Not sure if anyone actually thought of it that way, maybe this is the path for GUT. We need to examine how waves behave in certain space/time configurations, and I'm sure that all of the above will emerge from behavior of waves. Math of it will show all effect. After all - everything that exists is in fact wave in quantum fields.

[andrew s]

As I am sure you all know F=ma only holds in an inertial frame. In a rotating frame as in @vlaiv thought experiment you have to add the pseudo forces to make it work, hence, the centripetal force. 

Similar forces are needed to explain Foucault pendulum as viewed in the rotating frame of the earths surface.

Regards Andrew 

[andrew s]

10 minutes ago, vlaiv said:

Math of it will show all effect. After all - everything that exists is in fact wave in quantum fields.

Unfortunately,  the mathematical underpinnings  of GR and QFT are totally incomparable. This is why reconciling the two in a so called "quantum gravity" is proving, how should I put it, difficult. 

Regards Andrew 

[andrew s]

19 minutes ago, vlaiv said:

But rotation around one axis is strangest of all - if you are spun and left on your own - you will feel above mentioned forces, although same would happen in completely empty space with no reference point. You are rotating with respect to what? And you no longer even have energy expenditure unlike in accelerating case.

I think the scenario you propose is impossible to realise. If you were alone you could not set your self spinning. If one one were to set you spinning they would spin in an equal and opposite way to you. Your spin would be relative to theirs. You can't just spirit them away.

Regards Andrew 

Edited November 9, 2019 by andrew s

[johninderby]

Louis de Broglie has a lot to answer for. 😁

 

[vlaiv]

24 minutes ago, andrew s said:

I think the scenario you propose is impossible to realise. If you were alone you could not set your self spinning. If one one were to set you spinning they would spin in an equal and opposite way to you. Your spin would be relative to theirs. You can't just spirit them away.

Regards Andrew 

good point, but here is "counter argument".

First, you don't need someone else spinning for you to spin - you can eject particles and have your self spinning. Now imagine accelerating expansion of universe and the fact that one can disappear beyond the horizon. You shoot some photons from the flashlight attached to your head - like observing light - one that is directed perpendicular to your body. It sets you spinning and photons shoot off. At some point these photons will be causally disconnected from you once they cross event horizon. Now you are left alone in the space - spinning without reference point

I know - not really realistic scenario, but still...

[saac]

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

[vlaiv]

9 minutes ago, saac said:

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

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.

[andrew s]

35 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.

If I accept this then I think what you can tell is the you are not in an inertial frame. I am less clear that you could tell you were spinning as opposed to experiencing a radial force.

Only our previous referenced experience would allow a rotating interpretation. Another might be being stretched like a rubber band.

Regards Andrew 

[saac]

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 

[saac]

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 

[vlaiv]

40 minutes ago, saac said:

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 

Not sure about that one. In classical interpretation of gravity hollow sphere (or any other shape) will not have gravitational field inside as gravitational influences of small pieces all cancel each other out perfectly. Have no idea what would be the case in GR though.

[andrew s]

15 minutes ago, vlaiv said:

Not sure about that one. In classical interpretation of gravity hollow sphere (or any other shape) will not have gravitational field inside as gravitational influences of small pieces all cancel each other out perfectly. Have no idea what would be the case in GR though.

You need to look at the tidal effects. Maybe two large masses head and foot. Dark matter so you can't see them.

Regards Andrew 

[Paul M]

I can't remember where I read it, I possibly heard it at Hayden Planetarium's primary show presented and written by Neil deGrasse Tyson 5 years ago, that quite possibly everything we think we know about the large scale universe may be falsified by future understanding of dark matter and dark energy. That it, everything we think we know might well be wrong!

Those two are so whacky that they might never fit in our current models. Ergo, the models are fundamentally wrong.

Edited November 9, 2019 by Paul M

[Tiki]

On ‎09‎/‎11‎/‎2019 at 08:56, ollypenrice said:

I think Andrew has resolved your problem of curvature and perceived force. The tidal 'forces' only appear because parts of the moon encounter resistance from other parts of the moon because they are trying to fall at different rates. (I think?)

Tidal forces are in fact very real, as they cannot be made to vanish even for a free-fall observer.

 

On ‎09‎/‎11‎/‎2019 at 05:55, Sunshine said:

 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?.

 

Gravity is a force. In GR, gravity can be modelled as the curvature of spacetime. It is still a force.

 

In the case of Io, the disparate gravitational pulls on its near and far sides cause the whole moon to change shape. This deformation causes internal friction within  Io which consequently heats up.

Alternatively, you could say that owing to the curvature of spacetime, different parts of Io try and follow slightly different paths through spacetime. Since Io is a bound lump, there must therefore be internal forces which hold all the constituent particles together on an approximate parallel journey through spacetime.

 

 

Edited November 10, 2019 by Tiki

[andrew s]

1 minute ago, Tiki said:

Tidal forces are in fact very real, as they cannot be made to vanish even for a free-fall observer.

 

Gravity is a force. In GR, gravity can be modelled as the curvature of time. It is still a force.

 

 

Correct on tidal forces as different parts of a finite object want to free fall on different trajectories. 

In GR gravity is the curvature of space time  not just time and it is not a force. Free fall is the trajectory taken when no forces are acting on the object. The GR equivalent of Newtons first law. However, here the straight lines are geodesics. 

Regards Andrew 

[saac]

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  

[andrew s]

16 hours ago, Paul M said:

I can't remember where I read it, I possibly heard it at Hayden Planetarium's primary show presented and written by Neil deGrasse Tyson 5 years ago, that quite possibly everything we think we know about the large scale universe may be falsified by future understanding of dark matter and dark energy. That it, everything we think we know might well be wrong!

Those two are so whacky that they might never fit in our current models. Ergo, the models are fundamentally wrong.

Of course we might be wrong but the models we have today are the best fit we have to the data we have today.  

Strangely, we need dark matter and dark energy to make our current best cosmological model "LCDM"  work so I struggle to see how they are "so whacky that they might never fit in our current models".  They already do.

Regards Andrew 

Edited November 10, 2019 by andrew s

[Tiki]

6 hours ago, andrew s said:

 

In GR gravity is the curvature of space time  not just time

Regards Andrew 

Thanks Andrew. I meant spacetime. Interestingly though, Newtonian gravity can be formulated as the curvature of time.

 

6 hours ago, andrew s said:

 

In GR gravity is the curvature of space time  not just time and it is not a force. Free fall is the trajectory taken when no forces are acting on the object. The GR equivalent of Newtons first law. However, here the straight lines are geodesics. 

Regards Andrew 

I am unsure of what you mean by '..it is not a force.'  

Aren't tidal forces the part of gravity that can't be removed by invoking a free fall observer?

Can't tidal forces be used to calculate the curvature of spacetime? (More complicated than test particle trajectories I admit but equivalent)

 

 

[andrew s]

10 minutes ago, Tiki said:

Thanks Andrew. I meant spacetime. Interestingly though, Newtonian gravity can be formulated as the curvature of time.

 

I am unsure of what you mean by '..it is not a force.'  

Aren't tidal forces the part of gravity that can't be removed by invoking a free fall observer?

Can't tidal forces be used to calculate the curvature of spacetime? (More complicated than test particle trajectories I admit but equivalent)

 

 

If you have two non interacting test particles side by side free falling in a non uniform curved spacetime then they will drift apart. However,  if they are bound, say by covalent bonds, then they will resist this and it is this resistance that is felt as the tidal force.

Regards Andrew 

Edited November 10, 2019 by andrew s

[Paul M]

4 hours ago, andrew s said:

Strangely, we need dark matter and dark energy to make our current best cosmological model "LCDM"  work so I struggle to see how they are "so whacky that they might never fit in our current models".  They already do.

I think I'd have more trust in a cosmological model that predicted the existence of the two dark entities rather than a model that had been designed to accommodate them! 

Edited November 10, 2019 by Paul M

[andrew s]

10 minutes ago, Paul M said:

I think I'd have more trust in a cosmological model that predicted the existence of the two dark entities rather than a model that had been designed to accommodate them! 

Indeed, but science is about the interplay of observation/experiment and theory.  There are no theory free observations anymore than there are observation free theories (or at least there shouldn't be).

Regards Andrew