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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 noninertial 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/isgravityaforce.975552/#post6214766 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!

A Wrong Redressed  Hubble Law Renamed
Tiki replied to saac's topic in Physics, Space Science and Theories
Well said. 
Perhaps not. The em force that you refer to is a reactionary force to the gravitational force. Io's interior heats up because of work done by Jupiter's tidal forces. Also, if you had a frame of reference in which ALL of the gravitational forces disappeared then then there would be no curvature at all (in that frame the geodesics would remain parallel). In reality, the curvature of spacetime describes the tidal forces, therefore we can justifiably say 'The curvature of spacetime is gravitation'. Tidal forces are at the heart of GR. Really.

Yes. This tidal force is the 'real' force of gravity.

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)

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

Noone really knows what gravity is. Gravity is modelled in different ways and as such it is useful sometimes to think of gravity as a force and sometimes as the curvature of spacetime. Newtonian gravity makes much use of the concept of force and is an almost complete description of how us earth dwellers perceive the effects of gravity. All Newton 'knew' was force. Einstein, using the notion of a freefall observer (no force other than tidal forces) was able to extend Newtonian theory by accounting for the very few anomalies it contained. However, Einstein still needed force (tidal forces in fact) to account for the curvature of his spacetime. The curvature of spacetime and the path of freefall objects are one.

I was wondering if anyone here has read the John von Neumann biography by Norman Macrae. A friend of mine (who passed the book on) described it as being very dull and untechnical. I'd be pleased to hear to hear the opinions of any members who have read the book. Thanks.

Thanks Olly. Compulsive is good.

'Universe' by Freedman and Kaufmann. A bit pricey but well worth it. Packed full of all sorts of info and some basic formulas.

Somewhat astonishingly, for a Universe roughly 10 billion years old with a value of Omega not wildly different from 1, the value of Omega when the Universe is just 1 second old could not differ from unity by more than one part in 10^15. ( From 'Just Six Numbers')

I suppose it could be argued that it is not surprising since we are here talking about it at all. A slightly less flat Universe would have collapsed long ago or would have disallowed the formation of galaxies and stars. What is truly surprising though is that there are a whole bunch of constants at critical values (Omega being just one of them) that conspire to make a habitable Universe. (cf. 'Just Six Numbers' by Martin Rees or 'The Road to Reality' by Roger Penrose)

A little bit different to what you have in mind but you could use a piece of spray painted (matt black) aluminium foil and monitor its temperature as it is heated by the sun with an infrared thermometer. You could then shade the foil and measure the temperature drop. You could combine the two sets of results to obtain an approximation of the intensity of the sunlight.

Controversial time reversal, any thoughts?
Tiki replied to vlaiv's topic in Physics, Space Science and Theories
Thanks for calling me up on this. 'Pattern' is too loose a word to use in this context. 
Controversial time reversal, any thoughts?
Tiki replied to vlaiv's topic in Physics, Space Science and Theories
Having not yet read any of the links, there is one thought that springs to mind. If 'random' is defined as an absence of pattern then a random sequence (of random numbers for example) will be patternless when looked at from left to right, right to left or any other arbitrary ordering. If there is any pattern encountered then the sequence cannot be random. I am not sure whether it is relevant here but a random sequence can also be thought of as incompressible. Mind bending stuff but my hunch is that the arrow of time points in one direction only.