Tiki
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On 05/11/2019 at 09:25, ollypenrice said:
.... Zwicky, at his best, was one of the most penetrating astronomers in the history of science. His prediction of the neutron star some 35 years ahead of the Crab Pulsar discovery and his discovery of the missing mass/dark matter problem speak for themselves. Let's hear it for Fritz Zwicky!
Olly
For sure. I've just finished my copy this morning.
I feel it's also worth mentioning Zwicky's determination. He spent thousands of hours using relatively poor equipment hunting for the first 'looked for' supernovae. It took several years.
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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-6214766Regards 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!
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On 08/11/2019 at 18:06, saac said:
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 .
Jim
Well said.
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19 hours ago, andrew s said:
No in my example it is electromagnetic (em) just as it is the force that stops you free falling through the earth.
When you jump you free fall back to earth until em stops you.
Maybe, we will have to agree to disagree 😀.
Regards Andrew
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.
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4 hours ago, andrew s said:
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
Yes. This tidal force is the 'real' force of gravity.
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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)
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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.
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No-one 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 free-fall 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 free-fall objects are one.
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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 un-technical. I'd be pleased to hear to hear the opinions of any members who have read the book.
Thanks.
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On 03/11/2019 at 15:00, ollypenrice said:
..... No regrets though, it's compulsive.
Olly
Thanks Olly. Compulsive is good.
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'Universe' by Freedman and Kaufmann. A bit pricey but well worth it. Packed full of all sorts of info and some basic formulas.
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23 hours ago, andrew s said:
In the case of Omega I think it would be interesting to no what magnitude range (+ve or-ve) is allowable but I have not researched it.
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')
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8 hours ago, andrew s said:
Well why is it surprising?
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)
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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.
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I really had no idea. Thanks for posting.
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Impressive stuff. Very well done.
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Just a quick question to anyone who has this book....
Is it worth the money and would you buy it again?
Yes and yes. It has more information than my scopes and skies will ever warrant. It even goes into details of dark nebulae.
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I wonder if there is a revised Deep Field Guide to match the latest editions of the Uranometria ?
Yes there is. If chart 33 shows the Blinking Planetary in the top left hand corner of your single volume atlas then the corresponding guide book has ISBN 0943396735. For my modest scopes it is a complete reference.
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Thanks for the list. Feynman is always an inspiration.
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Winter Solstice Curiosity
in Physics, Space Science and Theories
Posted
If you were to go to a place on earth on the 22nd where local noon is at 0419 GMT then the length of the 21st and 23rd would be about the same (with the 22nd a tad shorter).
If you were to go to a place on earth on the 22nd where local mid-night is at 0419 GMT then the length of the 21st and the 22nd would be about the same.
If the sun were a better time-keeper then these compared day lengths would be exactly the same. The sun speeds up and slows down at times on its apparent journey along the ecliptic which obviously feeds into the measured length of day.
If the solstice were to fall on the 22nd at local-midnight when the earth is at aphelion (or perihelion) then the lengths of the preceding and following days will be virtually the same.