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Posts posted by 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 nonzero mutual acceleration is coordinateindependent 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 geometryas 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/isgravityaforce.975552/#post6214766Regards 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 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!
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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.

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.

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)

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 freefall 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.

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.
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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 untechnical. I'd be pleased to hear to hear the opinions of any members who have read the book.
Thanks.

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.

23 hours ago, andrew s said:In the case of Omega I think it would be interesting to no what magnitude range (+ve orve) 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)

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.

10 hours ago, vlaiv said:You are right in saying that random sequence is incompressible, but not in part where you say that it can't contain pattern.
Thanks for calling me up on this. 'Pattern' is too loose a word to use in this context.

18 hours ago, vlaiv said:It can be thought of time reversal, but it can also mean that we don't understand random  it's true nature or how it behaves.
Any thoughts?
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.

Straight from Wikipedia:
Physical magic numbers and odd and even proton and neutron count[edit]
Stability of isotopes is affected by the ratio of protons to neutrons, and also by presence of certain magic numbers of neutrons or protons which represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide. As in the case of tin, a magic number for Z, the atomic number, tends to increase the number of stable isotopes for the element.
Just as in the case of electrons, which have the lowest energy state when they occur in pairs in a given orbital, nucleons (both protons and neutrons) exhibit a lower energy state when their number is even, rather than odd. This stability tends to prevent beta decay (in two steps) of many eveneven nuclides into another eveneven nuclide of the same mass number but lower energy (and of course with two more protons and two fewer neutrons), because decay proceeding one step at a time would have to pass through an oddodd nuclide of higher energy. This makes for a larger number of stable eveneven nuclides, which account for 152 of the 253 total. Eveneven nuclides number as many as three isobar (nuclide)s for some mass numbers, and up to seven isotopes for some atomic (proton) numbers.
Conversely, of the 253 known stable nuclides, only five have both an odd number of protons and odd number of neutrons: hydrogen2 (deuterium), lithium6, boron10, nitrogen14, and tantalum180m. Also, only four naturally occurring, radioactive oddodd nuclides have a halflife over a billion years: potassium40, vanadium50, lanthanum138, and lutetium176. Oddodd primordial nuclides are rare because most oddodd nuclei are highly unstable with respect to beta decay, because the decay products are eveneven, and are therefore more strongly bound, due to nuclear pairing effects.^{[2]}
Yet another effect of the instability of an odd number of either type of nucleons is that oddnumbered elements tend to have fewer stable isotopes. Of the 26 monoisotopic elements (those with only a single stable isotope), all but one have an odd atomic number, and all but one has an even number of neutrons — the single exception to both rules being beryllium.
The end of the stable elements in the periodic table occurs after lead, largely due to the fact that nuclei with 128 neutrons are extraordinarily unstable and almost immediately shed alpha particles. This also contributes to the very short halflives of astatine, radon, and francium relative to heavier elements. This may also be seen to a much lesser extent with 84 neutrons, which exhibits as a certain number of isotopes in the lanthanide series which exhibit alpha decay.

On 26/12/2018 at 15:52, Stu said:Yes, I thought that one was a bit much, seeing as we can actually detect them now. I guess there are some many of the constantly streaming through the earth that by sheer chance one must hit a detector every so often. I wonder what the detection rate is.
Not very high.
SN 1987A let out a burst of about 10^58 neutinos (which in terms of energy is more than 100 times the amount of energy emitted by the sun in its lifetime). At 160,000 ly distance, the IMB and Kamiokande neutrino detectors combined, encountered a 12 second burst of about 10^16 (only) neutrinos from the SN. There were 20 detections (flashes of Cerenkov radiation).
Cerenkov radiation is pretty neat. https://en.wikipedia.org/wiki/Cherenkov_radiation

On 26/09/2018 at 15:45, andrew s said:"If a person were to fall into a black hole, time would come to a halt for them at the event horizon, so photons would not even reach their eyes"
Regards Andrew
Unfortunately not only wrong but also actively misleading.
Perhaps a correction will be published. After all, a scientific magazine has a responsibility to the public understanding of science. The consideration of objects falling into black holes are a great way of looking at GR, it is a shame that it has gone so badly awry in this most public case.

4 hours ago, JamesF said:He asks: "If an object starts moving as a result of being attracted to a magnet, where has the kinetic energy gained by that object come from? Has something else lost energy (so that energy is conserved), and if so, what, and how?"
James
As Captain Magenta says, potential energy. The objects initial potential energy ( this PE being a consequence of it's initial position relative to the magnet) is converted to kinetic energy as the object 'falls' towards the magnet.

Whilst it is impossible to visualize objects in greater than 3 dimensions it is simple enough to think about them. Objects with a fractional dimension (ie. noninteger) take a bit more effort to get your head round though.
Dimension is a tricky subject. For example, it is possible to specify any point inside a square with two numbers. Two numbers therefore two dimensions, easy. However, if you were to fill a square with a spacefilling curve ( eg. Peano curve, Hilbert curve etc.) then it is possible to specify any point within the square with just one number; the length along the curve until the desired point is reached. Plenty to think about with dimensions.

21 hours ago, JamesF said:..... what does he actually mean when he says "the universe forgets how big it is"?
Penrose is referring to a state of maximal entropy, ie. before the clock begins to tick. Time being necessary to define distance and hence 'size' is also rendered meaningless.
21 hours ago, JamesF said:He seems to be saying that the dimensions of space are everincreasing and time is always passing, up to a point when if we were able to wait long enough, suddenly they'd stop existing.
Looking into the distant future Penrose talks of a state of maximal entropy as the last blackhole evaporates. Don't hold your breath for this one.

On 20/08/2018 at 12:19, Macavity said:But I am intrigued by this one:
"The Strangest Man"  Paul Dirac. A biography by Graham Famelo.
Thanks for the reminder, this one is due for a reread. An interesting book that I'm sure you'll enjoy.
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Thanks for all the replies.
For reference, this was a very easy job. Everything unscrewed easily. The tube to lens cell thread was a bit tricky to start straight but other than that there were no difficulties whatsoever. It is now just a matter of waiting for the clouds to part......
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27 minutes ago, Stu said:If you proceed, I would maybe treat it as a two person job and keep the scope vertical, focuser downwards and unscrew carefully and slowly!!
Thanks Stu, that sounds like some good practical advice.
I just have to summon the necessary courage!
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Fritz Zwicky biography.
in History of Astronomy
Posted
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.