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# Tiki

Members

415

1. ## Another Graviitational Wave Results!

I used to wonder as to whether we would ever be able to observe such signals. A second discovery so soon after the first is fantastic. Not only does it help to establish functionality of the 'wave-detector' but would also indicate that coalescing-compact objects are commonplace enough in the cosmos for us to be able to gather some really useful data. Hopefully there will be more discoveries in the very near future.
2. ## Understanding the Crescent Nebula

Huge stars typically have huge convectional regions resulting in a rich stellar atmosphere of heavier elements.
3. ## How quickly could precession happen?

I would imagine that for a planet to have a very short precessionary period it would need to be very oblate (fat around the middle) , have a good sized moon nearby in just the right orbit and also be very close to the star. Such a planet, even if it was astrophysically feasible would be 1000's of degrees C and unlikely to have any meaningful sort of atmosphere. What do you mean by 'period of one year': one Earth year or the time it takes for said planet to orbit its star?
4. ## Warping of space-time

No-one really knows what gravity is. It has so far been explained as a force, as a force-field and as curvature of space-time. Hamilton also recreated Newtonian gravity but used cunning mathematics rather than physical reasoning (Hamilton's Principle). Newton thought that objects obeyed well defined laws of motion. He took these laws and guessed that the force of gravity F=Gm1m2/r^2, invented The Calculus and was able to explain the observed motions of the Solar System until Mercury's perihelion shift was detected over 200 years later. Newton explained gravity as a force. Gauss utilized (invented?) the notion of a force-field to obtain exactly the same result as Newton. Gauss's approach makes certain astronomical calculations much more simple. Gauss was very popular with the astronomers of his day. Einstein had the great insight (and also knew about Special Relativity and non-Euclidian geometry) to take a more elemental stance than Newton. Whereas Newton might consider an object either at rest or in uniform motion, Einstein took the notion of a free falling observer as elemental. The free-falling observer will see other free-falling particles close to him moving on straight lines. In the flat-space that the FFO inhabits, these straight lines are geodesics (geodesics are lines that go as straight as possible on a surface). In Newtonspeak: When you put the ramp and ball in space and leave them be they simply remain on their respective Keplerian orbits. When the ball is at the top of an earth based ramp, neither the ball nor the ramp follow Keplerian orbits because besides gravity, other forces are acting (the reaction force of the ramp on the ball and the reaction force of the earth on the ramp). In Einsteinspeak: When you put the ramp and ball in space and leave them be they simply remain on the geodesic that they happen to be on at the time of their release. On an earth based ramp the reaction forces mentioned above disallow both the ball and the ramp from remaining on a geodesic.
5. ## Kepler's second law.

Perhaps what you are looking for is the notion of 'effective potential'. 'Effective potentials' are quite intuitive. Try a google search.
6. ## M101 Pinwheel Galaxy

So do I. When M101 is very near the zenith I am sometimes (literally two or three times a year) able to glimpse it from home with my 4 inch scope. I always have a good search when the skies are promising but I am afraid it is only ever a slight smudge. I find my 9mm T6 giving a 1.7mm exit pupil is best for my eyes and skies.
7. ## Bridging the Gap

On ‎4‎/‎21‎/‎2016 at 22:19, Tiki said: There is quite a gap between 'rusty GCSE maths' and the excellent Stroud book mentioned above. Ahh, I was unwittingly referring to 'Advanced Engineering Mathematics' by KAStroud and not the book mentioned by Colinlp above. George's description of the book indicate that it would be useful to someone with rusty GCSE maths.
8. ## Bridging the Gap

There is quite a gap between 'rusty GCSE maths' and the excellent Stroud book mentioned above. You could perhaps try something like Vol.I of the Feynman Lectures on Physics with a view to filling in any gaps of your mathematical knowledge along the way. The book is quite wordy with much wisdom and a lot of original analogies along the way. Feynman excelled not only as a scientist. Mathematics is a dry subject to some and well motivated physical arguments leading to equations might be both interesting and helpful. QED for a fiver s/h delivered to the door has to be pretty good value too.
9. ## Singularities & Quantum Mechanics

Exactly. A free-fall frame's clock (ie. the probe) is momentarily in sync with everything it passes.
10. ## Angular position (age of nebula)

If you rank these distances and plot the results sequentially you will be able to achieve a decent estimate of radial velocity. Plotting the smallest distance on the left all the way to the largest distance on the right and then draw a line of best fit which resembles a sine curve (0 to pi/2) . ie. y-axis -> distance, x-axis -> 1 to 10 (or more). This curve should have an asymptote of the form x=constant. This constant is what you are after but you have to be careful with the units.
11. ## CCD/film/eyes

Eyes and film emulsions only react with light of certain colours. CCD's react with light of a wider spectrum of colours (hence UV/IR cut filters) . Or you could also say that ISO is like gain. High ISO, high gain and the signal becomes more noisy.
12. ## astronomy / astrophysics books

"Universe" by Friedman and Kaufmann is an excellent place to start.
13. ## Gravitational wave doubt

Exciting times indeed. I have always believed in gravitational waves I just thought that they might prove elusive. Cut and pasted the last paragraph of this tutorial: http://www.astro.cardiff.ac.uk/research/gravity/tutorial/?page=4blackholecollisions 'The detailed knowledge of the detected signal will allow us to infer the masses and spins of the binary components and its orientation in space. The knowledge so gained will enable strong field tests of general relativity such as the no-hair theorem which states that the geometry of a black hole is entirely determined by its mass and spin. Additionally, it will be a new observational tool to map and measure the Universe. For instance, being standard candles binary black holes will make it possible to measure the luminosity distance to sources at cosmological distances thereby providing a new tool for cosmology. In particular, Cardiff relativists have shown that by observing many such binaries at cosmological distances, it will be possible to measure the mean density of the Universe and its expansion rate very accurately. LISA observations of binary black holes will also help in measuring the dark energy equation of state (see the Section on super-massive black holes).'
14. ## Black Hole mergers

I hope this helps: http://www.astro.cardiff.ac.uk/research/gravity/tutorial/?page=4blackholecollisions
15. ## Self Same Photon

Therefore it doesn't make sense to talk of an individual photon since no-one knows what (or where) they are. Yes it is a sensible question , but the problems start to arise as soon as you try and specify 'which photon'.
16. ## Mathematics ?

Firstly guess, secondly you scrutinize. If the initial guess stands up to sufficient scrutiny then it becomes science. Eg. General relativity. https://www.youtube.com/watch?v=5v8habYTfHU (60 seconds of Feynman)
17. ## Mathematics ?

A good question with no clear answer. Personally, I believe that mathematics is discovered rather than invented. If mathematics was purely a human invention then we might expect some differences within the subject on purely cultural grounds. The very fact that Pythagoras's theorem (amongst others) has been independently discovered many times by different people with hugely varying socio-economic backgrounds is a strong argument against inventionism.
18. ## Magnetic Fields around a Black Hole

Here is a nice little page http://www.eventhorizontelescope.org/science/general_relativity.html that explains something about the Event Horizon Telescope's mission. There can never be enough strong-field gravity experiments.
19. ## Magnetic Fields around a Black Hole

Incredible. It says in the article that at 15micro-arcseconds you could resolve a golf ball on the moon!!!
20. ## Black holes

The mass of the initial star was spread over a much much larger volume than the mass of the subsequent black-hole. It is this super-concentration of matter which distorts space-time to such a high degree.
21. ## The 22" mapstar mirror

Impressive stuff. Very well done.
22. ## Two Questions

1) A vixen flip mirror should do the trick. 2) Don't know, but be careful you don't fry your mirror coatings or retina.