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Yes, even I put away the pencils and got out the camera!! Two images below taken on my IPhone.
Alex,
You raise an interesting point about what the Met office and BBC Weather class as a clear night. At the time of writing, both of them say it is clear outside (a single moon with no clouds on their websites) and yet at the moment it is at least 85% cloud cover- if only someone from these institutions could go outside and look up to check!
It would be interesting to know their definition of a clear night sky! Satellite 24 indicates a gap coming- we wait and see…
Cheers,
-PaulIt is rather depressing and I can’t recall it being this bad before- I had no clear nights at all in December 2023, and just a few nights in January to April. May so far has been a little better.
Like Gary, I am a visual observer so that don’t need pristine clear skies for observing. My interest hasn’t waned of course, but I have noticed just how poor the Met office is at predicting clear nights. I now use a combination of sources including satellite 24 to make my own forecasts, the official forecasts are simply to inaccurate.
Cheers,
-PaulPleased to report that T CrB is now accessible from my observatory! I shall be keeping a close watch of this star.
I’ve been through my records and overall the weather in Leicester has followed the same general pattern, namely April, May and June were the best months. Unusually November was fairly good but as usual December was terrible with no useable clear nights at all. The couple of clear nights we had in December were unusable due to high winds. Overall I managed 66 observing sessions in 2023- this does not mean there were only 66 clear nights, there would have been more than this but I was unable to use them due to being away. This was marginally better than the previous year.
What I have noticed this year is that the clear nights were very well dispersed by days and nights which were very cloudy and/or wet. Let us hope that 2024 is better.
Cheers,
-PaulHi all,
Interesting discussion. I am a bit of a way from honorary membership, however I can see that it is a nice tradition we should keep to mark the 50 years of someone’s membership. I do think Nick, Jeremy and Gary make a good point about the membership fee and it does seem that younger members effectively pick up the tab.
I suggest we adopt Martin’s suggestion and keep the honorary membership to mark 50 years but membership fees have to continue to be paid.
Cheers,
-PaulI made it magnitude 9.3 this evening! It was quite unmistakable at x38, even though the sky wasn’t completely dark at 1730UT.
How many more times- I am not a cosmologist part time or otherwise! 😉 Seriously though, I am pleased it was a successful meeting and many of the people who attended have told me how much they enjoyed it! As you say the speakers and the venue were excellent and I’m particularly pleased that all of the speakers on the Saturday were experienced BAA members.
My thanks to everyone who helped make the meeting a success- I think we should have definitely have another practical astronomy meeting in the near future!
I just hope it happens when CrB is accessible- perhaps Jeremy can check the delivery note to make sure that it arrives after December?! By the way, will we have some naked eye comparison charts ready?
It’s interesting what you said about a nova getting you interested in variables Gary, my first was the supernova in M82 a few years back- you suggested I do a few variables and now I have over 10 which I follow regularly!
Cheers,
-PaulLooks like the meeting went very well; I was planning on attending but alas the rail strikes on Friday and Saturday made it almost impossible!
Well I’ve now lost it from my observatory so I can guarantee that it will now do something interesting!!
Hi Chris, it’s best to let the meetings secretary (Hazel Collett) know. Her email address can be found in the Journal.
Cheers,
-PaulThere are many subtleties to this but it sounds like you haven’t included the Bianchi identities, in particular the first and perhaps the second Bianchi identities. There are reasons why this tensor has these properties. There are also the anti-skew symmetries and one of them is to do with defining a suitable inner product on the tangent space T(M) which is induced by the metric tensor g_ab.
If a curvature tensor satisfies the skew symmetry, anti skew symmetry and first Bianchi identities then it is trivial to show that there exists a curvature tensor at each point p in M (a pseudo-Riemmannian manifold) which has n^2(n^2-1)/12 components.
It’s all in two standard texts:
-Introducing Einstein’s Relativity by D’Inverno
-The Mathematical theory of Black Holes by Chandrasekhar.Due to cloudy skies I have only managed six observations- the one I made last night is included on the plot. Although it’s only six points it is enough to see the basic type 2 supernova light curve.
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Hi Ken,
I think an deep understanding of the mathematics involved is essential- there are some popular books which give overviews of GR and particle physics but without understanding topics like differential geometry, exterior calculus, group theory and Lagrangian dynamics it’s almost impossible to accurately convey the subjects properly. It’s rather like learning a foreign language by repeating constructed sentences but not knowing the meaning of the words.
I’ve been teaching our General Relativity course at Leicester University which our 4th year MPhys students can opt to take and I would recommend:
-‘Introducing Einstein’s Relativity’ by Ray D’Inverno
-‘A short course in general relativity’ by Foster and Nightingale.Not convinced about a variable Planck’s constant for a variety of theoretical and experimental reasons at the moment.
Cheers,
-PaulThere are indeed other approaches but these are the two main attempts so far, and the history of their developments allows a fairly simple introduction to the area without too much technical detail. In any case, many of them are similar in approach albeit with different wave equations (Dirac, Yang-Mills+gravity, DeWitt-Wheeler) and don’t as yet add much to the general picture I was describing for Ken.
Yes, I am familiar with Wolfram’s idea- I have to say I am not convinced at the moment. I have no issues with spacetime geometry being an emergent phenomenon, nor indeed time being an emergent phenomenon (which I’m sure it is and I wouldn’t be surprised if it has more than one dimension), but as yet I haven’t seen anything which really deals with the problems discussed here or other problems like the violation of unitarity.
cheers,
-PaulHi Ken,
So the difficulties between the quantum world and general relativity are actually more profound that just the uncertainty principle and the whole problem hinges on finding the much sort quantum theory of gravity. Let me explain…
First up, quantum mechanics. QM as governed by the Schrodinger equation is non-relativistic. We can see that because in this equation we differentiate twice with respect to position, but just once with respect to time, In relativity space and time are treated equally and that clearly isn’t the case with Schrodinger’s equation. Now the relativistic version of QM is quantum field theory. This is quite a change of quantum mechanics- in QFT underlying fields are fundamental objects not particles- and when these fields are stimulated under certain conditions, they give rise to particles- in much the same way as striking the chord A minor on a piano keyboard generate the musical sound of A minor.
Now QFT when applied to condensed matter physics is very accurate- there are problematic infinities in the equations (in order words, instead of a finite numerical value you get an infinity) but these can be renormalized away quite safely with predictions that agree very accurately with results from condensed matter physics experiments. The Fourier transform is a key part of this as it describes how the field works in terms of annihilation and creation operators. Excite the field with the creation operator and you get particles from the field, you can de-excite it until you reach the ground state (also known as the vacuum state). Since the particle is written in terms of annihilation and creation operators the particle is uniquely defined. The theory is also relativistic and can be applied to Maxwell’s equations to give a relativistic version of electromagnetism. So far so good- now to General Relativity.
General Relativity (GR) is a classical field theory and are governed by the Einstein Field Equations. On the left hand side we have quantities like the Reimann tensor which describe the curvature of spacetime, and on the right hand side we have the energy-stress tensor- this represents the matter and energy. In other words then, if we have something heavy like a star or a black hole, its matter-energy is represented by the stress tensor, and this tells spacetime how to distort and curve. Solutions to Einstein’s equations are metric tensors- these tell us (with the aid of a line element) how to measure distances in points between spacetime. The flat spacetime of special relativity is one example, the Schwarzschild spacetime is another. There are also cosmological solutions like the Robertson Walker spacetime and de Sitter spacetime solutions. These are all vacuum solutions though, and this means that outside the event horizons, the stress tensor is zero. Also the stress tensor doesn’t care what the matter and energy is made of, and has no quantum description.
So, this brings us to Hawking radiation. Clearly, all objects in the Universe have to obey all the laws of physics, not just some of them. in particular, they all have to obey the second law of thermodynamics: entropy increases in a closed system. If an object has entropy then it has heat and is coming into thermal equilibrium with its surroundings. As it turns out, black holes have a measure of entropy- this is their event horizons. As a result, black holes must have a temperature and this was what Hawking discovered. How does this help? Well thermodynamics are statistical physical processes which are governed by quantum phenomena- so in the vicinity of the event horizon, QFT and GR have to agree and work together.
The first attempts to do this properly is to put QFT into GR. This is done by replacing the stress tensor with something called the expectation value of the stress tensor- now it is composed of quantum fields rather than a generic ‘lump of matter’. This is where the problems really come in. Firstly, the vacuum (or ground state). The crux of this is the Fourier transform as I mentioned earlier. If you put the Schwarzschild back hole into QFT instead of flat spacetime, you find that there is more than one vacuum state! So the annihilation operator which de-excites you field before the star collapses to become a black hole, might no longer do so after the black holes has formed. As a result the notion of a particle is now deeply ambiguous. Worse still, the infinities!!! In flat spacetime we can renormalize them away, but energy contributes to the stress tensor- it is a potential source of gravity in general relativity and so we can’t just renormalize it! There is a process for doing so in one dimension but there is no general procedure for doing it 4 dimensions, and the processes in 4 dimension involve a mathematical technique called ‘point splitting’ which may or may not be Lorentz invariant!
So where does this leave us? Stuck, frankly! There are two approaches: string theory which has taken up a lot of money and time but delivered very little. We thought a few vacuum states was bad enough but in string theory there are something like 10^235 vacuum states and it is not clear which (if indeed any) are physically meaningful. The other approach is loop quantum gravity- here the idea is that spacetime is made up of tiny course grains down at the Planck level- however great problems remain here, for example it is not possible to recover ordinary flat space results in a low energy limit as one would expect.
The real paradigm shift will be the solution to the problems I have outlined here, they are conceptual and not just computational, and they require a deep understanding of the mathematical and physical tools we use in physics. I suspect at some point, someone will come up with a new understanding of space, time matter and energy- as far removed from general relativity as Newtonian mechanics is from special relativity. There are also many dep problems with quantum mechanics which have yet to be resolved and these do carry over to QFT. To sum up- the physics we have is very successful- it allows to build semi conductor devices and sat navs but there still remain many deep problems to solve.
Cheers,
-Paul
