Archive for July 4, 2019

Wolfram Alpha and the Principle of Astrogeometry

Posted in The Universe and Stuff on July 4, 2019 by telescoper

Regular readers of this blog (both of them) may remember the basically tedious and offensive but occasionally (accidentally) hilarious troll who keeps attempting to post comments like this:

I thought you might like to see the result of feeding the expression found in the above rant into Wolfram Alpha:

This is exactly the expression described above but produces nothing like the claimed value of the Hubble constant, and it’s in the wrong units too.

Update for the benefit of the extremely hard of thinking (especially Mr Hine):

π21 ≈ 2.75 × 1010 (dimensionless).

One parsec = 3.086 ×1016 m so one Megaparsec is 3.086 ×1022 m. Hence 2 × `a Mpc’ × c ≈ 2 × 3.086 ×1022 m × 3 × 108 m s-1 ≈ 1.83 × 1031 m2 s-1.

Thus the full expression is obtained by dividing this by the value for π21 obtained above giving a value approximately 6.7× 1020 m2 s-1 as demonstrated by Wolfram Alpha.

The correct value for the Hubble constant is about 2.2 × 10−18 s−1.


UPDATE: It’s interesting how the Megaparsec appears in the numerator in Mr Hine’s expression, but magically transfers to the denominator as far as the units are concerned:


I think I may have cracked it. I believe Mr Hine’s calculation involves using light-years instead of Mpc or SI (for some reason) the calculation is in which case the calculation becomes:

π21 ≈ 2.75 × 1010 (dimensionless) as before

One parsec = 3.26 light years so one Megaparsec is 3.26 ×106 million light years. Hence 2 × `a Mpc’ × c ≈ 2 × 3.26 ×106 m × 3 × 105 m s-1, using c in km/s.

When divided by the value of π21 this gives a number around 71 (I couldn’t be bothered with the extra decimal places).

However, although it is a number around 71 the units are then km/s times light years, not the correct units which are km/s divided by Megapersecs. The fact that the number comes out close to 70 is just a numerical artefact of Mr Hine’s basic misunderstanding of units and dimensions. In other words, it’s gibberish. I know you’ll all be shocked by this revelation, but it’s true.


A Head Again!

Posted in Biographical, Maynooth with tags , on July 4, 2019 by telescoper

Today is Independence Day – on which all joint probabilities P(A,B) can be expressed in the form P(A)P(B) – and by coincidence I received a letter that I’ve been expecting from the President. No, not Michael D. Higgins (nor Donald Trump for that matter) but the President of Maynooth University, Professor Philip Nolan.

Despite it being marked Strictly Private and Confidential I have actually read it, and it says that I have been appointed as Head of the Department of Theoretical Physics at Maynooth University, with effect from 1st September 2019.

The appointment is for three years in the first instance, with the possibility of renewal for another two years `subject to satisfactory performance’. So just the three years then.

The current Head of Department is taking a sabbatical next semester (from September to January) and just this morning we have been interviewing candidates for a temporary to provide teaching cover for his absence. Now we officially begin the handover (including, I suppose, moving offices…).

It’s about three years now since I stepped down as Head of School at the University of Sussex at which point I didn’t imagine I would be stepping up to be Head of Anything again, but to be honest this position has a smaller and much better defined set of responsibilities than the one I used to hold so I’m actually quite looking forward to it.

But first I’m going to take tomorrow off.