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Isn't cosmogony caught between the Scylla of creation out of nothing or the Charybdis of an infinite regress of initial conditions, and existence tout court, which would attract the question: why should
anything be?
Akhtar Said - Pakistan
I think the answer is yes! However, faced with this problem all we can do is continue to search for a reason for the the initial conditions for creation out of nothing (as you put it), or continue to work our way back along the 'infinite regress' of initial conditions.
Chris Lintott
I have just finished preparing a talk for my local astronomy society and was reading Bang! I was surprised at the figure on p31 which states that the (observable) Universe is 10^25m. I would like to get this right as the talk involves a journey from the Planck length to the size of the universe!
Is the observable universe not much larger than this? The light that reaches us has travelled for 13.7 billion years, but during this time, the Universe has been expanding, so the territory it has already covered represents a greater distance than its travel time would suggest.
The article: http://en.wikipedia.org/wiki/Size_of_the_universe
gives a figure of about 100 billion light years diameter. Which is roughly 10^27m.
NOTE: even if we use the 13.7 billion light years figure, I make the diameter roughly:
2.8x10^10 x 10^16 = 2.8 x 10^26
Many thanks for any clarification.
Steve Hill - Bath, UK
Oops. You are correct, and we are wrong. Sorry!
Chris Lintott
I believe that the escape velocity of a star is based on the gravity derived from the star's mass, the more mass the higher the escape velocity. On page 137 it says that as the density of a neutron star goes up, the escape velocity increases. Why is this? Surely the density increases because the star's diameter shrinks, but the mass remains about the same, so why does the escape velocity increase if the mass and hence the pull of gravity doesn't change?
Chris Hill - Swanland, E. Yorks, UK
The strength of the attraction of any body also depends on the distance one is from its surface and so a shrinking star will have an increasing escape velocity.
Chris Lintott
Hi Brian, i was wondering what are your views on pluto been excluded as a planet. I feel its disrespecting to Clyde Tombaugh and months of hard work. I have been into astronomy since i was 12 so talking to a rock star with a love for astronomy is a unique thing.
Jonathan
Well, yes, I see what you mean, Jonathan. I'm afraid it doesn't bother me much. I just think that as we learn more and more, many old ideas have to make way for new ones. Pluto is still there, and just as interesting!
But now we know about lots more strange boulders out there!
I get more worried about sending troops to Iraq! But, yes, know what you mean....
All the very best, and enjoy your astronomy!!
Cheers
Brian May
I would like to compliment you on 'bang' as it is by far the best astronomy book i've seen and is very easy to follow for novices like me. In your opinion, which telescope is better out of reflecting and
refracting? Also, the book mentions viewing the sun through a telescope, I would like to know if there is a filter strong enough to view the sun safely.
Thanks,
Love your work.
David Meredith - Darwin, Australia
Dear David
We have just received a nice new batch of BANG ! questions - all interesting - but, because your question embodies an important safety issue, I wanted to give it priority.
Using a telescope to study the Sun is a highly dangerous occupation. I am not kidding. Or over-reacting. Even experienced astronomers have been known to make terrible mistakes. The problem is that as soon as an instrument of this kind is pointed anywhere near the Sun, there is a risk of a huge amount of light and heat radiation being channelled into it. If ANY part of your body is near the exit pupil of the telescope, far less your eye, you are in severe danger. This is true even of a small (only 1 inch diameter, maybe) 'finder scope', which might be piggy-backed on to a larger instrument.
Once this amount of heat energy has arrived at the eyepiece of the 'scope, there is NO filter which is safe enough to use as protection. Even a miner's glass can shatter, and if your eye is there at that moment, you will be blinded for life in that eye in an instant.
The general rule is - stop and think very carefully before turning a telescope to even the vicinity of the Sun during the day. As a first option, choose projection on to a white card as the method. Even then, be careful not to get your face near the eyepiece while lining up. Projection is a highly satisfactory way of viewing the Sun, especially if you can devise a way of keeping the whole assembly rigid. You can line up the telescope itself by looking at the shadow of the tube on the ground behind it, or on a piece of card. Do NOT use the finder telescope to line up. IF a filter is to be used, it must be a large one on the front of the scope ... covering the entire object lens or mirror. This stops the dangerous radiation before it is magnified in intensity. The only filter we regard as safe is the new kind of interference filter which allows only one narrow spectral line of radiation through ... the H alpha line.
These instruments are quite expensive, but the view is absolutely safe, and absolutely spectacular. You will be able to see prominences on the edge of the disc of the Sun, and faculae, sunspots and granulation on the surface, with amazing clarity. If you want information on these telescopes - try our friend Ninian Boyle.
I say again .... this is the ONLY kind of telescope which it is safe to use to look at the sun directly.
Cheers !
Brian May
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