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Practical Relativity

01/03/2011. Contributed by Geoff Willmetts

Buy Practical Relativity: From First Principles To The Theory Of Gravity in the USA - or Buy Practical Relativity: From First Principles To The Theory Of Gravity in the UK

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Practical Relativity: From First Principles To The Theory Of Gravity by Richard N. Henriksen. pub: Wiley. 262 page illustrated indexed small enlarged paperback. Price: GBP 34.95 (UK). ISBN: 978-0-470-74141-2.

check out website: www.wiley.com

Wiley publishes a wide variety of technical books, mostly for the education market. I keep more than half an eye on what they produce for anything pertinent for background for our genre. Considering that the other month I had a go at a writer who obviously didn’t grasp how relativity works, my eyes lit up with a book explained how it is applied in real life.

Richard Henriksen’s book ‘Practical Relativity’ is described on the back cover as being an ‘advanced textbook’ but that depends more on how much of a background in maths and physics you have. Although there are a lot of formulas and examples are shown, the text does enough to explain the nature of co-ordinates and why, using the survey vehicles currently on Mars, as to why time there and here is different. If you come away from the first chapter understanding that time is relative place to place then it makes you better prepared for the next five chapters. Henriksen works his way up from first principles, even if they look befuddling, so you get a decent grounding, even if you don’t understand all the mathematical formulas involved. For those of you not up on your sciences, if something can be calculated to work in the same way all the time, then it makes it possible to give precise calculation as to when things happen. If you don’t think that has any relevance to us on Earth, the last chapter points out the importance of this for those of you who use satellite navigation aka SATNAV in working out where you are going.



If you get into the mathematical aspects of this book, the realisation that amongst the first equations shows co-ordinates in three positions then it does actually make sense why everything is done in threes as this progresses because it makes a progression of logic and why differential calculus is used to simplify things down to the moment. Even if you’re only familiar with a moderate level of science, once you hit on Newton’s gravitation laws you should be able to understand why movement has to be taken into account. Diagrams often point things out where text doesn’t and on page 29 the one there puts the symbols from the equations into physical context.

An interesting detail from chapter 2, page 56, is that a light second is 80% of the distance between the Earth and the Moon. Of even more interest to us in chapter 3 on page 101 is noting that a spaceship leaving at the speed of light will be seen to have vanished from all angles except the forward one although Henriksen fails to point out that you would need an even faster spaceship to observe the phenomenon. There won’t be any other distractions neither because as you’re going faster than starlight, the universe will appear black which kind of destroys ‘Star Trek’s portrayal of specks of light whizzing past their spaceships. I’m pointing out these particular points in case you were wondering what you can learn specifically from this book for your own stories about the real thing.

Something I wasn’t aware of until reading this book was the nature of light echoes which are the distortion of light sources by other light sources or even non-light sources like black holes. Again, it always pays to keep up with real science than other fiction in that regard.

In the final chapter, Henriksen points out that our sun’s mass was less than 3km in radius then it would collapse on itself and create a black hole and if you ever wondered what Schwarzschild had to do with this other than having the safe zone around a singularity named after him, then you find his maths involved. There is also some discussion at how the mass of stars can influence beams of light and bend it and why the cosmos is actually curved.

If you have to apply relativity to a story then a lot of the time, you might find someone who will either give you a simplified version that you can apply or you can sort out from your knowledge from this book. A lot of the time, you don’t have to show the maths, just apply the information. As long as you remember the principle that time here and on another planet is not on the same plane and the distance a message has to pass takes time, then you can work out how long messages take to get there in regular time. Even if you have a faster-than-light device like an ansible, I can dig a hole in its use simply because you would still have to work out where the planets are going to be rather where they are now in sending an instantaneous message.

Although I’m not sure if the entire book is going to be much use to the novice but for those of you with a grasp of differential calculus and want to use it in calculating something practical in cosmological terms then you will have a field day. From a text point of view, as you can see from some of the points above, I have definitely learnt something from this book that should help me avoid some silly mistakes.

GF Willmetts

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