23 February, 2021
Special look at Relativity 001: Spacetime - Setting the Stage
21 February, 2021
Special look at Relativity 00: Prequel - How we do science
The journey ahead
At the end of the 19th century many people thought that physics was pretty much 'in the bag'. Newton had given us a seemingly universal set of laws describing motion and forces, while Maxwell had done the same for electromagnetism. It didn't really seem like there was much left for the apprentice boffins to get their teeth into.
There were some annoying little problems. One was a small incompatibility between the laws of Newton and Maxwell, and in 1905 a young patent clerk proposed a solution to this problem that was to usher in a new era of physics. That clerk was Albert Einstein and the solution was Special Relativity and it changed fundamentally the way we think about time and space.
If you look at any high school or undergraduate physics textbook and you
will find an introduction to Special Relativity based around light
clocks and a presumption that the speed of light is constant. I want to
take a completely different approach which I hope will lead to deeper
understanding while actually making it easier to solve relativity
problems. This approach will be based on Minkowski diagrams,
also called spacetime diagrams, and will hopefully lead you to an
understanding of why the speed of light is constant for all observers,
and why \[ E = mc^2 \]
How Science works
Before we develop the spacetime framework I want to indulge in a few words about how science works. Perhaps one of the most succinct summations of science at work is given by Richard Feynman in a lecture to class at Cornell University in 1964.
In summary, when looking for a solution to a problem we make an educated guess, compute the predictions of that guess, then test those predictions against experiment and observation.
In Einstein's case the problem was the incompatibility between the laws
of Maxwell and Newton. I want to leave the discussion of the nature of
this incompatibility until later. To begin, I just want to introduce
Einstein's solution, which was to suggest that perceptions of time and
space vary according to the relative velocity of the observers. In other
words, if Prof Stick is moving relative to Albert, then they will
measure space and time differently. Our quest in this series is to
develop a framework which combines time and space in such a way that
Prof Stick and Albert can agree on measurements of 'distance' regardless
of their relative velocities. We'll call this framework spacetime.
To develop this framework we are going to make a series of educated and not-so-educated guesses and compute their consequences. This is going to take a fair bit of work but at the end we will have a set of predictions that we can compare observation and experiment.
All aboard!
Before we depart, let's pause, look around and get our bearings. Our starting position has 2 reference points:
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Galileo's Principle of Relativity - which states that all motion is relative and that there are no special or absolute frames of reference. This is important because it means that if two observers are moving relative to each other they both agree on their relative velocity.
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Einstein's solution to the Newton/Maxwell incompatibility that suggests that observers with relative velocities will measure space and time differently.
OK, so now we know where we are starting from, follow me...and watch your step!
05 November, 2014
Planck, Einstein and the birth of quantum physics
(or the man who created but didn’t believe, and the man who believed but wished it wasn’t so)
One of the disappointing things about the treatment of the birth of quantum physics in most textbooks is that they present an ahistorical story in which Planck and Einstein react to detailed and precise experimental evidence about blackbody radiation and the photoelectric effect. This certainly produces a very clear storyline, but at the expense of making one of the greatest scientific feats of all time appear practically mundane. In reality the experimental evidence Planck and Einstein had to work with was meagre and their brilliance in piecing it together to develop the foundations of quantum physics was astounding.
04 November, 2014
How to deal with loonies
29 November, 2012
Science behind the news: Perfect Poison?
04 December, 2011
Same, same...but different! Einstein, Planck and the role of science in society
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process information to discuss Einstein’s and Planck’s differing views about whether science research is removed from social and political forces
This 'dot point' from the NSW Physics Syllabus is at best woolly and ambiguous and at worst misleading because it seems to imply that a great gulf existed between Einstein and Planck when in fact they were lifelong friends and seemed to have much in common and a great respect for each other.
03 November, 2011
How Clean is Coal Seam Gas?
CSG Well (source: http://www.glng.com.au/ ) |
Those in favour of expanding the coal seam gas industry promote it as a green alternative to coal for use in power stations. They argue that coal seam gas, which is mainly methane, results in 70% less carbon dioxide emissions for the same amount of energy.
The 70 per cent figure relates to greenhouse gas generation at the power plant and comes from a comparison between a modern combined cycle gas turbine (CCGT) power plant and a traditional power station burning brown coal, which is the lowest grade of coal. If we make the comparison with a traditional plant burning higher grade black coal the difference falls to about 50%. If we make the comparison with a modern ultra-super critical coal plant the difference drops even further, though it is still significant.
But what happens at the power station is only part of the story. If we really want to know if CSG is greener we need to do a ‘cradle to grave’ comparison, that is, we need to look at greenhouse gas contributions from the coal deposit to the smoke stack. Coal is relatively easy and cheap to dig up and put on a truck and send to the power station. The energy input needed to mine and process the coal is about 2-3% of the amount of energy released by burning it. Coal seam gas is trickier. It needs to be captured, processed, compressed and transported at an energy cost of 20-25% of the energy that is released by burning it.
But the real villain in this story is fugitive emission. If we drop a few lumps of coal between the mine and the power station it is no big deal. But if we leak a bit of methane it’s a very big deal because methane is 20-30 times more potent as a greenhouse gas than carbon dioxide (methane breaks down more rapidly in the atmosphere than carbon dioxide and is about 25 times more potent as a greenhouse gas over a 100 year period, but over a 20 year period methane is over 70 times more potent than carbon dioxide). These leaks are known in the industry as ‘fugitive emissions’ and while the CSG industry argues that they are negligible, the truth is that there doesn’t seem to be any good published data on fugitive emissions from CSG in Australia. The most similar data we have at present comes from a paper by Robert Howarth on fugitive emissions from shale gas production in the United States. Howarth estimated fugitive emissions of 4‑8%. But shale gas production is not the same as CSG production, and to be fair there is also some leakage of methane from coal mining operations (just ask the canaries), but if the rate of leakage for CSG was comparable to that reported by Howarth, then it would be more than enough to wipe out any greenhouse benefit from switching to coal.
To this complicated picture we need to add another very cruel irony. CSG is definitely cleaner than coal in the sense that it produces much lower amounts of sulphur dioxide and particulates which contribute to acid rain and smog. But sulphur dioxide and particulates reflect solar radiation and also help cool the planet. In other words, this pollution from coal may actually help fight global warming.
So you can see that the science is quite complicated. A recent article in the peer reviewed journal Climate Change by Tom Wigley from Adelaide University tried to put all these pieces together and found that a switch in power generation from coal to CSG would most likely result in an increased greenhouse effect out till about 2100 and only a negligible decrease after that.
So does this mean that CSGs future as a ‘green fuel’ is dead. Maybe not. There is a great deal of uncertainty about the level of fugitive emissions associated with CSG in Australia, and fugitive emissions are, in theory, controllable. What we really need is some comprehensive independent scientific studies to give us the information to determine whether CSG really is the green transitional fuel we would all like it to be.