Surely mirror matter would have more effects
Michael Brooks's fascinating article on the concept of mirror matter presents it as a candidate for dark matter (8 June, p 34). That offers a natural explanation for the 80 per cent of the universe that we can't see, other than through its gravitational effects. It would, for example, account for galaxies rotating the way they do.
Brooks further speculates that this matter could otherwise be like ours, forming its own atoms, stars and planets just as normal matter does, all “out there” and around us but invisible.
But if this were indeed the case, it would suggest that there are five times as many “mirror” or “dark” stars than familiar ones within our galaxy, interacting with normal matter only through gravity. In that case, wouldn't we see lots of stars orbiting an invisible binary partner? Far more, that is, than those we currently observe, which appear to be partnered with black holes. And in the accretion process through which planets and stars form, why wouldn't the mirror matter accrete gravitationally with normal matter, giving, for instance, stars that otherwise appear equivalent randomly different masses?
Fake news detection can be made simpler than that
I read with interest Donna Lu's article on Rowan Zellers and his colleagues using a machine learning system that can generate fake news in order to detect it (15 June, p 15). Its reported accuracy is impressive, but I note that their Grover model has been tested only on a sample of fake news articles that it had itself generated.
I have recently done some work using public data sets and have achieved similar levels of accuracy with a much simpler model (available at ). I trained it on a sample of 13,000 fake news reports published on the data science website, and on the , which is a sample of 10,788 articles from the press agency's trusted newswire. To avoid bias and to future-proof the system, I didn't train the model on the articles' content.
Instead, it looks at sentence structures and function words (pronouns, prepositions, conjunctions and auxiliaries). A simple logistic regression machine learning model classified the documents, using 70 per cent for training and 30 per cent for testing. It was able to distinguish between real and fake news with 93 per cent accuracy.
Geography teachers do deal with population
Contrary to what Graham Lawton seems to suggest, population isn't a fringe or a taboo issue, at least not in schools in England (25 May, p 24). Geography deals with it in depth. It is discussed as “population change” rather than “population growth”. Students examine various models for predicting future population change and factors which may influence it.
They are left in no doubt that high fertility rates in some regions and rises in others with already substantial populations will account for a growth in world population to around 10 billion by the mid-century. They are also made aware that improvements in reproductive health and women's education may well slow down and even reverse that growth by the end of the century. When population does appear on the global policy agenda, there will be many geography students who will ask: what took you so long?
Setting the controls for the heart of the sun is hard
Leah Crane says that a probe that missed Mercury would be “trapped by the sun's powerful gravity and dragged to its doom” (22 June, p 42). This suggests the misconception that it would meet a fiery end.
A Mercury probe would try to meet the planet at the perihelion of its trajectory, when it is nearest the sun, since that is most fuel-efficient. If it missed, its orbit would take it away from the sun.
A result of this misconception is the idea that the sun would be a good place to dump nuclear waste, as if anything escaping Earth would slide straight down into the flames. In fact, the direct route would take vast amounts of fuel and has never been done. The easiest way to the sun is via Jupiter, as the Ulysses solar probe demonstrated. If you must shoot radioisotopes into space, the moon is far easier to reach.
Demonstrating the density illusion of different weight (1)
Thomas Patrick Reid wonders whether an object’s density fools our perception of its weight (Letters, 22 June). I sometimes carry bags of cement and of hydrated lime, both with a mass of 25 kilograms. The lime is approximately half as dense, so its bags are twice as large. Against all knowledge to the contrary, I find them much less heavy.
Demonstrating the density illusion of different weight (2)
Yes, density does affect our perception of weight. When I worked in concrete technology, I was told about a laboratory open day for which staff made small concrete cube specimens using different mixes.
At one extreme, they replaced the coarse gravel aggregate with lead shot and the fine sand with crushed barium sulphate; similar mixes are used in concrete for radiation shielding. At the other extreme, the aggregate was polystyrene beads and the fine aggregate hollow glass spheres from power station ash. These cubes floated on water.
All looked like regular concrete, as did cubes of intermediate density. Visitors who picked a high-density cube initially couldn’t move it, because it was unexpectedly heavy. Those choosing a low-density cube felt their arms shoot upwards because it was unexpectedly light.
How drones should avoid collisions with other craft
You report a system to avoid collisions between drones and aircraft, in which a drone that detects a potential crash drops by 50 metres, cuts its speed and starts circling (1 June, p 10). That avoidance action is inadequate in the real world.
Last year, a New Zealand court of flying a drone within 50 metres of fire-fighting helicopters and, at times, above them. The judge described the tourist's actions as moderately serious; the defendant thought he was being careful.
I suggest the safest option is for the drone to land. Less good would be to remain below the maximum legal altitude for remotely piloted drones, which in New Zealand is 120 metres above ground level. This airspace should normally be devoid of piloted craft.
I wonder how many pilots, whether on board or remote, would trust any automated system. The world is too complex, and we have seen such systems fail tragically.
Watch where you tread while getting to the stars
Julius Wroblewski offers a vision of robots going down mineshafts to extract minerals until the day they are swallowed by our sun, then grown into a red giant (Letters, 22 June). I find this a little optimistic.
Personally, I am looking forward to the day when robots do our recycling for us and there will be no need for mineshafts at all.
But, if I may update the four horsemen of the apocalypse, I fear nuclear war, plagues spread by air travel and floods and famines brought about by climate change may prevent this.
We are also now presented with a fifth problem: the worldwide drop in species diversity, including that of our internal microbiomes. Keep an eye on the stars by all means, but first, let's be careful where we put our size nines.
Free will is all sorted now… isn't it?
The question of free will exercises readers like Tony Spottiswoode (Letters, 27 April). But it was pretty much answered by Simon Ings, reviewing The Importance of Small Decisions (13 April, p 43). We probably all know people who can be overcome by indecision when compelled to make a choice.
It can happen to anyone, but it is a serious problem for some. This can't possibly happen without free will. If our responses were deterministic, there would be no dilemma.
This superstition survey doesn't convince me
You report a survey that seems to show belief in the supernatural is still alive, even among people who don't believe in a god 8 June, p 14. Only at the end do you note that the survey questions could be interpreted in a secular way.
Take the statement “significant life events are meant to be”. You might choose to have a child, and in that sense, the child is “meant to be”. Or agreeing that there are “underlying forces of good and evil”: underlying what? Politics? History? Psychology? I concur, despite having no supernatural beliefs at all.
I note that was funded by the John Templeton Foundation, “utilization of scientific methods in understanding the work and purpose of the creator”, “research on studying or stimulating progress in religion” and “research on the benefits of religion”.
There are better ways to harvest metal from the sea
David Adam reports a proposal to repurpose gas or oil production platforms to harvest cobalt from seawater (11 May, p 15). Why not extract it from the outlet streams of reverse osmosis desalination plants or of power stations that use seawater for cooling? Disused production platforms would better be used in carbon capture and storage.
Unintended consequences of environmental action
Chris Packham is now on a mission to prevent the culling of “pest” birds to protect crops (4 May, p 13). This will probably result in lower yields, more expensive food or expansion of farms to make up for the loss.
What bees do during a total eclipse of the sun
Rereading Leah Crane's account of bees in the US suddenly stopping buzzing during the 2017 solar eclipse reminded me of the last UK total eclipse, on 11 August 1999 (20 October 2018, p 18). Researcher Candace Galen says it wasn't clear whether the bees flew back to their hives or stayed put.
I am a biologist and beekeeper. In my garden taking eclipse photos and watching and listening to the wildlife, I observed that birds and insects went quiet. The bees didn't fly home, but rested on the flowers as they do when sudden cloud reduces light levels. Once the light returned, they continued working.
For the record 鈥� 06 July 2019
鈥� UK TV sports presenter Dickie Davis is 86 (Almost the last word, 15 June).
鈥� Several archaeological sites in Turkey are called Yaz谋l谋kaya (“inscribed rock”): for a photo of the one with a possible Hittite calendar, see bit.ly/NS-Hittite (22 June, p 8).
鈥� Human rights charity the International Coalition to End Transplant Abuse in China set up a China Tribunal to consider evidence of forced organ harvesting from prisoners of conscience (22 June, p 6).
鈥� Foiled again: the toast notifier switch is made with kitchen foil (How to be a Maker, 18 May).