Science helps us understand the universe we find ourselves in and scientists help us become more knowledgeable as a species. Obviously we don’t know everything and most of us are aware of the “big questions” that remain unanswered. Is the universe infinite or just really big? What happened before the big bang? Why does time seem to move forwards? These are huge cosmological questions so we shouldn’t feel bad for not being able to answer them yet.
However some of the most baffling mysteries are frustrating because they feel like they should be explained by now. It should be difficult to explain what happened before the big bang, but surely we know how bikes work. Right? Wrong. Here are 5 everyday mysteries scientists still can’t explain.
1. Why do bikes stay upright?
If you run along with your bike then let it go, it will happily cycle away into the distance on its own until it slows down. Some of us might worry about learning to ride bikes and we put stabilisers on for children but without us the bikes do a fine job staying upright when they’re moving. In the early 1900s, physicists thought they had figured out how it works.
The thinking was that a whole bunch of properties in bikes come together to cause the gyroscopic effect that keeps them upright. These properties include the positions and angles of the forks and other parts that allow the front wheel to self-correct when the bike begins to topple. It seemed like an intuitive explanation but physicists always had issues with the details because the maths didn’t always add up, so to speak.
It took years of careful engineering but in 2011 a team of scientists created a bizarre bike with a complex system of counter-rotating wheels that countered the gyroscopic effect. When you push this bike it isn’t affected by the gyroscopic effect like normal bikes are. Surprisingly, it stayed upright just as well as any other bike despite the fact the the gyroscopic effect wasn’t at play. The gyroscopic effect might help bikes stay upright but doesn’t explain it entirely.
2. What is glass?
This is a really contentious one and you will find completely different answers from equally reliable sources. We can be in agreement about one thing: glass is weird. The mystery involves its classification as a solid or a liquid. In some ways it’s obviously a solid, otherwise your window would have turned into a pool of glass by now. On the other hand, glass doesn’t look like a solid at the molecular level. A popular line of evidence is that old stained glass windows in cathedrals are sometimes thicker at the bottom than the top, which is claimed by some to be evidence that the glass is slowly drifting to the bottom as a liquid. That’s not actually true and it’s caused by the way they made windows centuries ago in Europe.
So is glass a solid or liquid? We’re not sure. Let’s consider the options. It’s easy to identify solids and liquids around us. The wooden table? Solid. The water in your glass? Liquid. The molecular structure of a solid is highly organised. If you look at a crystal, like salt, the atoms are in neat lines. The atoms in your water aren’t as organised and can move around allowing the liquid to flow. Gases take this even further. This explains the difference between frozen and liquid water. The molecules in solids are organised and rigidly bound but in liquids are disorganised and aren’t rigidly bound? And glass is the weird one, with disorganised molecules that are rigidly bound.
If you speak to people who know their thermodynamics and molecular dynamics, you’ll still get different answers. Good arguments can be made for glass being an amorphous solid, an extremely viscous liquid, or an entirely different state of matter altogether. The big difference appears to be that going from liquid to solid is usually an issue of thermodynamics; above or below a specific temperature, a material either melts into a liquid or crystallises into a solid because it’s the most energetically favourable thing to do. With glass it seems to be simpler and there just isn’t enough energy to move the atoms past each other and allow the glass to flow. Does that make it a weird solid, a weird liquid, or a weird something else? It’s difficult to say.
3. Why is ice slippery?
If you want to explain what friction is to a child, a great place to start would be the difference between the pavement and an ice rink. What’s weird is that as sure as we are that ice doesn’t have much friction, we’re not actually sure why that’s the case. Since the 1850s, the consensus was that there’s a thin layer of unfrozen water on the surface of ice and that’s why ice skates can glide along on the rink or a frozen pond but stop dead when they hit the surrounding area. This theory came from holding two ice cubes together and them becoming a single block. Perhaps an extremely thin layer of water on the surface was then freezing. The thinking was that pressure on the ice created the thin layer of liquid that reduces friction.
The problem? Skaters aren’t heavy enough to provide the friction required to melt the top layer of ice into liquid. The layer of water sounds like a good idea, but the assumption that pressure causes the melting just doesn’t make sense mathematically. Friction itself has been proposed as an alternative, suggesting that the friction of the ice skate on the ice melts the surface and creates the water layer but ice is slippery even when someone is standing still.
Some scientists argue that ice water is weird material with some bizarre properties that allow it to have so little friction. One hypothesis is that the water layer occurs naturally because there is a layer of unstable water molecules and they move rapidly as they search for other molecules to stabilise with. This rapid movement of molecules could result in the slipperiness of ice. Changqing Sun thinks that the answer might not involve a liquid layer at all and that there’s a tiny degree of levitation at work. Instead of liquid, he believes there’s a supersolid layer of ice with molecules behaving in an abnormal way that generates force on the the ice skater. Maybe when we go skating we’re like MagLevs or hovercraft.
4. Why is the Sun’s atmosphere hundreds of times hotter than its surface?
The Sun is quite warm. The surface, that generates almost all of the visible light, is a toasty 5500°C. Inside it’s even hotter as gravity creates extreme pressure and temperature bringing the core to about 15 million °C. The further out you go, the cooler it gets and the surface should really be the coolest part of the Sun. It makes perfect sense. What makes less sense is that if you go a little further out from the surface, into the Sun’s atmosphere, the temperatures shoot from 5500°C to 1 million °C. It’s like lighting a match and the air around the flame being 200 times hotter than the flame itself.
The corona is an aura of plasma around the Sun. Lots of hypotheses have been proposed. The most favoured involve magnetic fields. The incredible weather we see on the Sun’s surface is generated by extremely strong magnetic fields all over the surface so perhaps it has something to do with that. The orientation and location of thousands of “tornadoes” on the surface have hinted that magnetic field lines twist around the Sun in ways that build pressure and then break, releasing huge amounts of energy. Other ideas involve extremely powerful magnetic fields inside the core somehow bringing energy up through the surface and into the corona.
The idea that magnetic fields are involved is probably a safe bet. What we don’t know is exactly how the magnetic fields are generating so much heat. It seems odd that astronomers know so much about the universe, including galaxies billions light years away, yet they still can’t explain one of the most familiar objects in the world.
5. Why do we have sex?
OK, good point, but it is a biological mystery nonetheless. In an evolutionary sense, sex just isn’t that good an idea. There was a time on the Earth when all species reproduced asexually but now we have a mix meaning sex is a trait that has evolved. There are lots of proposed explanations for why sex would evolve but none of them can really justify it. Because sex is commonplace and enjoyable to so many, its biological weirdness is overlooked. It’s a system that means that (usually) half of the members of a species don’t provide any offspring yet they use up the same amount of resources. The best way to have your genes survive is to clone yourself, so what’s the point of having males and females?
The proposed explanations are really good and make a lot of sense but never quite satisfy as an answer when we do the maths. A very obvious explanation could be that sex helps protect against harmful mutations. If you’re asexual and have a dodgy gene, your offspring will inherit that gene. But if your offspring’s genome is a mix of genes from two parents then some of those offspring might be the best of both parents. It means your offspring has a chance of obtaining a healthy version of the gene. Sex might weed out the harmful mutations.
However, when we do the maths it doesn’t really justify how this system could evolve. It’s not that sex doesn’t clean out the gene pool; it just doesn’t do it so well that it cancels out the downsides of reproducing sexually. A few decades ago the idea seemed to have more merit because it could make sense if mutation rates in populations were very high. Thanks to modern genomics we now have a better idea of mutation rates and they simply aren’t enough to explain how sex could be a better system when it’s otherwise so incredibly inefficient.
There are lots of other ideas that make some sense, such as sex creating offspring more capable of surviving environmental change, but when it comes to proving them they all fall flat. It’s not that sex doesn’t have advantages, it’s just difficult to justify why it would evolve as an alternative to asexual reproduction in the first place.
Scientists are investigating all of these everyday mysteries and most have working hypotheses so we’ll hopefully come to understand them better in the future. It really is amazing that we can learn about galaxies that formed early in the universe’s history or see how planets form in other solar systems yet we’re not totally certain about why bikes stay upright. But we’ll get there eventually.
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