Answers to last issue's Do You Know?

1. Do radioactive things glow in the dark?

Ans: The short answer to your question is "no," radioactive things do not glow in the dark, not by themselves. Radiation emitted by radioactive materials is not visible to the human eye. 
But then, why do so many people think of glowing things when they think of radioactive materials? This is because we are used to "radium dials" on watches and clocks, that make the dial visible in the dark, and we know that radium is radioactive.
Even if radition is invisible, there are ways to "convert" this invisible energy to visible light. Many substances emit visible light if "stimulated" by the ionizing radiation from radioactive material. These materials are known as "fluors" or "scintilators." So, by mixing some radioactive material with such a fluor, you can make a substance that glows. This is the kind of material used in the radium dials on clocks and instruments on ships and airplanes to make them visible in the dark. 
It is also possible to "trick" radioactive material into creating visible light. This is called Cherenkov radiation. This happens when the radiation from the radioactive material goes into a material such as glass or water. Because the speed of light in this material is relatively slow (compared to the maximum speed of light in a vacuum), the radiation is actually traveling faster than light can travel in that material, and so it gives off light as it slows down. But to actually see this glow, it usually takes something which is very radioactive, such as the internal parts of a nuclear reactor. Weak Cherenkov light can also be made from smaller amounts of radioactivity, but very sensitive devices have to be used to detect it.

2. Is vacuum matter, or a state of matter?
Ans: We all think of vaccum as space with no matter in it. A partial vacuum is a vacuum with low amounts of matter enclosed. A total, perfect, or absolute vacuum has no matter enclosed. Sometimes this type of vacuum is referred to as "free space." But it is hard to imagine such an empty spatial area. How do we even find absence of matter in the universe?
In the laboratory, since it is really hard to get all the matter out of any space, it is defined in an experimental way: vaccum us space in which the pressure is so low that any particles in the space do not affect any processes being carried on there. Thus it is defined by an experimental limit, called the "ultrahigh vacuum" (UHV). The UHV is obtained when reaching the low pressure 10^-7 Pascal. This space is created with vacuum pumps, but this vacuum is still filled with millions of particles per cubic millimeter. Even in deep space between the stars, which we consider "empty", the vacuum contains millions of particles per cubic metre. 
Even though it seems an abstract concept, vacuum is very important and useful in everyday life and in technology. Your lungs take in air when your diaphragm drops, creating a partial vacuum in the alveoli of the lungs, causing air to rush in. A vacuum cleaner sucks up debris because it creates a pressure difference between the area to be cleaned and the suction tube. Vacuum tubes are devices, usually made of glass, that contain very low gas pressures inside the tube.
Many modern devices (like the integrated circuit chips that make everything from cars to computers work), have to be fabricated in a vacuum. In many science laboratoties, equipment only work at extremely cold temperatures, that need a vacuum. 

3. If you jumped into a pool of liquid oxygen, would your body instantly crystallize?
Ans: It will of course be immensely cold and lead to death, but nothing happens instantly. The first thing would be frostbite to the skin followed by the onset of hypothermia to the internal organs. No doubt everything would "freeze up" with time. 
What this really brings up is safety issues with cryogenic fluids, that is, those substances that are normally gases (like oxygen, nitrogen, carbon dioxide, hydrogen, or helium) at room temperature but can be changed to liquid form through the use of very low temperatures. When you have them in liquid form we do need to be extremely careful.
Besides the usual danger of handling anything that cold, there is something much more dangerous with these liquids. If they are exposed to the much warmer conditions in which we live, they will convert very quickly back into gas form. Then they will expand out to hundreds of times their liquid volume. In some situations, this can mean that the air (and oxygen) around you will be displaced by this gas. If it contains no oxygen then you may end up breathing an atmosphere almost devoid of any oxygen. The worst thing about this is that you will NOT realize that you are not breathing proper air and you will pass out within seconds. Extreme injury and death can result all too quickly.
In the case of oxygen, you have another danger. If the liquid oxygen is converting back into a gas, then you will have too much oxygen around you. Oxygen is what is needed to keep a fire burning. So with extra oxygen around, substances that are normally almost non-flammable can suddenly be very flammable! 
So don't worry too much about jumping into liquid oxygen, but do worry about handling any liquid oxygen with great care.

4. What is the brain doing when we are at rest?
Ans: What do you mean by "we"? Don't you include your brain in you? So the question already suggests that the brain is active when resting as a person.
The important thing to realise is that taking rest is not being inactive, but engaging in "restorative" activity. It is not a completely passive state. In fact, different people consider different activities as restorative. Some even claim long walks to be relaxing. Some climb mountains on vacation. Such activities might be vigorous but yet restorative becase they offer a complete break from "normal" working life. 
The critical thing to recognize is that when we are letting our minds wander, when our minds do not have any particular thing they have to focus on, our brains are quite active. When you do things like go for a long walk, your subconscious mind keeps working on problems. The experience of having the mind slightly relaxed allows it to explore different combinations of ideas, to test out different solutions. Once it has arrived at one that looks promising, you might have an "Aha" moment of realisation.
The work of Marcus Rachle, a celebrated neuroscientist, showed something very special in 2014 (for which he was awarded the prestigious Kavli prize in 2015). His team studied the brains of persons taking rest quietly without falling asleep. They found a unique fronto-parietal network in the brain. It has come to be known as the "default mode network", whose regions are more active during rest than during physically active states. They also studied chemical receptors in the brain and showed that the brain could actually use more energy during these default modes than in attention demanding tasks! 
So the correct answer is that the brain is certainly working (in many senses of the term) when we are resting, and science is only now beginning to understand what it is doing. 

5. Why do we swing our arms when we walk?
Ans: One way of asking this question is to wonder whether it is *natural* to swing your arms when you walk, because if it is natural, there is probably some reason for it. Well, try walking without moving your arms. Better yet, try walking while swinging your left arm when you step with your left foot and vice versa. Both of these practices are unlike the natural method of arm swinging while walking, which is for your left arm to swing forward as you step forward with your right foot. 
Experts believe that the ancestors of modern humans began walking upright at least 3.6 million years ago. Yet the reason we move our arms out of synchronization with our legs has only recently been solved.
For years, the answer was thought to be simply balance. Research published in 2010 by a team led by Dr Sjoerd Bruijn in the Netherlands confirmed this, but with a twist. Swinging our arms does not make us much more stable when walking normally. But it does help *restore* our balance if we suddenly lose our footing while walking over uneven ground.
His group also found another benefit: swinging our arms while walking is more energy efficient than keeping them still. The found that people who hold their arms still while walking use 12% more energy than people who swing their arms normally. Even more surprisingly, they found that when they had people walk with their arms synchronized with their legs (left arm swing forward when stepping forward with the left leg), those people used 26% more energy compared to normal walking.
Normal arm swinging actually does not use much energy from the arm muscles. Simply walking causes the body to sway in a way that makes the arms move naturally, like a pendulum. While it does take energy to move our arms, this is more than compensated for by the reduced energy needed by the rest of the body to propel itself forward.
When you walk, your arms begin to swing naturally without much effort from your arm muscles. The natural movement of your arms also helps to offset a part of the force caused by your legs hitting the ground, keeping your torso and hips from wobbling and twisting too much. This results in your legs using less energy!
Sources: Jefferson Labs, Science focus, Scientific American