Recent comments in /f/askscience

Redingold t1_j9jmpip wrote

Reply to comment by [deleted] in How did Paul Dirac predict the existence of a positron before the actual discovery? by bazongaenthusiast

I want to expand on the bit about negative energy there.

The issue with having negative energy states allowed is that, generally, particles try to occupy the lowest energy states available to them. Electrons in an atomic orbital for example, will emit photons and move onto lower energy orbitals if those orbitals are empty. Having negative energy states, and, in particular, having negative energy states with no lower limit (you can see that as p gets larger and larger, -sqrt(p^(2)c^2 + m^(2)c^(4)) gets lower and lower without bound) means that electrons should all be shedding as much energy as they can as they cascade down towards negative infinity energy. This doesn't resemble the reality we live in, so something else must be happening.

What Dirac reasoned was that you could make it work if all the negative energy states were already filled, all the way down to the bottom. The positive energy states would be free and available for all the conventional electron physics we already know about, and the negative energy states would be blocked, so electrons won't have room to go barrelling down towards negative infinity.

This has an interesting implication, though. There's a gap between the positive and negative energy states: when p = 0, the energy jumps from E = mc^2 to E = -mc^(2) (so the gap is 2mc^2 wide, or wider if p is not 0), and there are no states at all in this region. If, however, you supplied that 2mc^2 worth of energy (or more), you could make an electron jump up from an occupied negative energy state, into a free positive energy state. This would leave a hole in the negative energy states, and this hole would act like it had a positive charge (because when an electric field is applied, the electron next to it moves into that space, leaving a new space where it was before, and then the next electron moves into that space, and so on, like how a bubble rises in water by the water falling down and pushing the bubble up. The hole would move in the opposite direction of the electrons, and so would have the opposite charge, i.e. positive). So by supplying at least 2mc^2 of energy, you could make an electron, and a hole that behaves like a positively charged electron. That's a positron.

Nowadays, modern physics doesn't really use this Dirac sea model, as it's called. Quantum field theory lets us treat the ground state as a vacuum containing no particles, not an infinite sea of negative energy particles, and it treats positrons as real particles, not holes where an electron isn't in the negative energy sea. The Dirac sea model also struggles to explain why we don't see the effects of having infinity negatively charged particles everywhere in the universe (although the energy of the vacuum isn't a solved problem in quantum field theory, either, see the cosmological constant problem). That said, the two are mathematically compatible to an extent.

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GeriatricHydralisk t1_j9jklgp wrote

Reply to comment by Tarantio in What are more accepted hypotheses that similarly explain the aspects of hominid evolution that the "pseudoscientific" aquatic ape theory does? by KEVLAR60442

To be fair, though, catching fish doesn't necessarily mean swimming or diving. With a good spear and some practice, you don't even need to go knee-deep.

The other problem with extensive aquatic behavior in early humans is that basically every body of water in Africa bigger than a puddle probably has at least one Nile crocodile in it. That doesn't preclude aquatic behavior, but certainly discourages it.

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Elout t1_j9jkj4v wrote

Reply to What makes bats a good disease vector? by JustJustinInTime

Bats are mammals and they have a big overlap with humans regarding virus affinity. They get the viruses because they eat parasites and insects carrying these viruses. I saw a post yesterday where they were talking about some cave in Africa that people just cant go in because there are too many crazy viruses in bats in there.

Also as mentioned in other comments. Bats can just host a virus without getting sick from it. Except for rabies. Bats can still give you rabies, but they cant host that without being affected by it.

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fatguyfromqueens t1_j9jj6k1 wrote

Reply to AskScience AMA Series: I'm Dr. Christine Wilkinson, National Geographic Explorer, carnivore ecologist, human-wildlife interactions specialist, and performer. Want to know why a coyote wanders through your city? What happens when hyenas chew your tires during research? How to get into SciComm? AMA! by AskScienceModerator

Well since cats are carnivores, is it cruel to have a cat chase a laser pen mousey when they can never actually catch the mouse?

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pauvre10m t1_j9jivxm wrote

Reply to comment by 911derbread in when a limb gets amputated, how do they stop the flow of blood? by EnchantedCatto

As I was instructed when I had received first aid courses, you should first try to use pressure. If it's not enough to stop the bleading you should consider using a tourniquet.

Please note also the time where the tourniquet where put in place, it's importent for doctors. also please note that she tourniquet should only be removed by a doctor ;).

In any case, the tourniquet should be the last attempt

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