Recent comments in /f/askscience

iayork t1_jd114s9 wrote

Reply to Has the HIV virus become less deadly? by shaun3000

Possibly HIV might have become less virulent, but you absolutely have not detected that. What you are seeing is improved treatment of HIV.

There are a handful of ambiguous and marginal studies that hint that, in some regions and some populations, HIV might be becoming slightly less virulent. Again, you personally have not seen this because you don’t live in those areas and you personally don’t have the opportunity to test viral blood load in tens of thousands of stage-matched, strain-matched infected people.

> In support is evidence that SPVL, and hence virulence, has declined in some African HIV subtypes, even accounting for the use of antiviral therapy, and that this reflects a trade-off between virulence and transmissibility

—The phylogenomics of evolving virus virulence

(The reference cited here is Blanquart F, et al. A transmission-virulence evolutionary trade-off explains attenuation of HIV-1 in Uganda. eLife. 2016;5:e20492.)

There’s a widely believed myth that viruses inevitably evolve to reduced virulence over time. In spite of the great confidence with which this is claimed, it is not true, there are many counterexamples, and there are 60 years worth of observation of theory (with math) demonstrating why it’s not true.

> For example, in the case of the second virus released as a biocontrol against European rabbits in Australia — rabbit haemorrhagic disease virus (RHDV) — there is evidence that virulence has increased through time … Similarly, experimental studies of plant RNA viruses have shown that high virulence does not necessarily impede host adaptation and, in the case of malaria, higher virulence was shown to provide the Plasmodium parasites with a competitive advantage within hosts.

>Theory therefore tells us that natural selection can increase or decrease pathogen virulence, depending on the particular combination between host, virus and environment

—The phylogenomics of evolving virus virulence

So there’s no particular reason to expect HIV to evolve to reduced virulence.

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drhunny t1_jd0wicd wrote

Reply to What the hell is the actual difference between an isotope and a nuclide?? by amypinecone

Isotopes are nuclides with the same number of protons. C-12,. C-13, C-14 are on the "6 proton isotope". Isotones are nuclides that have the same number of neutrons. H-3 and He-4 are on the n=2 isotone. Similar to how places at the same atmospheric pressure are on an isobaric line.

Weirdly, "isobar" is also a term in nuclear physics where it means same number of total neutron plus protons. I think it comes from from "baryon" whereas in meteorology it comes from "barometer"

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ozymandian___ t1_jd0eij6 wrote

Reply to comment by BlueParrotfish in If the universe is constantly expanding, how is the law of conservation of energy obeyed? by Pristine_Pop_7818

I just want to point out that the fact that the universe is expanding does not mean that it breaks time symmetry in a way that is relevant for Noether’s theorem. Systems can change over time and conserve energy. Furthermore, Noether’s theorem describes local physics, and local physics is unchanged by general relativity. Also, if you consider the energy of spacetime itself, then energy is conserved in the universe. There is a caveat that in GR the energy of spacetime curvature can’t be localized in the way energy usually is. This makes some people uncomfortable, and they choose to say energy is not conserved for the universe (seemingly ignoring that spacetime carries energy as in gravitational waves). You can just as well say that energy is conserved and that the energy of spacetime is only well defined for the universe as a whole.

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viscence t1_jd0ehya wrote

Reply to Do we know where is the center of big bang located presently in reference to earth? by MagnetCarter

Every experiment you can do to find out will tell you the same thing: "right here". If you measure how fast a galaxy is moving, you find that they all tend to move away from "here," and moreover they tend to move away from "here" faster the further they're away.

So did the big bang happen here and we're just spectacularly privileged to live in exactly that place? No. It doesn't matter where you perform the experiment, the answer is always "here". The explanation for this is that space itself is expanding. We're not seeing debris being thrown away from a central location where an explosion happened into some pre-existing space, what we're seeing is the metric expansion of space itself. All the distances between things are just increasing, not because the things are actually moving but rather because the space between them is literally growing. If you extrapolate time backwards far enough, all the distances become zero, and we call that singularity where our understanding of physics breaks down "big bang".

If you have two objects that are too far apart to affect each other, but that randomly happen to be stationary with respect to each other today, then tomorrow they will be further apart. There is no centre to this. The universe isn't expanding away from one spot, it's expanding everywhere like an infinite sponge soaking up water. And really, with the universe being infinite as far as we know, there can't really be another answer. It's not like the centre could have been "three quarters of an infinity" away from here.

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mutandis57 t1_jd018fx wrote

Reply to What happens if you throw 2 balls of equal mass and equal velocity at each other and they combine into one object? by minecraftmanyt

> ... > > 1=1/2*2*v_net^2 > > v_net=0.707

You missed the extra factor of "*2" at the end and took the square root of 1/2 by mistake. Had you taken the square root of "1/2*2" you would have gotten v_net=1, i.e. the speed of the two balls together is the same as each ball separately. But that's because, as other commenters pointed out, you used conservation of energy for a perfectly-elastic collision. Using conservation of momentum in a perfectly-inelastic collision v_net will be zero and all the kinetic energy will be destroyed (converted into thermal energy of the atoms inside the ball bouncing around randomly).

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bad_bird_karamaru t1_jczvl4k wrote

Reply to What happens if you throw 2 balls of equal mass and equal velocity at each other and they combine into one object? by minecraftmanyt

As the others have mentioned, momentum is conserved in a perfectly inelastic collision, but kinetic energy is not. Assuming that not all of the missing energy is lost to heat, sound or the deformation of the balls, a force will be present to hold the two balls together, so that they stick. The missing kinetic energy is converted to the potential energy associated with the force, which you might call the "binding energy" for the system.

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amypinecone OP t1_jczsxk7 wrote

Reply to comment by RobusEtCeleritas in What the hell is the actual difference between an isotope and a nuclide?? by amypinecone

does that essentially mean that nuclide is more used generally as just describing something by its number of protons and neutrons (as opposed to just describing an element it with it's atomic number for example), and then the term isotopes would more often be used when talking about variations of something (such as carbon having carbon-12, 13, and 14)? sorry if I have it confused still

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