This makes me wonder. So if photons travel through a non-vaccum medium by being absorbed and re-emitted, how the heck does the information travel through that medium? Who tells the emitting atom to generate photons of exactly this frequency and polarisation in exactly that direction? How does it actually generate that frequency, e.g. the 432.1THz of a ruby laser when passing through a pane of glas? If one adds unspecific energy to the same piece of glass, i.e. melts it, it glows in yellow or white. Is there any way to make that glass emitting photons of a certain frequency except shining the right frequency into it?

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In a near vacuum water is going to sublimate off of whatever you store in that environment. Some organisms like tardigrades are able to survive exposure to vacuum, but they don't "live" in it and they definitely don't reproduce.

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I'm curious to hear more about the story. Were the US Legionnaires killed from breathing in what is essentially toxic fumes?

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>I always say, if you want to get some physicists to fight, ask them why light propagates slower through a non-vacuum.

I still remember an argument in my intro QM class about why photons slow in a medium lol

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> So, first answering your main question- elementary particles are all fungible. That means, they are truly identical, and they are impossible to label. So, if a photon is absorbed and then remitted, it doesn't really make sense to say "is it the same photon or a different one?" There aren't really "same" or "different" photons, there's just photons, unlabeled.

Isn't there any sense in which, say, a photon flying through space at time t and then a moment later at time t+1 is "the same photon", and in which two photons flying in opposite directions at the same moment and the same point in space (with different energies, even) are "different photons"?

All things being equal (with no regulation etc.) two copies of a gene are going to make twice as many mRNA transcripts and twice as many proteins as a single copy.

So gene dosage essentially.

Having too many copies can be damaging or lethal to an organism, especially in complex delicate situations during development.

This is a gross oversimplification (and not something that actually happens), but what if your brain was trying to make itself two times bigger than what your skull could contain?

You can imagine all sorts of processes going out of whack in a single cell, let alone when they have to interact in complicated ways.

There are other ways evolution could have dealt with it, but we have evolved to have around two functional copies of a gene on the autosomes (with a few recessive completely non-functional genes here are there), but that is quite a bit different than having a two fold difference across the entire X- chromosome.

Once the system we have evolved, it would be very difficult (practically impossible) to change the regulation of almost every single gene on the X-chromosomes, let alone change the regulatory scheme of every genes on the autosomes.

So yeah it is conceptually weird, but the barriers to doing something different in evolutionary terms are too high or too unlikely for it to have occurred in humans. Biology does not have to make sense from the standpoint of how a reasonable person might design a thing.

Even in tightly regulated genes, it can take more energy to regulate for two copies than one, which would be exacerbated across the entire X-chromosome. Again, we have evolved in such a manner that we are regulating our two copies of autosomal genes appropriately, which is evident by the fact that we are here.

Edit: this is over generalized for all animals. There are some other methods of dosage compensation in animals. Drosophila just doubles expression on an X chromsome, instead of inactivating one. There are some other methods of dosage compensation in other animals, especially those with different type of sex chromosomes.

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This was Lamarck’s thesis. That the reason we see life at different levels of complexity is that life originates continually, then evolves, so lower life forms of life are members of younger lineages. Bacteria would be only slightly removed from the latest origination, for example. That said, no, it is not a current hypothesis of Biology that life is continually emerging from non living precursors. We haven’t observed any extant natural ecosystems that contain “proto-life,” nor something transitional between life and non-living potential precursors, nor chemical environmental conditions similar to those surmised to have existed 3.5 billion years ago on earth.

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Location is indeed a way to distinguish identical particles, however if two identical particles share the same location then there is no way to distinguish them. You can not say "which is which", only that two particles came in and two went out.

Thank you for clarifying. I was looking at my left and right hand thinking "Certainly the electrons in my left hand are not the electrons in my right hand."

Your first point is new to me, and weirdly offputting. The idea that "identity" stops being a thing when talking about subatomic particals is oddly disconcerting. Why is that the case? Is it part of the uncertainty principle? What words should I google if I want to learn more?

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It isn't clear that there was a specific entity that could be widely considered alive that suddenly appeared one day at all.

There could have been millions of years of complex processes going on which was sort of a gradient from "definitely not alive" to "definitely alive".

Microbes can accept genetic information much more readily than animals do with unrelated forms, and all sorts of genes have probably disappeared in the last billion years. How would one define not ancestral to modern life vs ancestral.

Even if all the genes of some creature are no longer extant, they could arguably have shaped the evolution of genes that still are,so there is still some remaining influence.

If the two liquids are the same (C_Liquid1 = C_Liquid2) that's exactly what you do get. But the OP asked for a generalized answer for any two fluids.

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Why does it have to be this complex? Can't it just be calculated like averages are? If mix 1 part 100 degree water and 2 parts 200 degree water, can't I just do the following? (100 + 200 + 200)÷3=166 degrees. Assuming both waters came from the same source of course.