The abiogenesis "event" took billions of years to occur to get a single cell organism.

As far as my knowledge goes, which is very little - earth had a reducing atmosphere early on in the development of life, which would make abiogenesis much easier by making the production of macromolecules more generally spontaneous. Instead, we now have an oxidizing atmosphere, making breakdown tend to be spontaneous.

Moreover, it’s possible that circumstances were never favorable for abiogenesis. Rare events do happen, after all, and if at some point the circumstances were truly favorable, then we might expect it to have happened more than just once in Earth’s history (or the history of all of the planets we’ve investigated for life). But we haven’t found evidence for multiple abiogenesis events.

Combining those two notes, although it certainly isn’t impossible that abiogenesis continues to happen every now and then, it was probably never likely, and may be even less likely now.

Work can turn out to be one of the less intuitive aspects of physics. For example, magnetic fields cannot do any work because they act orthogonally to the direction of motion, yet it certainly looks like work when you use an electromagnet to lift an object and make it float. I wish I could remember the explanation my Electromagnetism professor gave me for what is actually doing the work in that scenario.

[removed]

If it induces the charged particles to move due to its field then those moving charged particles will also produce em waves.

[removed]

Someone below pointed out the gravity analogy. Another is a spring, where an object can do work on the spring, and then the spring can return the energy by doing work on the object.

As Feynmann's lecture points out, the electrons that are worked by the applied electric field of the photon can be considered to be "fastened" to the atoms of the materials by a "spring". So the incoming photon moves the electron against a "spring", and the spring returns the energy by working on the electron, which produces a new oscillating electric field to add to the field of the incoming photon.

This can't be understood without math.

The wave doesn't lose energy as long as the crystal lattice is perfect, because the physics of our universe allows it to spread in a special way. (The photons don't interact with the particles the medium is made of.)

[removed]

Re “dosage compensation”, is it generally true that more copies of a gene means more gene expression? Aren’t most genes regulated by homeostatic feedback systems?

And what about the many, many plant species that get along just fine with duplicates or triplicates of their entire genome?

[removed]

Leyman: since ley is the Spanish word for law, is a leyman a lawman?

[removed]

[removed]

Thanks this is the answer I was looking for! The smearing effect is what I was anticipating, I see now why it doesn’t happen for stars and the like.

[removed]

I think number 2 is often overlooked. The oxygen catastrophe completely changed Earth’s environment. Oxidation tends to lead to a lot of water unstable molecules. It could be that for life to originally form you need longer more stable molecules and now the environment is just not conducive to the molecules being stable enough to start new life.

This seems somewhat unlikely because of compatibility of chirality. Many molecules aren't the same as their mirror image (just like your right and left hands), and chirality is what distinguishes these molecules from their mirror image (a "right-handedness" or "left-handedness").

All life on Earth uses a set of compounds with the same chirality. Enzymes are built to catalyze reactions that exclusively produce the correct chirality of product. In fact, for some of these molecules, their mirror images are actually toxic.

In contrast, non-biological reactions generally don't have such a strict chirality preference. In general, you produce a mix of both "handednesses" in most reactions not involving something extremely selective like an enzyme.

So in abiogenesis conditions, the selection of our current chirality rather than a different one was likely due to chance. If it happened multiple times, it's unlikely that it would have given the same chirality every time.

[removed]

A few things to clear up:

HIV has a strong tropism for CD4+ cells so provirus integration within egg and sperm cells is fairly rare but can occur preconception:

The Integrated HIV-1 Provirus in Patient Sperm Chromosome and Its Transfer into the Early Embryo by Fertilization

When it comes to perinatal HIV infection, there are many other available modes of vertical transmission: breastfeeding, placenta, etc.... Children that don't receive ART typically don't live past 2 years old:

Prevention of vertical transmission of HIV-1 in resource-limited settings

So this more traditional transmission method can obfuscate the origin of HIV. But even then, the risk for vertical transmission is between 15% and 45%.

All this means that there are indeed plenty of mechanisms to transmit HIV perinatally but that doesn't mean it is an absolute certainty .

If anyone is interested in infectious disease news: r/ID_News

> Sure, it's not individual photons being absorbed by individual charges,

You are correct overall but dismissing this specific point as if most people understand it easily is pedagogically dangerous -- based on my teaching experience that's exactly what a lot of people who hear "photon absorption and emission" end up thinking of.

This is where the analogy breaks down though. A water wave lifting a ping pong ball and returning it to it's initial position has lost energy. The ping pong ball pushes against air as it moves upwards, and the resulting ball-air collisions generate heat which is lost to entropy.

That's a really good analogy. Thanks for explaining it that way!

> a mirror is also an absorption and re-emission situation

I already addressed the use of "absorption and re-emission" in a previous reply to you, so I won't repeat that -- but for anyone reading, this is another case where you must not confuse "classical particle absorption and re-emission" (photon comes in, is absorbed, is gone for a moment, is emitted) with the collective scattering of probabilities/wavefunctions that we calculate in QM.

The law of reflection does not (cannot) come from a classical particle scattering model. It can be derived from either a classical wave model or a properly quantum mechanical model (wavefunctions are waves after all).

> The answer comes down to conservation of momentum and interference. Since the incoming photon has momentum, there is a higher probability that the photon will be emitted in the same direction to conserve momentum.

We've already discussed why you should not imagine the photon being completely absorbed by an atom when trying to derive the laws of optics. But let me address the momentum part of this answer (interference part is fine) as well because it's also misleading. Why should an atom prefer to return exactly to its previous state of "zero momentum"? Answer: It doesn't. Further: First of all the momentum we are talking about is tiny when a photon is absorbed by an atom, so even the thermal motion of the atoms is already going to wash out any momentum change. Second the atom is free to exchange momentum with its neighbours via phonons, so there's no reason why it has to return to zero by itself. Third, we measure single photon/single atom absorption and re-emission behaviour experimentally and it truly is isotropic (well, kinda), while your answer effectively claims it is not (contradicting your very first statement). (Incidentally, when 1 and 2 above are not true any more, like for a very cold gas, we can do fun stuff like laser Doppler cooling.)

The real answer is that we're not dealing with classical absorption/emission of photons by one atom, we're dealing with a collective behaviour and scattered probabilities/partial waves (see previous linked discussion, etc.) -- even in the case of few photons. So though all the scattered components are random in direction, we add them coherently (interference, as you said). The newly constructed wave/wavefunction has a clear direction of travel obeying the law of reflection.

/u/dack42

It was my understanding that most Legionella that lives within such environments are actually inside of free-living amoebas, like Acanthamoeba. These cells can phagocytose other bacteria for sustenance, and can ingest nutrients from their surroundings as well. They are much tougher than the more fastidious Legionella bacteria.