No. The changing electric and magnetic fields are the wave. A electric and magnetic field can change without a medium, although until semi-recently many scientists thought that a medium was required for a wave to propagate. They hypothesized an all pervasive static medium called the aether. The Earth and all heavenly bodies traveled through it.

In the late 19th century the michael-morley experiment showed that there wasn't a static aether. It determined that light travels at the same speed in perpendicular directions. If light was propagating through a stationary aether then the light would have taken different times to travel one way vs its perpendicular since the Earth, the galaxy, and everything else is moving.

Relativity further poke a hole in the bottom of the boat for an aether since it doesn't accept a static absolute reference frame for anything. And a medium would not support everyone everywhere agreeing that the speed of light is the same.

You take the longest day of the year (the summer solstice) and then count the days until the next longest day. You then find it's consistently 365 days.

If you live in a cloudy place, this might seem difficult and inaccurate. If you lived in Egypt, you'd have lots of cloudless days to base it on.

This is one of those issues with using language to express rigorous concepts, because "medium" has a lot of meanings in English, but it really just means one thing in physics.

For ELI5 purposes a key difference is that in a medium the propagation of a wave is impacted by the movement of the medium. Imagine a tank of water with a whirlpool in it, rotating clockwise. If you generate a wave in that flow it's velocity through the medium is going to be impacted by the velocity of the flow of that medium.

Fields don't work that way.

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A subatomic particle is just a particle "smaller than" an atom (hence sub + atomic). The three you're familiar with are the three component particles that make up atoms: protons, neutrons, and electrons. It turns out that protons and neutrons (but not electrons, so far as we know) have further internal sub-structure; protons and neutrons are each made of three smaller particles called quarks. The only other example you "see" in everyday life is the photon, the particle of light. Others exist, but none of them are long-lived or seen in everyday life under normal conditions.

What they "really are" is more a matter of philosophy than it is of physics. Physics is interested in describing how they behave. And in modern physics, we model the behavior of particles through quantum mechanics.

It turns out that the way we tend to think of particles in our minds - tiny little spheres bounding around and exerting forces on one another - just doesn't correspond to the way particles actually behave.

In quantum mechanics, the underlying "reality" of a particle is something called its wavefunction. Rather than a particle "being in one position", a wavefunction takes every possible position and assigns a single number to each of them. These numbers are complex numbers (that is, they look like something like 0.2 - 0.4i), and the Schrodinger equation tells us how they evolve over time.

Exactly how we interpret these wavefunctions as corresponding to any of the things we observe day to day is a topic of some debate. In a sense, the ultimate underlying reality is always the wavefunction, and the Universe as a whole has one wavefunction that has just been evolving according to the Schrodinger equation since the beginning of time. But since we often want to consider particles in isolation so that we can talk about an electron without talking about the whole Universe, we have to think about how we can "cut off" the electron from the cosmos. In this sense we try to talk about "the electron's" wavefunction, but in reality, even the existence or non-existence of the electron is a statement about the wavefunction of the entire Universe, so by talking about "an electron" at all we are going to introduce some weirdness.

We do this by thinking in terms of probabilities. The electron, if measured, will appear to be in one of several locations, and the probability of it being in each of them is equal to the square of the magnitude of the number the wavefunction assigns to that location. For example, if a position has wavefunction value 0.1 - 0.4i as mentioned earlier, the electron will be in that location with probability (0.1)^2 + (0.4)^2 = 0.17. Since the wavefunction has non-zero values at many points, in this sense the electron "can be in more than one place at once", or more properly, could be measured to be in any one of several different places at random.

Exactly how this corresponds to our conventional notions of position, velocity, etc is a topic of considerable debate, but this approach to modeling particles works well and describes our world accurately (and no non-probabilistic model can do so).

Iron costs 2 ore and has a 50% chance to smelt. Steel costs 1 iron ore and 2 coal and has a 100% chance to smelt. Silly question.

Wouldn't that make the electromagnetic field the medium?

My guess is that you're thinking of some sort of opponent-process model or something like the CIELAB space. But neither of those corresponds to the physical response curves of cones.

There's two different things we call waves because they have similarities. The wave you are thinking of is a behavioral pattern of matter, like water. This is a word to describe the motion of something else. A wave of light isn't motion of something. It's an actual thing itself, that when viewed through a sensor, moves in a wave-like pattern, so we named it a wave after the waves of the ocean. It's radiation. The difference between light and radiation that's invisible is simply that our eyes didn't adapt to see the invisible radiation. When people say infrared light, they are talking about radiation that has a lower frequency than what our eyes pick up, and when they say ultraviolet light they are talking about radiation that has a higher frequency than our eyes can pick up.

Light isn't a wave, and it isn't a particle, it's something which has properties of waves and particles. As a result looking at light as just one or the other leads to misunderstandings, such as the assumption that there must be a medium for light to travel through.

If you prefer you could also imagine light as being a localized excitation of the electromagnetic field, which would be more accurate, but less ELI5.

I’ve seen different data presented, I’m not sure I can explain the discrepancy.

I'll add that the seemingly imperfect cuts that the skirting board covers are often intentional, as a small space between the floorboards and the wall will allow for seasonal expansion and contraction.

> That said, even returning the rays to parallel does not mean recombining them. At the first surface they are given different angles, splitting one beam. Returning this set of beams to parallel just means that the colors do not spread even further once they've left.

This is true, but the effect is pretty small, much narrower than a typical light beam, and dominated by diffraction, so you just get fringes of very slight color.

> Each of the three has a color it’s best at detecting, and we name them based on that. One for red, one for green, one for blue.

This is incorrect. The three cones in your eye are most sensitive to violet, green, and yellow-green, and they're not called red, green, or blue cones. The usual name is S, M, and L, for short, medium, and long wavelengths.

> But for whatever reason, way down in violet, the red cone has a little bump in sensitivity again, after not seeing green or blue at all. So violet literally is the blue cone sending a strong signal, but a little bit of the red cone too.

Also incorrect. The L cone does not have any particular sensitivity to violet light.

Can we use the algae as fertilizer? As in, direct ask runoff to large ponds with non-harmful algae, allow it to absorb the leftover nutrients, harvest abs dry the algae, and then use it as fertilizer later?

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It's worth noting that CO2 levels are far lower than oxygen levels in the atmosphere, so it's much easier to change the (relative) amount of CO2 than it is to change the (relative) amount of O2.

It isn't completely constant over geologic time. Earth's oxygen has been as low as 0 and as high as about 33% at different points during Earth's history.

The oxygen in the air is the result of an equilibrium between plants and other photosynthetic organisms (it's actually mostly microbes in the ocean) producing oxygen as a by-product of photosynthesis. This equilibrium is pretty stable, since organisms are short-lived and organisms will be slightly more successful in environments that are more favorable to them. (As an example, global plant growth has accelerated pretty significantly as humans have added carbon to the atmosphere, although not by enough to compensate for the rate at which we're adding it.) This part is basically a closed cycle: plants produce oxygen in the process of capturing carbon to use in their "bodies", animals eat the plants and burn the plants' "bodies" with oxygen from the air.

Over the very long term, though, there's another important effect: geology. Or more specifically, geology driven by living things. Earth is old enough, and living things dominant enough in its surface chemistry, that life on Earth and Earth's own geology are intimately linked over the longest timescales. (This is, I think, incredibly cool. In a very real sense, the very stones of Earth are part of the biosphere.)

In the processes described above, some organisms manage to die and decompose without returning their carbon to the atmosphere. The corresponding oxygen that was liberated during the capture of that carbon, then, ends up sticking around in the atmosphere over the long term. This is, in effect, what happened during the burying of the organic material that became fossil fuels, and it's part of why the era in which those fuels were produced had the highest oxygen levels in the history of Earth to date.

On the other hand, sometimes geological processes expose this buried carbon to the atmosphere, where it reacts with oxygen. This is the same kind of chemical cycle as the regular biological one, just dominated by geological processes over much, much longer times.

There are a few other minor contributors, like serpentization of certain rocks, but the ones described above are the big drivers.

The net effect of all of this in the Earth we have today is very close to zero if you ignore humans. So, human activity aside, Earth's oxygen levels aren't changing quickly over any timescale remotely relevant to humanity.

But thanks to the fact that we're burning an absolute ton of buried carbon (effectively accelerating one side of the geological processes described above), the oxygen content of the atmosphere has very slightly declined due to human activity. Not by anything significant - a few parts in 10,000 - but some.

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> So does the algae itself consume oxygen (aerobic respiration?), or produce oxygen (photosynthesis?) in order to survive? Sorry, I study inorganic geochemistry but don't know much of anything about biology or botany. > >

The Algae both consumes and produces oxygen - at first it's necessarily producing more than it consumes, in order to get the carbon it needs to grow in size, but when it's as big as it's going to get those numbers start balancing out.

Add in the fact that dead algae decaying take oxygen solely from the water, while living algae photosynthesising put their oxygen both into the water below and the air above, and you get a net decrease in oxygen over time (barring gas transfer, which as mentioned is blocked by the matting effect)

Another part of the problem is that the algae only produces oxygen in the daytime, and consumes it constantly, so at night the oxygen level in the water drops - and if something can't survive at night-time oxygen levels then it dies, and starts decaying, further lowering the oxygen levels.

You have three types of cells in your eye that see color, called cones because they’re cone-shaped.

Each of the three has a color it’s best at detecting, and we name them based on that. One for red, one for green, one for blue.

But, it’s not that they see one color and that’s it. Any color you could pick will hit all three, and the ratio between the three signals they send to your brain is what turns into a color perception. Also, the color they’re best at isn’t a narrow sliver, it’s a big hump of the spectrum that falls off to either side of the middle color.

Yellow activates a bit of the red cone and a bit of the green cone, but not as much for either as a red color or green color would have.

Same for colors near blue, a nice teal will activate the blue cone and a bit of the green cone.

But for violet, that’s where it gets a bit odd. The red cone has its big hump in red, falling off in sensitivity toward orange and yellow on one side and toward infrared on the other (which none of the cones see). But for whatever reason, way down in violet, the red cone has a little bump in sensitivity again, after not seeing green or blue at all. So violet literally is the blue cone sending a strong signal, but a little bit of the red cone too.

Pigments and computer displays take advantage of this in exactly the same way: add pure red and pure green, and your brain sees it as yellow, because it can’t tell the difference between actual yellow and an exact mix of green and red.

It did take hundreds of millions of years to form. When Earth first formed 4.5 billion years ago there were just a carbon dioxide and nitrogen atmosphere. Then about 3.5 billion years ago the earliest oxygen producing microorganisms came about and we started getting a trickle of oxygen in the atmosphere generated by combining water, carbon dioxide and sunlight. But levels were still low until about 850 million years ago when the oxygen levels in the atmosphere shot up and most of the carbon dioxide was converted to oxygen and organisms. This was right about when the first plants developed, and we started getting multicellular organisms. Since then the plants have been able to maintain current oxygen levels. Or at least somewhere between 15% and 35%. Without plants we would not have oxygen.

Not all of the sides on a cube are parallel. Specifically, any two adjacent sides.

That said, even returning the rays to parallel does not mean recombining them. At the first surface they are given different angles, splitting one beam. Returning this set of beams to parallel just means that the colors do not spread even further once they've left.

Plants and algae are constantly converting carbon dioxide to oxygen (and sugar) using water and sunlight. This more than makes up for the oxygen they use to stay alive; the extra oxygen is what we rely on to breathe.

But just because of the size of the Earth and the atmosphere, it would take a long time for any changes to propagate through it. This also means that any attempts to fix a problem with the atmosphere (like excess carbon dioxide) would also take a long time to take effect.

Algae does both. During the day it contributes highly to oxygenating the water, but it consumes it at night. Since it also limits he surface area to replenish the oxygen, the water goes anaerobic during night time.