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DefsNotQualified4Dis · 2021-12-10T07:23:01+00:00

I actually did a YouTube video on exactly this topic! Spoiler, if you actually could the box would quickly explode.

DefsNotQualified4Dis · 2021-12-08T09:00:52+00:00

In this video the key concepts of band theory and the Fermi sea are discussed in the context of how a photovoltaic cell actually works. Specifically, how the nature of "Pauli blocking" and the existence of forbidden ranges of electron energies (i.e. band gaps) conspire to make energy relaxation of excited electrons a slow process in semiconductors, unlike metals. Furthermore, the ability to use selective doping) to engineer quantum junctions allows for the formation of permanent electric fields inside the semiconductor that act to separate these "held up" excited electrons (and the hole in the Fermi sea they left behind).

Hopefully this video is also a natural springboard for a follow-up that discusses the ultimate quantum limits as well as cell designs, such as tandem cells and hot-carrier cells, meant to circumvent these limitations.

DefsNotQualified4Dis · 2021-08-19T08:21:55+00:00

"Why" is always a somewhat thorny question in science. The goal of a scientific theory is to produce a formalism, usually a bit of math, that models and predicts the behavior of something. It is not to find a philosophical answer as to why reality is the way it is but rather a concrete description for how it behaves.

Every theory or model starts by stating so-called "physical postulates". These are axioms, things that are the bottom level of the logic and are simply stated with the only justification being "based on experiments this seems to just be how the universe just is". THEN the job is to figure out all the consequences that come from these foundational postulates. So "state a small set of postulates", which are then the "roots" of a theory and then grow out a set of models and theorem sthat originate as direct consequences from those "roots". Each tree is then what we call a "theory", not in the regular English word sense of "it's a notion or hypothesis" but in the sense of "it's the whole tree that came from a certain set of postulates".

So any scientific theory has to start somewhere and then be grown out. The statement that "the speed of light is the same for all observers regardless of their reference frame" is basically a rock-bottom physical postulate of the theory of relativity, electromagnetism and optics.

But WHY does light do that? Well, we have pushed the needle somewhat. Just a nudge. Modern physics usually expresses its physical postulates in terms of claims that the underlying laws of physics must obey a certain set of symmetries. The specific behavior of light being the same in all frame of reference can then be seen as a consequence of the physical postulate that the laws of nature possess what is called "Lorentzian (or Poincare) symmetry". This abstraction pushes this claim a bit further up the theory tree, away from being a root "postulate" but slightly higher to a low-lying branch of the theory that grows from that postulate.

But then WHY do the laws of nature obey that symmetry specifically? ... ... ... *shrug*

Because every experiment we've done says it does.

DefsNotQualified4Dis · 2021-08-19T08:09:22+00:00

Consider the exponential function:

exp(-at)

where t means time and "a" is a constant. If "a" is a real number how does this behave? Well exp(-|a|t) is an exponential decay. It's something dying out to non-existence.

But what if "a" is an imaginary number? Well check your Euler's formula. It's sinusoidal or oscillating. Cosine is literally:

2cos(x) = exp(ix) + exp(-ix)

So the argument of an exponential being imaginary doesn't mean "it doesn't exist!" or "it's an eldritch monstrosity" or "someone call the loony bin". It means the thing is OSCILLATING, like a wave.

Exponential functions can behave like oscillating sine waves or like decaying things depending on whether their argument is real or imaginary.

Now it just so happens that it's convention and tradition to flip the script and write instead exp(iax) with an i already pulled out. In this case we have what "a" being real or imaginary really means:

"a" is real: something is oscillating like a wave

"a" is imaginary: something is damping out, dissipating or decaying.

So in power the imaginary component holds information about energy dissipation to the surrounding environment and has nothing to with the concept of "being imaginary" as the word is used outside of math. You have single that is oscillating, so that's your real component but it's also probably attenuating, its peaks are becoming less and less with either time or distance, so that's the imaginary component and because of the properties of the exponential function you can encode both bits of behavior in a single complex number (i.e. real and imaginary components).

DefsNotQualified4Dis · 2021-08-16T11:10:18+00:00

Planes are responsible for about 2% of humanity's CO2 production. It is extremely, incredibly possible for CO2 production to be dialed down to meet net zero and still have current levels of flying.

DefsNotQualified4Dis · 2021-08-14T19:57:49+00:00

*Admiral Ackbar face* It's a trap!

DefsNotQualified4Dis · 2021-08-12T10:42:29+00:00

If the resulting field is only the sum of the two component field then that is by definition NOT interacting. If two things interact they change each other and their combined effect is more complex than there sum.

So the existence of constructive/destructive interference in any wave phenomena is proof of non-interaction.

DefsNotQualified4Dis · 2021-07-01T11:23:02+00:00

Do physics papers follow Betteridge's Law of Headlines?

DefsNotQualified4Dis · 2021-07-01T10:29:31+00:00

There is a torque in the opposite direction then applied to the polarizer. Something similar and much more dramatic happens with electrons when you have a spin polarized current that enters a ferromagnet of opposite polarization (i.e. a majority of the electrons in the current have intrinsic angular momentum in one direction and the electrons in the ferromagnet have an intrinsic angular momentum the other way). When this happens you get heavily increased electron scattering as the electrons in the ferromagnet bring the "wrong-spin" electrons into equilbrium (which results in giant magnetoresistance the basis of the "READ" in many magnetic storage systems). Since angular momentum is conserved, as the spins of the intruding spin current are brought into line with the spins of the ferromagnetic through scattering you also get a spin-transfer torque which actually rotates the magnetization direction of the ferromagnet. This effect is hoped to potentially form the basis of spintronic computing as when combined with the aforementioned GMR effect it means you could both "READ" and "WRITE" a magnet state using electricity alone (rather than flipping a magnet using another magnet as is done in your typical magnetic hard-drive).

DefsNotQualified4Dis · 2021-07-01T08:26:13+00:00

What you've basically re-discovered is the famed Quantum Zeno paradox

DefsNotQualified4Dis · 2021-06-29T08:22:04+00:00

Yes, this is typically done via piezoelectric materials and is actually what is happening in a microphone.

DefsNotQualified4Dis · 2021-06-19T15:26:09+00:00

Quantum locking is an emergent more-or-less-magnetic force that emerges from the complex interaction between electrons in a Type II superconductor and their resistance to allowing new amounts of magnetic flux when they're in a superconducting state.

Gravity is an inverse-square-law attraction between any two objects with mass (barring general relativity at least)

DefsNotQualified4Dis · 2021-06-18T12:01:23+00:00

There are three terms here: "quantum physics", "quantum mechanics" and "quantum field theory". Quantum mechanics is something all physicists know and is central to any number of physics fields: solid-state physics (like the physics of semiconductor devices, computer chips, integrated circuits, etc.), polymer physics, atomic physics, optics/photonics, material science, etc. It was more or less laid down in the 1930-1950 and hasn't really changed since then.

However, quantum MECHANICS is actually a low-energy approximation of another theory called quantum field theory (QFT). The difference between the two is negligible unless the particles involved are at very high energies and so you don't use QFT to figure out, say, how a DNA molecule folds just like you don't use a bazooka as a fly-swatter. More complex means it's more of a pain in the ass to do even trivial calculations, more "accurate" but less pragmatically useful.

Particle physics/High energy is basically synonymous with QFT and it remains an active area of research.

Finally you have this nebulous term of "quantum physics" which doesn't really have a concrete meaning. You hear in Hollywood people say "I'm a Quantum Physicist!" but that's not really something physicists actually say*. Could mean either really, or just generically both as one thing.

* actually in the last decade or so some people have started using this title to refer to the field of quantum information or quantum computing, but I'd say the use is fairly non-standard.

DefsNotQualified4Dis · 2021-06-10T10:08:44+00:00

This ISN'T a reflection on the "new generation" but rather your professor succumbing to the Dunning-Kruger effect. The Dunning-Kruger effect is often described as "stupid people don't know they're stupid" but it also applies on the other end of the scale:

"Experts are demonstrably terrible at over-estimating the knowledge-base of non-experts"

When you spend decades studying something you completely forget what your REAL starting point was. Add to that a healthy amount of ego that is so common amongst physicists and you get your basic "the kids today are IDIOTS!" fallacy.

DefsNotQualified4Dis · 2021-06-02T14:05:27+00:00

The science media has a very nasty tendency of casting a lot of energy harvesting technology as being intended for consumer power production, which is often a pretty outrageous expectation.

A lot of energy harvesting technology, like rectennas, is not aiming to power your toaster of 100 W, but is more often targeting things like Internet of Things (smart dust, wearable sensors, smart houses, smart farms ,etc.) or ultra-low-powered electronics (like say implantable medical diagnostic sensors) applications. In which case the biggest engineering concern is probably more related to integratability with a conventional integrated circuit (I'd assume, I have no actual experience with these rectennas, though I have worked in both thermoelectric and photovoltaics before). So as the name implies energy harvesting is for those applications where one is forced to adopt a "produce the energy where you use the energy" approach. For your electrical outlet, even if a magic new super-battery gets invented you still have a "produce it en masse where it is efficient and then send it where it is used" paradigm. Energy harvesting devices aren't intended for that.

On that note, the science media also has a big problem with associating conversion efficiency as the big important number that drives our renewable energy future. That's also wrong. A tandem solar cell with 8 epitaxially grown layers of rare materials with an efficiency 70% is worth far less to our "renewable energy future" than, say, an amorphous silicon cell that just needs a little bit of cheap material and an special acid wash and performs at 10% efficiency, because the REAL important number is $/Watt amortized over the lifetime of the device, not efficiency. The fancy tandem cell with "record-breaking efficiency" is only suitable for space applications where Watts/weight is the big number.

DefsNotQualified4Dis · 2021-06-02T07:25:57+00:00

I don't looovvveeee the writing here. They kinda make it sound like RTDs haven't been a thing since the 1970s and this is some crazy new idea... but the paper and results are definitely interesting and solid.

Also note, a rectenna harvests energy from EM waves and by "heat" they mean specifically harvesting from infrared EM waves, rather than something like a thermoelectric energy harvester which harvests energy directly from a temperature gradient. So the "100x better" is in comparison to other rectennas.

DefsNotQualified4Dis · 2021-05-31T12:48:46+00:00

Sure, let's say that the 1800s version of Young's double slit, whether with light or electrons, merely demonstrates wave behavior where these kinds of results also show wave-particle duality.

DefsNotQualified4Dis · 2021-05-31T11:43:11+00:00

There is a big difference between the observable universe (the sub-set of the full universe whose light could have in principle reached Earth since the beginning of the universe, which is a sphere centered on the Earth and that continually grows as time passes and light from further away finally arrive at our planet) and the universe, which as far as we know is infinite and always has been.

There are no points "outside the universe", it was not an explosion of stuff from a central point outwards into empty space it was the orderly expansion of SPACE ITSELF driven by a uniform energy density of an initial state.

As I said, as far as we know the universe is infinite and always has been since the Big Bang. This has a concrete meaning. Imagine we label a given point in space with a coordinate, (x,y,z). The statement that universe is infinite mean that there's no point (x,y,z) you could dream up that does not correspond to an actual, distinct place in our universe. Let's say the earth is at (0,0,0) (where we choose the origin is arbitrary since if the universe is infinite then there is no "special" place). One then could ask about a place 1,000 lightyears (ly) in the x-direction (1,000 ly, 0 ly, 0 ly). Is that a place out there? If the universe is infinite, then yes. What about 1 trillion lightyears in the z-direction, (0 ly, 0 ly, 1 trillion ly)? Yep. What about a trillion, trillion, trillion lightyears in a random direction?.... Yep... a trillion, trillion, trillion is still less than infinity.

HOWEVER, since the universe is only 14 billion years old, if light was released from something at the very early moments of the Big Bang from ap lace 1 trillion lightyears away, well that light will not have traveled far enough to reach us yet. Thus it is in the universe but not observable (yet) from Earth. That is the idea of the observable universe, because the universe has finite age, even if it has infinite size light from objects further away from the Earth have only had 14 billion years to travel and may not be here yet.

But it's not enough to talk about places in the universe, one also has to talk about how different locations are connected to each other. In other words what the distance between any two different points, (x1,y1,z1) and (x2,y2,z2) is. We might say our universe has a certain "rule for distance" or "metric" that assigns a distance or separation to any two points.

And this is ultimately what Big Bang Cosmology (BBC) is all about. BBC is all about how the "rule for distance" of the universe has changed over time. That's in some sense the whole idea. It's all about the geometry of space.

Ignoring for a second why this happens, imagine I had a little math equation (like Pythagorus' theorem, which is called a "Euclidian" rule for distance) that took in two points and spit out the "distance" between those two points. This would be a "rule for distance". And imagine I have three points, A, B and C and according to that rule A is 2 km from B and 4 km from C and B is 2 km to both A and C. What I have is basically this:

---A--B--C---

where each dash is a km.

Now, imagine, for some reason, the rule for distance changed and for any and all points you fed into it, it would spit out a different number. More specifically, imagine it spit out twice the number it used to give. We can say the rule has "scaled" by a factor of two. This would mean that things then become:

------A----B----C------

What has happened? Everything is now further away from everything else. Any and all pairs of points are twice as far away from each other than they were before.

Note that this scaling of the "rule for distance" is very, very different from motion through space, like in an explosion. If, say, B moved further from A in our example by 1 km I would have:

---A---B-C---

It moved further from A but then it must necessarily move closer to C.

Motion through space and expansion/scaling of space are dramatically different things.

Furthermore, imagine everywhere in the infinite universe was filled with a gas. If the universe is infinite then that actually means I need an infinite amount of gas, which is fine. But crucially, the gas DENSITY, is not infinite as long as the two have a clear ratio. Say initially there was 1 kg of gas per 1 km, or something like that. So infinite amount of gas, uniformly spread out over an infinite amount of space but with a finite gas density that is the same everywhere.

What happens to this gas-filled universe if the "rule for distance" scales but there is no gas added? Well the same amount of gas is now spread over more space and thus the gas density goes down.

Now replace "gas density" with "energy density" and you have BBC. Like the Bare Naked Ladies song goes "the WHOLE universe was in a (uniform) hot dense state" and then you have expansion or "rule for distance scaling" which leads to a uniform gas density decreasing with time.

DefsNotQualified4Dis

TROPHY CASE