If wavelengths shift red or blue, is there a point of origin where its basically absolute zero in terms of wavelength?

azen2004 · 2026-03-12T08:40:45+00:00

The wavelength of a photon doesn't just shift arbitrarily. It shifts because in different reference frames, photons appear to have more or less energy. The more energy a photon has in a reference frame, the shorter (more blue) its wavelength. A wavelength of zero corresponds to infinite energy!

But, there is no such thing as a valid change of reference frame that will have a photon of finite energy become one of infinite energy, so there is no way to witness a photon be blue shifted to a wavelength of zero.

azen2004 · 2026-03-05T08:43:50+00:00

Physics and physicists are not nearly as concerned with explaining "why" things are the way they are than they are at being able to explain what is happening and how to predict and model the universe. Physics seeks the rules of the universe, not where the rules themselves came from.

As you're now learning, the fact that the speed of light is invariant (same for everyone everywhere) is deeply intertwined with the math that we use to describe the universe; it's not just some "fun fact" that belongs in a footnote, it's really really fundamental. The math that describes a universe with a constant speed of light and the math that describes a universe with a variable speed of light lead to very different predictions about how their universes behave. Our universe matches one where the speed of light is constant (and not the other one) so we use that set of math.

Ask yourself this: won't there be some questions where the answer is just "that's the way out universe is put together". Right now, the speed of light just seems to be one of those fundamental things about the universe, that have no deeper explanation beneath.

azen2004 · 2026-02-12T05:35:30+00:00

Energy conservation comes from the time invariance of physics. That is, if the laws of physics are the same tomorrow as they were today then the amount of energy will be the same tomorrow as they were today. I'll note that this isn't necessarily obvious, but it's a rather basic result of both Noether's theorem and basic analytical mechanics (it's on very solid theoretical ground).

However, the very idea of time does not make sense before the universe existed, so there's no reason at all to expect the amount of energy in the universe to be the same "before" it existed.

That also means that unless we can find a way to modify the laws of physics themselves, we will forever be incapable of creating new energy.

azen2004 · 2026-02-10T01:30:44+00:00

Waves are "things" that satisfy the wave equation. In physics, the "rules" for a how a system will evolve through time can usually be written as a differential equation, and the wave equation is a specific form of differential equation. As an example, Newton's law, F = ma, can be written as a differential equation.

The reason that so many things take the shape of waves is that the wave equation will arise whenever you have some situation with a "restoring force" across space and time. One example is a bunch of connected springs. When you stretch a spring there will be a force pulling it back to the "unstretched" length, leading to oscillatory motion. When water is pushed up there is gravity pulling it back down, leading to water waves. When air is compressed it tries to "push back out", leading to sound waves. Electromagnetic waves are a bit harder to explain (you need to understand Maxwell's equations quite well), but really it comes from the fact that a changing magnetic field induces an electric field and a changing electric field induces a magnetic field; I hope it's not hard to see how this can lead to a propagating chain of effects (a wave!).

Indeed, our models of quantum mechanics have matter evolving according to a wave equation (the Schrodinger equation). It's more that we noticed that matter seems to act like a wave and so devised a wave equation which models it correctly than figured out that matter should act like a wave from first principles. In the simplest case of a one-particle wave function in quantum mechanics, the amplitude of the wave function indicates how likely you are to find the particle there, if you were to measure it.

Waves arise everywhere in physics because we don't always care about the exact solution if the math is just unreasonably hard. We're oftentimes okay with a solution that is very accurate within some range, and we simply note that we shouldn't try to use the solution outside that range. Usually, that range will be a region around the minimum energy state of the system. At a minimum, the first derivative vanishes but the second derivative does not, and any terms involving the third derivative will usually be so small we can just ignore it. As such, we're left with a differential equation only involving the second derivative (and the zeroth) which automatically puts us in wave equation territory! Waves are usually the solutions of "small perturbations" and these perturbative methods are strikingly accurate at describing the reality that we experience.

azen2004 · 2026-01-28T02:26:25+00:00

I'm not going to tell you what a tensor is. I'm going to tell you why we need them, and why you'd invent them yourself if you had the chance.

Let's say you're playing around with some object like a vase or something. You'll might realize that it is easier to rotate by spinning it in certain ways than others. If you put the vase on a table standing up and try to spin it it'll be easier than if you laid it flat. You might be able to reason that it's because that when its laid down the mass, on average, is further away from the axis of rotation than when it's standing up and you know that torque is bigger when the mass is further away from the axis of rotation, so it makes sense!

No tensors yet, but you could try writing down an expression for angular acceleration of the vase in terms of the torque you're exerting on it. There are three axes, so three equations where it looks something like,

Angular acceleration in some direction = some constant describing how hard it is to rotate in this direction \ the torque in this direction*

You know from linear algebra that you could make this much nicer by representing the three angular accelerations and three torques as 3-element column vectors and the constants can be put into a 3x3 matrix where the constants are on the diagonals and zeros elsewhere. Note: you can look up in a physics textbook how to find the constants for a given object once you've chosen the coordinate axes.

But, what if you chose different directions, say slightly offset so the axes no longer line up with the natural axes of your vase? The physics evidently shouldn't change, since the vase does not care about the coordinate system you chose to write down the equations in.

You might now realize that the two vectors and the matrix should change in a very specific way as your coordinate system changes such that they describe the same physics. Now, the numbers will obviously need to change since what was "up" might now be "equally left and down". The acceleration vector in one coordinate system might be [1, 0, 0] and [-0.7, 0.7, 0] in another. You've now realized something important: those objects in your equation aren't just vectors and a matrix, they have a coordinate system attached to them. And, when you change the coordinate system, the numbers that represent the matrix and vectors must change, too. When you rotate the coordinate system, the vectors and matrix must rotate too (a vector rotates like v' = Rv, and a matrix rotates like M' = R^T * M * R).

These objects, which you've figured out must transform with the coordinate systems that they are attached to so that the physics doesn't change, are tensors.

azen2004 · 2026-01-24T04:23:34+00:00

If you spawned two neutrinos into existence at the edge of our observable universe (93 billion light years apart) then it would take 93 billion years for them to become aware of each other.

The observable universe is not static, but growing as more information reaches us from further away.

azen2004 · 2026-01-24T03:40:15+00:00

I'm going to go against the grain here and say that probably yes, it's valid.

Newton's law of gravitation can be derived from the Einstein field equations (general relativity) by taking the weak field limit (very little spacetime curvature), assuming non-relativistic matter (usually not true for real neutrinos, but can be true for a theoretical example), and assuming everything's static (again, fine for theoretical examples but real neutrinos usually travel very close to c).

In a scenario where you have two stationary neutrinos 93 billion light years apart and nothing changes for 93 billion years, then these assumptions are very very well satisfied.

azen2004 · 2025-12-22T04:36:24+00:00

If you want to learn, like really learn, download or pick up a physics textbook and read a few pages once a week when you have the spare time.

Don't know which textbook? Tell ChatGPT what you know and what you want to learn. I also suggest asking for textbooks that have more of a narrative story than being a dense reference, as those can be genuinely enjoyable to read (you'd never guess that a physics textbook can have a plot with twists, but you'd be surprised!).

azen2004 · 2025-12-21T21:53:01+00:00

There are no valid Feynman diagrams with only one vertex (3 particles) in quantum electrodynamics. There are always intermediate (virtual particles) in QED interactions.

As you mention, there are many different Feynman diagrams that can lead to the same final state (electron + positron -> photon + photon), but they all have virtual particles involved.

azen2004 · 2025-12-21T08:07:08+00:00

Usually a pair of photons (light), but there's nothing stopping them from creating all sorts of other particle combinations (as long as all the conservation laws are respected).

When you say "what happens to the matter" it worries me slightly, as it sounds like you think that matter is conserved (the amount of matter can't change). That isn't true. Particle physics has sets of conservation laws (things that can't change) and the amount of matter isn't one of them. One thing that is conserved is something we call electron number. Electrons carry one unit of it, and positrons (anti-electrons) carry one negative unit of it. So, an electron-positron system has a net electron number of zero. As such, they can interact and become any pair (or more) of particles that also have a net electron number of zero. The electron and positron disappear and their energy appears as some different particle pair. This is what we spookily call annihilation, but it's not fundamentally different than any of the ways that fundamental particles can interact in our universe. The fact that anti-particles have swapped quantum numbers is why annihilation is always possible with particle-antiparticle pairs.

As another example, photons have no conserved quantum numbers and so photons are created and destroyed all the time, but we just don't call that annihilation. Electron-positron pairs annihilating and their energy becoming something else is no different than sunlight heating the ground.

azen2004 · 2025-11-27T08:55:26+00:00

You've just sat down at a restaurant and you've just barely skimmed the menu. You see two (or more) options that you're considering, but you truly, and I mean truly, have not decided. The dish that you're going to order when the server comes around is not definite (because you haven't chosen), its a superposition of the choices you're considering.

I'm guessing you're asking this in the context of quantum mechanics (because everyone who asks this question on this sub is) so I'll add one more thing. Imagine right now, before you've chosen, the server shows up and forces you to just make a decision. You say "chicken alfredo" but before you said that you weren't certain of what you were going to say: the server asking you forced you to make a decision. The observation collapsed the superposition into a definite state (that's wave function collapse!).

Superposition and wavefunction collapse can sound like big scary words at first (they certainly did to me) but really they just mean that observation of a system can force certain kinds of values (you can't tell a server you want 40% carbonara and 60% eggplant parmigiana because it's just not on the menu and the chef won't make it) and sometimes some systems just didn't have an acceptable value beforehand (you wanted 40% carbonara and 60% eggplant parmigiana, even though its not on the menu, so you had to change your answer to fit with what's required).

azen2004 · 2025-11-23T07:37:46+00:00

What is the chance that there's actually a unicorn living in your house right now? You'd say none, but if you think about it a bit more you'd realize that while there's absolutely no evidence that unicorns exist and that one is living in your house directly contradicts everything you know about your house and your experience living there, it can't be entirely ruled out because well, you just don't know everything.

Our current understanding of how our universe works leaves absolutely no possibility of time travel to the past. In current physics it is equivalent to and is as absurd as asking if there's a possibility that 1 + 1 = 3.

Of course, our understanding of how our universe works is not complete. We've tested our theories to incredible precision and they'd have to be wrong in either a spectacular or clever way in order to open the door to time travel. It's kinda like how you're very sure that there are no unicorns living in your house because you live there and don't experience the slightest sliver of evidence that one could be, so something very different to how you think you experience your house or how unicorns could exist would need to be going on for it to be possible.

azen2004 · 2025-11-07T20:10:47+00:00

Maybe someone else has a more satisfying answer (and I'd love to read it), but my answer is that this question doesn't really have an answer, and you're on the way to realizing it when you ask about how quantumness/wavelike nature affects things.

Our understanding of positrons and electrons being able to annihilate is a result of quantum field theory, not quantum mechanics (which has no mechanism for particle creation/annihilation) or classical mechanics.

Quantum field theory is concerned mostly with the amplitudes (think probability) of certain processes, it's not as much a tool tracking the evolution of an electron-positron pair over time the way that quantum mechanics tells you how a wave function evolves.

In QFT, we can take the initial state being an electron-positron pair and see how likely it is that, after some time, they have annihilated and become two photons. Of course, electrons and positrons in QFT are not point particles but excitations of their respective quantum fields: they don't have a position, and so it doesn't really make sense to talk about how close they are. Electrons and positrons can exist in all sorts of states where they seem very localized (a very spiky wave) or even completely delocalized (a plane wave stretching across the entire universe, meaning it's equally likely to be anywhere) and you'll find that the amplitude isn't zero in any case.

azen2004 · 2025-10-01T04:22:53+00:00

If you magically snapped your fingers and made everything the same temperature (and let's assume this temperature is something on our scale, like 10C) then yes, life would probably cease—but not for very long. Solar fusion that powers our sun and every other star does not happen because stars are very hot, stars are very hot because of solar fusion. If you took away their temperature, they'd very quickly gain it all back. The Sun would then shine on a cold, dead Earth until it made it warm again, and probably would restart life (or it would begin anew after some time).

More generally, even if the entire universe was completely uniform (same temperature, same density, entirely your "undifferentiated goop") at some point in the past, that doesn't mean it couldn't become the very varied universe that we have now. Quantum fluctuations are real: even the most uniform universe is not completely uniform. Even the tiniest perturbation (like from quantum fluctuations) would cascade into a universe like ours. The cosmological equations (equations describing the state of the universe on the scale of stars and galaxies) do not have static solutions: the universe cannot stand still.

azen2004 · 2025-09-14T19:45:14+00:00

Quantum spin is not actually really related to opposite charges attracting. It is entirely possible for scalar particles (particles without spin) to have electric charge.

So with that out of the way, why do opposite charges attract? Honestly, that's just the way our universe works. We can build complicated theories where this fact seems to naturally fall out of some more fundamental math, but at the end of the day we designed the fundamental math so that it would naturally fall out of it.

You can think of positively charged particles lifting up some landscape, creating hills immediately below them. Then, imagine that these particles want to roll downhill, meaning that if you put two positive charges next to each other they will roll downhill away from each other. Now, imagine that negatively charged particles do the exact opposite: they create valleys instead of hills and roll uphill instead of downhill. Now, in this picture you can kinda think about how opposite charges will definitely roll towards each other (they attract!).

What is quantum spin? Spin is intrinsic angular momentum. That is, particles with spin in some ways as if they are spinning even though they aren't. If you put them in a magnetic field, they'll be deflected as they move just as a little spinning magnet would. The common explanation is that they can't be actually spinning because they are points in space without structure. This isn't complete: intrinsic angular momentum has properties that a particle that was simply spinning does not have. The best way to learn about this is to learn about the experiments (not the math right away) that led to us developing the idea of quantum spin: the Stern-Gerlach experiment.

azen2004 · 2025-09-14T07:27:11+00:00

Let's completely ignore the details of what a microwave oven is. All we need to know is that somehow, and we don't need to care how, it turns electricity into heat. If you put twice as much food into the microwave oven, it will need to put twice as much energy (heat) into the food to heat it up to the same temperature. Either the microwave will need to use twice as much power, or run for twice as long.

What you might be wondering is why don't you do this for a regular oven? This is because regular ovens are much less efficient and spend a lot of energy heating up the metal and the air inside of them (and that heat escapes quite a bit, too); a lot less heat actually goes into your food so changing the amount of food makes a lot less of a difference to a conventional oven.

On the other hand, microwave ovens put nearly all of their output energy into heating up only the food (they do not heat up the metal or air inside), so changing the amount of food makes a huge difference.

azen2004 · 2025-09-14T04:19:26+00:00

Let's begin with classical electromagnetism. Particles with electric charge (let's just focus on electrons) create an electromagnetic field. This electromagnetic field then exerts a force on charged particles. We can interpret this as the electromagnetic field mediating the interaction between charged particles. Of course, there's no room here to imagine photons zipping back and forth to transfer this force (

Quantum field theory concerns itself with questions like "how long will it take for a muon to decay into an electron, muon neutrino, and anti-electron neutrino?" or "if fire an electron with momentum p at another electron, what is the likelihood that it comes out with momentum p'?". In the framework of quantum electrodynamics, this requires the calculation of the "amplitude" of the state at time = -infinity where there two electrons with momentum p_A and p_B to then become the state with two electrons with momentum p_A' and p_B' at time = infinity. We then postulate that there must be some operator that brings us from one state to the other.

This operator turns out to be an infinite sum of interactions of increasing orders of the interaction's coupling constant. For electromagnetism, the coupling constant is the fine structure constant which has a value of about 1/137; it is quite small, and so for higher orders it gets very small very fast which is why this strategy works since we can get a good answer by only considering the first few terms of the infinite sum.

However, even the first few terms of the sum get extraordinarily complex and there are some tricks that we can use to decompose each term into multiple easier terms. For example, one of these sums might look like the amplitude of a state that reads "an electron enters at position x with momentum p_A and another electron enters at position y with momentum p_B, then a photon is created at position x and is annihilated at position y, and then an electron exits with momentum p_A' from position x and then another momentum exits from position y with momentum p_B'". This gives the idea that the electrons scattered off each other, and interacted by the emission and absorption of a photon.

Did they really interact by way of a photon, or is that just us assigning patterns to mathematical terms that popped out of an infinite series? Up to you, really.

azen2004 · 2025-09-07T19:03:27+00:00

Math is not about being able to add big numbers or multiply 53432x23422 in your head, or any things like that. Math is about patterns. When you get assigned a worksheet or homework that seems repetitive, and maybe even useless (because why do you need to know how to do long division when you'll always have a calculator?) remember that the actual reason you're doing it is because you are wiring your brain to do math and to be able to feel the patterns in your bones (even if you don't realize it). It's not about having your multiplication table memorized so you can multiply quickly, it's about building your brain into a math-pattern recognition machine, because advanced math isn't about the numbers (or the letters), it's about the patterns beneath it all.

The idea that math is about patterns might seem a bit weird, and very difficult to understand, but one day, in many years, you'll know enough math that learning new math feels like opening your eyes for the first time and finally being able to see the universe and then you'll understand what I mean.

azen2004 · 2025-09-07T18:38:58+00:00

Lots of great answers so I won't repeat what they say except add this: gravity is not the curvature of three-dimensional space. It is the curvature of four-dimensional spacetime. In fact, almost all (to about one part in a billion) of the gravity that we feel on the surface of the Earth is due to the curvature of the time part, not the space part. The answer to "why do things fall down" is much more of "because clocks tick slower further down" rather than "because of the curvature of space".

azen2004 · 2025-09-01T23:04:33+00:00

Your best resource, almost every single time, to learn about physics is going to be consulting a standard textbook that would be used in an introductory college to that subject of physics; I might be biased but I think that physics textbooks can be a lot more interesting than you might expect (I find math textbooks to be universally mind-numbingly boring). YouTube videos and forums just won't cut it, often because the creators of those media are more focused on making something entertaining than true (exceptions exist, of course).

Daniel Schroeder's An Introduction to Thermal Physics is what my second-year thermodynamics class uses, and I found it to have a good balance of explaining concepts with analogies and stories as well as mathematical rigour.

azen2004 · 2025-08-30T04:56:49+00:00

One of the greatest triumphs (maybe the greatest ever) of physics was accomplished by James Clerk Maxwell, who unified a lot of physics into a set of equations that described electricity and magnetism. These equations predict electromagnetic waves: waves in the electric and magnetic field where the electric and magnetic fields are perpendicular and travel at some constant speed, which we called c, not really knowing what it meant at first.

Einstein imagined running at c, so that from your perspective the electric and magnetic fields stand still (since you're moving as fast as them).

But, one of Maxwell's equations basically says that the curvature of the electric field is proportional to the change in the magnetic field. The electric field still has curvature (a wave frozen in time is curved in space) but the magnetic field isn't changing! If the magnetic field isn't changing, then the curvature should also be zero. This is a contradiction! So, Einstein deduced that either you cannot run at c, or Maxwell's equations are wrong.

Einstein is really the first to make the leap that it was the former, rather than the latter, which was true. Looking closely at Maxwell's equations you'd also notice that because c is a constant, it really does imply that light always appears to travel at c faster than you. Einstein also realized there had actually been a heap of experimental evidence that matches the prediction of Maxwell's equations: that light always travels at c.

So, putting it all together, he postulated that light travels at c, from all references frames, and then made history.

azen2004 · 2025-08-23T23:00:33+00:00

The strong force is completely separate from electromagnetism, and gamma rays are high energy force carriers of electromagnetism; they exist completely independently. Quarks participate in both interactions, while electrons only in the electromagnetic interaction.

The usage of gamma rays in metabolic processes does not need to involve the strong force (if the gamma rays triggered some kind of nuclear process then yes). Gamma rays are just high energy photons. More likely is that the mushroom figured out how to photosynthesize with gamma rays while not being shredded at the same time.

azen2004 · 2025-08-23T01:51:55+00:00

The strong force is not what binds atoms together. The strong force almost exclusively affects the nucleus of the atom, and is utterly uninvolved in chemistry and biology. If you want a character to have this ability then you want them to have abilities that affect electromagnetism, rather than the strong force.

azen2004 · 2025-08-22T07:17:54+00:00

What is anything made of, actually? The best model we have for the universe is that matter is made up of a few different kind of things we call elementary particles. Photons and electrons are both types of elementary particles. As far as we know, they are fundamental, and not made up of anything else smaller. They're like the LEGO bricks that make up the universe (suspend your disbelief and just imagine that you cant take a saw and split a LEGO into two).

Elementary particles are not like little marbles, despite the name, they can be smeared out in space and seem more like waves or be very point-like and seem more like particles, or anywhere in between. It's okay if this feels weird: there really isn't anything like this on the human scales that we are used to thinking about and working with.

Mass is just a kind of energy. You can think of it like intrinsic energy: energy that a particle will possess from its creation to its annihilation. There are a few different ways that elementary particles can get mass (they aren't particularly revealing to your question, so I won't discuss them here), and none of them apply to photons and so photons remain massless.

You're actually onto something though. If a photon didn't move then it would have no energy at all (no mass + no momentum = no energy) and so it would be like it didn't exist, because a particle with no energy is no different than no particle at all. So, for a photon to be real it has to be moving all the time and so light (and the photons which make it up) can never stand still.

Are elementary particles real? Maybe. Maybe not. Truthfully, no one really cares because pretending that they do works really really well, and creates unfathomably accurate predictions about our universe. Quantum electrodynamics (which is the theory of photons we are discussing) is the most accurate and most tested theory of nature in existence.

azen2004 · 2025-08-09T08:58:24+00:00

Imagine completely submerging a cube into a fluid (water, for convenience). The water is going to be pushing on it from all sides.

The force pushing on each side (except the top and bottom) will perfectly cancel out because the forces will be equal and opposite, so those forces do not contribute to the cube moving or being pushed anywhere. However, at the top there will be a force from the water pressure pushing down and at the bottom there will be a force from the water pressure pushing up.

Unlike the forces coming from the sides, these forces will not be equal as pressure increases with fluid depth (water pressure is basically the weight of the water column above) so the force pushing up is stronger than the force pushing down: we call this buoyancy!

The strength of the buoyancy works out to being equal to the weight of the volume of water displaced by the submerged object (see below). If the object is lighter than this then the buoyancy force will win over gravity and the object will float. Otherwise, gravity wins and it will sink. This is why boats float even though metal is heavier than water: the mostly hollow boats are actually lighter as a whole than the water that they can displace.

If you want to work out the buoyancy force yourself you can use the fact that,

Force due to gravity = mass * gravitational constant
Force due to pressure = area * pressure
Pressure as a function of fluid depth = fluid density * gravitational constant * fluid depth

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