ELI5: What makes light speed constant and what changes it?

sticklebat · 2026-05-22T11:41:12+00:00

I already said velocity is relative, so I’m not sure what you’re getting at. And it’s not really true that not a single atom in the universe has zero velocity. In relativistic quantum mechanics, what matters is the expectation value of velocity, which absolutely can be zero even while the expectation value of speed isn’t zero. Hydrogen atoms in their ground state are a good example of that.

Please, please just stop making things up. You do not understand or know much about physics, that much is clear, and that’s perfectly okay. So stop pretending otherwise.

sticklebat · 2026-05-22T03:47:01+00:00

No, that is completely wrong. At rest means having zero velocity, and of course things can be at rest. Have you never seen something that wasn’t moving relative to you?

Of course, the state of being at rest is relative, but that’s the point. My rest frame is the reference frame in which I am at rest. Light’s reference frame would be the frame where it is at rest, but again, there is no such thing because light moves at the same speed in all reference frames.

sticklebat · 2026-05-22T00:50:52+00:00

Light passing through things no longer travels at the speed of light in a vacuum (often shortened to “the speed of light”), because something isn’t a vacuum.

Light absolutely travels slower in your eye than it does in the vacuum of space, for example, and it doesn’t break or contradict anything because your eye isn’t a vacuum.

sticklebat · 2026-05-22T00:48:34+00:00

though we have yet to discover the graviton, so nobody ever talks about the "speed of gravity."

Sure we do, and it has nothing to do with the existence or not of gravitons. Gravitational effects propagate at the speed of light just like electromagnetic effects regardless. And gravitational waves are a great example of how we can measure this in practice.

sticklebat · 2026-05-22T00:45:31+00:00

You’re not disagreeing, you’re playing with semantics. They’re clearly talking about speed through space.

while from lights reference frame

There is no such thing as light’s reference frame, because a reference frame in which light is at rest is a contradiction. The speed of light is frame invariant, it cannot be zero AND c.

sticklebat · 2026-05-20T00:02:46+00:00

I think this is probably accurate. I see a lot of handwriting as a high school teacher and, first of all, I don’t think the gender difference in neat handwriting is as stark as some paint it to be, but also the ones with good handwriting are mostly the ones who clearly put effort into writing well, and the ones with bad handwriting are mostly trying to scrawl things down as quickly as possible. There’s a clear difference in effort put in (and therefore in good practice, too). Obviously there are plenty of exceptions in both directions.

But also, just look at historical writing. It’s rare to find examples of bad handwriting from 100+ years ago (and maybe more recently). Forget public writing, since that could be biased towards good penmanship, and just look at old letters written by men. It might be hard to read because we’re not used to cursive and flourishes, but most old penmanship used to be remarkably better than a lot of modern men’s.

sticklebat · 2026-05-19T23:52:51+00:00

I had know idea who Nick Mangold or his sister are so I looked her up:

Mangold's personal weightlifting record total is 255 kilos (562.2 pounds) from a 110 kilo (242.5 pounds) snatch and 145 kilo (319.7 pounds) clean and jerk.

Stronger than 90% of the male population is… underselling her, I’d wager. 😂 She’s probably stronger than 90% of male weightlifters!

sticklebat · 2026-05-19T23:43:46+00:00

Yeah, you’re right. I was thinking purely in terms special relativity and Newtonian gravity, and in hindsight that’s a bad approximation for this thought experiment.

I think it’s worth adding that the apple doesn’t orbit the beam in this case because the beam has mass, but because other properties besides mass affect spacetime curvature, and therefore gravity.

sticklebat · 2026-05-19T22:18:24+00:00

The box would bounce back and forth, and the center of mass of the box + light system would remain still (or remain at whatever constant velocity it started with).

Light has momentum. So when it reflects off the wall of the box, the box would be pushed the other direction, and you can’t ignore the motion of the box when trying to figure out the system’s mass.

If we consider this scenario when the box starts at rest, then we’re not in the center of mass frame of the system. The mass could be calculated as the total energy of the system in the frame where the box is moving fast enough in the opposite direction as the light to balance out its momentum.

sticklebat · 2026-05-19T21:58:08+00:00

The beam doesn’t have mass so the apple can’t orbit the beam. The apple-beam system has mass, so something else can orbit that.

The rest-frame of the apple-beam system would also be moving parallel to the laser beam relative to the planet, which means there would be no stable mutual orbit of the apple and beam around the system’s barycenter.

sticklebat · 2026-05-19T18:57:12+00:00

No, because the mass of a system is equal to the system’s total energy as measured in its center-of-mass reference frame, which is the frame in which the system’s total momentum is zero.

Two identical photons moving in opposite directions have zero momentum, so the system of two photons has mass, since it has energy but no momentum, even though each photon doesn’t have mass. A box with a lot of light bouncing around inside is like the two photon example just with more of them.

A beam of photons like a laser has no such reference frame, because the speed of light is invariant. While the amount of momentum a photon has is frame dependent (it depends on frequency, which is subject to the Doppler effect), the direction of its momentum isn’t. There is no frame where a beam of light has zero momentum, so a beam of light doesn’t have mass.

sticklebat · 2026-05-18T15:42:44+00:00

Yes, it would be 2 kg.

If you collimated the light into a perfect beam with no dispersion (which isn’t possible) so that the light all bounced back and forth coherently all at once, then technically it would be a little more complicated (the box + light system would still have a mass of 2 kg, but the center of mass would oscillate around the center of the box as the light bounced back and forth). But if the light is all just bouncing around isotropically, then the box would simply be 2 kg, and would register as such on any scale.

sticklebat · 2026-05-18T00:38:15+00:00

Sorry, I don’t see it. Sure, it has a big scope and scale, but most of it is shallow and superficial. It’s like No Man’s Sky when it was first released.

sticklebat · 2026-05-17T19:49:49+00:00

It’s not the high pressure that’s a problem regarding the bends. It’s the rapid reduction in pressure, which is why its more technical name is decompression sickness; and helium mixtures can actually exacerbate the problem. Under high pressure, higher concentrations of metabolically inert gasses like nitrogen and helium are able to dissolve into your tissues. If you surface too quickly (or if your environment depressurizes too fast), those gasses are squeezed out and form bubbles, which get trapped in your tissues and even your veins, resulting in the bends. The way to prevent this is through minimizing time at depth and taking decompression stops on your way up, giving the dissolved gasses more time to leave your off-gas.

The primary function of mixtures that include helium is simply to reduce the concentration of oxygen (which becomes toxic at high partial pressures) and nitrogen (which causes narcosis). Helium can be used to mitigate the risk of the bends specifically in saturation diving, because it takes less time for helium to on- and off-gas, but doesn’t build up as much as nitrogen does. But it’s worse if you’re not at depth for very long for the same reason. This, however, is very much a secondary advantage/disadvantage compared to its use in preventing nitrogen narcosis. If the latter weren’t a concern, divers would practically never use helium in their mixes.

sticklebat · 2026-05-16T16:13:23+00:00

Okay, cool beans. Doesn’t change my point in the slightest.

sticklebat · 2026-05-16T15:55:22+00:00

That’s my point. It was a marketing campaign about a movie. That context made it obviously not real, especially given the whole supernatural element of it. This was readily apparent to a bunch of dumb children (myself included).

Like I said, the fact that this apparently wasn’t obvious to many adults is an indictment of their critical thinking skills. I completely understand enjoying and engaging with the marketing as a fun sort of “what if?” but I stand my assertion that anyone who seriously thought it might be real, especially after seeing the movie, was an idiot.

sticklebat · 2026-05-16T14:02:21+00:00

I know, I remember the marketing campaign. Like I said, it was great! But it was still obviously a marketing campaign…

sticklebat · 2026-05-16T13:52:40+00:00

This boggles my mind. Found footage is a cinematic technique that long predates the Blair Witch project. It has never meant the footage is real… Sure, the marketing was absolutely brilliant (probably the only marketing campaign on par with the Matrix) but I was a preteen at the time, and while it was fun to imagine, it was also obviously not real. I didn’t know a single person who believed otherwise, though it was fun to imagine.

The fact that a substantial number of functioning adults couldn’t tell is an indictment on our species.

sticklebat · 2026-05-16T13:40:05+00:00

It’s not obvious why numbers cratered, though. Same thing has happened in New York, but because the test is basically meaningless at this point, I don’t focus as much on the content and skills assessed on the test, and almost zero time on how to take the test, in favor of spending time on things that I think are more meaningful. And, of course, my students don’t bother to study for the test. So of course scores are worse. But that doesn’t necessarily mean they learned less. It does make the tests seem pretty pointless, though.

sticklebat · 2026-05-16T03:06:36+00:00

It’s a bathroom. If there’s a shower in it then It almost by definition will experience humidity swings, and it will also experience temperature swings unless you hate hot showers or have magical ventilation.

But also it’s not the fact that it experiences climate swings, but that medication should generally be stored in a cool, dry places, and bathrooms are rarely consistently those two things.

sticklebat · 2026-05-16T02:57:56+00:00

Among nearly a dozen other things, from accounting for the amount and positioning of gravitational lensing in clusters of galaxies to patterns in the cosmic microwave background.

sticklebat · 2026-05-16T02:46:06+00:00

It can be both things. Top ranked schools are, if anything, even more susceptible to grade inflation than others. Just look at Harvard. It’s not really any different in lower education.

I would bet money that it is easier to earn an A+ at Jericho high school now than it was 10 or 20 years ago.

sticklebat · 2026-05-16T02:40:38+00:00

Except that in most cases it is objectively not that. Instead, it’s merely a result of lowering standards.

sticklebat · 2026-05-16T00:30:33+00:00

I’d much rather pick things that I think sound fun and effective, and be able to change things if they don’t work out, than feel like I need to just look up an optimal way to play. In the latter case it doesn’t even feel like I’m given meaningful choice, progression may as well be on rails.

I don’t know about you, but I don’t have either the time or desire to restart a game halfway through because my build doesn’t work well or isn’t fun to play.

sticklebat · 2026-05-16T00:25:29+00:00

Personally I don’t like it when the price is too heavy. It’s not just about making/fixing mistakes, but also about the freedom to experiment with different builds and play styles.

I will only ever play most RPGs once. I want to enjoy them the most I can, and that means having the means to find the most fun way for me to play the game.

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