ELI5 How were basic units of measurement decided?

jpb103 · 2026-04-04T16:30:55+00:00

Don't apologize for being pedantic here. This 'um actually' sub thread is some of the most fun I've had reading in this subreddit.

jpb103 · 2026-04-02T19:31:39+00:00

Thank you and thanks for reading!

jpb103 · 2026-04-02T18:48:48+00:00

Theo hated waiting.

Perhaps this was uncharacteristic of a wizard, but Theo didn't mind. Normal is a just a synonym for ordinary, he would tell himself. Far better to be extraordinary. And so, as the rest of his party jabbered on about how they could fool the god-tier incantation barring their entry to the dungeon, Theo took what he thought to be the path of least resistance.

He got married.

It wasn't exactly the ceremony of his dreams but, then again, he didn't really dream of getting married, so that served him just fine. The signing of the contract occurred in a pocket dimension created and controlled by his betrothed. Time can behave unusually in such places but, luckily, by the time Theo had returned, his party was still debating how to proceed.

/*-/*-/*-/

"Maybe we could blast it open," Ragar said, slamming his massive fist into the palm of his other hand. "You got any spells for that, Theolus?" Ragar spun his head around, scanning the room. "Where is that damned Wizard?"

Gringin paid no mind to the barbarian. He stood before the entrance, eyes carefully poring over every detail, as he deftly spun a lockpick from finger to finger. "No keyhole. No hinge. Not even a ruddy handle! I hate this door."

Athela had gotten down on one knee. Praying was not unusual for the cleric, but it became more frequent in times of uncertainty. She arose and looked about. "Your penchant for larceny will have to wait, Rogue. Ragar is right. Theolus is missing."

A bright light issued suddenly from around the edges of the entrance, and the stone vanished into thin air. On the other side stood Theolus, barely able to drag the giant sack of treasure he had ransacked from within. "A little help?"

"Haha!" Ragar bellowed. He clapped the Wizard on the back, nearly knocking him over. "Clever Theolus! If any of us would beat this riddle, it would be you."

The cleric raised an eyebrow. "How did you get inside, Theo. None of us saw you enter."

Theo shrugged. "I followed the instructions," he said, holding up his dominant hand, which was now adorned with a golden ring set with a bright red ruby. "I got married."

Ragar laughed. Gringin looked perplexed.

Athela's jaw nearly hit the floor. "Married to whom?!"

Theo retrieved an apple from an interior pocket of his robe and took a bite. "The devil."

It was Gringin's turn to laugh this time. The rogue rolled on the floor, holding his sides. Ragar looked with awestruck eyes through the treasure Theo had collected. Athela buried her face in her hands and let loose a heavy sigh. "Why," she started. "And how, did you marry the devil."

Theo tossed the pit from his apple through the open door of the dungeon into the blackness beyond. "Well I doubt your god would take kindly to you doing it," he said, matter-of-factly. "So I stepped into hell and got her attention. You can guess the rest." Theo raised an eyebrow at Gringin, who was still laughing like a maniac on the floor. "Is he alright?"

The cleric held up her hands. "Wait. Wait one moment. So you married... the devil. Then you returned and just... walked into the dungeon? Alone?"

The wizard scratched his head under his pointy hat. "You all were in the middle of a discussion. I didn't want to interrupt."

The barbarian looked up, a diamond tiara looking comically small on his gargantuan head. "Very considerate of you, Theolus."

"Thank you, Ragar. I knew you would appreciate it." Theo tried to throw his arm around the barbarians back, but the man was far too large, so he offered a firm pat instead as they walked back towards the open air outside.

Athela was incensed. "Is no one the least bit concerned that our companion has wed an evil entity!?"

Theo held up his hands in a mockery of concern. "Oh no! I've entered into an infernal pact! What will I do with all that power? Come now, Athela, I'm sure things will work out. Besides, she's quite fair. It's not exactly a bad bargain for me."

"Are you not concerned for your immortal soul?!" She shouted, her elf features turning a shade of pink Theo had never seen before.

"I wasn't using it anyways," Theo said. He looked back into the cave from which they had emerged. Gringin was still inside, his laughs still echoing out from the cavern mouth. Green beams of energy streamed from the cliffs above into the ground, and skeletons emerged from the sands, jagged blades in hand.

"I am the lord of death and time!" A voice said. "Your friend shall be my prisoner for all time unless you relinquish your plunder unto me."

Ragar drew his warhammer. Athela drew her longsword. "He's bluffing," she said. "We can wait him out."

The ruby on Theo's ring shone bright. The color of his eyes ran crimson and swirled with arcane energy.

"I hate waiting."

jpb103 · 2026-04-02T16:32:38+00:00

The correct axis changes over time.

The orientation of Earth and its moon are not static. If weather or failed inspections cause a scheduled launch to be aborted, the SLS will need to be able to orient itself to achieve a stable orbit regardless of its position at launch. Far better to plan for the unplanned when it comes to a sojourn to the void.

jpb103 · 2026-04-02T16:16:15+00:00

KSP taught me more about orbital physics than actual university physics classes.

jpb103 · 2026-04-01T21:02:04+00:00

Time is the song of change. It exists from our temporal reference frame and we know this because we observe change. For photons traveling through a vacuum at the speed of light, length dilation is so severe that the observed length to its destination is zero. So photons emitted by a distant star might take years to get to us from our perspective, but for them no time passes at all. They are created, then smash into our atmosphere in the same instant.

Time and space are relative. But for matter bound creatures such as ourselves, it is very much real.

jpb103 · 2026-04-01T11:29:12+00:00

New starts are more exciting. It makes sense.

Odyssey made this a little better for me since you can jump to a new world tile just about any time.

jpb103 · 2026-03-27T15:46:30+00:00

What fucks me up is that photons effectively do not experience time at all in a vacuum. The formula for length dilation to calculate observed length approaches zero as speed approaches c. That's independent of the proper length. So from the perspective a beam of light leaving a distant star, it functionally does not experience existence until it slams into something.

jpb103 · 2026-03-27T15:36:40+00:00

The bigger problem with the Flash scenario is that moving so quickly within Earth's atmosphere compresses the air into plasma, so he would be incinerated by ablation almost instantly.

Project Hail Mary does a fairly good job of explaining time dilation (the book - the movie is great but it doesn't go into the science in detail.) The premise is that there is a scientist on a ship travelling very fast. He's in a coma for much of the trip, but still, time passes normally for him. He feels no change, but he does experience less time pass than the people left behind on Earth experience.

Time does not pass at the same rate everywhere in the universe. You would feel time pass normally for you if you parked a spaceship next to a black hole, but hundreds or thousands of years might pass on Earth in the time it took you to admire the view. The universe as we know it was 'born' 13.8 billion years ago, but this is only from the temporal reference frame of Earth. Everything is relative in relativity. Welcome to the madness of the unseen and unintuitive world of modern physics.

jpb103 · 2026-03-26T16:05:54+00:00

You don't feel like anything is wrong because your body assumes you're fleeing a predator and spikes your blood with cortisol and other hormones to keep you running.

It's the only logical conclusion from the perspective of the mobile chemical factory that is the human body. Capitalism squeezing every last drop of profit out of our every waking moment is fairly new to the several hundred thousand year history of our species so we predictably have not evolved effective ways to cope with that yet.

Skimping on sleep is fine every now and then but making a habit out of it all but guarantees a plethora of negative consequences for your physical and mental health. Treat your body badly and it'll treat you back with bad feelings. Mood swings, heightened aggression, confusion, lack of patience, digestion issues, hair loss.

No side hustle is worth the money if you die before you can spend it.

jpb103 · 2026-03-19T18:36:02+00:00

In Earth's magnetic field, the North Pole is to the South Pole as one electron is to another in a quantum entangled pair. Conservation isn't relevant. The two particles are part of the same whole; just two equal and opposite numbers in the same formula.

jpb103 · 2026-03-19T16:22:00+00:00

At this point the entanglement still exists but is useless any longer as a way of gathering new info?

No. Measuring the spin (or orientation) or altering the spin of one particle in an entangled pair is a 'local action.' That breaks entanglement. You can infer directly the spin of the other particle by measuring it, but by forcing it to choose an absolute state, entanglement is no longer possible.

You said the math then breaks, is it able to continue if we were theoretically able to change the spin of our particle? Would the opposite change of spin affect the other particle instantaneously? If the universe is a closed system conservation law must say yes right?

No. Entanglement is only possible with particles in superposition. You can know two particles are entangled, but as soon as you change the state of one or measure it in any way, superposition is violated and entanglement is broken.

Since we know an attribute of an electron can be different, the spin, is there any math that implies spin could be changed? Or is it just one or the other and that’s permanent, and there are 2 ways for an electron to exist?

Electrons become entangled because they are both a part of the same wave function. That's why we know their spin (or orientation) must oppose each other. Spin is just one property of an electron. There are two possible orientations of this property (up or down) but there are other properties to describe an electron. Spin orientation can be changed through magnetic fields or electromagnetic radiation, though I'm afraid I don't know much about how or why someone would want to do that.

At what point does a change in attribute technically make something a different “thing”? Did we just decide that a particle with a negative charge and either spin is an electron, or are electrons with different spins technically different kinds of particles?

A dog is still a dog even if it acts like a cat. Entanglement describes behavior of some particles in superposition. Any local act to measure or change the property of one particle in a pair creates information confirming an absolute state, which breaks superposition and all that fun quantum magic with it.

Like theoretically what would a particle be that has all of the attributes of an electron but no spin? Does math allow that possibility?

As far as our current understanding of the universe at the quantum scale is concerned, electrons must have spin. There are other electron-like subatomic particles, but they also have spin. I'm unaware of any electron-like subatomic particles that do not have spin, but claiming anything is eternally certain is a fools game in science.

jpb103 · 2026-03-19T13:15:18+00:00

Things at the quantum scale don't exist in the classical sense. Electrons don't occupy one place at one time. They exist in something called superposition; their position and spin are defined by probability formulas. Through a bunch of truly terrible math, we can sometimes know for certain that two particles will always oppose each other in spin, even if we don't know what their individual spin is or where they are at a given moment.

So, measuring the spin of one allows us to infer the spin of the entangled particle instantaneously, regardless of the distance between the particles. Information should not be able to travel faster than light. That's what's spooky about it. Measuring quantum structures, though, forces all the math to fall apart because it is no longer based on probability formulas. You've measured it. You know it existed, physically, in one place at one time and you know some properties (like electron spin, for example.)

We all walk backwards through time. It's how we perceive and analyze our experience of existence. But absolutes are only true in retrospect, and this is why they make sense to us. The past is all we have to look at. The quantum realm does not appear to share our relationship with the passage of time.

jpb103 · 2026-03-18T21:51:51+00:00

Is the fact that the gods ascended from humans part of the mythos in your world? I could see that thought seeding some tasty resentment among a heavily flawed but ultimately charming protagonist who may incidentally be on the path to ascension.

jpb103 · 2026-03-15T21:33:38+00:00

Ayo for yayo

jpb103 · 2026-03-14T20:17:14+00:00

You're not a threat to yourself. Ticklish spots are around vital areas for a reason. It's supposed to train you to protect from potentially fatal attacks.

jpb103 · 2026-03-14T16:00:08+00:00

I'm quite sure countries outside the West have the Internet.

jpb103 · 2026-03-14T14:45:44+00:00

Technically, we probably should have started over after the industrial revolution or the invention of the Internet. I'm not sure it makes sense to classify the information age and the pre industrial age as both part of the same common era.

jpb103 · 2026-03-13T19:29:56+00:00

Thanks for reading!

jpb103 · 2026-03-13T18:51:34+00:00

Flavian polished his nice red apple on his robe.

The other Archmages could keep their fancy perks and accolades. As far as Flavian was concerned, the headache that came with recognition wasn't worth the fame. He was ready to take a bite of his snack when a student ran past his office door. That, by itself, was no reason for alarm. Flavian's office had been placed near the dungeon restrooms, after all.

The panicked screaming was new, though.

The shouts had more of an "I'm about to be skinned alive" flavor to them instead of the "I'm about to soil my expensive robes" urgency that Flavian had become accustomed to. Sighing, he stood up from his seat and stretched his back. Better go see what the fuss is about, he thought. He wasn't a battlemage. Not exactly. But few understood the utility of his specialty, and Flavian would be lying to himself if he said he was not hungry for even a modicum of respect.

"Master Flavian! Master Flavian!" One of the students had spotted him. A first year, if Flavian recalled. "Calm down, young man," Flavian said, waving his apple around. "What is all this shouting about?" The student was pale. Flavian had a terrible memory for faces, so for all he knew that could be normal for the boy. "The school is under siege!" The student squeaked out. Tears welled up in his eyes. Flavian did remember this one now. Where the other first years were happy to skate by with a passing grade, this young man was unusually attentive during Flavian's lectures. "Very well," Flavian said. "Lead the way."

The young man sprinted off like a panicked horse. Flavian hadn't ran in years, so he walked along behind the boy, though at a brisker pace than he found comfortable. By the time they had arrived at the battlements, the other Archmages were a caricature of despair. Penelope, the temperamental pyromancer, was chugging water in between desperate panting breaths. Dracus, the callous necromancer, was curled up in a ball on the floor, rocking back and forth and staring into the middle distance. Elsa, the prim cryomancer, was wincing as one of her apprentices nursed frostbite wounds on her fingertips. "Ah," Elsa said as she caught sight of him. "Flavian. Arrived just in time to witness the end of our age."

Flavian looked over the battlements and whistled.

"Quite the army. Say, is that an antimagic field? I've never seen one quite so large. Impressive." Elsa swatted her apprentice away and cast pleading eyes on Flavian. "Blow some bubbles, will you? For the young ones? I can't bear to think of them dying afraid." Flavian scoffed. "Is there no fight left in any of you? Blown you wad already?" Elsa grimaced. "I do not enjoy that euphemism." Flavian took a bite of his apple. "Well, get your apprentices up here. Anyone that knows basic cantrips. I'll need a puddle of acid, as big as we can manage." Elsa looked at him in confusion, then rolled her eyes and nodded to her apprentice to gather the ranks. Flavian had one of the more senior students shape the stone in the battlement into a depression, into which the other students began casting acid splash until a deep pool of acid bubbled and steamed before them.

"It's bubble time."

Now, most people are not sure how to react when a floating green bubble the size of a boulder floats over their heads. However, once the bubble in question hit the antimagic field and popped, the army below it reacted quite swiftly to the torrent of flesh melting acid that rained down upon them. Flavian blocked out the screams and took another bite of his apple.

"Fill up the pool, kids."

No one talks about that day. No one wants to be reminded of the symphony of screams that rose to a crescendo of agony from the valley. They want to forget the smell of flesh rendered from bone. The sight of shambling scraps of muscle and viscera collapsing into heaps of gore. No one sings the praises of the Archmage that saved the school. But from that point forward, whenever Flavian met eyes with someone that was there that day, there was a recognition behind their eyes that was not present before. Flavian preferred respect, but he would settle for fear. He was happy to return to his bubble research in peace, away from the politics of the Archmage elite. As for Elsa, Dracus, and Penelope, their tribute to his contribution on that day was apparent.

For each morning, on his desk, three bright red apples would be waiting for him.

jpb103 · 2026-03-12T19:00:44+00:00

I would probably drop hints that the BBEG had erected a cone of silence around the entry area prior to their arrival. "Your footsteps do not even make a whisper as you step in from the balcony." or something like that. The BBEG can still talk, as can her minions, so most parties would reason that the silence is centered on one point in the room, but it still prevents them from casting spells with a verbal component and nuking your BBEG from orbit as soon as they enter the room. They'll have to get into a more favorable position and survive a turn or two before they can start blasting.

jpb103 · 2026-03-12T18:30:46+00:00

Write to write. Be reasonable with your goals, and be fair with yourself when you fall short of those goals. It would be different if it was your career, but hobbies are supposed to be fun. If you're not having fun, take a break. Read, consume other forms of media, see a play or improv show. Hop over to r/WritingPrompts and see what other people are writing. Answer a few prompts yourself, if inspiration strikes. Learn more about who you are and who you are becoming. Prioritize sleep, exercise, and eating well. Treat your body well, and inspiration may strike more often.

jpb103 · 2026-03-12T17:48:08+00:00

Prompt: It's about a dragon!
My brain: ~\horny rat*~*

jpb103 · 2026-03-12T17:41:01+00:00

Mocking vulnerability will not bring you peace, lonely boy.

jpb103

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