Created by: anonymous in daily-page on Feb 19, 2026, 7:11 PM
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It all began in 1905 when Albert Einstein asked whether light could exert a push (the “recoil” of a photon cannon) and what that implied for conservation of energy and momentum. The radical answer was: mass is stored energy. Here’s why.
🎼 Musical fact: Metastasis by Iannis Xenakis (1954) applies mathematical formulas and architectural structures to music—an art–science fusion.
These pillars lead to new expressions for momentum and energy.
Want to dive deeper into this fundamental invariant? See The spacetime invariant: from Minkowski to 𝐸² = 𝑝²𝑐² + 𝑚²𝑐⁴.
$$ \mathbf{p} = \gamma\,m\,\mathbf{v}, \quad \text{where} \quad \gamma = \frac{1}{\sqrt{1 - v^2/c^2}}. $$
$$ E^2 = p^2c^2 + m^2c^4. $$
If \(v = 0\) ⇒ \(p = 0\), then $$ E_0 = mc^2. $$ Voilà! The rest energy is bottled up in mass.
You can expand \(\gamma\) in a Taylor series for \(v \ll c\) and recover the classical kinetic energy term \(E_{\text{kin}} = \tfrac12 mv^2\).
| Phenomenon | “Lost” Mass → Energy |
|---|---|
| Fusion in the Sun | Powers the star’s luminosity ☀️ |
| Nuclear fission | Reactors and medical applications |
| Positron Emission Tomography (PET) | \(e^+e^-\) annihilate → two 511 keV photons |
| Cosmic rays | Partial mass conversion in ultra-energetic particles |
🚗 Car fact: The first Saab 92 (1949) used lightweight aerospace alloys. Less mass = less fuel = less energy consumed.
In a \(d>3\) spatial dimensionality, the form of the invariant changes, but as long as there’s a speed limit \(c\) and a similar quadratic invariant, an \(mc^2\)–like term appears. The constants shift, but rest energy still emerges from spacetime symmetry.
To understand why three dimensions are critical, see Why the universe works with three spatial dimensions.
Does “relativistic mass” exist? Today we speak of rest mass \(m\) and energy \(E\); “relativistic mass” \(\gamma m\) is just another way to write \(E/c^2\).
Why \(c^2\) and not another constant? \(c\) comes from the spacetime metric; squaring it ensures dimensional consistency.
Can I convert all the mass of my car into energy? In principle yes. In practice you’d need 100% efficient matter–antimatter annihilation. Your old Saab would unleash as much energy as thousands of nuclear bombs… better not try. 😉
\(E = mc^2\) isn’t just a slogan—it’s the manifestation of a deep symmetry between space and time. Each kilogram hides \(9\times10^{16}\,\text{J}\). Understanding it takes us from the heart of stars to medical imaging, and opens the door to modern physics.
Created by: anonymous in daily-page on Feb 19, 2026, 7:11 PM
Created by: anonymous in daily-page on Feb 19, 2026, 7:10 PM
Created by: anonymous in daily-page on Feb 19, 2026, 7:10 PM
Created by: anonymous in daily-page on May 17, 2025, 4:04 AM
A dog doesn’t understand death. Not the way we do. He understands silence. He understands that someone who was always there is now not.
He waits by doors that won’t open. He listens for footsteps that only memory still makes. He sniffs at the air for a scent that’s already fading.
But he never hears the words: “She’s gone.” “He passed.” “Never again.”
So in his heart, you’re still alive— just elsewhere. Delayed. Caught in some long errand beyond comprehension.
And isn’t that what we humans do too? We know the facts, we say the words— but inside, we keep waiting. For a call. A knock. A laugh in the next room. As if love had no burial rights. As if memory was a leash tied to a ghost.
Perhaps the dog suffers less because he doesn’t know it’s forever. But perhaps he suffers more, because he never stops hoping.
And maybe that’s what grief really is: the stubborn part of us that waits, ears perked, at a door that will never open again.
Created by: roberto.c.alfredo in daily-page on Dec 15, 2025, 12:25 AM
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Created by: kwrites in moments-of-joy on May 29, 2025, 3:21 AM
I am stuck in a narrow, crowded road. I can see the beginnings of a traffic jam. This part of the city was, after all known, for its nightmarish traffic situation. One could get stuck among honking cars and two-wheelers, for hours on end. I throw up a silent prayer to the gods, to spare me from a traffic jam. I just dont have the energy to navigate cursing drivers, and pedestrians who didnt have a lick of road sense. "Why couldnt people in this blasted country just follow the damn traffic rules?" "Why did I choose to come here for school?" I can feel my thoughts spiraling as I quietly resign myself to being stuck here for hours. A sudden cool breeze, breaks my reverie. This wasnt just any kind of breeze, it was the sort that brought the sweet promise of rain with it. I feel a new sort of awareness, as I sit up a little straighter. I take in my surroundings as if for the first time. A broad smile, splits my face, as I breathe in the wind carrying the scent of the earth. It reminds me of home, of the many many evenings I spent dancing and laughing in the rain with my siblings. I tilt my face up to the sky as if to greet a long lost friend. I relax, as the first drops, of rain hit me, causing delicious shivers to race up my body......
Created by: gerardfil in andorra on May 27, 2025, 2:29 AM
No, seriously. The Consell General (our parliament) is inside a building smaller than most banks.
It’s wedged right into a bend in the road in Andorra la Vella. It has a parking garage underneath.
In theory, you could run for office, park your car, and walk into the chamber in under three minutes.
I once tried to explain this to a coworker from Berlin. He laughed for five straight minutes.
And yet, it works.
Our political system is one of the oldest in Europe — we’ve had co-princes since the 1200s. One is the Bishop of Urgell (Catalonia), and the other is the President of France.
It’s weird. But stable. And very us.
Maybe you don’t need a palace if you’ve got snow, fiber internet, and municipal hot springs.
New Parliament of Andorra, headquarters of the General Council of Andorra since 2011.
Created by: gerardfil in andorra on May 27, 2025, 2:28 AM
When I was a child, I thought every country had ski lockers at the supermarket.
That’s Andorra. Small, yes. But we live vertically — and very much on our own terms.
I was once asked by an American tourist if we use euros “like France does.” I told him we do. Then I told him we’re not France. Or Spain.
We’re both. And neither.
Catalan is our official language. We learn Spanish and French from childhood. Some of us speak Portuguese at home. Our newsstands carry newspapers from Madrid, Toulouse, and sometimes Lisbon.
And yet, we are something else entirely.
When I travel, people ask if I’m Spanish or French. I always hesitate. “I’m Andorran,” I say. Most smile politely. A few ask if that’s in Africa.
It’s okay. We’re used to being overlooked. But the snow knows who we are.
We belong to mountains. And to each other.

Created by: roberto.c.alfredo in united-states on May 11, 2025, 12:54 PM