with Ethan Siegel • May 3, 2025
Greetings readers,
Have you ever wondered about the deepest, most fundamental questions of all? Questions like, what are the ultimate limits of the Universe? At some point, there's a maximum energy scale, minimum distance scale, and shortest timescale at which the laws of physics make sense; beyond that, a fundamental breakdown occurs. This scale, the Planck scale, has never been achieved but remains a frontier that we cannot even make sensible predictions about. There's also the question of why our Universe is made of matter and not antimatter. We know that it is, but we don't know how it got to be this way. A new collider, usually talked about in the search for beyond-the-Standard Model particles, may be the tool we need to help answer that question.
There are also the questions surrounding dark energy: What is it, what is its nature, and — for this week's Ask Ethan — can we even be sure that it exists, or are we just measuring the "leftover" expansion from the hot Big Bang? On a more practical note, what are the hardest, strongest, toughest materials we can make in nature? As it turns out, six known materials exceed even the vaunted diamond in terms of hardness, and a few of them have nanotechnology applications that could revolutionize how we live.
Of course, all of that is predicated on the assumption that we don't destroy ourselves (and our society) by succumbing to fascism, a major concern here in 2025, particularly in my own country of the USA. Scientists have stood together against it before, and we can learn the lessons needed from them on how to stand against it today. It's your world and your Universe, too, and you deserve to be a part of it just as much as anyone else. I'll keep doing all I can to bring it to you, to the best of our understanding.
All the best,
Ethan
THE LIMIT
Why does physics break down at the Planck scale?
Our laws of physics apply at all times and in all places throughout the Universe, from the Big Bang to black holes and everywhere in between. However, these laws only apply within a certain regime. They break down at high enough energies, on short enough timescales, or at small enough distances. But why do those laws break down? Let's explore.
NEW TECH
A new collider can teach us about the origin of matter
The laws of nature are almost perfectly symmetric between matter and antimatter, yet our Universe is made ~100% of matter only. Could a new collider finally give us the answer we need and show us how the Higgs symmetry breaks? Our Universe depends on the answer.
ASK ETHAN
Ask Ethan: Is dark energy just leftover momentum from the Big Bang?
Since the late 1990s, we've recognized that the Universe isn't just expanding but that the expansion is accelerating, leading us to conclude that some mysterious form of "dark energy" dominates the energy budget of the cosmos. But what, exactly, is dark energy? Could what we've been interpreting as evidence for dark energy be the leftover momentum from the initial "Big Bang" event that started it all? Here's what physics has to say.
If you have a burning question about the Universe, send me an email.
STANDING UP FOR SCIENCE
This 1938 pro-science manifesto defended democracy against fascism
In the USA in 2025, hundreds of thousands of federal civil servants have been terminated and had their legal protections stripped from them, including in the science and health sectors, threatening the scientific future of our entire world. Many similarities parallel the state of affairs in 1930s-era Nazi Germany, but in 1938, over 1000 American scientists struck back with a democracy-defending manifesto. Its lessons still matter, perhaps now more than ever.
THE HARD TRUTH
The 6 strongest materials on Earth are harder than diamonds
As an atom, carbon's bond structure has some special properties that allow it to form extremely hard materials when bound together in a lattice. Although diamonds, crystalline structures known since antiquity, are extraordinarily hard, they're not the hardest possible configuration. They only rank at #7 on the current list. Can you guess which material is #1?
Ethan Siegel, Ph.D., is an award-winning theoretical astrophysicist who's been writing Starts With a Bang since 2008. You can follow him on Twitter @StartsWithABang.
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Singularities in General Relativity: Einstein’s general relativity predicts that at certain points—such as inside black holes or at the very beginning of the universe (the Big Bang)—the curvature of spacetime becomes infinite. These “singularities” represent conditions where the equations deliver infinities or undefined values, meaning the theory itself can’t predict what actually happens. In these extreme situations, our classical understanding of gravity just falls apart, pointing directly to the necessity of a theory of quantum gravity.
The Quantum Gravity Regime: At the tiniest scales imaginable (on the order of the Planck length, about 10^-35 meters) and at correspondingly high energies, the familiar separation between gravity and quantum mechanics dissolves. Our quantum field theories work brilliantly for the three fundamental forces (electromagnetism, and the strong and weak nuclear forces), yet when we try to incorporate gravity at these scales, the mathematical tools we use start to yield nonsensical results. It’s here that our current theories “break down,” suggesting that a novel framework—perhaps string theory, loop quantum gravity, or an entirely new paradigm—is needed to reconcile the two.
Limitations Within the Standard Model: Even in regimes where our theories are powerful, there remain phenomena that escape our grasp. Dark matter and dark energy, which seem to dominate the universe’s mass-energy budget, aren’t accommodated by the Standard Model of particle physics. Although this isn’t a “breakdown” in the mathematical sense, it does indicate that there are gaps in our understanding of the fundamental constituents and forces of nature. These discrepancies encourage us to look beyond established theories for answers.
Conceptual Hurdles and the Measurement Problem: Quantum mechanics itself, though incredibly successful in its predictive power, carries with it conceptual puzzles like the measurement problem and the nature of wavefunction collapse. These aren’t breakdowns in the equations per se but rather areas where our interpretation of the theory is unsettled. They remind us that even our most successful models might not fully capture the underlying reality without a more complete theoretical framework.
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