Category: theoretical physics

This Is Why Dark Energy Is The Biggest Unsolved Problem In The Universe

“The true fact of the matter is that, observationally, dark energy is behaving as though it’s a form of energy inherent to the fabric of space itself. WFIRST, NASA’s flagship astrophysics mission of the 2020s (after James Webb), should allow us to reduce the measured constraints on w down to the 1-or-2% level. If it still looks indistinguishable from a cosmological constant (with w = -1) then, we’ll have no choice but to reckon with the quantum vacuum itself.

Why does empty space have the properties that it does? Why is the zero-point energy of the fabric of the Universe a positive, non-zero value? And why does dark energy have the behavior we observe it to have, rather than any other?

There are an infinite number of models we can cook up to describe what we see, but the simplest model — of a non-zero cosmological constant — requires no additions or modifications to match the data. Until we make progress on understanding the quantum vacuum itself, dark energy will remain the biggest unsolved puzzle in all of modern theoretical physics.”

Since 1998, astronomers have known that the Universe isn’t just expanding, but that the more distant a galaxy gets from us, the faster it appears to recede away from us. The reason for this isn’t because of motion, but rather because there’s more than just matter and radiation in the Universe; there’s also a form of energy that appears to be inherent to space itself: dark energy.

While it may be theoretically fashionable to concoct new fields, modifications to gravity, or other forms of new physics, it’s unnecessary. What we really need to do is understand the quantum vacuum, and we don’t. Here’s the story so far.

Ask Ethan: Why Do Gravitational Waves Travel Exactly At The Speed Of Light?

We know that the speed of electromagnetic radiation can be derived from Maxwell’s equation[s] in a vacuum. What equations (similar to Maxwell’s – perhaps?) offer a mathematical proof that Gravity Waves must travel [at the] speed of light?

If you were to somehow make the Sun disappear, you would still see its emitted light for 8 minutes and 20 seconds: the amount of time it takes light to travel from the Sun to the Earth across 150,000,000 km of space. But what about gravitation? Would the Earth continue to orbit where the Sun was for that same 8 minutes and 20 seconds, or would it fly off in a straight line immediately?

There are two ways to look at this puzzle: theoretically and experimentally/observationally. From a theoretical point of view, this represents one of the most profound differences from Newton’s gravitation to Einstein’s, and demonstrates what a revolutionary leap General Relativity was. Observationally, we only had indirect measurements until 2017, where we determined the speed of gravity and the speed of light were equal to 15 significant digits!

Gravitational waves do travel at the speed of light, which equals the speed of gravity to a better precision than ever. Here’s how we know.

Ask Ethan: What Does It Mean That Quantum Gravity Has No Symmetry?

“What does it mean that quantum gravity doesn’t have symmetry?“

Last month, a scientific paper was published in a prestigious journal, entitled: Constraints on Symmetries from Holography. It states that three long-standing conjectures about quantum gravity, including one stating that quantum gravity does not allow global symmetries of any type, have just been proven if the holographic principle, and by extension the AdS/CFT correspondence that’s fundamental to string theory, is correct.

This is really, really big news. If there are no global symmetries, then there are no absolute conservation laws. Energy is not conserved, momentum and charge are not conserved, and additionally all sorts of things that are not forbidden but not observed, like magnetic monopoles, must necessarily exist. 

Paradoxically, if string theory is right, our expectations about hidden symmetries revealing themselves at a more fundamental level are not only wrong, but that nature has no global symmetries at all.

Come get the full, fascinating story, with all the deep implications that come along with it, today.

Fine-Tuning Really Is A Problem In Physics

“In science, our goal is to describe everything we observe or measure in the Universe through natural, physical explanations alone. When we see what appears to be a cosmic coincidence, we owe it to ourselves to examine every possible physical cause of that coincidence, as one of them might lead to the next great breakthrough. That doesn’t mean you should credit (or blame) a particular theory or idea without further evidence, but the possible solutions we can theorize do tell us where it might be smart to look.

As always, we have strict requirements for any such theory to be accepted, which includes reproducing all the successes of the previous leading theory, explaining these new puzzles, and also making new predictions about observable, measurable quantities that we can test. Until a new idea succeeds on all three fronts, it’s only speculation. But that speculation is still incredibly valuable. If we don’t engage in it, we’ve already given up on discovering new fundamental truths about our reality.”

Imagine you observe something that seems like a coincidence: you have two quantities that should be unrelated, but they appear to balance perfectly. Why would this be the case? It’s possible it’s just a coincidence, and it’s also possible that someone deliberately caused this apparent balance. But it’s also possible that there’s a physical mechanism that compelled these two quantities to balance so well, and that there’s an underlying cause to the effects we observe. That last possibility is the whole point of doing theoretical physics, and represents our most powerful approach to solving these fine-tuning problems that would otherwise go unaddressed.

When we see coincidences like these, we call them fine-tuning problems in physics. Here’s why we take these puzzles so seriously.

Fine-Tuning Really Is A Problem In Physics

“In science, our goal is to describe everything we observe or measure in the Universe through natural, physical explanations alone. When we see what appears to be a cosmic coincidence, we owe it to ourselves to examine every possible physical cause of that coincidence, as one of them might lead to the next great breakthrough. That doesn’t mean you should credit (or blame) a particular theory or idea without further evidence, but the possible solutions we can theorize do tell us where it might be smart to look.

As always, we have strict requirements for any such theory to be accepted, which includes reproducing all the successes of the previous leading theory, explaining these new puzzles, and also making new predictions about observable, measurable quantities that we can test. Until a new idea succeeds on all three fronts, it’s only speculation. But that speculation is still incredibly valuable. If we don’t engage in it, we’ve already given up on discovering new fundamental truths about our reality.”

Imagine you observe something that seems like a coincidence: you have two quantities that should be unrelated, but they appear to balance perfectly. Why would this be the case? It’s possible it’s just a coincidence, and it’s also possible that someone deliberately caused this apparent balance. But it’s also possible that there’s a physical mechanism that compelled these two quantities to balance so well, and that there’s an underlying cause to the effects we observe. That last possibility is the whole point of doing theoretical physics, and represents our most powerful approach to solving these fine-tuning problems that would otherwise go unaddressed.

When we see coincidences like these, we call them fine-tuning problems in physics. Here’s why we take these puzzles so seriously.

Why Supersymmetry May Be The Greatest Failed Prediction In Particle Physics History

“There are two very different camps of scientists when it comes to the issue of SUSY. On the one hand, we have a large group of people, both theorists and experimentalists, are are following the evidence closely, seeking alternative explanations for these various puzzles, and responsibly constraining the viable scenarios to progressively tighter bounds. To rule out a theory that has dominated a sub-field of physics for nearly two generations would be a tremendous advancement for science.

But on the other hand, there’s a large and powerful group of (mostly) theorists who will go to their graves as true believers in not only SUSY, but electroweak-scale SUSY, regardless of what the evidence says. Yet with every new proton the LHC collides, we see the same answer again and again: no SUSY. No matter how often we fool ourselves, nor how many scientists get fooled, nature is the ultimate arbiter of reality. The experiments do not lie. As of today, there is no experimental evidence in favor of SUSY.”

Supersymmetry, or SUSY for short, is an incredibly compelling idea. You should know why, and I’m happy to tell you in this article. But it also doesn’t appear to describe our Universe, and not only should you know that, but so should many of the theorists still clamoring for more research into it.

Supersymmetry is, for my money, the greatest failed prediction in the history of particle physics. The experimental data says it’s time to move on.

There’s Almost No Antimatter In The Universe, And No One Knows Why

“So how did we get here today, with a Universe made of a lot of matter and practically no antimatter, if the laws of nature are completely symmetric between matter and antimatter? Well, there are two options: either the Universe was born with more matter than antimatter, or something happened early on, when the Universe was very hot and dense, to create a matter/antimatter asymmetry where there was none initially.

That first idea is scientifically untestable without recreating the entire Universe, but the second one is quite compelling. If our Universe somehow created a matter/antimatter asymmetry where there initially wasn’t one, then the rules that were at play back then should remain unchanged today. If we’re clever enough, we can devise experimental tests to uncover the origin of the matter in our Universe.”

The laws of physics, to the best of our knowledge, are highly symmetric between matter and antimatter. There are no known interactions that have ever resulted in the net creation or destruction of matter particles versus antimatter particles; you cannot make one without making the other. Yet, quite clearly, our Universe is made of matter and not antimatter! Where did all our matter come from? Or, conversely, where did all our antimatter go?

This is one of the greatest unsolved mysteries in physics, but one we’ve got a tremendous shot at figuring out. Here’s the story behind it.

This Simple Thought Experiment Shows Why We Need Quantum Gravity

“The description that General Relativity puts forth — that of matter telling space how to curve, and curved space telling matter how to move — needs to be augmented to include an uncertain position that has a probability distribution to it. Whether gravity is quantized or not is still an unknown, and has everything to do with the outcome of such a hypothetical experiment. How an uncertain position translates into a gravitational field, exactly, remains an unsolved problem on the road to a full quantum theory of gravity. The principles that underlie quantum mechanics must be universal, but how those principles apply to gravity, and in particular to a particle passing through a double slit, is a great unknown of our time.”

Perhaps the greatest holy grail in theoretical physics is the quest for a quantum theory of gravity. For all the gravitational phenomena we’ve ever measured, observed, or subjected to a test, General Relativity has come through with predictions that match what we’ve seen exactly. For all the other physical phenomena in the Universe, the rules of quantum field theory and the Standard Model of particle physics match up perfectly. But what would happen if we tried to apply General Relativity to an inherently quantum phenomenon? In particular, what happens if we fire a single particle, like an electron, through a double slit? What happens to that particle’s gravitational field?

Believe it or not, measuring that (or something analogous to it) would tell us whether gravity is a fundamentally quantum force or not! Come learn why this is arguably the most important, first stop on the road to quantum gravity.

Is Theoretical Physics Wasting Our Best Living Minds On Nonsense?

“The book is a wild, deep, thought-provoking read that would make any reasonable person in the field who’s still capable of introspection doubt themselves. No one likes confronting the possibility of having wasted their lives chasing a phantasm of an idea, but that’s what being a theorist is all about. You see a few pieces of an incomplete puzzle and guess what the full picture truly is; most times, you’re wrong. Perhaps, in these cases, all our guesses have been wrong. In my favorite exchange, she interviews Steven Weinberg, who draws on his vast experience in physics to explain why naturalness arguments are good guides for theoretical physicists. But he only manages to convince us that they were good ideas for the classes of problems they previously succeeded at solving. There’s no guarantee they’ll be good guideposts for the current problems; in fact, they demonstrably have not been.”

There are a slew of brilliant ideas in physics that have now become the dominant, accepted theory of what describes reality: the Standard Model. the Big Bang, General Relativity, etc. These theories are, in many ways, beautiful. They have an elegant mathematical structure, they have strong predictive power, and most importantly, they match reality. It’s that last criteria that separates them from other beautiful theories that have fallen by the wayside, such as the beautiful (but incorrect) Sakata Model. theory of Technicolor, Steady-State Model, and more. Without the experimental evidence to support them, however, are we wrongly investing our energy, intellect, and resources into beautiful, promising dead-ends? In particular, are supersymmetry, grand unification, string theory, and the multiverse exactly those dead-ends, and is following them the reason (or a symptom of) why progress has been so scarce in recent decades?

Come learn about naturalness, this possibility, and why you should buy Sabine Hossenfelder’s new book, Lost In Math, to learn more!

The Multiverse Is Inevitable, And We’re Living In It

“It’s important to recognize that the Multiverse is not a scientific theory on its own. It makes no predictions for any observable phenomena that we can access from within our own pocket of existence. Rather, the Multiverse is a theoretical prediction that comes out of the laws of physics as they’re best understood today. It’s perhaps even an inevitable consequence of those laws: if you have an inflationary Universe governed by quantum physics, this is something you’re pretty much destined to wind up with.”

It sounds like an unprovable fantasy: the idea that our Universe is just one of countless others, dotted across an eternally expanding empty space separating them. That’s generally how we picture the Multiverse, with each Universe having its own hot Big Bang distinct from every other Universe. But this isn’t simply pure speculation, but the result of a few simple facts combined: our Universe is quantum in nature, inflation gave rise to the Big Bang, and quantum fields spread out in value over time. Put those pieces together, and you’ll find that no matter how small of a region inflation starts off in, so long as you demand you get enough inflation to stretch our Universe flat, it will continue on for an eternity into the future. In some locations, inevitably, it will come to an end, giving rise to a hot Big Bang, but in many others, it will continue forever, separating the regions where inflation ends from one another for all time.

The Multiverse itself may not give rise to any observable, testable predictions, but arises as a direct consequences of other physical theories that have already been validated. Find out today why it’s inevitable.