String Theory Doesn't Describe Our Universe, But It Isn't Dead
String theorists themselves admit it: string theory does not describe the universe we live in, and on that point there is no ambiguity. But it has been demoted from theory of everything to a tool in the theoretical physicist's toolbox — ADS-CFT means the 10-dimensional string theory and a 4-dimensional quantum field theory carry the same information, and a hard problem on one side is easy on the other.
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The fuel math for chemical rockets doesn't work
To reach the nearest star in 20 years with a chemical rocket, the fuel mass required is 10 to the 6000th times the mass of the observable universe. This is the tyranny of the rocket equation: the fuel rides on the ship, so going a little faster means carrying exponentially more fuel, and the extra fuel itself needs fuel to push it. Starshot's solution is to leave the "engine" on Earth and push a solar sail with a ground-based laser, which is why it can do what a chemical rocket never could.
— Bob NovellaThe laser array is itself a strategic weapon
Bob raises an underrated implication: a 200-gigawatt ground-based laser array is not just a launcher. Any vehicle entering its airspace could be vaporised instantly; put a mirror in orbit and the beam can be aimed at almost any point on Earth. So once this thing is built, other countries will inevitably demand it be dismantled — and whoever spent half a trillion dollars won't use it to push one probe. It could launch solar sails across the entire solar system. Geopolitics may be the project's real cause of death.
— Bob NovellaString theory was demoted from theory of everything to toolbox
Brian is a professional string theorist, and his judgment is blunt: string theory does not describe the universe we live in, and on that point it is "unambiguous." But it isn't dead — it has transformed, from "this might be the theory of the real universe" into one tool in the theoretical physicist's toolbox. His example is the ADS-CFT conjecture from the late 1990s: 10-dimensional string theory was found to carry exactly the same information as a 4-dimensional quantum field theory, so a hard problem on one side is easy on the other, and string theory and geometry can be used to study hard problems in quantum field theory. None of those universes corresponds to the one we live in, but quantum field theory is a theory we know describes the universe, so the tool is worth playing with.
— Brian WechtThe backlash is rooted in funding, not just science
String theory's hype was so extreme that the backlash it drew was extreme too. Brian acknowledges that many scientists think string theory doesn't deserve that kind of attention, but he points to a direct cause: a lot of funding was diverted to create string theory positions, money that might otherwise have gone to other departments. Academic funding is scarce and hiring is highly herd-driven, so when the hot trend isn't your field, resentment comes easily. He says this doesn't mean those objections don't hold up, but the source of the emotion is worth stating clearly. He also stresses that the string theorists he has worked with are extremely careful and work very hard to do good science.
— Brian WechtRational choice theory traced the exact same arc
Cara says this was an aha moment for her: rational choice theory in political science followed the same path. In the 2000s and 2010s, political scientists assumed rational actors and derived behaviour from there — why rational voters vote the way they do, why two rational leaders would slaughter each other — but anyone who watches the news knows nobody is rational. It wasn't a correct model of the universe, but at the time people were excited that it was "useful enough," and now that the heat has faded and the field has turned to large-scale data processing, the framework is still a useful tool: first explain why rational people do things that look irrational, then layer on self-esteem, madness and risk aversion. Brian thinks this arc recurs throughout science, and that it's a feature, not a bug.
— CaraInjecting an obesity drug intravenously into 6- to 10-year-old boys
Cara digs up a disturbing study: in 1990s New York, 100 to 150 Black and Hispanic boys aged 6 to 10 who had criminal older brothers were given intravenous fenfluramine, on the reasoning that the drug acts on serotonin, that serotonin levels might predict criminal behaviour, and so serotonin could be raised with the drug and the differences compared. The New York State Psychiatric Institute conducted a year-long investigation and concluded there was no misconduct. The study was published in 1995; the drug was withdrawn from the market in 1997. Cara's question: ethical review mechanisms were already fully in place in the 1990s, so how did this get past an IRB?
— CaraPlatinum means hydrogen can never scale
PEM fuel cells need platinum as a key element. By the episode's account, converting 10% of the cars on the road to hydrogen fuel cells would use up the entire world's annual platinum production — and platinum has plenty of other uses, electronics among them. A heavy truck needs 10 times the platinum of a passenger car, and a passenger car needs six or seven times that of an internal combustion engine's catalytic converter; battery EVs use no platinum, and gasoline cars use it because of the catalytic converter. The conclusion: unless we mine asteroids, hydrogen fuel cells cannot reach appreciable scale.
Without solving radiation, Mars is just talk
Every Mars narrative dodges the real roadblock: radiation. With no atmosphere as a shield, a person cannot spend a year or two in space. The episode cites a NASA person they interviewed: you just have to get there as fast as possible and then deal with the medical problems that follow — that's the whole plan. Musk has never addressed radiation head-on. So "build a slightly better rocket" doesn't solve this at all; talking about terraforming Mars before solving how to get there puts the cart before the horse.
A tachyon isn't a faster-than-light particle, it's a signal the theory is broken
In science fiction a tachyon is a faster-than-light particle, but Brian says physicists don't think of it that way at all: a tachyon means the theory's vacuum state has a fundamental problem. It corresponds to a negative mass squared, and once it exists it can keep producing tachyons that drop to lower energy states — and physics always tends toward lower energy states, so this is bad news. He also notes that theories with more than one time dimension naturally produce tachyons in the particle spectrum. As for Bob's question about "two time dimensions," Brian's response is that the 2T program is not viewed favourably by most physicists.
— BrianIn their own words · checked verbatim
If you are carrying your fuel on board your rocket, if you just want to go a little bit faster, it takes exponentially more fuel to move the fuel that you just added.
Bob Novella21:33
there are a very, very large number of such possible shapes for the extra dimensions, and to this day, we don't know any mechanism for deciding which of those a priori should be correct.
Brian Wecht36:13
And I don't think that's, you know, obviously it would be amazing if we found the unified theory of everything. That's the best thing that could happen. That is not happening with string theory, and I think it's unambiguous that that's the case.
Brian45:39
I learned to speak a language. It's like you learn to speak another language. I just got used to talking to that language.
Brian50:44
Later, research showed that if you took this drug for fewer than three months, you had something like a 3% chance of developing, which is still pretty high, right? Which is why they pulled it from the market. But if you took this drug for upwards of two years, like 40% of people had irregular or abnormal EKGs.
Cara56:53
they don't fail because the technology fails. They fail because there's no killer app or use case or they just couldn't predict how people would react to the technology.
So the bottom line, when you hear about tachyons in science fiction, it's bullshit. That is not the way any physicist thinks about tachyons.
Brian2:05:58
And as you go to arbitrarily high dimensions, the volume of that sphere goes to zero.
Brian2:08:01
Figures
| Starshot spent so far | $4.5 million (originally planned at $100 million) | 25:45 |
| Fuel mass needed for a chemical rocket to reach the nearest star | 10 to the 6000th times the mass of the observable universe | 21:33 |
| Total monthly Fen-Phen prescriptions (1996) | 18 million | 55:48 |
| Probability of heart problems after less than three months of use | about 3% | 56:53 |
| Share with abnormal electrocardiograms after two or more years of use | about 40% | 56:53 |
| Round-trip efficiency of hydrogen fuel cell vehicles | about 40% | 1:15:27 |
| Battery electric vehicle efficiency | 75% to 90% | 1:15:27 |
| Global annual hydrogen production | 100 million tonnes | 1:21:36 |
| Share of green hydrogen | less than 1% | 1:22:37 |
Glossary
- ADS-CFT
- The conjecture that 10-dimensional string theory and 4-dimensional quantum field theory carry the same information, so a hard problem on one side can be solved on the other.
- Calabi-Yau
- The shapes into which string theory's extra dimensions might curl up — astronomically numerous and impossible to pick out a priori.
- metasurface
- A nanostructured surface; Starshot's solar sail uses one to be almost perfectly reflective so it can withstand a 100-gigawatt laser.
- PEM
- A type of hydrogen fuel cell whose key element is platinum, so its scale is hard-constrained by platinum production.
- tachyon
- In science fiction, a faster-than-light particle; physicists actually use the term for a signal that the theory's vacuum state has a problem.
How to listen
Listeners interested in the current state of string theory, the engineering constraints on deep-space propulsion, and cases like Fen-Phen and hydrogen cars where the technology didn't lose — something else did.
The chit-chat around 16:28 about transporting the Endeavour space shuttle and transplanting trees, which has nothing to do with any thread of this episode.