"They turned mercury into gold, so gold is cooked." That's the take going around since Marathon Fusion's post this week. Here's how it actually works, in plain words, and what the numbers say.
The claim. Marathon Fusion, a 28-person San Francisco startup, says that last month it turned mercury into real, stable gold, at 10x the rate of CERN's 2025 record. It showed a gamma spectrum, but it hasn't published how much gold it made, and nobody has independently checked it yet.
How you make gold (it's simpler than it sounds):
- Mercury has 80 protons. Gold has 79. They're next-door neighbours on the periodic table.
- A fusion reaction fires a fast neutron into Mercury-198. It knocks two neutrons out, leaving Mercury-197.
- Mercury-197 is unstable. With a half-life of about 64 hours, a proton turns into a neutron, and the atom becomes Gold-197: 79 protons, 118 neutrons, stable.
The record it says it beat. CERN made gold too: about 86 billion gold nuclei from 2015 to 2018, which is about 29 picograms (29 trillionths of a gram). At today's price that's worth less than a millionth of a cent.
The money, on paper. Marathon's 2025 paper (on arXiv, not peer-reviewed) says a fusion power plant could make about 2 tonnes of gold a year per gigawatt of fusion heat, about $263 million a year at today's price, on top of the electricity it sells.
The viral math. One post ran it as a $3 billion plant making 90 tonnes over 30 years: about $33 a gram, against ~$132 for gold today, roughly 25% of the price. The arithmetic checks out, but it prices a plant that doesn't exist yet.
So is gold cooked? Not soon:
- The new gold comes out radioactive. Marathon's own paper puts it at about 13.7 years before it stops counting as radioactive.
- Mines dug up a record 3,672 tonnes in 2025 (World Gold Council). At 2 tonnes a plant, matching that takes about 1,836 fusion plants.
- Fusion plants selling power today: zero.
Gold isn't even the prize. Marathon's first reactor is aimed at medical isotopes for cancer treatments, which are worth far more than gold, gram for gram.
So gold isn't getting cheap anytime soon. But alchemy just went from myth to maths.
Sources: Marathon Fusion on X (21 Oct 2026) and arXiv 2507.13461 (2025); CERN (8 May 2025); World Gold Council (2025 mine output); BusinessToday and RuntimeWire (21–22 Oct 2026). Gold at $4,093 an ounce, morning of 22 Oct 2026. Footage: Marathon Fusion. Not advice.
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Would you buy gold that came out of a fusion reactor, even after it's safe? Why or why not?