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Five Seconds: The Feud Where Both Sides Agree on the Number
Yann LeCun derived about five seconds of Roadster thrust from first principles on October 17, 2026. Tesla's engineering chief told Top Gear one fill is about five to six seconds. The argument is over what five seconds means, not over the arithmetic.
The full exchange, post by post, is on the LeCun page. This page is about the number at the center of it, because the number is the rare thing in a public feud that both sides signed.
LeCun’s derivation
On October 17, 2026, Yann LeCun — Meta’s chief AI scientist and, on this subject, a physicist doing arithmetic in public:
“Okay, so it’s just compressed air. Easier. But crappy specific impulse (less than 50 seconds). Even assuming that 50% of the mass of the car is the compressed air, that gives you less than 10s of useful thrust (with thrust-to-weight ratio larger than 1.0). Probably more like 5 seconds, actually. So the car must be an empty, lightweight shell containing big compressed air tanks. Cute demo, but totally impractical. But of course, you knew that.”
Label: Primary, his post. Note the structure: a derivation, a number, then a verdict. The number and the verdict are separable, and the week since has separated them.
Tesla’s number
Lars Moravy, the engineer who ran the demo, told Top Gear that one fill is about five to six seconds of thrust. That is LeCun’s number, from the other side of the argument, in an interview arranged by Tesla.
The explainers land in the same band: about 2 seconds at full blast, 7–9 seconds merely holding a hover — indexed on the video index. The system’s hardware is on the package page.
What the feud was actually over
On October 18, Mr. Musk answered the derivation with a Monty Python taunt — “I fart in your general direction” — which drew 3.2 million views by Monday. LeCun answered the answer:
“Come to think of it, that’s exactly how your car is powered: a 700 bar, 600 liter, 3 second, noisy fart.”
And roughly 21 minutes after Mr. Musk’s post, a user asked Grok to check the math. Grok — Mr. Musk’s own AI — answered that “LeCun’s physics holds up”: about five seconds of hover or hop, and the impractical verdict accurate.
Label: All primary posts, as logged on the LeCun page.
The desk’s reading
Strip the theater and the record is this: a critic computed five seconds, the builder’s engineer confirmed five to six, and the builder’s AI confirmed the critic. The remaining disagreement is a values question wearing a physics costume — whether five seconds of flight is a party trick or the first five seconds of something. Roger Connor, the Smithsonian’s vertical-flight curator, put the expert version plainly: “We’re not talking about sustained flight.” Mr. Musk, in a sentence the explainers keep quoting back, said the same thing from the other direction: “We are literally talking about hopping.”
A hop is what was demonstrated. What a hop is worth is the argument. The arithmetic is settled, and it was settled by both sides.