# Extrapolating into the past, we’ve missed three increasingly implausible opportunities to do this before.

On July 5 2010, Total Film fooled a lot of people into believing that that was the day Marty McFly and Doc Brown visited in Back To The Future Part II. In fact it was October 21 2015.

Yesterday, on June 27 2012, Simply Tap fooled a lot of people into believing that that was the day Marty McFly and Doc Brown visited in Back To The Future Part II. In fact it was still October 21 2015.

To punish people for falling for such nonsense, I propose more of these, increasing in frequency as we approach the actual date, so that when it actually happens, nobody believes it. But when?

The first of these errors was 1934 days premature. The second was 1211 days premature. (I’m not counting the copycat hoax on July 6 2010.) I think the reason these hoaxes are so seductive are that $\frac{1934}{1211}=1.60$, and that’s very close to the golden ratio, $\phi$. $\phi\approx1.62$, and has the lovely property that $\frac{1}{\phi}=\phi-1$ (or, $\phi^2=\phi+1$). It’s the only number of which that’s true, and it often appears in nature, art and architecture.

(Or, if you prefer, reports of $\phi$ appearing by accident are mostly coincidence and optimistic rounding, and so is this. I’ll leave that decision to you.)

Continuing the Golden Cascade of Back To The Future Hoaxes, we should have the next on September 23 2013. There will be two in 2014: on July 4 (when the alien mothercraft destroys the Hill Valley town hall) and December 28. The hoaxes will have to come thick and fast in 2015: April 18, June 27 (like this year), August 10, September 6 and 23, and October 4, 10, 14, 16, 18, 19 (twice), and then 25 separate hoaxes on the day before Future Day.

To be honest I suggest we use areweinthefutureyet.com for those ones.

## 3 thoughts on “Extrapolating into the past, we’ve missed three increasingly implausible opportunities to do this before.”

1. Okay, yes.

I don’t think it’s important but since people are focussing on it, yes, $\frac{1-\sqrt{5}}{2} \approx -0.62$ has similar fun properties to $\phi$ (but is harder to build into a shell or a building).

$\phi = \frac{1+\sqrt{5}}{2}$ but I thought this definition less useful than $\phi^2=\phi+1$ for understanding what the number is about and less useful than “1.62″ for understanding how big the number is, so I didn’t bother to look it up and include it.

2. I think Fenn’s point was specifically that (1-sqrt(5))/2 also satisfies x^2=x+1, so that’s not a definition of phi. (Indeed in general no quadratic equation will uniquely define a real number.)