NO VACANCY there's always room
It's late.
You're tired.
and the only hotel for miles is full
Hilbert's Hotel
infinitely many rooms, all of them taken
Everybody: one room up.
1 → 2    2 → 3    3 → 4    …
There is no last room.
Room 1 is free.
One bus.
Infinitely many passengers.
Double your room number.
1 → 2    2 → 4    3 → 6    …
Every odd room: empty.
the whole bus checks in
∞ × ∞
infinitely many buses, each one infinitely full
Prime numbers.
guests → 2, 4, 8, 16 …
bus one → 3, 9, 27 …
bus two → 5, 25, 125 …
Room 6: nobody.
full hotel + ∞ × ∞ guests = spare rooms
One last bus.
every endless name you can spell with A and B
Flip the diagonal.
a guest who is not on the list
A bigger infinity.
1891
Georg Cantor

The guest list

Room 1ABBABA…
Room 2BBAABB…
Room 3AAABAB…
Room 4BABBAA…
Room 5ABABBB…
Room 6BAAABA…
New guestBABAAB…
NO ROOM
A thought experiment
David Hilbert, 1920s
★★★★★

“They woke me up three times to change rooms.”

A guest, room 2
Always full.
Always room.
unless your name is very, very long

Three minutes about infinity. Captions on.

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Room ∞. It does not exist; every room has an ordinary number.

Always room.

The hotel holds as many guests as there are counting numbers. The endless names outnumber them, and that is Cantor's theorem.

Read about the hotel The diagonal argument
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