For it to tell something of value
The Standard Model assumed in the eighties
That neutrinos had no mass. Hence, as
Einstein showed, they would
Travel at no less than
The speed of light.

Yet experience showed only a third
Of the expected population of neutrinos
Coming from the sun. An idea from the sixties
Was that they changed form on their way to Earth.
In other words, we were carefully watching
Only a chosen fraction, ignoring alter egos.

And there’s the riddle :
On reaching the speed of light, time stops,
So how could neutrinos experience change,
Which is another word for the passing of time ?
For if they did, they would have to be
Slower and hence
Carry mass.

Two years before the next century,
Someone showed changes indeed
Occurring on the way from the Sun :
The neutrino slowed in its track
For us to wonder still
How much it exactly weights.