How old is the light that woke you this morning?

The light that woke you flew eight minutes through space. Before that, its energy climbed from the Sun's core to the surface. Pick an answer. The verdict appears after you commit.

In the Sun's core, light does not fly straight. It is absorbed and sent out again in a random direction, step after step. Choose the step length and see how long the escape takes.

The model here uses equal steps. In the real Sun, the shortest steps are right at the centre, where light spends most of its time. That is why the full calculation comes out larger than the simulation shows.

Each step is short: under a millimetre on average. But the Sun's centre is nearly 700,000 kilometres from the surface, and after every step the direction changes at random. The 1992 calculation counts that many steps, a 1 followed by 25 zeroes.

Spread over the whole time, the average outward drift is 0.97 centimetres per second: about 35 metres an hour. The energy climbs toward the surface very slowly.

The rule is merciless: a hundred times more steps take you only ten times further. After 100 random steps you are some 10 metres from the start, after 10,000, 100 metres. That is why leaving the Sun takes so many steps.

On the strip below, each step forward multiplies the time by ten. Otherwise eight minutes could never fit beside 170,000 years.

In January, when the Earth is nearest the Sun, the light arrives some 17 seconds earlier than in July.

For a sense of the proportion: if the whole walk lasted a year, the sprint across space would last three milliseconds.

The same reaction also makes neutrinos: particles that pass through matter like air. Guess when one reaches you, then watch the race.

A neutrino born with the energy in today's light reaches you…

Neutrinos are born in the core's reactions and barely touch matter: they cross the Sun, the Earth and you without stopping. That is why they escape in two seconds while the light's energy wanders for 170,000 years.

There are so many that 65 billion pass through every square centimetre each second. You feel nothing: they go through you like air.

To grasp why light escapes so slowly, look at where it starts.

Every second, the Sun turns 600 million tons of hydrogen into helium, and 4 million tons never reappear: they have become energy, the light and warmth we receive.

The core is over 15 million degrees, and one cubic centimetre of it holds 162 grams. Newborn light is a gamma ray, full of energy. Each collision takes a little away until it leaves the surface as ordinary light.

The answer depends on the step length you assume. The curve shows the escape time for each step. The dots show what the textbooks assumed and what the layer-by-layer sum gave.

The old textbooks assumed a one-centimetre step everywhere and a constant density, and got thousands or tens of thousands of years. The 1992 calculation used the real step in every layer of the model: under a millimetre across almost half the radius. Hence the 170,000 years.

And the millions of years in some textbooks are mistakes copied from book to book, with nobody redoing the sum. The only real time of that order, some 30 million years, measures something else: how long the Sun would take to shrink if its core reactions went out.

Three limits, stated plainly.

• No single number is nailed down. Careful calculations give between 10,000 and 170,000 years. Of these, 170,000 comes from the most careful one.
• Nobody has tracked one ray's journey. Every number is calculated from models of the Sun's interior.
• There is no published number for the convection zone. All that is known is that the rising gas crosses it much faster than light would by random steps.

Eight statements about the ancient light. Some sound like stories and are true.