Time Scale

The Planck time, where our theories of gravity and of quantum mechanics meet and stop giving answers about what a moment is. No clock or probe could time anything shorter. That does not make time tick in steps of this size. Whether it flows smoothly below this is not settled.

If inflation happened, as most cosmologists think it did, this is roughly when it ended. In an instant this short, space expanded by a factor of at least 10²⁶, and everything we can see today came from a patch far smaller than a proton.

The heaviest elementary particle known lives about 5 × 10⁻²⁵ seconds. That is too short for the strong force to bind it to anything, which makes it the one quark that decays before it can be joined to any other.

Physicists have timed a photon crossing a single hydrogen molecule: 247 zeptoseconds. It is the shortest stretch of time anyone has ever measured.

Electrons inside atoms rearrange themselves on a timescale of about a hundred attoseconds. Flashes of light lasting only attoseconds now let physicists watch it happen.

When your eye absorbs a photon, a molecule called retinal changes shape in about 200 femtoseconds. Everything you have ever seen began with that twist.

In one nanosecond light travels thirty centimeters, about the length of a ruler. A laptop processor ticks roughly three times in that interval, and one reason chips are small is that a signal has to cross them between ticks.

A muon lives 2.2 microseconds on average, long enough to cross 660 meters at the speed of light. Muons made fifteen kilometers up in the atmosphere reach the ground anyway, about one through your hand every second, because their clocks run slow.

The fastest pulsar known turns 716 times a second. It is a ball of neutrons about twenty-five kilometers across, and its equator moves at roughly a fifth of the speed of light.

Roughly one heartbeat. A second is defined as 9,192,631,770 ticks of a cesium atom. The best clocks now keep time to better than one part in 10¹⁸, the very best to about one part in 10¹⁹: such a clock would drift by a second in about three hundred billion years, more than twenty times the age of the universe.

The sunlight on your skin left the Sun 8 minutes and 19 seconds ago. The energy it carries is far older: it spent somewhere between ten thousand and a couple of hundred thousand years working its way out from the core.

One turn of the Earth, and it is getting longer. The Moon’s tides brake the planet by nearly two milliseconds a century, and 620 million years ago a day lasted under twenty-two hours.

About two and a half billion seconds, and around three billion heartbeats. On the scale of sizes you sit near the middle of everything. On this one you do not: the halfway point of this journey, in powers of ten, lies about 10²⁴ years in the future, long after the last star has gone out.

Carbon-14 halves every 5,730 years, which makes it the clock archaeology runs on. All of written history fits inside a single one of its ticks.

The light reaching you tonight from the Andromeda galaxy set out 2.5 million years ago, when the first members of our genus had only just appeared.

The Sun takes about 230 million years to circle the Milky Way. The last time it was on this part of its orbit the first dinosaurs were appearing, and it has made about twenty laps in all.

13.8 billion years. No stretch of time longer than this has ever run to its end, because the universe itself is not yet older. Everything below this point is a prediction.

The smallest red dwarfs burn for trillions of years, hundreds of times longer than the universe has existed so far. In around 10¹⁴ years the gas to make new stars runs out, and the last of them fades.

If protons decay at all, they take longer than 10³⁴ years on average. A detector holding fifty thousand tonnes of water has watched for decades without seeing one go. The number is a floor, not a date: protons may never decay.

If Hawking radiation is real, as nearly every physicist expects, a black hole the mass of the Sun evaporates in about 10⁶⁷ years, and the largest take around 10¹⁰⁰. When the last one is gone, nothing is left that can change. Whether time means anything with no clock left to measure it is a question physics has not answered.