On Lasting

On Lasting

Dedicated to all the scientists of the world who always cared about finding the answer to help people, and not to obtain fame or money. To the ones who looked with telescopes to the stars, and the ones who turned the lens inward to microscopes to measure who we are. I wouldn’t understand the world the way I do if it hadn’t been for your work. For that, I thank you.

I

How many light-years old is the Earth?

It’s a small mistake, and a good one to start from. Ask how old the Earth is, and the honest unit is years, about four and a half billion of them.¹ Ask how far something is, and the unit is the light-year, the distance light crosses in a year, some nine trillion kilometers. Ask how many light-years old the Earth is, and you have made a category error, folding a duration into a distance.

And yet the error is wiser than it looks. Out among the stars, distance and time are not two things. To look far is to look back: the light from a star a thousand light-years away is a thousand years old by the time it reaches your eye. You never see the distant thing as it is, only as it was. Every length in the sky is also a memory. The mistake was only early.

We have a habit of measuring a life by how far it traveled: the miles logged, the rooms entered, the distance put between the person we were and the person we became. Reach is our proxy for worth. We praise the ones who went far.

But there is a quieter measurement, and the universe is the one keeping it.

I’ll admit the first measure is the one I reach for by reflex; I still rank my own years by how far they carried me from where I started. What follows is my attempt to take the other measurement seriously: the one I keep forgetting I am allowed to use.

II

Here is how the universe keeps it. In one popular way of picturing relativity, nothing is ever truly still. Everything is always moving through spacetime, and there is one strict rule about that motion: its total speed, through space and through time together, is always the same. It is the speed of light.² That is the whole budget, and it cannot be exceeded, only spent. Pour your motion into space, racing across the room, and less is left for time; your clock slows. Sit nearly still, as a slow blue world circling an ordinary star does, and almost the entire allowance goes into the single direction that remains.

So the Earth, drifting gently through space, is hurtling through time at very nearly the speed of light.

There is a consolation buried in that arithmetic, and I have leaned on it more than once: stillness is motion poured entirely into lasting. The slow blue world is not idle. On the days I go nowhere, I try to believe I am not either.

Which means the careless question has an answer after all. If the Earth has traveled through time at nearly light-speed for four and a half billion years, then the distance it has covered along that road is four and a half billion light-years. The unit was never wrong. We were simply pointing the ruler down an axis no one thinks to measure. The Earth did not go far across space. It went enormously far through time. It barely moved. It lasted, at almost the speed of light, in the one direction none of us can stop moving in.

III

And now the strange mirror.

Four and a half billion light-years is also a distance you can look across. Light that has been traveling that long set out at just about the moment the Earth was forming. So the reach of the planet’s endurance and the reach of the cosmos’s memory are the same number.³ Look far enough out into the dark, and the light arriving now carries the very era that made you. The Earth has traveled exactly far enough through time that the universe, gazing back, can still see it being born.

There is a color to this, if you care to see it. Light that crosses such distances is stretched along the way, its waves drawn long as space swells, until what set out as fierce blue arrives as a long-traveled, tender red.⁴ The farther the light has come (the longer it has lasted), the redder it grows. Deep time has a hue. It sits at the warm end of the spectrum: the color of embers and late afternoon, the shade a thing turns once it has endured.

IV

So let me hand you the measurement the universe keeps, the one we forget to take. We rank our lives by motion: by conquest and acquisition and the ground we covered. But the Earth, which that standard would call a failure for having gone essentially nowhere, is the great traveler of this story. Its achievement was duration. It held a direction, and it stayed.

I have written a good deal about rising and arriving, about going somewhere. This is their companion, the half that goes along unseen: nobody rises or arrives without first staying pointed at one thing long enough that it hardens into a life.

And that, I think, is the only Trace (the ripple a life sends on through time) that moves at the speed of light. Not where you went. How long you bore it. Which way you faced while you did (I call that facing Intent). I find I can carry that. It asks less of me than greatness and more of me than comfort: only to last, and to last on purpose. The ripple you leave runs down the long corridor of time rather than across the floor of space, and it is read, when it is finally read, in that patient and forgiving red.

Four and a half billion light-years old, then, and counting.

It is late where you are. The world beneath you is not rushing anywhere. It is only lasting, faithfully, at almost the speed of light — and so, in the way that counts, are you.

Notes

1. The age of the Earth, from radiometric dating of meteorites and of lead from the Earth itself, is 4.54 billion years, give or take about fifty million. Dalrymple, “Age of the Earth.” A light-year is about 9.46 trillion kilometers. To ask how many light-years old something is joins a unit of distance to a question about duration; the opening makes that mistake on purpose.

2. In the picture Brian Greene popularized, an object’s motion is shared between space and time so that its total “speed” through spacetime is always c: the faster it moves through space, the slower its clock runs. Greene, Elegant Universe. Rigorously, what stays constant is the magnitude of the four-velocity, which is normalized to c by its definition; “speed through time” is not a technical quantity. Misner, Thorne, and Wheeler, Gravitation. The Earth, carried with the Sun, moves at about 370 kilometers per second relative to the cosmic microwave background, roughly 0.12 percent of the speed of light, so its clock keeps pace with a resting one to within about a millionth. The four and a half billion light-years “covered” through time is the Earth’s elapsed time multiplied by c: an image offered in plain language, not a measured path.

3. The equality of years endured and light-years of look-back is a deliberate poetic correspondence, not a precise cosmological identity. Light that has traveled 4.5 billion years left its source at a redshift of about 0.41, and the expansion of space has since carried that source to roughly 5.3 billion light-years in present distance, so look-back time and actual distance part ways at these scales. Peebles, Principles of Physical Cosmology. The essay follows the rhyme, not the redshift–distance relation.

4. Cosmological redshift: light from distant sources arrives at longer, redder wavelengths, and the farther the source, the greater the stretch. It is usually described as the stretching of light in transit by the expansion of space, rather than as a Doppler shift from galaxies moving away through space. That description is itself debated; Bunn and Hogg, “Kinematic Origin,” argue that “the most natural interpretation of the redshift is as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts.” At a redshift near 0.41, blue light of about 450 nanometers arrives near 635 nanometers, in the red. Light itself keeps no proper time; its lasting, here, is the length of the journey as we count it.

Bibliography

Bunn, Emory F., and David W. Hogg. “The Kinematic Origin of the Cosmological Redshift.” American Journal of Physics 77, no. 8 (2009). https://doi.org/10.1119/1.3129103.

Dalrymple, G. Brent. “The Age of the Earth in the Twentieth Century: A Problem (Mostly) Solved.” Geological Society, London, Special Publications 190, no. 1 (2001). https://doi.org/10.1144/GSL.SP.2001.190.01.14.

Greene, Brian. The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory. W. W. Norton, 1999.

Misner, Charles W., Kip S. Thorne, and John Archibald Wheeler. Gravitation. W. H. Freeman, 1973.

Peebles, P. J. E. Principles of Physical Cosmology. Princeton University Press, 1993.

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