Starry Night Over The Rhone (V. van Gogh, public domain)

An unusual question raises an intriguing idea.

At a party a few nights ago a friend approached me with a dilemma. A relative of theirs had died, and the spouse was trying to understand if it was at all possible that there was still 'something' of their partner in existence; a tangible part of their being, an imprint of form or self left in the cosmos. They wanted nothing mystical, just to know if there were any rational, scientific ways in which a human being who was gone could nonetheless continue to be. My friend, a caring and decent person, didn't want to just dismiss this idea, but was stumped for an answer to give the bereaved.

My first instinct was to say that the end was the end. But the question persisted. Was there no scientific escape clause? Could any of us leave more than mere molecules and atoms behind when we die - bits and pieces slowing being carried off and repurposed by industrious bacteria or the relentless drift of thermodynamics? For example, might there conceivably be some deep quantum memory, a fairy-tale ghost of entanglement - the wavefunctions of our being forever permeating space? Such possibilities seem awfully unlikely.

Except, I realized standing there, there is something that we all do that creates a fingerprint of our brief time on this world. It happens every time that you or I go outside on a clear day. At the surface of the Earth, roughly a hundred thousand trillion (10 to the power of 17, or a hundred quintillion) solar photons arrive per second in every square centimeter. A person standing out in the sunlight will absorb some of these and reflect the rest (or re-emit in the infrared), I don't know what the precise reflectivity (albedo) of a human is, but I'm guessing it's close to 30-50% (depending on clothing and so on).

Suppose someone lays on the ground, the cross-sectional area of a typical adult is about 0.7 square meters. (How do I know this? Well I Googled it and found: 'On the everyday effects of wind drag on people' in the journal Weather - I kid you not - and this lovely paper has a reasonable-sounding estimate of human cross-section). Thus, if you lounge on the beach or on a picnic blanket it means that every second you'll be reflecting something like a hundred million trillion photons (10 to the power of 20). Where do these photons go? Lots will go back into the sky, and even after passing through the Earth's atmosphere there will still be tens of millions of trillions per second streaming out into the cosmos.

In that sense we of course are simply another piece of the Earth's daylight surface, a teensy tiny part of the blue marble. But, in principle, somewhere across interstellar space, on another world or in its orbit, a civilization could have built a telescope beyond anything we've ever considered. An instrument capable of not only imaging distant planets, but of seeing them to such fine resolution that your form could be noticed, studied, gawped at. Except, of course, this would take place hundreds or thousands of years from now.

And even if no such telescope exists, the simple fact is that some of the photons that have bounced off you on any clear day here on Earth are right now passing steadily through the void. In the future, when you and I are no more, there will still be a lifetime's worth of us carried by this light, passing through distant systems, and eventually out to intergalactic space. Photons will get absorbed and scattered by gas and dust, but the odds are good that of the octillions (10 to the power of 27) that once bounced from us (over a human lifetime), some will continue to propagate unimpeded for long enough that it is only the stretching expansion of the universe that eventually diminishes them to near nothingness.

I said this to my friend, and asked if they felt that such an ethereal human fingerprint, one that would eventually run out with the darkening cosmos a hundred billion years hence, would perhaps bring their relative a little solace.

They said that they thought it would.