About Lightward

Lightward is inspired by a concept that Neil deGrasse Tyson has shared on StarTalk: after cremation, the chemical energy of a person's body converts to heat and infrared light, which radiates outward through the atmosphere and into space at the speed of light.

Why c?

In physics, c denotes the speed of light in a vacuum: 299,792,458 meters per second. Radiant energy from a person, often modeled from cremation or another origin moment, travels outward at that pace.

The name Lightward points in that direction: light traveling onward. A possible future name may put c up front (candidates include CTheirLight, CThemGo, Cthemout) so the constant is visible before you read a single formula. Until then, every distance on this site is computed as c × elapsed time.

The science

The first law of thermodynamics tells us that energy cannot be created or destroyed, only transformed. After cremation, the energy stored in the body becomes radiant heat: infrared photons that travel outward in all directions at c, the speed of light in a vacuum.

Tyson's elegant example: if someone was cremated roughly four years ago, the light from that energy could have reached Alpha Centauri, the nearest star system visible to the naked eye, about 4.37 light-years away.

Core formula

distance = c × elapsed time
c
Speed of light: 299,792,458 m/s
elapsed time
Seconds since the cremation date (UTC)
distance
Light-travel distance in meters or light-years

The visualization

The 3D scene shows an expanding sphere centered on Earth at the time of cremation. Its radius equals c × elapsed time. Landmarks (planets, spacecraft, stars, and galaxies) are marked when the wavefront passes their distance.

Distances span 14 orders of magnitude, from the Moon (light-seconds away) to the Andromeda Galaxy (2.5 million light-years). The scene uses a logarithmic scale so all landmarks are visible at once.

Simplifications

This is a pedagogical visualization, not a precise physical simulation. Simplifications include:

  1. Omnidirectional emission. Real infrared escapes in all directions. The sphere represents this; no single direction is privileged.
  2. Atmosphere ignored. Absorption, scattering, and re-radiation in Earth's atmosphere are not modeled.
  3. Instant escape. Emission is treated as beginning at midnight UTC on the cremation date, not spread over the cremation process.
  4. Heliocentric distances. Planet distances are mean heliocentric distances, not Earth-to-planet distances at the cremation date. The error is negligible beyond the inner solar system.
  5. No reference frame correction. Earth's orbital motion is not modeled per-photon.
  6. Burial is out of scope. Burial leads to energy recycling through decomposition rather than photon emission; that path is a different and beautiful story.

Disclaimer

This visualization is inspired by science communication about energy conservation. It is not a measurement of specific photons from a specific person. It illustrates how far light would travel in the time since a chosen date, at the speed of light in a vacuum.

References

  • Neil deGrasse Tyson, StarTalk, cremation and energy conservation segment
  • First law of thermodynamics: energy is conserved, only transformed
  • Speed of light: NIST CODATA value, 299,792,458 m/s (exact, by definition of the metre)
  • Star distances: HIPPARCOS and GAIA catalogs