Electromagnetic waves do not “run out” of energy just by traveling, but their energy can become harder to detect because it spreads out, and in some situations it can genuinely be lost to other forms like heat or cosmological redshift. In an ideal, perfectly empty space with no expansion and no matter, the total energy of the wave is conserved as it moves.

Quick Scoop

  • In empty space near everyday sources, EM waves conserve total energy; what drops is intensity (energy per area), not total energy.
  • Intensity falls roughly with the inverse square of distance because the same energy spreads over a larger spherical surface.
  • In real media (air, glass, water, interstellar gas), part of the wave’s energy is absorbed and turned into other forms (mostly heat).
  • Over cosmological distances, light also loses energy because the expanding universe stretches its wavelength (cosmological redshift).

How EM waves carry energy

Electromagnetic waves are oscillating electric and magnetic fields that propagate through space, and the energy they carry is stored in those fields. The energy density is proportional to the square of the field amplitudes, so stronger fields mean more energy packed into a given region.

  • A beam of light or radio wave carries energy and momentum, which is why it can exert radiation pressure on matter.
  • For a given frequency, each photon has energy E=hνE=h\nu E=hν, so if the number of photons per second is fixed, the total carried power is fixed.

Why intensity drops with distance

From a point-like source radiating in all directions, the wavefronts are roughly spherical, and their area grows like 4πr24\pi r^24πr2. The same total power must then be spread over that growing area, so intensity (power per area) falls like 1/r21/r^21/r2.

  • The field amplitude of the wave typically falls like 1/r1/r1/r, and since intensity goes like amplitude squared, that gives the 1/r21/r^21/r2 law.
  • This weakening does not mean the wave “lost” energy; it just means any detector with fixed size grabs a smaller fraction of the total energy as it gets farther away.

When EM waves really lose energy

Outside the ideal vacuum picture, waves can truly lose energy, in the sense that it is transferred to other forms.

  1. Absorption in a medium
    • In air, glass, water, or tissue, atoms and molecules can absorb EM radiation and convert it to internal energy (usually heat).
 * This is why radio signals fade in buildings and why sunlight warms surfaces: the wave’s energy ends up in the material instead of remaining in the propagating wave.
  1. Scattering and attenuation
    • Even if not fully absorbed, waves can be scattered, redirecting energy away from the original beam direction and making it appear weaker along a straight line path.
 * Communication engineers describe this overall reduction in amplitude and intensity with distance and medium effects as attenuation.
  1. Cosmological redshift
    • Over galaxy-scale distances, the expansion of the universe stretches the wavelength of light, lowering each photon’s energy.
 * Here, even if photon number stays the same, the total energy of that light beam decreases as its spectrum shifts to redder (lower-energy) wavelengths.

Forum-style wrap‑up (with multiple viewpoints)

In physics discussions and forums, you’ll often see two statements that sound contradictory but are both valid in context:

“Light doesn’t lose energy as it travels; it just spreads out.”

“Waves attenuate and lose energy in real media or due to cosmic expansion.”

Both perspectives hinge on what is being held ideal:

  • In a perfect, non-expanding vacuum , total wave energy is conserved; intensity falls only because of geometric spreading.
  • In the real universe , matter, fields, and expansion provide ways for the wave’s energy to be transferred or reduced, so over long distances, the wave can genuinely carry less energy than it started with.

TL;DR: In a perfect vacuum with no cosmic expansion, EM waves do not lose total energy as they travel; they just spread out, so they seem weaker to a detector. In real situations, absorption, scattering, and the expanding universe can drain energy from the wave and convert it into other forms.

Information gathered from public forums or data available on the internet and portrayed here.