Earth 1

Light beyond violet · X-rays · gamma rays

X-rays & Gamma Rays

Radio, light and gamma rays are the same thing: waves of electric and magnetic field. Shrink the wave and each packet of light carries more punch.

One light, every colour

Slide from radio to gamma. The wave gets shorter and each photon carries more energy. Visible light is about 2 eV a photon; a dental X-ray is around 60 000 eV; gamma rays from atomic nuclei carry millions.

  • E: the energy of one photon
  • h: Planck's constant, 6.63 × 10⁻³⁴ J·s
  • c: the speed of light
  • λ: the wavelength

501 nm

wavelength

2.5 eV

per photon

visible

band

The thin slice eyes evolved for, because the Sun is brightest here.

Seeing through skin

X-rays are absorbed mostly by heavy atoms. Bone is rich in calcium, so it stops far more than flesh does, and the film behind it stays unexposed: a shadow of the skeleton.

  • I₀, I: X-rays going in and coming out
  • μ: how strongly the material absorbs, high for bone, falling fast with energy
  • x: how thick the material is

33.4 %

through 3 cm flesh

14.9 %

through 1 cm bone

30 keV

photon energy

Bright means the X-rays were stopped. Soft X-rays are swallowed by everything; hard ones pass through bone too. Radiographers pick the energy in between that makes bone stand out, as Röntgen first saw in his wife's hand in 1895.

Gamma-ray bursts

The most energetic light comes from the most violent events: massive stars collapsing to black holes and neutron stars colliding. Their jets point a burst of gamma rays at us that can outshine a whole galaxy for seconds. Earth's air absorbs X-rays and gamma rays, which is why we are alive, and why we watch them with satellites.

Timeline

  1. 8 Nov 1895

    Wilhelm Röntgen sees a screen glow across his lab from a covered tube. Weeks later he X-rays his wife Anna Bertha's hand.

  2. 1896

    Henri Becquerel finds uranium gives off rays by itself; Marie and Pierre Curie go on to find polonium and radium.

  3. 1900

    Paul Villard finds a third, most penetrating kind of radiation from radium; Rutherford names it gamma.

  4. 1962

    Riccardo Giacconi's rocket finds Scorpius X-1, the first X-ray star.

  5. 1967

    The Vela satellites, watching for secret nuclear tests, see flashes of gamma rays from deep space. Published in 1973.

  6. 1999

    NASA's Chandra X-ray Observatory launches.

  7. 9 Oct 2022

    GRB 221009A, the brightest gamma-ray burst ever recorded: "the BOAT".

Sources

  • W. C. Röntgen, “Über eine neue Art von Strahlen”, Sitzungsber. Würzburger Phys.-Med. Ges. (1895)
  • R. W. Klebesadel, I. B. Strong, R. A. Olson, “Observations of gamma-ray bursts of cosmic origin”, ApJ 182 (1973) L85
  • R. Giacconi et al., “Evidence for X rays from sources outside the solar system”, Phys. Rev. Lett. 9 (1962) 439
  • LHAASO Collaboration, “A tera-electronvolt afterglow from a narrow jet in an extremely bright gamma-ray burst”, Science 380 (2023) 1390