From Einstein's idea to the ruby laser
In 1917 Albert Einstein predicted "stimulated emission": when light strikes an already excited atom, the atom can emit a second photon with the same direction, wavelength and phase. The idea stayed theoretical for almost 40 years. In 1954 Charles Townes built the maser, which amplified microwaves using ammonia molecules, and in 1958 he and Arthur Schawlow showed how the principle could work for visible light. In May 1960 Theodore Maiman at Hughes Research Laboratories made the first laser from a ruby crystal and a flash lamp; later that year Ali Javan built the helium–neon gas laser, and in 1962 Robert Hall and others made semiconductor lasers. Townes shared the 1964 Nobel Prize in Physics; Maiman never received one. Early on, the laser was famously called "a solution looking for a problem".
How laser light differs from ordinary light
A light bulb emits many wavelengths scattered in every direction, while laser light is monochromatic, directional and coherent. A laser pumps energy into a medium until more atoms are excited than not — a population inversion — and mirrors bounce light back and forth so that stimulated emission keeps multiplying it. All of this rests on quantum mechanics, which describes atomic energy levels, and on the insight from the photoelectric effect that light comes in photons. Even low-power laser pointers can damage the retina if shone into the eye, so never aim one at people or aircraft.
Lasers today
In 1966 Charles Kao proposed that pure glass fibres could carry light over long distances, winning the 2009 Nobel Prize; today most internet traffic travels as semiconductor-laser light through optical fibre. Lasers also run barcode scanners, laser printers, Blu-ray players, metal cutting and welding, LASIK eye surgery and dermatology. Chirped pulse amplification, developed by Donna Strickland and Gérard Mourou in 1985, made ultra-intense ultrashort pulses possible and earned the 2018 Nobel Prize. In 2015 LIGO's four-kilometre laser interferometers detected a jiggle smaller than an atomic nucleus — the first direct observation of gravitational waves, confirming a prediction of relativity a century later.
FAQ
- Why does a laser beam stay narrow?
- Because coherent light is amplified into a tight beam. It still spreads slightly: a laser aimed at the Moon widens to a few kilometres across.
- How dangerous are laser pointers?
- Anything above about 1 mW can injure the retina if it enters the eye directly, and some imported pointers are far stronger than labelled. Never aim one at people, animals, vehicles or aircraft.
Sources reviewed: 2026-10-26
References
- Einstein, A. (1917). Zur Quantentheorie der Strahlung. Physikalische Zeitschrift, 18, 121–128.
- Gordon, J. P., Zeiger, H. J., & Townes, C. H. (1954). Molecular microwave oscillator and new hyperfine structure in the microwave spectrum of NH₃. Physical Review, 95(1), 282–284.
- Schawlow, A. L., & Townes, C. H. (1958). Infrared and optical masers. Physical Review, 112(6), 1940–1949.
- Maiman, T. H. (1960). Stimulated optical radiation in ruby. Nature, 187(4736), 493–494.
- Javan, A., Bennett, W. R., & Herriott, D. R. (1961). Population inversion and continuous optical maser oscillation in a gas discharge containing a He-Ne mixture. Physical Review Letters, 6(3), 106–110.
- Hall, R. N., et al. (1962). Coherent light emission from GaAs junctions. Physical Review Letters, 9(9), 366–368.
- Kao, K. C., & Hockham, G. A. (1966). Dielectric-fibre surface waveguides for optical frequencies. Proceedings of the IEE, 113(7), 1151–1158.
- Strickland, D., & Mourou, G. (1985). Compression of amplified chirped optical pulses. Optics Communications, 56(3), 219–221.
- Abbott, B. P., et al. (LIGO Scientific Collaboration and Virgo Collaboration). (2016). Observation of gravitational waves from a binary black hole merger. Physical Review Letters, 116(6), 061102.
- Siegman, A. E. (1986). Lasers. Mill Valley, CA: University Science Books.