For the first time, researchers have seen how light behaves during a mysterious phenomenon called 'imaginary time'.

Imaginary time is a mathematical concept used in physics that represents time multiplied by the imaginary unit i (the square root of -1)[1]. While it may sound like science fiction, it’s a legitimate scientific tool used in special relativity, quantum mechanics, and cosmology.

Stephen Hawking explained imaginary time using a spatial analogy: “One can think of ordinary, real, time as a horizontal line. On the left, one has the past, and on the right, the future. But there’s another kind of time in the vertical direction. This is called imaginary time, because it is not the kind of time we normally experience”[2].

Recent experimental work has given physical meaning to this abstract concept. In 2025, physicists Isabella Giovannelli and Steven Anlage demonstrated how imaginary time manifests in the real world by measuring frequency shifts in microwave pulses[3]. Their groundbreaking experiment showed that imaginary time delays correspond to measurable changes in wave frequencies, proving that these mathematical constructs have observable physical effects[4].

Key applications of imaginary time include:

  1. Quantum Mechanics
  • Used to calculate quantum states and predict system behavior
  • Helps solve complex quantum mechanical equations
  • Essential for understanding particle behavior at microscopic scales
  1. Cosmology
  • Helps remove singularities in models of the universe
  • Used in Hawking’s “no boundary proposal” for the origin of the universe
  • Allows scientists to model the Big Bang without mathematical breakdowns[1:1]
  1. Special Relativity
  • Appears in calculations involving spacetime intervals
  • Helps describe the relationship between space and time
  • Used in the Minkowski spacetime model[1:2]

Hawking noted that imaginary time is not merely a mathematical trick: “Imaginary time may sound like science fiction… But nevertheless, it is a genuine scientific concept. In fact, imaginary time is really the real time, and what we call real time is just a figment of our imaginations”[2:1].

The mathematical representation of imaginary time (τ) is obtained from real time (t) through what’s called a Wick rotation: τ = it, where i is the imaginary unit[1:3]. This transformation helps physicists solve complex problems that would be difficult or impossible to address using only real time.

  1. Wikipedia - Imaginary time ↩︎ ↩︎ ↩︎ ↩︎

  2. Stephen Hawking Estate - The Beginning of Time ↩︎ ↩︎

  3. Science Alert - Physicists Catch Light in ‘Imaginary Time’ in Scientific First ↩︎

  4. Physics APS - Imaginary Time Delays Are For Real ↩︎