Quantum Physicist Links Time's Arrow to Quantum Decoherence

British physicist Jim Al-Khalili proposes a quantum explanation for time's one-way direction. He outlines his view in a new book titled "On Time" and in an interview for the Fiction Science podcast.

Most physicists point to the Second Law of Thermodynamics as the source of time's arrow. This law states entropy, or disorder, must always increase in an isolated system. Some have even speculated about universes where time flows from high to low entropy.

A Quantum Foundation for Time

Al-Khalili, a professor emeritus at the University of Surrey and a BBC science presenter, specializes in quantum physics. His explanation centers on quantum entanglement and decoherence. "I argue that the arrow of time is indeed real, and it's baked into the structure of the universe," he says. He contends the direction of time emerges from the quantum area, not from thermodynamics.

In his view, entanglement is fundamental. When quantum entities interact, their properties become linked. Observation then causes quantum decoherence. This is the irreversible leak of "quantumness," like determining the fate of Schrödinger's cat. "You can't undo decoherence once you've observed," Al-Khalili states. "Once you've opened Schrödinger's box to see if the cat is dead or alive, that's it. You fixed it."

Observation and Irreversibility

Every observation increases the universe's overall entanglement and causes decoherence. Al-Khalili writes that measuring something requires partitioning it from the universe, leading to entanglement with the observer's macroscopic environment. This act causes irreversible decoherence. "Observing anything is what gives time its direction," he concludes.

This raises a science-fiction staple: does a conscious observer create reality? Al-Khalili dismisses this. Observation does not require a human or consciousness. "The air molecules surrounding an electron can do the observing, can cause the decoherence," he explains. The state is recorded in the environment regardless of a conscious observer.

Many Worlds and Time Travel

The source mentions the many-worlds interpretation, where all quantum outcomes branch into separate realities. Al-Khalili acknowledges this view but suggests we are too entangled in our universe to jump streams. From within the universe, entanglement only increases.

In a bonus chapter of his book, Al-Khalili assesses time travel. Current physics allows a narrow possibility via closed timelike curves in general relativity. However, he notes time travel to the past leads to paradoxes. "Philosophically and for common sense, we would argue that time travel to the past surely must be impossible," he says. Yet, the door remains technically open, pending discoveries in quantum gravity and negative energy.

If he could ask a future physicist one question, Al-Khalili would want to know the correct interpretation of quantum mechanics. He would also like to know if his quantum arrow of time theory is correct, admitting he might be "completely wrong."