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The ‘Quantum Zeno Effect’ Explained

Written by alexander zhang

Greek philosopher Zeno of Elea once had an interesting thought (as philosophers do): if an arrow in flight is examined at an instant in time, the arrow appears to be motionless. However, if the arrow is motionless at every instant in time, and time is entirely composed of instants, then the idea of “motion” is complete bogus. This posed an interesting mathematical problem.

Thankfully, “motion” was later un-bogused with the power of calculus. An intuitive refutation of Zeno’s paradox was provided by Aristotle, who noted that “time is not composed of indivisible nows any more than any other magnitude is composed of indivisibles.” That is, instantaneous moments in time do not exist because it is impossible to complete a task (such as firing an arrow from one point to another) with infinite steps. Bertrand Russell further criticized Zeno’s model of motion, arguing that, while it is true that an arrow cannot be “in motion” at an infinitely small duration of time, all motion requires is for the arrow to be at a point at one time, another point at another time, and at the appropriate points during the intervening time. 

Zeno’s paradox may not be successful in dismantling the foundations of motion, but Zeno may have been pleased to know that his ideas helped to name an effect observed in the quantum realm. To preface this, it’s important to acknowledge that slapping the word “quantum” in front of a disproved paradox doesn’t necessarily make it any more sensible, but the quantum Zeno effect—a feature of quantum systems with an idea similar to Zeno’s arrow paradox—is a very real and observable phenomenon.  

To put the quantum Zeno effect into understandable (yet nowhere near accurate) terms, take a common example: waiting for something to load. Suppose Zeno wanted to watch a YouTube video, but it was taking a long time to load and he was growing impatient. Then an idea struck him: if he closed his eyes while waiting for the video to load, periodically opening them to check on the progress, he might make it seem to speed up! He does this, measuring the state of the video frequently. However, he finds that his video—which usually finishes loading after a short while—doesn’t load at all. In fact, everytime he glances at the loading icon, it seems like nothing changed! That’s because, according to the quantum Zeno effect, nothing—or at least very little—did change. By measuring the system frequently enough at specific intervals, Zeno was able to arrest his video’s time evolution (that’s assuming that his video is actually a particle abiding to quantum laws), keeping the video at its initial conditions. Put into simple terms, the quantum Zeno effect describes how a quantum system can be “frozen” by measuring it frequently enough. 

A better metaphor to explain this phenomenon is Schrodinger’s cat. Schrodinger’s famous thought experiment hypothesized that a cat locked in a box with a 50% of dying (usually due to a radioactive atom) will be half-dead and half-alive, a victim of the duality of conditions that exist within quantum physics. What’s curious is that the quantum Zeno effect can actually ensure that the cat always lives. Assume the fate of the cat relies on a radioactive atom which has a 50% chance of decaying and triggering the destruction of a poison vial. Plotting the probability that the survivability of the cat over an x-value of time, a negatively sloped graph arises. This—while tragic—is to be expected; a greater value of time leads to a higher probability that the atom has decayed and, thus, a lower survivability of the cat. However, if the atom is “observed” over the course of its decay, a more lively graph takes form: 

The “bounces” in the graph correspond with the times at which the system is measured, while the dashed line depicts the survivability of the cat without measurements. 

         Everytime the system is measured, and the atom still hasn’t decayed (in other words, it’s still in its initial conditions), it’s as if nothing happened at all. As a result, the system “resets” in a sense, and the process begins again. So, Schrodinger’s cat lives on, and Zeno’s quantum YouTube video loads in an endless spiral. As strange as this may seem, the quantum Zeno effect has been experimentally proven in the real world. Researchers from Cornell found ways to use the quantum Zeno effect to freeze the tunneling of atoms—a phenomenon that usually occurs when atoms are exposed to extremely cold temperatures. Instead of “tunneling” place to place as a result of having nearly 0 velocity, the atoms exhibited reduced tunneling when measured. 

Thankfully, the quantum Zeno effect isn’t only understood under the context of “magic.” The active action of measuring—that is, in the Cornell researcher’s case, shining an imaging laser on atoms—served as an outside force, disrupting the sensitive balance of quantum systems. This caused for the reduction of tunneling. As measurements by the imaging laser became brighter and/or more frequent, the tunneling was dramatically reduced.

It’s evident that the quantum Zeno effect isn’t yet readily applicable for everyday life. It may be a long time until this phenomenon, like many other quantum phenomena, is fully understood and ready to be taught in high school classrooms. Even so, it’s valuable to take a look at what really is reality and explore the stranger parts of it. After all, that’s what Zeno of Elea did when he took an aspect of reality—motion—and thought about it for a little while, even if his work is unfounded today. Quantum physics is still a relatively new and unexplored realm of physics, and it’s up to the thinkers of today to un-bogus it. 

Felix Pollock – The quantum Zeno effect: how curiosity can save Schrodinger’s cat. (2015, May 28). Retrieved from

Řeháček, J., Hradil, Z., Peřina, J., Pascazio, S., Facchi, P., & Zawisky, M. (2008, May 30). Advanced Neutron Imaging and Sensing. Retrieved from

Steele, B. (2015, October 22). ‘Zeno effect’ verified: Atoms won’t move while you watch. Retrieved from

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