Revised Schrödinger's cat experiment challenges reality

A classic experiment gets an update that contradicts key assumptions of quantum mechanics.

  • Physicists revise the Schrödinger's cat thought experiment.
  • The new version leads to contradictions in quantum theory.
  • Scientists are stumped by the implications.

Quantum mechanics has produced its share of weird ideas, not least of which is what's probably the world's most famous thought experiment devised by physicist Erwin Schrödinger in 1935. It described the uncertain fate of a cat trapped in a box with a deadly substance. Now the experiment got an update from two physicists, leading to conclusions that threaten to undermine the foundations of the whole field.

By replacing the cat in the box with multiple physicists doing experiments, the duo of Daniela Frauchiger and Renato Renner of the Swiss Federal Institute of Technology (ETH) in Zurich has caused heated debates among physicists for the past two years.

Schrödinger's original idea proposed that if you put a cat in a box, along with a possibly decaying radioactive substance which would release a killer acid, the cat could be both alive and dead until that box was opened, fixating its state. Schrödinger devised this scenario to point to inconsistencies in the so-called Copenhagen interpretation of quantum mechanics, created by Niels Bohr and Werner Heisenberg in the 1920s.

The interpretation states that a quantum particle can exist in all possible states until an observer forces what's called "the wave function collapse", making the particle choose one probable state. Unfortunately, as Schrödinger showed, this theory may work on the quantum level but when applied to larger objects like cats, it becomes somewhat ridiculous and impossible—the cat cannot be both alive and dead.

Still, the Copenhagen interpretation has persisted, in part due to saying that while quantum objects may exist in uncertain states, experimental observation can give certain results. It is that certainty which the new thought experiment has attacked.


Diagram for the Schrödinger's cat thought experiment, showing the radioactive substance and the hammer that will potentially be dropped to spill the acid, thereby killing the cat.

Credit: Dhatfield/Wikimedia Commons.

The conceptual experiment by the scientists from Zurich involves putting two physicist cats into boxes. One cat would toss a coin and using its knowledge of quantum physics send a message to the other cat. That second cat, in its turn, would also employ quantum theory but to detect the message from the other cat and guess the coin toss. If two outside observers were to open these boxes, they would some times be able to guess with certainty how the coin landed but on occasion their conclusions would not agree.

"One says, 'I'm sure it's tails,' and the other one says, 'I'm sure it's heads,'" described that eventuality Renato Renner.

That's like implying reality can split in two on occasion.


This paradox has stumped other scientists as well. "I think this is a whole new level of weirdness," said Matthew Leifer, a theoretical physicist at Chapman University in Orange, California to Nature magazine.

There is a potential way this experiment can actually be carried out, but it would involve quantum computers that are not in existence yet.

The scientists originally published their argument online in 2016. Check out their final paper "Quantum theory cannot consistently describe the use of itself" from September 2018 in Nature magazine.

3D printing might save your life one day. It's transforming medicine and health care.

What can 3D printing do for medicine? The "sky is the limit," says Northwell Health researcher Dr. Todd Goldstein.

Northwell Health
Sponsored by Northwell Health
  • Medical professionals are currently using 3D printers to create prosthetics and patient-specific organ models that doctors can use to prepare for surgery.
  • Eventually, scientists hope to print patient-specific organs that can be transplanted safely into the human body.
  • Northwell Health, New York State's largest health care provider, is pioneering 3D printing in medicine in three key ways.
Keep reading Show less
Big Think Edge
  • "I consider that a man's brain originally is like a little empty attic, and you have to stock it with such furniture as you choose," Sherlock Holmes famously remarked.
  • In this lesson, Maria Konnikova, author of Mastermind: How to think like Sherlock Holmes, teaches you how to optimize memory, Holmes style.
  • The goal is to expand one's limited "brain attic," so that what used to be a small space can suddenly become much larger because we are using the space more efficiently.

Active ingredient in Roundup found in 95% of studied beers and wines

The controversial herbicide is everywhere, apparently.

(MsMaria/Shutterstock)
Surprising Science
  • U.S. PIRG tested 20 beers and wines, including organics, and found Roundup's active ingredient in almost all of them.
  • A jury on August 2018 awarded a non-Hodgkin's lymphoma victim $289 million in Roundup damages.
  • Bayer/Monsanto says Roundup is totally safe. Others disagree.
Keep reading Show less
Big Think Edge
  • Our ability to behave rationally depends not just on our ability to use the facts, but on our ability to give those facts meaning. To be rational, we need both facts and feelings. We need to be subjective.
  • In this lesson, risk communication expert David Ropeik teaches you how human rationality influences our perception of risk.
  • By the end of it, you'll understand the pitfalls of your subjective risk perception system so that you can avoid these traps in the future.