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.
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.
We seem to perceive time as passing in one direction. After all, we can't just just forward to the future or revisit our past if we felt like it. Every minute of every day appears to move us ahead, pulling us through our lives towards an inexorable demise. At least that's what the conventional experience of time tells us. But what if your present, past, and future all existed already? Time, from that point of view, would not flow.
The block universe theory says that our universe may be looked at as a giant four-dimensional block of spacetime, containing all the things that ever happen, explained Dr. Kristie Miller, the joint director for the Centre for Time at the University of Sydney.
In the block universe, there is no "now" or present. All moments that exist are just relative to each other within the three spacial dimensions and one time dimension. Your sense of the present is just reflecting where in the block universe you are at that instance. The "past" is just a slice of the universe at an earlier location while the "future" is at a later location.
So, is time just an elaborate mind trick? And more importantly - is time travel possible?
Dr. Miller's answer to that is "yes". Of course, just hypothetically, since we'd need to figure out first how to travel at "some reasonable percentage of the speed of light". Going to the past would entail using wormholes, like "short cuts through space-time".
Block Universe diagram.Credit: ABC Science
Now, if you did manage to get back in time, you won't be able to change it. This is because your past is always simultaneously someone else's future. So if you travel to the past, you're just making that future the way it is. So don't worry about "grandfather paradoxes" - your time machine has already been incorporated into the scheme of things.
'If I travel to the past, I am part of the past," said Miller. "Importantly, I was always part of the past."
What's more - maybe the past has already been altered by time travelers. How would we be able to tell if it hasn't? "For all we know, the reason the past is the way it is, is in part due to the presence of time travelers," added Miller.
By that logic, what you do tomorrow will make it the way it is, with you fulfilling a certain destiny writ in time, which is in itself more of an illusion than a fundamental property of nature.
Certainly, with such claims, the block universe theory has its detractors. One big criticism is that the future shouldn't exist yet. Physicist Lee Smolin wrote that "The future is not now real and there can be no definite facts of the matter about the future." Furthermore, as he added at a 2017 conference, what is real is just "the process by which future events are generated out of present events."
Another negative of this idea is if the block universe is static, what is the point of anything? Can you have progress? Answering that is the "evolving block universe" model which sees the block of the universal space-time growing rather than staying the same. The surface of such a volume would represent the present moment. It's when "the indefiniteness of the future changes to the definiteness of the past," as described it cosmologist George Ellis. Under that model, the changing part would be the future.
While the debates are going to continue, the block universe theory is one of the most promising approaches that can reconcile the cosmological view of time with our everyday experience. What may be certain - time is much more than what it appears to be. Unraveling its mysteries is integral to understanding the human experience.
In 1928, physicist Paul Dirac predicted that every fundamental particle has an antiparticle - a twin that has an opposite charge. If a particle and antiparticle were to meet, they would be annihilated while releasing a burst of energy. But in 1937, physicist Ettore Majorana added the prediction that a class of particles exists known as fermions, which would include particles that are their own antiparticles.
Now the researchers from Stanford University and University of California found the Majorana fermion in a series of lab experiments on exotic materials. They were led by UC-Irvine Associate Professor Jing Xia and UCLA Professor Kang Wang, and followed a plan proposed by the Stanford physics professor Shoucheng Zhang.
Professor Zhang, one of the senior authors of the paper, put their finding in perspective:
“Our team predicted exactly where to find the Majorana fermion and what to look for as its ‘smoking gun’ experimental signature,” said Zhang. “This discovery concludes one of the most intensive searches in fundamental physics, which spanned exactly 80 years.”
Want to learn more about antimatter? Let Michio Kaku explain:
The complexity of the experiments necessary to find the Majorana fermion makes this work a “landmark in the field,” said Tom Devereaux, director of the Stanford Institute for Materials and Energy Sciences.
Nobel Prize-winning theoretical physicist Frank Wilczek, who was not involved in the research, also praised the importance of the discovery, calling it “a really clean observation of something new” and a “real milestone”.
The fermion observed by the team is known as a “chiral” fermion as it moves in just one direction along a one-dimensional path.
The experiment that resulted in the find involved looking for a special half-speed signature of Majorana quasiparticles, which are “excitations” that come from how electrons behave in superconducting materials. While these are special entities that have some properties of particles but are not actually particles found in nature, quasiparticles are still regarded as real Majorana fermions.
The researchers applied electricity to stacks of superconducting materials and topological insulators in a chilled chamber. A topological insulator conducts current along its surface or edges only, not through the middle. A magnet was then used to control the behavior of generated electrons as they sped along the edges of the surface. The electrons were slowed down, stopped and their direction changed. This cycle continued until the scientists identified the Majorana quasiparticles that emerged by by their unique speeds.
The practical implications of this discovery are well in the future, with potential for use in quantum computing, where it can help overcome environmental noise. Zhang calls the newly-found particle the “angel particle,” referring to Dan Brown’s bestseller “Angels and Demons” which featured a matter/anti-matter time bomb as part of the plot.
Read the study here, in Science magazine.
You can watch Professor Zhang discuss the search and discovery of the Majorana Fermion here:
