All light and radiation around us is made up of photons. Elementary particles that cannot be split in two. Or can they?
Three researchers at the University of Oslo asked themselves a fairly fundamental question: What happens if we try to cut off the tail of a photon?
The answer is pretty wild.
It turns out that anything from zero to a whole bunch of new photons can appear.
Thinks physicists will be surprised
Annonse
The study, which has been published in the scientific journal Physical Review Letters, is theoretical. The researchers carried out a mathematical thought experiment.
In their experiment, they send a photon towards a mirror in a vacuum. Since photons behave both as waves and as particles, we can picture them as waves with a certain length.
The researchers then remove the mirror in the middle of the wave. It's like trying to cut the photon in two.
The result has not been demonstrated before. Johannes Skaar believes most physicists will be surprised.
"I think a lot of physicists would expect to get a combination of zero and one photon," he says. Skaar is a professor of physics at the University of Oslo and one of the researchers behind the study.
But that turned out not to be the case. Instead, the result can be thought of as a mixture of many different states, such as zero, one, ten, or a thousand photons.
To understand what is happening, you have to accept that a vacuum is not empty, says researcher Johannes Skaar.(Photo: University of Oslo)
"It's exciting that they're asking such a simple question"
Even so, you have to know where to look in order to detect this strange effect, according to the researchers.
On either side of the mirror, the situation appears much simpler. Measurements made there would indicate either zero or one photon.
But this conceals the truth about what is happening in the exact location where the mirror used to be.
“In the middle, there's chaos. That might be the strangest and coolest part,” says Skaar.
Asle Sudbø, a physics professor at NTNU, thinks it's a cool study.
“I think it's quite exciting that they're asking such a simple question and end up uncovering something so non-trivial,” says Sudbø.
A vacuum is not truly empty
Annonse
But how can a whole bunch of photons appear when there was only one to begin with?
It's actually not that strange, according to Skaar.
To understand what is happening, you first have to accept that a vacuum is not actually empty. Instead, you can picture it as small ripples on the surface of a sea.
This is what physicists call quantum fields. These fields are always present. And the elementary particles that make up the world are better thought of as waves moving across this ever-present sea than as tiny billiard balls.
So what happens when the mirror in the experiment is removed? New energy is injected into the closed system.
And it is this energy that creates the photons in the vacuum, Skaar explains.
“You can picture the vacuum as a sea filled with disturbances, and the time-dependent mirror causes them to become actual photons,” says the professor.
The photons would have been created anyway
But that's not all.
Both Skaar and Sudbø point out that you actually don't need to send in a photon to create these new photons.
Even in a vacuum, photons could be created simply by removing the mirror very quickly.
One of the unusual properties of photons is that they have no mass, Sudbø points out. They only have energy.
“You have to put energy into the system to remove the mirror. And when you do that, you create photons out of a vacuum,” he says.
In the study, the researchers refer to a well-known phenomenon called the Casimir effect. It demonstrates that quantum effects can influence objects in a vacuum, according to the Encyclopedia Britannica.
Asle Sudbø points out that the experiment needs to be replicated in the real world.(Photo: NTNU)
Could become important for quantum technology
But even if something similar could occur in a vacuum, it is important to establish that this is actually what happens when you try to cut off a photon’s tail, Sudbø points out.
An increasing number of technologies make use of the quantum properties of photons. Two examples are quantum sensors and quantum communication.
"What these authors have discovered is essentially a new rule for how photons behave when you manipulate them rapidly. And this new rule has to be taken into account when designing these kinds of quantum-optical machines," says Sudbø.
Could be difficult to carry out
At the same time, Sudbø points out that the experiment needs to be replicated in the real world. He believes it will be difficult to accomplish.
Sudbø also notes that the University of Oslo researchers demonstrate something in the new study that has little connection to reality. If the mirror is removed infinitely fast, the thought experiment suggests that an infinite number of photons could be created.
Skaar agrees that an infinite number of photons will never actually be produced.
"The expected number of photons produced is finite. And it only becomes very large if the mirror is extremely fast," says Skaar.
However, he disagrees that the experiment would be very difficult to carry out.
"Cutting off the tail could actually be easy, as long as the photon is long. The challenging part is measuring what you end up with," says the professor.
Puzzled by the findings along the way
But even though the new study is only a thought experiment, it was not easy to carry out.
Johannes Skaar had been pondering the problem for two years before master's student Isak Cecil Onsager Rukan joined him. And even after the master's thesis was completed, they had still not reached the finish line.
Along the way, Skaar, Rukan, and researcher Jan Gulla were puzzled by many of their findings. Then, after another six months, they got there.
"I was surprised along the way, but I wasn't surprised by the result once we got there," says Skaar.
