Wednesday, July 12, 2017

A Study About Nothing

A vacuum is a space absolutely devoid of matter, at least according to the Merriam-Webster dictionary. But if you talk to a physicist you may get a different answer. According to quantum physics, even vacuums are not completely empty. Constant fluctuations in energy can spontaneously create mass not just out of thin air, but out of absolutely nothing at all.

"It's like a boiling sea of appearing and disappearing particle pairs," said James Koga, a theoretical physicist from the National Institutes for Quantum and Radiological Science and Technology in Kyoto, Japan. The pairs, made up of one particle and one antiparticle, exist for only moments. Koga is investigating the subtle effects caused by these fluctuations.

This peculiar nature of vacuum, sometimes referred to as "quantum vacuum," is not just theoretical speculation. It has real, measurable effects on our physical reality. Although these effects are usually far too small to impact even the most sensitive instruments of today, scientists think the picture will change for the miniaturized technologies of tomorrow.

"In the macroscopic world, we don't care about these forces at all. You wouldn't care about it when you are driving a car for instance. It's totally negligible," said Alejandro Manjavacas, a physicist specializing in photonics at the University of New Mexico in Albuquerque. "But in the context of nanotechnology or nanophotonics—at a super small scale, these effects will start playing a role."

Although the concept of a fluctuating vacuum was theorized and proven during the first half of the last century, scientists are still grappling with the implications. Two recently published papers explore two separate aspects of the same mystery—what happens when there is nothing at all?

A glistening ocean

The energy fluctuation in vacuum can be explained by the uncertainty principle of quantum physics. The principle, first introduced by German physicist Werner Heisenberg, states that at any definite point in space, there must exist temporary changes in energy over time. Sometimes this energy is converted into mass, generating particle-antiparticle pairs.

Most of the time these newly born pairs recombine and vanish before interacting with anything. Because of this, physicists like to refer to these pairs as "virtual particles," but this doesn't mean they aren't real—they just need something to interact with to make their presence felt.

For this, Koga and his team envision a way to observe this boiling sea of vacuum the same way we see glistening waves in the ocean—with light. In their latest paper, published in Physical Review Letters, they lay down the theoretical groundwork needed for the experiment. Specifically, they want to study photons that bounce off an atomic nucleus in a distinctive way that wouldn’t happen without the "boiling" vacuum acting as the middleman. This peculiar light phenomenon is known as Delbrück scattering, predicted by German-American physicist Max Delbrück in 1933. The effect was later observed experimentally in 1975—but just barely.

"[Scientists] could kind of guess that the Delbrück scattering was there, but it was like if you include this effect in your calculation then it agrees more with the data," said Koga.

Koga and his team hope to take Delbrück scattering to another level by characterizing the phenomenon’s effect. It is as if scientists knew about air resistance, but still needed to study it further so that engineers could use the knowledge to build an airplane.

The task is tricky. To measure Delbrück scattering, one must shine light onto trillions of atomic nuclei, which creates a problem. Photons bounce off nuclei, electrons and even each other in all directions, via all kinds of different interactions. How can one distinguish which photon is scattered from what?

Koga's team suggests that we use polarized gamma rays. Just like polarized sunglasses can help you see better by filtering out unwanted solar glares, polarized gamma rays can help scientists sift through the gazillions of photons based on their polarization, in addition to energy and scattered angle. As long as one knows where to look for the specific photons that are the results of Delbrück scattering, one should be able to pick them out from the lineup.

"The point that we are trying to make in our paper is by using a new polarized source, you can almost see the signal isolated," said Koga.

But there is just one problem—such an instrument doesn't exist. At least not yet.

Enter the future Extreme Light Infrastructure in Măgurele, Romania. This facility will not only provide the polarized gamma rays Koga proposed, but will make some of the brightest gamma rays in the world. This is important because just like a brighter ambient light can shorten the exposure time for taking a photo, a brighter gamma ray can shorten the run time for Koga's proposed experiment.

Credit: ELI-NP Romania

Kazuo Tanaka, the scientific director of the Nuclear Physics division of the future facility, is pleased with Koga's team's proposal.

"I think their proposal is very crystal clear. They calculated how many days of shooting they need for the experiment, and came up with 76 days," he said. "I think if they do the experiment we can have a very definitive measurement for Delbrück scattering."

While the facility is still under construction, and will not be ready for the experiment at least until 2019, a different group of physicists are studying the same nothingness of vacuum, but with a different set of eyes. Instead of beaming light into the vacuum and looking for a glint, physicist Alejandro Manjavacas and his group at the University of New Mexico want to know if the fluctuations of vacuum can actually exert an invisible force on physical objects -- as if they were being moved by Jedis.

An invisible force

The video shows two plates moving towards each other in a vibrating pool of water, an analogy to the Casimir effect that exist in a fluctuating vacuum. Credit: Denysbondar

The Casimir effect, named after Dutch physicist Henrik Casimir, describes the force that pushes two objects together due to surrounding waves. The effect exists for two beads on a vibrating string, or two boats in a wavy ocean, as well as two particles in a fluctuating vacuum. Much like Delbrück scattering, the Casimir effect was theorized in 1948 and has already been confirmed, in 1996. So, what is left to be discovered?

"Most of the work that was done on Casimir effect was for systems that weren't moving, or if they were moving, they were moving in a uniform motion," said Manjavacas.

In a paper published in Physical Review Letters, Manjavacas and his colleagues calculated how the Casimir effect can nudge objects that are already spinning and moving. Through calculations, they discovered that when a tiny sphere spins near a flat surface, it will move as if it is rolling down the surface, despite never making contact with it.

"If you try to make a nanostructure that involves moving parts that are very close together, it is crucial to know what is going to be the effect from these type of forces. You'll need to know whether it is going to cause the moving parts to get stuck," said Manjavacas. "Or we can use these forces to our advantage, such as using them to move objects or to force them to do the things that we want."

In their study, the researchers evaluated the effect for spheres with diameters ranging from 50 to 500 nanometers, much less than one hundredth the width of a human hair. As expected, the relationship between the spinning and the lateral movement isn't straightforward—it depends on the speed that the sphere is spinning, as well as the size of the sphere and the distance between the sphere and the surface. These minute effects may soon be relevant on the frontier of technology, for example when engineers design medical nanobots.

A real virtuality

Even though the quantum interpretation of vacuum—complete with strange particles popping into and out of existence—accurately describes our reality, how can we tell that this isn't just another placeholder theory? Will the theory eventually fail just like the geocentric model, or the flat earth model, or perhaps most relevantly, the famous failed theory of ether from the 19th century?

The theory of ether was proposed by physicists to explain how light waves can propagate through the vacuum of space. Based on intuition, scientists back then believed that a medium was necessary for light waves to travel, just like the waves in the ocean travel through the medium of water. This hypothesis was disproved in 1887 by Albert Michelson and Edward Morley, in a famous experiment, in which they measured the speed of light in perpendicular directions and found no difference. Albert Michelson was later awarded a Nobel Prize in 1907 for his achievements, and became the first Nobel laureate from the United States.

So, will the quantum model of vacuum also be proved wrong? Most physicists today do not think so. In fact, Nobel laureate Robert Laughlin from Stanford University has written in his book "A Different Universe: Reinventing Physics from the Bottom Down" specifically about this comparison: "The word [ether] has extremely negative connotations in theoretical physics because of its past association with opposition to relativity. This is unfortunate because, stripped of these connotations, it rather nicely captures the way most physicists actually think about the vacuum."

Because unlike the ether theory, the quantum model of vacuum, with all its fluctuations and peculiar features, has since been thoroughly tested and proven.

"We see pair creation all the time actually, like in particle accelerators," said Koga. In fact, it happens so often that for certain experiments scientists actually have to consider the phenomenon as "noise" that could obscure the signal they are looking for, according to Koga.

"We now have experimental evidence of all kinds of particles coming in and out [of the vacuum]," said Toshiki Tajima, a physicist from the University of California, Irvine. "Muons and anti-muons, protons and anti-protons, and even quarks and anti-quarks."

In 1665, Robert Hooke and Antoni van Leeuwenhoek discovered microbes when they pointed their microscopes at "nothing." In 1964, Arno Penzias and Robert Woodrow Wilson discovered the cosmic microwave background when they pointed their telescopes at "nothing." Vacuum is perhaps the ultimate "nothing," so if history is any indication, "nothing" is an interesting place, especially if you want to look for something.

Yuen Yiu, Inside Science News

Yuen Yiu covers the Physics beat for Inside Science. He's a Ph.D. physicist and fluent in Cantonese and Mandarin. Follow Yuen on Twitter:

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