The Unruh effect: Probing the structure of a vacuum


The impact of virtual particles on accelerating objects in a vacuum has never been observed, a new thought experiment aims to rectify this.

Is the vacuum of space truly empty? If quantum field theory is correct, then the answer is no. According to this theory — used to describe the physics of elementary particles or field quanta — vacuums are filled with continuously forming annihilating particles with observable manifestations; for example, they force structureless quarks to be confined within composite particles, like protons neutrons.

Moreover, the mathematical machinery of quantum field theory shows that even the number of stable particles that we see in the vacuum is never fixed but dependent on the motion of the observer — moving at a constant speed, then there are no observable particles, but at an accelerated rate, electromagnetic radiation will be observed, similar to the radiation of a heated body. The more rapid the acceleration, the higher the radiation temperature. This prediction of quantum field theory is called the Unruh effect.

The acceleration needed to observe Unruh radiation with modern photon detectors is about one quadrillion times bigger than the free fall acceleration at the Earth’s surface, making it very difficult to confirm experimentally.

In a new study, a team of physicists at MIT the University of Waterloo have perfected a classical thought experiment, paving the way for a possible future observation of Unruh radiation. The general idea is to rapidly accelerate an atom which, if the Unruh hypothesis is correct, would then “observe” the flow of electromagnetic field quanta — photons. Interaction with electromagnetic radiation can put an atom into an excited state in which its electrons occupy higher energy orbitals. If such an excitation of an accelerated atom could be found experimentally, this will confirm the existence of the Unruh effect.

As mentioned, the necessary acceleration is unattainable, to avoid this complication, the physicists suggested studying a stimulated Unruh effect by placing the atom into an electromagnetic field before accelerating it.

This idea is based on a famous effect put forth by Albert Einstein about a century ago where the more intense the electromagnetic field around an atom, the higher the probability that the atom will be excited. Due to this phenomenon, the acceleration required to excite the atom is much smaller than the acceleration required for the atom in a vacuum could maybe be achieved experimentally in the near future.

The problem here is that if an atom is placed in electromagnetic field, then there is a chance that it will be excited even without the Unruh effect, which makes it difficult to disentangle the contribution of the latter from that of the atom’s interaction with electromagnetic field.

However, if an atom’s acceleration is non-uniform, it is possible to completely suppress this competing effect, reducing the corresponding excitation probability to zero. What is even more important, by analyzing the interaction of the atom with electromagnetic field using quantum electrodynamics, the scientists found the exact law of atom’s motion, which guarantees this suppression.

In their paper, the researchers did not propose a specific experimental set up for testing the theoretical results they obtained, but the work on this problem is ongoing.

There is a very important application of the study, which relies on a mathematical similarity between the Unruh effect the Hawking radiation of black holes. If Hawking’s theory is correct, then the temperature radiation of a black hole are extremely small, as a result, there is almost no chance of detecting them, even with space-based telescopes. A study of the Unruh effect could be the only chance to understthis aspect of their bizarre physics.

Reference: Barbara Šoda, Vivishek Sudhir, Achim Kempf, Acceleration-Induced Effects in Stimulated Light-Matter Interactions, Phys. Rev. Lett. (2022). arXiv:2103.15838

Feature image: Fred Moon on Unsplash



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The Ocean’s Biggest Garbage Pile Is Full of Floating Life


There are several nonprofit organizations working to remove floating plastic from the Great Pacific Patch. The largest, the Ocean Cleanup Foundation in the Netherlands, developed a net specifically to collect concentrate marine debris as it is pulled across the sea’s surface by winds currents. Once the net is full, a ship takes its contents to lfor proper disposal.

Dr. Helm other scientists warn that such nets threaten sea life, including neuston. Although adjustments to the net’s design have been made to reduce bycatch, Dr. Helm believes any large-scale removal of plastic from the patch could pose a threat to its neuston inhabitants.

“When it comes to figuring out what to do about the plastic that’s already in the ocean, I think we need to be really careful,” she said. The results of her study “really emphasize the need to study the open ocean before we try to manipulate it, modify it, clean it up or extract minerals from it.”

Laurent Lebreton, an oceanographer with the Ocean Cleanup Foundation, disagreed with Dr. Helm.

“It’s too early to reach any conclusions on how we should react to that study,” he said. “You have to take into account the effects of plastic pollution on other species. We are collecting several tons of plastic every week with our system — plastic that is affecting the environment.”

Plastic in the ocean poses a threat to marine life, killing more than a million seabirds every year, as well as more than 100,000 marine mammals, according to UNESCO. Everything from fish to whales can become entangled, animals often mistake it for food end up starving to death with stomachs full of plastic.

Ocean plastics that don’t end up asphyxiating an albatross or entangling an elephant seal eventually break down into microplastics, which penetrate every branch of the food web are nearly impossible to remove from the environment.

One thing everyone agrees on is that we need to stop the flow of plastic into the ocean.

“We need to turn off the tap,” Mr. Lecomte said.



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A Psychedelic Trip to Timothy Leary’s Catalina Resort in Mexico


Their Mexican neighbors were bemused. The Zihuatanejo historian Rodrigo Campus Aburto, a young teen in the 1960s, recalls that the community thought the mostly American trippers were lunatics. He also remembers older teens sometimes attended fiestas that IFIF hosted on the beach. “Moon, fire beer,” is how he describes the parties. Some smoked marijuana (Guerrero state was then, still is, a major marijuana producing area), but “the sacrament,” as the IFIF people called their LSD, was not shared with the locals.

It was decades before the rise of the narco-trafficking that has wreaked murderous violence havoc on Mexico. The one rule of IFIF was that people on LSD were not to leave the compound, by all available accounts, that seems to have been followed.

One or two individuals did wind up in Mexico City hospitals with breakdowns, according to a Saturday Evening Post article published in the fall of 1963, titled “Mind-Distorting Drugs: The Weird Saga of LSD.”

On June 13, 1963, the Mexican government formally gave the group 20 days to leave the country. It’s unclear exactly what prompted the expulsion. “They were breaking the law,” Mr. Aburto said. The Saturday Evening Post reported Leary got the group deported after he read a paper on LSD at the National Autonomous University of Mexico’s Institute of Biomedical Research, as it is now known. The scandalized director deemed his talk “absurd, confused, valueless,” protested to the Mexican government.

Besides the Mexican federales, the group faced a more primeval challenge. The group was 60 percent male, Dr. Downing, the California psychiatrist ever the empirical observer, dryly noted that “marital instability characterized many.”

Mr. Weil, the psychologist, brought his wife to the community was among the few participants whose marriage survived. “I do remember a kind of loosening of sexual bounds,” he said. “It was like a love fest.”

Did the Zihuatanejo Project achieve its goals? Mr. Weil isn’t sure. “The intent, as I reflect now, was to form a more concentrated network, a more concentrated group who could carry on the work. How naïve we were in terms of our belief that we could change the world overnight!”



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Astronauts Have Distinct Brain Changes Even Months After They Return to Earth


Flung into freefall for months on end, our bodies adjust in ways that makes for a long list of health concerns for space travelers.

The latest evaluation of microgravity’s warping effect on our biology focuses on the spaces surrounding the blood vessels that weave through our brain, revealing concerning changes that remain with astronauts between missions.

 

Researchers from across the US compared a series of magnetic resonance image (MRI) scans of 15 astronaut brains taken prior to a six-month stay on the International Space Station, up to six months after their return.

Using algorithms to carefully assess the sizes of perivascular spaces (gaps in brain tissue thought to facilitate the balance of fluids), the team found time spent in orbit had a profound effect on the brain’s plumbing. For the first-timers, at least.

Among the pool of veteran astronauts, there appeared to be little difference in the sizes of perivascular spaces in the two scans taken prior to the mission the four taken after.

“Experienced astronauts may have reached some kind of homeostasis,” says Oregon Health & Science University neurologist Juan Piantino.

The findings might not be all that surprising given what we already know about how the brain distorts when the constant tug of gravity is canceled out.

Previous studies on brain tissues their fluid volumes have found they’re slow to recover from a stint in space, with some changes persisting for a year or more.

 

Right now, astronauts rarely make more than a few trips into space in their lifetime, typically hanging around for roughly six months at a time. Yet as commercialization of a space industry ramps up, this could all change.

It will pay to know whether repeat trips compound harm, or if changes experienced in that first trip temporarily adapt astronauts to a new kind of normal.

“We all adapted to use gravity in our favor,” says Piantino.

“Nature didn’t put our brains in our feet – it put them high up. Once you remove gravity from the equation, what does that do to human physiology?”

Even in the context of expanded perivascular spaces, it’s not yet fully clear if the change comes with any appreciable health risks.

We tend to make the most use of this neurological drainage system when we sleep. The flush of fluids around our grey matter seems to play an important role in removing waste products that accumulate during our more active hours.

Without these channels functioning efficiently, disruptive materials might accumulate, potentially contributing to increased risks of neurodegenerative disorders like Alzheimer’s.

 

It’s too soon to tell if microgravity has any impact at all on the circulation of cerebral spinal fluid around our noggins, let alone if changes in the shapes of the network of channels is significant. It might not even become evident until researchers have a good sized sample of veteran astronauts with a substantial career under their belt.

Knowing more about these small adjustments goes beyond the potential harms of working off world in a space industry.

“It also forces you to think about some basic fundamental questions of science how life evolved here on Earth,” says Piantino.

Gravity’s ever-present pull isn’t just something we fight against, after all. It’s a force we’ve evolved to utilize, assisting in the flow of blood shedding of waste, potentially a variety of other functions we’ve barely considered.

By studying the subtle changes in health anatomy under conditions we never evolved to endure, we’re almost certain to learn more about diseases disorders our bodies have been forced to weather down here.

This research was published in Scientific Reports.

 



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Sales Are Booming, But Activity Is Plummeting. Why?


Worldwide sales of fitness trackers increased from US$14 billion in 2017 to over $36 billion in 2020. The skyrocketing success of these gadgets suggests that more people than ever see some value in keeping tabs on the number of steps they take, flights of stairs they climb, time they spend sitting, calories they burn.

 

The manufacturers of these devices certainly want consumers to believe that tracking fitness or health-related behaviors will spur them on to increase their activity levels make them healthier.

Our analysis of research published over the past 25 years suggests otherwise.

We are professors of kinesiology – the science of human body movement – at Boise State, the University of Tennessee, the University of North Florida. To learn whether how physical activity has changed in the years since fitness trackers became popular, we analyzed more than two decades of research from several industrialized nations – all conducted before the COVID-19 pandemic.

Our systematic review of data from eight developed nations around the world shows that despite the surge in sales of fitness trackers, physical activity declined from 1995 to 2017.

What’s more, we discovered that this was not an isolated effect in one or two countries, but a widespread trend.

Reviewing the research

To conduct the study, we first searched for published research that tracked physical activity such as walking, household activities, or playing sports throughout the day. We wanted studies that obtained two ‘snapshots’ of daily activity from a population, with the measurements separated by at least one year.

We found 16 studies from eight different countries that met these criteria: Canada, the Czech Republic, Denmark, Greece, Japan, Norway, Sweden, the United States. The studies were conducted between 1995 2017.

 

It is important to note that these snapshots did not track specific individuals. Rather, they tracked samples of people from the same age group. For example, one Japanese study of physical activity among adults ages 20 to 90 collected data each year for 22 years from people in each age group.

Scientists tracked the participants’ physical activity using a variety of wearable devices, from simple pedometers – step counters – to more sophisticated activity monitors like accelerometers.

The study groups ranged from large, nationally representative samples numbering tens of thousands of people to small samples of several hundred students from a few local schools.

After identifying the research studies, we calculated an ‘effect size’ for each study. The effect size is a method of adjusting the data to allow for an ‘apples-to-apples’ comparison. To calculate the effect size, we used the data reported in the studies.

These include the average physical activity at the beginning end of each study, the sample size, a measure of the variability in physical activity. Using a technique called meta-analysis, this allowed us to combine the results of all studies to come up with an overall trend.

 

We discovered that overall, researchers documented fairly consistent declines in physical activity, with similar decreases in each geographical region in both sexes.

Overall, the decrease in physical activity per person was over 1,100 steps per day between 1995 2017.

Our most striking finding was how sharply physical activity declined among adolescents ages 11 to 19 years – by roughly 30 percent – in the span of a single generation.

For instance, when we compared the studies reporting physical activity in steps per day, we found the total steps per day per decade declined by an average of 608 steps per day in adults, 823 steps per day in children, 1,497 steps per day in adolescents.

Our study doesn’t address why physical activity has declined over the past 25 years. However, the studies we reviewed mentioned some contributing factors.

More staring at screens, less walking or bicycling

Among adolescents, declines in physical activity were associated with increases in ownership use of smartphones, tablets, video games, social media.

In the U.S., for example, screen time increased dramatically in adolescents, from five hours per day in 1999 to 8.8 hours per day in 2017.

 

At school, most of the physical activity that adolescents perform has traditionally come from physical education classes. However, the changes in the frequency of physical education classes during the study period are inconsistent vary from country to country.

All of these factors may help to explain the decline in physical activity that we observed in our study.

In addition, fewer adults children are walking or bicycling to school or work than 25 years ago.

For instance, in the late 1960s, most U.S. children ages 5 to 14 rode a bicycle or walked to school. Since then, this ‘active transportation’ has largely been replaced by automobile trips. Rates of travel by school bus or public transportation have seen little change.

So why use a fitness tracker?

So if levels of physical activity have dropped at the same time that the popularity of fitness tracking has grown, what makes these gadgets useful?

Fitness trackers can help to increase people’s awareness of their daily physical activity. However, these devices are only part of the solution to addressing the problem of sedentary lifestyles. They are facilitators, rather than drivers, of behavior change.

When a person’s physical activity goes down, it opens the door to overall reduced fitness levels other health problems such as obesity or diabetes.

On the other hand, physical activity has a dramatic positive impact on health well-being.

The first step to increasing active movement is to measure it, which these devices can do. But successfully increasing one’s overall physical activity requires several additional factors such as goal-setting, self-monitoring, positive feedback, social support.

Scott A. Conger, Associate Professor of Exercise Physiology, Boise State University; David Bassett, Professor Department Head of Kinesiology, Recreation Sport Studies, University of Tennessee, Lindsay Toth, Assistant Professor of Kinesiology, University of North Florida.

This article is republished from The Conversation under a Creative Commons license. Read the original article.

 



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