A hypothetical “fifth force” could attract asteroids, if it exists. But asteroid Bennu shows no signs that its orbit has been altered by such a force. This puts a ceiling on how strong a possible fifth force could be, physicists report Sept. 20 Physics of communications.
Scientists know of four forces in the universe: electromagnetism, gravity, and the strong and weak nuclear forces. But some scientists suspect that a fifth force may exist. For one thing, a fifth force could help explain a great unsolved puzzle of physics: the identity of the unknown source of mass in the entire universe known as dark matter.
So scientists are looking for unknown forces. And Bennu is a particularly suitable target, realized physicist Yu-Dai Tsai and colleagues. The asteroid’s trajectory was closely monitored by NASA’s OSIRIS-REx spacecraft, which collected a sample of the asteroid and returned it to Earth in 2023 (SN: 15.2.24). “Bennu has been tracked with amazing precision over a long period of time, so even small deviations from its predicted path can provide evidence for new physics,” says Tsai, of Los Alamos National Laboratory in New Mexico.
In physics, forces go hand in hand with particles transmitting the force. For example, photons, particles of light, mediate electromagnetism. If there is a fifth force, its corresponding particles may play the role of dark matter. Bennu’s data allowed scientists to search for a fifth force associated with particles that are very light, perhaps a millionth of a trillionth of an electron volt. (For comparison, an electron has a mass of about 500,000 electron volts.)
Measurements of additional asteroids could help scientists expand the search to cover a range of possible masses. So while the space rocks returned by Bennu can tell us about the matter that makes up the objects in our solar system, its trajectory, and that of asteroids like it, can tell us about the dark side of matter.
An experiment to grow new forests in central Mexico offers hope that crucial winter habitat for millions of migrating monarch butterflies may survive into the next century.
When scientists decided to plant hundreds of small oyamel firs (Abies religiosa) about 100 kilometers from their native habitat, they weren’t sure how many trees would survive. Today, most of the seedlings are flowering, researchers report on September 17 Forest boundaries and global change. Even at an altitude of 3,800 meters, above where the trees normally grow, almost 70 percent of the seedlings survived at least three years.
While moving an entire forest may sound like a drastic measure, “desperate times call for desperate measures,” says Karen Oberhauser, a conservation biologist at the University of Wisconsin-Madison, who was not involved in the research. “If we don’t help organisms move, you know, we’re just going to lose a lot of ecosystems.”
Every fall, after the monarchs (Danaus plexippus) migrate from the milkweed meadows of southern Canada to the mountains of central Mexico, they hibernate exclusively in oyamel fir. Thousands can crowd onto a single branch, causing it to collapse under their collective weight. But the forests—and the butterflies that hibernate within—are at risk (SN: 4/4/11). Monarch butterfly populations continue to decline. And climate change projections predict that the oyamel fir will disappear almost entirely by 2090.
“I know this sounds crazy, but we need to move the forests to a higher altitude,” says Cuauhtémoc Sáenz-Romero, a forest geneticist at the Universidad Michoacana de San Nicolás de Hidalgo in Morelia, Mexico. The Oyamel fir, which grows from about 2,400 meters to 3,500 meters, needs cool mountain air to survive. The high-altitude cold also works to slow the butterflies’ metabolism, allowing them to survive the long winter. As central Mexico gets warmer, new generations of oyamel fir are likely to creep higher up their native slopes. They may soon run out of mountains to climb.
Sáenz-Romero wants to move the trees to higher mountains, but he knows they won’t get there on their own. “Unfortunately, the scene in lord of the ringswhere the trees are marching to battle – it’s just fiction. It doesn’t happen.”
His team collected oyamel fir seeds from elevations between 3,100 meters and 3,500 meters within the Monarch Butterfly Biosphere Reserve in Michoacán state and temporarily grew them in a tree nursery. Then, in partnership with the indigenous community in Calimaya, the team planted about 960 trees at four different elevations in the community forest on the Nevado de Toluca volcano.
Some saplings were planted at 3,400 meters – similar to the typical oyamel fir that lives inside the butterfly tank. But Sáenz-Romero wanted to determine how high an oyamel could get. Other trees were planted higher up, in cooler climates, at 3,600, 3,800 and 4,000 meters above sea level.
Local foresters from the indigenous community in Calimaya planted oyamel fir seedlings on the Nevado de Toluca volcano in central Mexico as part of an experiment to see if trees essential to the survival of monarch butterflies can thrive in new locations. Here, planters control young spruces that have been planted at 4,000 meters above sea level, higher than their normal habitat.Cuauhtémoc Sáenz-Romero/UMSNH
If the spruce can take root at higher-than-normal elevations, the trees may thrive there in the future as temperatures warm, Sáenz-Romero hoped.
Three years after planting, the team found that the young spruces were smaller and shorter the higher they sat in Nevado de Toluca. However, many survived their first year, which may indicate long-term survival. On average, 80 percent of seedlings that were moved to locations 2.3 degrees Celsius colder than their regions of origin survived at least three years.
Turning the experiment into reality will likely face many hurdles, including community and government support. And even if the trees can survive longer, another question remains: Will the monarch butterflies find them?
During the winter of 2023–2024, several large monarch colonies did not hibernate within the boundaries of the Monarch Butterfly Biosphere Reserve. They flew to other forests. “My guess is that monarchs are already looking for cooler places,” says Sáenz-Romero.
