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Why This Universe? Maybe It’s Not Special—Just Probable
Cosmologists have spent decades striving to understand why our universe is so stunningly vanilla. Not only is it smooth and flat as far as we can see, but it’s also expanding at an ever-so-slowly increasing pace, when naive calculations suggest that—coming out of the Big Bang—space should have become crumpled up by gravity and blasted apart by repulsive dark energy.
To explain the cosmos’s flatness, physicists have added a dramatic opening chapter to cosmic history: They propose that space rapidly inflated like a balloon at the start of the Big Bang, ironing out any curvature. And to explain the gentle growth of space following that initial spell of inflation, some have argued that our universe is just one among many less hospitable universes in a giant multiverse.
But now two physicists have turned the conventional thinking about our vanilla universe on its head. Following a line of research started by Stephen Hawking and Gary Gibbons in 1977, the duo has published a new calculation suggesting that the plainness of the cosmos is expected, rather than rare. Our universe is the way it is, according to Neil Turok of the University of Edinburgh and Latham Boyle of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, for the same reason that air spreads evenly throughout a room: Weirder options are conceivable but exceedingly improbable.
The universe “may seem extremely fine-tuned, extremely unlikely, but [they’re] saying, ‘Wait a minute, it’s the favored one,’” said Thomas Hertog, a cosmologist at the Catholic University of Leuven in Belgium.
“It’s a novel contribution that uses different methods compared to what most people have been doing,” said Steffen Gielen, a cosmologist at the University of Sheffield in the United Kingdom.
The provocative conclusion rests on a mathematical trick involving switching to a clock that ticks with imaginary numbers. Using the imaginary clock, as Hawking did in the ’70s, Turok and Boyle could calculate a quantity, known as entropy, that appears to correspond to our universe. But the imaginary time trick is a roundabout way of calculating entropy, and without a more rigorous method, the meaning of the quantity remains hotly debated. While physicists puzzle over the correct interpretation of the entropy calculation, many view it as a new guidepost on the road to the fundamental, quantum nature of space and time.
“Somehow,” Gielen said, “it’s giving us a window into perhaps seeing the microstructure of space-time.”
Imaginary Paths
Turok and Boyle, frequent collaborators, are renowned for devising creative and unorthodox ideas about cosmology. Last year, to study how likely our universe may be, they turned to a technique developed in the ’40s by the physicist Richard Feynman.
Aiming to capture the probabilistic behavior of particles, Feynman imagined that a particle explores all possible routes linking start to finish: a straight line, a curve, a loop, ad infinitum. He devised a way to give each path a number related to its likelihood and add all the numbers up. This “path integral” technique became a powerful framework for predicting how any quantum system would most likely behave.
As soon as Feynman started publicizing the path integral, physicists spotted a curious connection with thermodynamics, the venerable science of temperature and energy. It was this bridge between quantum theory and thermodynamics that enabled Turok and Boyle’s calculation.
The South African physicist and cosmologist Neil Turok is a professor at the University of Edinburgh.Photograph: Gabriela Secara/Perimeter Institute
New measurements of galaxy rotation lean toward modified gravity as an explanation for dark matter
Although dark matter is a central part of the standard cosmological model, it’s not without its issues. There continue to be nagging mysteries about the stuff, not the least of which is the fact that scientists have found no direct particle evidence of it.
Despite numerous searches, we have yet to detect dark matter particles. So some astronomers favor an alternative, such as modified Newtonian dynamics (MoND) or modified gravity model. And a new study of galactic rotation seems to support them.
The idea of MoND was inspired by galactic rotation. Most of the visible matter in a galaxy is clustered in the middle, so you’d expect that stars closer to the center would have faster orbital speeds than stars farther away, similar to the planets of our solar system. What we observe is that stars in a galaxy all rotate at about the same speed. The rotation curve is essentially flat rather than dropping off. The dark matter solution is that galaxies are surrounded by a halo of invisible matter, but in 1983 Mordehai Milgrom argued that our gravitational model must be wrong.
At interstellar distances, the gravitational attraction between stars is essentially Newtonian. So rather than modifying general relativity, Milgrom proposed modifying Newton’s universal law of gravity. He argued that rather than the force of attraction as a pure inverse square relation, gravity has a small remnant pull regardless of distance. This remnant is only about 10 trillionths of a G, but it’s enough to explain galactic rotation curves.
Of course, just adding a small term to Newton’s gravity means that you also have to modify Einstein’s equations, as well. So MoND has been generalized in various ways, such as AQUAL, which stands for “a quadradic Lagrangian.” Both AQUAL and the standard LCDM model can explain observed galactic rotation curves, but there are some subtle differences.
This is where a recent study comes in. One difference between AQUAL and LCDM is in the rotation speeds of inner orbit stars vs. outer orbit stars. For LCDM, both should be governed by the distribution of matter, so the curve should be smooth. AQUAL predicts a tiny kink in the curve due to the dynamics of the theory. It’s too small to measure in a single galaxy, but statistically, there should be a small shift between the inner and outer velocity distributions.
So the author of this paper looked at high-resolution velocity curves of 152 galaxies as observed in the Spitzer Photometry and Accurate Rotation Curves (SPARC) database. He found a shift in agreement with AQUAL. The data seems to support modified gravity over standard dark matter cosmology.
The result is exciting, but it doesn’t conclusively overturn dark matter. The AQUAL model has its own issues, such as its disagreement with observed gravitational lensing by galaxies. But it is a win for the underdog theory, which has some astronomers cheering “Vive le MoND!”
The research is published on the arXiv preprint server.
More information:
Kyu-Hyun Chae, Distinguishing Dark Matter, Modified Gravity, and Modified Inertia with the Inner and Outer Parts of Galactic Rotation Curves, arXiv (2022). DOI: 10.48550/arxiv.2207.11069
Journal information:
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Underground Italian lab searches for signals of quantum gravity
For decades, physicists have been hunting for a quantum-gravity model that would unify quantum physics, the laws that govern the very small, and gravity. One major obstacle has been the difficulty in testing the predictions of candidate models experimentally. But some of the models predict an effect that can be probed in the lab: a very small violation of a fundamental quantum tenet called the Pauli exclusion principle, which determines, for instance, how electrons are arranged in atoms.
A project carried out at the INFN underground laboratories under the Gran Sasso mountains in Italy, has been searching for signs of radiation produced by such a violation in the form of atomic transitions forbidden by the Pauli exclusion principle.
In two papers appearing in the journals Physical Review Letters (published on September 19, 2022) and Physical Review D (accepted for publication on December 7, 2022) the team reports that no evidence of violation has been found, thus far, ruling out some quantum-gravity models.
In school chemistry lessons, we are taught that electrons can only arrange themselves in certain specific ways in atoms, which turns out to be due to the Pauli exclusion principle. At the center of the atom there is the atomic nucleus, surrounded by orbitals, with electrons. The first orbital, for instance, can only house two electrons. The Pauli exclusion principle, formulated by Austrian physicist Wolfang Pauli in 1925, says that no two electrons can have the same quantum state; so, in the first orbital of an atom the two electrons have oppositely pointing “spins” (a quantum internal property usually depicted as an axis of rotation, pointing up or down, although no literal axis exists in the electron).
The happy result of this for humans is that it means matter cannot pass through other matter. “It is ubiquitous—you, me, we are Pauli-exclusion-principle-based,” says Catalina Curceanu, a member of the physics think-tank, the Foundational Questions Institute, FQXi, and the lead physicist on the experiments at INFN, Italy. “The fact we cannot cross walls is another practical consequence.”
The principle extends to all elementary particles belonging to the same family as electrons, called fermions, and has been derived mathematically from a fundamental theorem known as the spin-statistics theorem. It has also been confirmed experimentally—thus far—appearing to hold for all fermions in tests. The Pauli exclusion principle forms one of the core tenets of the standard model of particle physics.
Violating the principle
But some speculative models of physics, beyond the standard model, suggest that the principle may be violated. For decades now, physicists have been searching for a fundamental theory of reality. The standard model is terrific at explaining the behavior of particles, interactions and quantum processes on the microscale. However, it does not encompass gravity.
So, physicists have been trying to develop a unifying theory of quantum gravity, some versions of which predict that various properties that underpin the standard model, such as the Pauli exclusion principle, may be violated in extreme circumstances.
“Many of these violations are naturally occurring in so-called ‘noncommutative’ quantum-gravity theories and models, such as the ones we explored in our papers,” says Curceanu. One of the most popular candidate quantum-gravity frameworks is string theory, which describes fundamental particles as tiny vibrating threads of energy in multidimensional spaces. Some string theory models also predict such a violation.
“The analysis we reported disfavors some concrete realizations of quantum gravity,” says Curceanu.
It is traditionally thought to be hard to test such predictions because quantum gravity will usually only become relevant in arenas where there is a huge amount of gravity concentrated into a tiny space—think of the center of a black hole or the beginning of the universe.
However, Curceanu and her colleagues realized that there may be a subtle effect—a signature that the exclusion principle and the spin-statistics theorem have been violated—that could be picked up in lab experiments on Earth.
Deep under the Gran Sasso mountains, near the town of L’Aquila, in Italy, Curceanu’s team is working on the VIP-2 (Violation of the Pauli Principle) lead experiment. At the heart of the apparatus is a thick block made of Roman lead, with a nearby germanium detector that can pick up small signs of radiation emanating from the lead.
The idea is that if the Pauli exclusion principle is violated, a forbidden atomic transition will occur within the Roman lead, generating an X-ray with a distinct energy signal. This X-ray can be picked up by the germanium detector.
Cosmic silence
The lab must be housed underground because the radiation signature from such a process will be so faint, it would otherwise be drowned out by the general background radiation on Earth from cosmic rays. “Our laboratory ensures what is called ‘cosmic silence,’ in the sense that the Gran Sasso mountain reduces the flux of cosmic rays by a million times,” says Curceanu. That alone is not enough, however.
“Our signal has a possible rate of just one or two events per day, or less,” says Curceanu. That means that materials used in the experiment must themselves be “radio-pure”—that is, they must not emit any radiation themselves—and the apparatus must be shielded from radiation from the mountain rocks and radiation coming from underground.
“What is extremely exciting is that we can probe some quantum-gravity models with such a high precision, which is impossible to do at present-day accelerators,” says Curceanu.
In their recent papers, the team reports having found no evidence for violation of the Pauli principle. “FQXi-funding was fundamental for developing the data analysis techniques,” says Curceanu. This allowed the team to set limits on the size of any possible violation and helped them constrain some proposed quantum-gravity models.
In particular, the team analyzed the predictions of the so-called “theta-Poincaré” model and were able to rule out some versions of the model to the Planck scale (the scale at which the known classical laws of gravity break down). In addition, “the analysis we reported disfavors some concrete realizations of quantum gravity,” says Curceanu.
The team now plans to extend its research to other quantum-gravity models, with their theoretician colleagues Antonino Marcianò from Fudan University and Andrea Addazi from Sichuan University, both in China. “On the experimental side, we will use new target materials and new analysis methods, to search for faint signals to unveil the fabric of spacetime,” says Curceanu.
“What is extremely exciting is that we can probe some quantum-gravity models with such a high precision, which is impossible to do at present-day accelerators,” Curceanu adds. “This is a big leap, both from theoretical and experimental points of view.”
More information:
Kristian Piscicchia et al, Strongest Atomic Physics Bounds on Noncommutative Quantum Gravity Models, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.131301
Kristian Piscicchia et al, Experimental test of noncommutative quantum gravity by VIP-2 Lead, Physical Review D (2022). journals.aps.org/prd/accepted/ … 182249cd253e38bf3406
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Wild New Hypothesis Suggests IBS Could Be a Form of ‘Gravity Intolerance’ : ScienceAlert
There’s an invisible and relentless force acting on your bowels right now, and it might be causing some people serious irritation.
No one really knows how or why irritable bowel syndrome (IBS) develops, but gastroenterologist Brennan Spiegel from Cedars-Sinai hospital in Los Angeles has outlined a weighty new hypothesis.
In a paper published in The American Journal of Gastroenterology, Spiegel argues IBS is triggered by the body’s inability to manage gravity.
Our bowels, Spiegel explains, are like a big sack of potatoes that we have to carry around our whole lives.
If our body’s usual management of gravity fails for whatever reason, our diaphragm can slip down and compress our intestines, possibly causing motility issues and bacterial overgrowth.
“Our nervous system also evolved in a world of gravity, and that might explain why many people feel abdominal ‘butterflies’ when anxious,” says Spiegel.
“It’s curious that these ‘gut feelings’ also occur when falling toward Earth, like when dropping on a roller coaster or in a turbulent airplane. The nerves in the gut are like an ancient G-force detector that warns us when we’re experiencing – or about to experience – a dangerous fall. It’s just a hypothesis, but people with IBS might be prone to over-predicting G-force threats that never occur.”
The nice part about Spiegel’s hypothesis is that it’s easily testable and doesn’t exclude other theories of IBS.
Currently, there is no definitive test for IBS, and its symptoms are extremely variable from patient to patient. As a result, the syndrome is usually made as a diagnosis of exclusion.
Once other disorders that can cause gut symptoms – such as pain, bloating, cramping, constipation, or diarrhea – have been ruled out, patients are usually told they have IBS.
Today, about 10 percent of people worldwide are thought to suffer from the syndrome, and Spiegel is one of many scientists working to figure out why.
Gravity, he argues, might be the grounding force that pulls all these different symptoms together.
Under Spiegel’s framework, a disordered response to gravity might also trigger a gut-to-brain interaction disorder. By squashing the intestines, it might even impact the gut microbiome, causing hypersensitivity, inflammation, or discomfort.
“There’s such a variety of explanations that I wondered if they could all be simultaneously true,” says Spiegel.
“As I thought about each theory, from those involving motility, to bacteria, to the neuropsychology of IBS, I realized they might all point back to gravity as a unifying factor. It seemed pretty strange at first, no doubt, but as I developed the idea and ran it by colleagues, it started to make sense.”
If IBS is caused by the body struggling to grapple with gravity, then it could explain why physical therapy and exercise can prove so beneficial in relieving its symptoms.
It could also explain why serotonin tends to be elevated in IBS patients.
Serotonin is a neurotransmitter that is primarily produced in the gut to regulate our bowel movements and also our mood, but too much of it can trigger diarrhea. It is also involved in the regulation of our blood pressure in response to gravity.
Without serotonin, Spiegel says, your body might not be able to stand up, maintain balance, or continue circulating blood.
“Dysregulated serotonin may be a form of gravity failure,” argues Spiegel.
“When serotonin biology is abnormal, people can develop IBS, anxiety, depression, fibromyalgia, and chronic fatigue. These may be forms of gravity intolerance.”
Chronic fatigue syndrome/ myalgic encephalomyelitis (CFS/ME) is another chronic and debilitating sickness without a cause or cure, and it often crosses over with IBS. Many CFS/ME patients also struggle with standing up, which can cause a sudden drop in blood pressure, fatigue, dizziness, and a racing heart.
Other symptoms that cross over with IBS include lower back pain, headaches, dizziness and postural tachycardia syndrome (POTS), which is when blood pressure plummets after a person rises.
All of these conditions could be explained by the body’s inability to properly manage the force of gravity.
Without direct research, Siegel says the gravity hypothesis is just a “thought experiment”. But he hopes it encourages new ways of researching and treating IBS in the future.
“Our relationship to gravity is not unlike the relationship of fish to water,” writes Siegel.
“We live our entire life in it, are shaped by it, yet hardly notice its ever present influence on the nature of our existence.”
Perhaps it’s about time we considered it.
The study was published in The American Journal of Gastroenterology.
What causes IBS? Gravity allergy to blame, scientist theorizes
People with irritable bowel syndrome (IBS) may actually be allergic to gravity, scientists have suggested.
The true cause of IBS is not known, but one scientist thinks it could be due to gravity’s pull on intestines in the body.
The abdomen is kept in place by muscle and bones, but if the body cannot handle gravity’s force, it could squash the spine and cause organs to shift downwards.
This could lead to symptoms of IBS including pain, cramping, lightheadedness and back problems, according to Dr Brennan Spiegel, director of Health Services Research at Cedars-Sinai in California.
Some people are better equipped to deal with gravity’s pull down on our organs, scientists have suggested
It could even cause an overgrowth of bacteria in the gut — another cause of IBS.
Between 25 and 45 million Americans are blighted by the condition, which is more common in women than men. Its main symptoms are stomach pain, gas, diarrhea and constipation.
Dr Brennan Spiegel theorizes that some people are just better at coping with gravity than others.
For instance, individuals might have a ‘stretchy’ suspension system where the intestines hang down.
Other people have spinal problems which cause the diaphragm to sag or the stomach to stick out, which results in a squashed abdomen and can set off mobility problems.
The theory might explain why exercise can help IBS, as exercise strengthens the support system holding up organs.
Dr Spiegel’s gravity theory extends beyond the intestines.
He said: ‘Our nervous system also evolved in a world of gravity, and that might explain why many people feel abdominal ‘butterflies’ when anxious.
‘It’s curious that these ‘gut feelings’ also occur when falling toward Earth, like when dropping on a roller coaster or in a turbulent airplane.
‘The nerves in the gut are like an ancient G-force detector that warns us when we’re experiencing — or about to experience — a dangerous fall. It’s just a hypothesis, but people with IBS might be prone to over-predicting G-force threats that never occur.’
People react differently to gravity, Dr Spiegel argued, leading to a spectrum of ‘G-force vigilance’.
Some will enjoy the hair-rising feeling of dropping on a rollercoaster, while others will be wishing it was over.
Dr Spiegel said other conditions may also be caused by gravity intolerance, including anxiety, depression and chronic fatigue.
He claims that a body that struggles to manage gravity may also struggle to pump serotonin – dubbed the ‘love’ hormone – and other neurotransmitters around the body.
He said: ‘Dysregulated serotonin may be a form of gravity failure.
‘When serotonin biology is abnormal, people can develop IBS, anxiety, depression, fibromyalgia, and chronic fatigue. These may be forms of gravity intolerance.’
Other theories are that IBS is a disorder arising from the interaction between the gut and the brain, because behavioral therapy and substances like serotonin can help.
Another idea is that IBS is down to harmful bacteria in the gut. Studies indicate the condition can be controlled with antibiotics and a diet with lots of eggs, meat, grains and fruit and vegetables.
Gut hypersensitivity, atypical serotonin levels or a dysregulated nervous system could also be to blame.
More research is required to test Dr Spiegel’s idea and look at potential treatments.
Dr Shelly Lu, the women’s guild chair in gastroenterology and director of the division of digestive and liver diseases at Cedars-Sinai, said the theory was ‘provocative’.
‘The best thing about it is that it is testable,’ she said.
She added: ‘If proved correct, it is a major paradigm shift in the way we think about IBS and possibly treatment as well.’
The hypothesis was published in the American Journal of Gastroenterology.
Are Newton’s Laws of Gravity Wrong: Observation Puzzles Researchers
Astrophysicists have made a puzzling discovery while analyzing certain star clusters. The finding challenges Newton’s laws of gravity. Instead, the observations are consistent with the predictions of an alternative theory of gravity. (Artistic concept of strange gravity.)
Finding cannot be explained by classical assumptions.
An international team of astrophysicists has made a puzzling discovery while analyzing certain star clusters. The finding challenges Newton’s laws of gravity, the researchers write in their publication. Instead, the observations are consistent with the predictions of an alternative theory of gravity. However, this is controversial among experts. The results have now been published in the Monthly Notices of the Royal Astronomical Society. The University of Bonn played a major role in the study.
In their work, the researchers investigated the so-called open star clusters, which are loosely bound groups of a few tens to a few hundred stars that are found in spiral and irregular galaxies. Open clusters are formed when thousands of stars are born within a short time in a huge gas cloud. As they “ignite,” the galactic newcomers blow away the remnants of the gas cloud. In the process, the cluster greatly expands. This creates a loose formation of several dozen to several thousand stars. The cluster is held together by the weak gravitational forces acting between them.
“In most cases, open star clusters survive only a few hundred million years before they dissolve,” explains Prof. Dr. Pavel Kroupa of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn. In the process, they regularly lose stars, which accumulate in two so-called “tidal tails.” One of these tails is pulled behind the cluster as it travels through space. In contrast, the other one takes the lead like a spearhead.
Prof. Dr. Pavel Kroupa of the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn. Credit: Volker Lannert / University of Bonn
“According to Newton’s laws of gravity, it’s a matter of chance in which of the tails a lost star ends up,” explains Dr. Jan Pflamm-Altenburg of the Helmholtz Institute of Radiation and Nuclear Physics. “So both tails should contain about the same number of stars. However, in our work we were able to prove for the first time that this is not true: In the clusters we studied, the front tail always contains significantly more stars nearby to the cluster than the rear tail.”
New method developed for counting stars
From among the millions of stars close to a cluster, it has been almost impossible to determine those that belong to its tails—until now. “To do this, you have to look at the velocity, direction of motion, and age of each of these objects,” explains Dr. Tereza Jerabkova. The co-author of the paper, who did her doctorate in Kroupa’s group, recently moved from the European Space Agency (ESA) to the European Southern Observatory in Garching. She developed a method that allowed her to accurately count the stars in the tails for the first time. “So far, five open clusters have been investigated near us, including four by us,” she says. “When we analyzed all the data, we encountered the contradiction with the current theory. The very precise survey data from ESA’s Gaia space mission were indispensable for this.”
In the star cluster “Hyades” (top), the number of stars (black) in the front tidal tail is significantly larger than those in the rear. In the computer simulation with MOND (below), a similar picture emerges. Credit: AG Kroupa/Uni Bonn
The observational data, in contrast, fit much better with a theory that goes by the acronym MOND (“MOdified Newtonian Dynamics”) among experts. “Put simply, according to MOND, stars can leave a cluster through two different doors,” Kroupa explains. “One leads to the rear tidal tail, the other to the front. However, the first is much narrower than the second — so it’s less likely that a star will leave the cluster through it. Newton’s theory of gravity, on the other hand, predicts that both doors should be the same width.”
Star clusters are shorter-lived than Newton’s laws predict
The team of astrophysicists calculated the stellar distribution expected according to MOND. “The results correspond surprisingly well with the observations,” highlights Dr. Ingo Thies, who played a key role in the corresponding simulations. “However, we had to resort to relatively simple computational methods to do this. We currently lack the mathematical tools for more detailed analyses of modified Newtonian dynamics.” Nevertheless, the simulations also coincided with the observations in another respect: They predicted how long open star clusters should typically survive. And this time span is significantly shorter than would be expected according to Newton’s laws. “This explains a mystery that has been known for a long time,” Kroupa points out. “Namely, star clusters in nearby galaxies seem to be disappearing faster than they should.”
However, the MOND theory is not undisputed among experts. Since Newton’s laws of gravity would not be valid under certain circumstances, but would have to be modified, this would have far-reaching consequences for other areas of physics as well. “Then again, it solves many of the problems that cosmology faces today,” explains Kroupa, who is also a member of the Transdisciplinary Research Areas “Modelling” and “Matter” at the University of Bonn. The astrophysicists are now exploring new mathematical methods for even more accurate simulations. They could then be used to find further evidence as to whether the MOND theory is correct or not.
Reference: “Asymmetrical tidal tails of open star clusters: stars crossing their cluster’s práh challenge Newtonian gravitation” by Pavel Kroupa, Tereza Jerabkova, Ingo Thies, Jan Pflamm-Altenburg, Benoit Famaey, Henri M J Boffin, Jörg Dabringhausen, Giacomo Beccari, Timo Prusti, Christian Boily, Hosein Haghi, Xufen Wu, Jaroslav Haas, Akram Hasani Zonoozi, Guillaume Thomas, Ladislav Šubr and Sverre J Aarseth, 26 October 2022, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stac2563
In addition to the University of Bonn, the study involved the Charles University in Prague, the European Southern Observatory (
The study was funded by the Scholarship Program of the Czech Republic, the German Academic Exchange Service (DAAD), the French funding organization Agence nationale de la recherche (ANR), and the European Research Council ERC.
How fast is gravity, exactly?
Of all of the fundamental forces known to humanity, gravity is both the most familiar and the one that holds the Universe together, connecting distant galaxies in a vast and interconnected cosmic web. With that in mind, a fascinating question to ponder is whether gravity has a speed. It turns out that it does, and scientists have precisely measured it.
Let’s start with a thought experiment. Suppose at this very instant, somehow the Sun was made to disappear — not just go dark, but vanish entirely. We know that light travels at a fixed speed: 300,000 kilometers per second, or 186,000 miles per second. From the known distance between the Earth and the Sun (150 million kilometers, or 93 million miles), we can calculate how long it would take before we here on Earth would know the Sun had disappeared. It would take about eight minutes and 20 seconds before the noon sky would go dark.
But what about gravity? If the sun disappeared, it would not only stop emitting light, but also stop exerting the gravity that holds the planets in orbit. When would we find out?
If gravity is infinitely fast, gravity would also disappear as soon as the Sun poofed into nonexistence. We’d still see the Sun for a little over eight minutes, but the Earth would already start wandering off, heading for interstellar space. On the other hand, if gravity traveled at the speed of light, our planet would continue to orbit the Sun as usual for eight minutes and 20 seconds, after which it would stop following its familiar path.
Of course, if gravity traveled at some other speed, the interval between when beachgoing Sun worshipers noticed the Sun was gone and when astronomers observed that the Earth was going in the wrong direction would be different. So, what is the speed of gravity?
Different answers have been proposed throughout scientific history. Sir Isaac Newton, who invented the first sophisticated theory of gravity, believed the speed of gravity was infinite. He would have predicted that the Earth’s path through space would change before Earth-bound humans noticed that the Sun was gone.
On the other hand, Albert Einstein believed that gravity traveled at the speed of light. He would have predicted that humans would simultaneously notice the disappearance of the Sun and the change of Earth’s path through the cosmos. He built this assumption into his theory of general relativity, which is currently the best accepted theory of gravity, and it very precisely predicts the path of the planets around the Sun. His theory makes more accurate predictions than Newton’s. So, can we conclude that Einstein was right?
No, we can’t. If we want to measure the speed of gravity, we need to think of a way to directly measure it. And, of course, since we can’t just “disappear” the Sun for a few moments to test Einstein’s idea, we need to find another way.
Einstein’s theory of gravity made testable predictions. The most important one is that he realized that the familiar gravity we experience can be explained as a distortion of the fabric of space: the greater the distortion, the higher the gravity. And this idea has significant consequences. It suggests that space is malleable, similar to the surface of a trampoline, which distorts when a child steps on it. Furthermore, if that same child jumps on the trampoline, the surface changes: it bounces up and down.
Similarly, space can metaphorically “bounce up and down,” although it is more accurate to say that it compresses and relaxes similar to how air transmits sound waves. These spatial distortions are called “gravitational waves” and they will travel at the speed of gravity. So, if we can detect gravitational waves, we can perhaps measure the speed of gravity. But distorting space in ways that scientists can measure is quite difficult and well beyond current technology. Luckily, nature has helped us out.
Measuring gravitational waves
In space, planets orbit stars. But sometimes stars orbit other stars. Some of those stars were once massive and have lived their lives and died, leaving a black hole — the corpse of a dead, massive star. If two such stars have died, then you can have two black holes orbiting one another. As they orbit, they emit tiny (and currently undetectable) amounts of gravitational radiation, which makes them lose energy and draw closer to one another. Eventually, the two black holes get close enough that they merge. This violent process releases enormous amounts of gravitational waves. For the fraction of a second that the two black holes come together, the merging releases more energy in gravitational waves than all of the light emitted by all of the stars in the visible Universe during the same time.
While gravitational radiation was predicted back in 1916, it took scientists nearly a century to develop the technology to detect it. To detect these distortions, scientists take two tubes, each about 2.5 miles (4 kilometers) long, and orient them at 90 degrees, so they form an “L.” They then use a combination of mirrors and lasers to measure the length of both of the legs. Gravitational radiation will change the length of the two tubes differently, and if they see the right pattern of changes of length, they have observed gravitational waves.
The first observation of gravitational waves occurred in 2015, when two black holes located more than 1 billion light years away from Earth merged. While this was a very exciting moment in astronomy, it didn’t answer the question of the speed of gravity. For that, a different observation was needed.
Although gravitational waves are emitted when two black holes collide, that’s not the only possible cause. Gravitational waves are also emitted when two neutron stars slam together. Neutron stars are also burned-out stars — similar to black holes, but slightly lighter. Furthermore, when neutron stars collide, not only do they emit gravitational radiation, they also emit a powerful burst of light that can be seen across the Universe. To determine the speed of gravity, scientists needed to see the merging of two neutron stars.
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In 2017, astronomers got their chance. They detected a gravitational wave and a little over two seconds later, orbital observatories detected gamma radiation, which is a form of light, from the same location in space originating in a galaxy located 130 million light years away. Finally, astronomers found what they needed to determine the speed of gravity.
The merging of two neutron stars emits both light and gravitational waves at the same time, so if gravity and light have the same speed, they should be detected on Earth at the same time. Given the distance of the galaxy that housed these two neutron stars, we know that the two types of waves had traveled for about 130 million years and arrived within two seconds of one another.
So, that’s the answer. Gravity and light travel at the same speed, determined by a precise measurement. It validates Einstein once again, and it hints at something profound about the nature of space. Scientists hope one day to fully understand why these two very different phenomena have identical speeds.