Category Archives: Space Exploration

Yes, The Multiverse Is Real, But It Won’t Fix Physics – Usman Abrar * Can Physicists Ever Prove the Multiverse Is Real? – Sarah Scoles.

“We are all agreed that your theory is crazy. The question that divides us is whether it is crazy enough to have a chance of being correct.”

Usman Abrar

Niels Bohr spoke these words to Wolfgang Pauli about the latter’s theory of elementary particles, but it could just as easily apply to many of today’s most controversial modern physics ideas.

One that’s gotten a lot of attention recently is that of a Multiverse. In short, it’s the idea that our Universe, and all that’s contained within it, is just one small region of a larger existence that includes many similar, and possibly many different, Universes like our own. On the one hand, if our current theories of physics are true, the Multiverse absolutely must exist. But on the other hand, as Sabine Hossenfelder rightly points out, it’s unlikely to teach us anything useful.

The observable Universe might be 46 billion light years in all directions from our point of view, but there’s certainly more, unobservable Universe, perhaps even an infinite amount, just like ours beyond that.

Why must the Multiverse exist? Quite simply: there must be more Universe than the part that is observable to us. If you look just at the portion of the Universe we can see, you can measure its spatial curvature, and find that it’s incredibly close to flat. No regions repeat; no locations connect or loop back on one another; no large-curvature regions show themselves on a scale approaching that of the Universe we can observe.

If the Universe were a hypersphere, the four-dimensional analogue of a sphere, it must have a radius of curvature hundreds of times the size of what we can observe. There must be more Universe out there than what we can access.

Inflation causes space to expand exponentially, which can very quickly result in any pre-existing curved space appearing flat. If the Universe is curved, it has a radius of curvature hundreds of times larger than what we can observe.

But this isn’t just a conclusion from observations; it’s the same conclusion that we’d draw from our leading theory of the Universe’s origin: cosmological inflation. Prior to the hot Big Bang, the fabric of the Universe was expanding at an exponential rate, where every 10-35 seconds or so, it would double in scale in all dimensions. Inflation went on for at least as long as 10-33seconds or so, but could have lasted far longer: seconds, years, millennia, trillions of years or an arbitrarily long length of time. When inflation ends, the Universe we’re left with is stretched flat, the same temperature everywhere, and far, far vaster than anything we can ever hope to observe. Considering the finite nature of all we can see, inflation is the natural way to create a Multiverse of possibilities.

Inflation set up the hot Big Bang and gave rise to the observable Universe we have access to, but we can only measure the last tiny fraction of a second of inflation’s impact on our Universe.

Without a solid knowledge of how inflation began, or if it ever had a beginning, we cannot know how much “Multiverse” there is out there beyond our actual Universe. But based on the properties of inflation that imprint themselves on the Universe we inhabit, we can draw a few conclusions about it. In particular:

– The lack of spatial curvature,
– The adiabatic nature and spectrum of fluctuations imprinted on the cosmic microwave background,
– The magnitude of imperfections that gave rise to the large-scale structure we see,
– The constraints on the gravitational waves inflation could have created,
And the superhorizon fluctuations that we observe (on scales larger than the visible Universe),

All give us some important constraints on the type of inflation that occurred, and teach us two very important lessons, if the implications of these verified and validated theories are correct, about our Multiverse.

The fluctuations in the CMB are based on primordial fluctuations produced by inflation. In particular, the ‘flat part’ on large scales (at left) have no explanation without inflation, and yet the magnitude of the fluctuations constrains the maximum energy scales the Universe reached at the end of inflation. It’s far lower than the Planck scale.

1.) Inflation did not occur at arbitrarily high energies. There’s an energy scale at which the laws of physics no longer make sense: the Planck scale, or about 1019 GeV. This is about 100 trillion times larger than the maximum energies the LHC achieves, and a factor of about 100 million higher than the highest energy cosmic particles we’ve ever detected in the Universe. From the imprints of inflation, we can conclude that the temperature at the start of the hot Big Bang never got higher than about 1015 or 1016 GeV, safely below the Planck scale. This implies that inflation likely occurred below that scale as well. If true, this would mean that the inflationary epoch obeyed the current laws of physics, as well as every region of the Multiverse that inflation created.

Artist’s logarithmic scale conception of the observable universe. Note that we’re limited in how far we can see back by the amount of time that’s occurred since the hot Big Bang: 13.8 billion years, or (including the expansion of the Universe) 46 billion light years. Anyone living in our Universe, at any location, would see almost exactly the same thing from their vantage point

2.) There are countless regions where inflation did not end, and still continues today. The idea that the Big Bang happened everywhere at once may apply to our Universe, but certainly ought not to apply to the vast majority of Universes existing in the Multiverse. Assuming that inflation is a quantum field, like all fields we know of, it must spread out over time, meaning that in any region of space, it has a probability of ending at a certain time, but also a probability of continuing on for a while longer.

If inflation is a quantum field, then the field value spreads out over time, with different regions of space taking different realizations of the field value. In many regions, the field value will wind up in the bottom of the valley, ending inflation, but in many more, inflation will continue, arbitrarily far into the future.

In the region that became our Universe, which may encompass a large region that goes far beyond what we can observe, inflation ended all-at-once. But beyond that region, there are even more regions where it didn’t end. Those regions grow and inflate as time goes on, and even though many of those new regions will see inflation end, the ones where it doesn’t will continue to inflate. Inflation, therefore, should be eternal to the future, at least in some regions of space. This is irrespective of whether it was eternal to the past or not.

Wherever inflation occurs (blue cubes), it gives rise to exponentially more regions of space with each step forward in time. Even if there are many cubes where inflation ends (red Xs), there are far more regions where inflation will continue on into the future. The fact that this never comes to an end is what makes inflation ‘eternal’ once it begins.

Accepting all of this leads to an inescapable conclusion: we live in a Multiverse, and our Universe is just one of countlessly many that exist within it. However, the standard predictions that come out of this are difficult to do science with. They include:

– That different regions where inflation ends should never collide or interact.
– That the fundamental constants and laws in different regions should be the same as they are here.
– And that unless inflation was truly eternal to the past, there isn’t enough “space” to contain all the parallel Universes that the many-worlds interpretation of quantum physics would require.

The idea of parallel Universes, as applied to Schrödinger’s cat. As fun and compelling as this idea is, without an infinitely large region of space to hold these possibilities in, even inflation won’t create enough Universes to contain all the possibilities that 13.8 billion years of cosmic evolution have brought us.

It’s always possible to construct a contrived model that defies these generic predictions, and some scientists make a career of doing so. Writing in NPR, Sabine Hossenfelder is right to criticize that approach, stating, “Just because a theory is falsifiable doesn’t mean it’s scientific.” But just because variants of the Multiverse are falsifiable, and just because the consequences of its existence are unobservable, doesn’t mean that the Multiverse isn’t real. If cosmic inflation, General Relativity, and quantum field theory are all correct, the Multiverse likely is real, and we’re living in it.

An illustration of multiple, independent Universes, causally disconnected from one another in an ever-expanding cosmic ocean, is one depiction of the Multiverse idea.

Just don’t expect it to solve your most burning questions about the Universe. For that, you need physics you can put to an experimental or observable test. Until that day arrives, the consequences of a Multiverse will likely remain in the realm of science fiction: where they presently belong. It’s okay to speculate, but if you insist on attributing a physics problem’s solution to an untestable feature of the Universe, you’re essentially giving up on physics. We all know that the mysteries of the Universe are hard, but that’s no reason to not even try to find a solution. The Multiverse is real, but provides the answer to absolutely nothing.

Sci-Tech Universe


Can Physicists Ever Prove the Multiverse Is Real?
Astronomers are arguing about whether they can trust this untested, and potentially untestable, idea.
Sarah Scoles

This is a hypothetical set of possible universes. (Detlev Van Ravenswaay/Science Photo Library/Corbis)

The universe began as a Big Bang and almost immediately began to expand faster than the speed of light in a growth spurt called “inflation.” This sudden stretching smoothed out the cosmos, smearing matter and radiation equally across it like ketchup and mustard on a hamburger bun.

That expansion stopped after just a fraction of a second. But according to an idea called the “inflationary multiverse,” it continues—just not in our universe where we could see it. And as it does, it spawns other universes. And even when it stops in those spaces, it continues in still others. This “eternal inflation” would have created an infinite number of other universes.

Together, these cosmic islands form what scientists call a “multiverse.” On each of these islands, the physical fundamentals of that universe—like the charges and masses of electrons and protons and the way space expands—could be different.

Cosmologists mostly study this inflationary version of the multiverse, but the strange scenario can takes other forms, as well. Imagine, for example, that the cosmos is infinite. Then the part of it that we can see—the visible universe—is just one of an uncountable number of other, same-sized universes that add together to make a multiverse. Another version, called the “Many Worlds Interpretation,” comes from quantum mechanics. Here, every time a physical particle, such as an electron, has multiple options, it takes all of them—each in a different, newly spawned universe.

A representation of the evolution of the universe over 13.77 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (NASA / WMAP Science Team)

But all of those other universes might be beyond our scientific reach. A universe contains, by definition, all of the stuff anyone inside can see, detect or probe. And because the multiverse is unreachable, physically and philosophically, astronomers may not be able to find out—for sure—if it exists at all.

Determining whether or not we live on one of many islands, though, isn’t just a quest for pure knowledge about the nature of the cosmos. If the multiverse exists, the life-hosting capability of our particular universe isn’t such a mystery: An infinite number of less hospitable universes also exist. The composition of ours, then, would just be a happy coincidence. But we won’t know that until scientists can validate the multiverse. And how they will do that, and if it even possible to do that, remains an open question.

Null results

This uncertainty presents a problem. In science, researchers try to explain how nature works using predictions that they formally call hypotheses. Colloquially, both they and the public sometimes call these ideas “theories.” Scientists especially gravitate toward this usage when their idea deals with a wide-ranging set of circumstances or explains something fundamental to how physics operates. And what could be more wide-ranging and fundamental than the multiverse?

For an idea to technically move from hypothesis to theory, though, scientists have to test their predictions and then analyze the results to see whether their initial guess is supported or disproved by the data. If the idea gains enough consistent support and describes nature accurately and reliably, it gets promoted to an official theory.

As physicists spelunk deeper into the heart of reality, their hypotheses—like the multiverse—become harder and harder, and maybe even impossible, to test. Without the ability to prove or disprove their ideas, there’s no way for scientists to know how well a theory actually represents reality. It’s like meeting a potential date on the internet: While they may look good on digital paper, you can’t know if their profile represents their actual self until you meet in person. And if you never meet in person, they could be catfishing you. And so could the multiverse.

Physicists are now debating whether that problem moves ideas like the multiverse from physics to metaphysics, from the world of science to that of philosophy.

Show-me state

Some theoretical physicists say their field needs more cold, hard evidence and worry about where the lack of proof leads. “It is easy to write theories,” says Carlo Rovelli of the Center for Theoretical Physics in Luminy, France. Here, Rovelli is using the word colloquially, to talk about hypothetical explanations of how the universe, fundamentally, works. “It is hard to write theories that survive the proof of reality,” he continues. “Few survive. By means of this filter, we have been able to develop modern science, a technological society, to cure illness, to feed billions. All this works thanks to a simple idea: Do not trust your fancies. Keep only the ideas that can be tested. If we stop doing so, we go back to the style of thinking of the Middle Ages.”

He and cosmologists George Ellis of the University of Cape Town and Joseph Silk of Johns Hopkins University in Baltimore worry that because no one can currently prove ideas like the multiverse right or wrong, scientists can simply continue along their intellectual paths without knowing whether their walks are anything but random. “Theoretical physics risks becoming a no-man’s-land between mathematics, physics and philosophy that does not truly meet the requirements of any,” Ellis and Silk noted in a Nature editorial in December 2014.

It’s not that physicists don’t want to test their wildest ideas. Rovelli says that many of his colleagues thought that with the exponential advance of technology—and a lot of time sitting in rooms thinking—they would be able to validate them by now. “I think that many physicists have not found a way of proving their theories, as they had hoped, and therefore they are gasping,” says Rovelli.

“Physics advances in two manners,” he says. Either physicists see something they don’t understand and develop a new hypothesis to explain it, or they expand on existing hypotheses that are in good working order. “Today many physicists are wasting time following a third way: trying to guess arbitrarily,” says Rovelli. “This has never worked in the past and is not working now.”

The multiverse might be one of those arbitrary guesses. Rovelli is not opposed to the idea itself but to its purely drawing-board existence. “I see no reason for rejecting a priori the idea that there is more in nature than the portion of spacetime we see,” says Rovelli. “But I haven’t seen any convincing evidence so far.”

“Proof” needs to evolve

Other scientists say that the definitions of “evidence” and “proof” need an upgrade. Richard Dawid of the Munich Center for Mathematical Philosophy believes scientists could support their hypotheses, like the multiverse—without actually finding physical support. He laid out his ideas in a book called String Theory and the Scientific Method. Inside is a kind of rubric, called “Non-Empirical Theory Assessment,” that is like a science-fair judging sheet for professional physicists. If a theory fulfills three criteria, it is probably true.

First, if scientists have tried, and failed, to come up with an alternative theory that explains a phenomenon well, that counts as evidence in favor of the original theory. Second, if a theory keeps seeming like a better idea the more you study it, that’s another plus-one. And if a line of thought produced a theory that evidence later supported, chances are it will again.

Radin Dardashti, also of the Munich Center for Mathematical Philosophy, thinks Dawid is straddling the right track. “The most basic idea undergirding all of this is that if we have a theory that seems like it works, and we have come up with nothing that works better, chances are our idea is right,” he says.

But, historically, that undergirding has often collapsed, and scientists haven’t been able to see the obvious alternatives to dogmatic ideas. For example, the Sun, in its rising and setting, seems to go around Earth. People, therefore, long thought that our star orbited the Earth.

Dardashti cautions that scientists shouldn’t go around applying Dawid’s idea willy-nilly, and that it needs more development. But it may be the best idea out there for “testing” the multiverse and other ideas that are too hard, if not impossible, to test. He notes, though, that physicists’ precious time would be better spent dreaming up ways to find real evidence.

Not everyone is so sanguine, though. Sabine Hossenfelder of the Nordic Institute for Theoretical Physics in Stockholm, thinks “post-empirical” and “science” can never live together. “Physics is not about finding Real Truth. Physics is about describing the world,” she wrote on her blog Backreaction in response to an interview in which Dawid expounded on his ideas. And if an idea (which she also colloquially calls a theory) has no empirical, physical backing, it doesn’t belong. “Without making contact to observation, a theory isn’t useful to describe the natural world, not part of the natural sciences, and not physics,” she concluded.

Multiverse (Standford University)

The truth is out there

Some supporters of the multiverse claim they have found real physical evidence for the multiverse. Joseph Polchinski of the University of California, Santa Barbara, and Andrei Linde of Stanford University—some of the theoretical physicists who dreamed up the current model of inflation and how it leads to island universes—say the proof is encoded in our cosmos.

This cosmos is huge, smooth and flat, just like inflation says it should be. “It took some time before we got used to the idea that the large size, flatness, isotropy and uniformity of the universe should not be dismissed as trivial facts of life,” Linde wrote in a paper that appeared on in December. “Instead of that, they should be considered as experimental data requiring an explanation, which was provided with the invention of inflation.”

Similarly, our universe seems fine tuned to be favorable to life, with its Goldilocks expansion rate that’s not too fast or too slow, an electron that’s not too big, a proton that has the exact opposite charge but the same mass as a neutron and a four-dimensional space in which we can live. If the electron or proton were, for example, one percent larger, beings could not be. What are the chances that all those properties would align to create a nice piece of real estate for biology to form and evolve?

In a universe that is, in fact, the only universe, the chances are vanishingly small. But in an eternally inflating multiverse, it is certain that one of the universes should turn out like ours. Each island universe can have different physical laws and fundamentals. Given infinite mutations, a universe on which humans can be born will be born. The multiverse actually explains why we’re here. And our existence, therefore, helps explain why the multiverse is plausible.

These indirect pieces of evidence, statistically combined, have led Polchinski to say he’s 94 percent certain the multiverse exists. But he knows that’s 5.999999 percent short of the 99.999999 percent sureness scientists need to call something a done deal.

The detailed, all-sky picture of the infant universe created from nine years of WMAP data. The image reveals 13.77 billion year old temperature fluctuations (shown as color differences) that correspond to the seeds that grew to become the galaxies. (NASA / WMAP Science Team)

Eventually, scientists may be able to discover more direct evidence of the multiverse. They are hunting for the stretch marks that inflation would have left on the cosmic microwave background, the light left over from the Big Bang. These imprints could tell scientists whether inflation happened, and help them find out whether it’s still happening far from our view. And if our universe has bumped into others in the past, that fender-bender would also have left imprints in the cosmic microwave background. Scientists would be able to recognize that two-car accident. And if two cars exist, so must many more.

Or, in 50 years, physicists may sheepishly present evidence that the early 21st-century’s pet cosmological theory was wrong.

“We are working on a problem that is very hard, and so we should think about this on a very long time scale,” Polchinski has advised other physicists. That’s not unusual in physics. A hundred years ago, Einstein’s theory of general relativity, for example, predicted the existence of gravitational waves. But scientists could only verify them recently with a billion-dollar instrument called LIGO, the Laser Interferometer Gravitational-Wave Observatory.

So far, all of science has relied on testability. It has been what makes science science and not daydreaming. Its strict rules of proof moved humans out of dank, dark castles and into space. But those tests take time, and most theoreticians want to wait it out. They are not ready to shelve an idea as fundamental as the multiverse—which could actually be the answer to life, the universe and everything—until and unless they can prove to themselves it doesn’t exist. And that day may never come.

Smithsonian Magazine

10 Weird Things You (Probably) Didn’t Know About the Milky Way –  Sci-Tech Universe. 

On a dark night, the dense plane of the Milky Way winds like a ribbon across the sky. On a really dark night, in areas free from light pollution, that ribbon becomes so intensely spangled with stars that it’s possible to see the dark, dusty clouds of dust and gas deep in space that blot out their light. Those clouds are so prominent that Australia’s Aboriginal people saw them create the shape of an emu.

Our galactic home is one of trillions of galaxies in the universe. Astronomers have been ardently studying them for almost a century, ever since Edwin Hubble discovered that neighboring Andromeda was not just another nearby dusty nebula, but a galaxy in its own right. And yet, humans are still trying to unravel the secrets of our galactic home and how it fits in the tapestry of the universe.

“I would love to see a movie in time of the assembly of the Milky Way,” says Jay Lockman of the Green Bank Observatory, who presented new observations about our galaxy this week at the 231st meeting of the American Astronomical Society in Maryland.

Here are some of the fun, weird facts and questions we have about the 13.6-billion-year-old space oddity we inhabit.

The Milky Way Is (Mostly) Flat

Our galaxy is, on average, a hundred thousand light-years across but only a thousand light-years thick. Within this flattened (though somewhat warped) disc, the sun and its planets are embedded in a curving arm of gas and dust, putting the solar system about 26,000 light-years away from the galaxy’s turbulent core. A bulge of dust and stars swaddles the galactic center, looking like a dollop of whipped cream plopped on both sides of a pancake.

Earth Is 18 Galactic Years Old

The solar system is zooming through interstellar space at around 500,000 miles an hour. Even at that rate, it takes about 250 million years to travel once around the Milky Way. The last time our 4.5-billion-year-old planet was in this same spot, continents fit together differently, dinosaurs were just emerging, mammals had yet to evolve, and the most profound mass extinction in the planet’s history—an event called the Great Dying—was in progress.

There’s a Monster Black Hole in the Galaxy’s Middle

Called Sagittarius A*, the supermassive black hole weighs in at more than four million times the mass of the sun. We’ve never seen this object directly—it’s hidden behind thick clouds of dust and gas. But astronomers have been able to follow the orbits of stars and gas clouds near the galactic center, which allowed them to infer the mass of the cosmic heavyweight hiding behind the curtain. It’s thought that supermassive black holes are parked in the cores of most galaxies, and some are feeding on nearby matter so greedily they shoot out jets of powerful radiation visible from millions of light-years away. 

The Milky Way Won’t Live Forever

In about four billion years, the Milky Way will collide with its nearest neighbor, the Andromeda Galaxy. The two spiral galaxies are currently hurtling toward each other at 250,000 miles an hour. When they do smash into one another, it won’t be as cataclysmic as you might imagine—Earth will likely survive, and very few stars will actually be destroyed. Instead, the newly formed mega-galaxy will offer a night skyscape with a spectacular blend of stars and streamers unlike anything we see today.

Our Sun Is One Star among Several Hundred Billion

There are a hundred billion stars in the Milky Way. Or is it 300 billion? Or 400 billion? That’s right—we don’t actually know how many stars are in our galaxy. Many of them are dim, low-mass stars that are hard to detect over vast cosmic distances, and there are massive clouds obscuring the bulge of stars nearest to Sagittarius A*. Astronomers have estimated the total number of stars based on the Milky Way’s mass and brightness, but more precise numbers are still elusive.

We’re Surrounded By a Dark Halo

The Milky Way is embedded in a clump of dark matter that is far larger and more massive than the galaxy itself. In the late 1960s, astronomer Vera Rubin inferred the presence of these invisible halos around galaxies when she observed that stars near the edge of Andromeda were whipping around the galaxy’s center at speeds that should send them flying off into space. And yet, they weren’t, meaning that some sort of cosmic glue held everything together. That glue, we now know, is dark matter.

We Hang Out With Ancient Stars

The Milky Way is also surrounded by more than 150 ancient groups of stars, some of which are among the oldest in the universe. Called globular clusters, these primordial stellar conglomerates live in the Milky Way’s halo and orbit the galactic center. Each is crammed with hundreds of thousands of stars. Also hanging around the Milky Way are dozens of satellite galaxies; most of these are tough to see, but the Small and Large Magellanic Clouds glisten each night in the southern sky.

The Galaxy Is an Island in a Stream of Stars

The Milky Way eats galaxies that come too close. Over the years, scientists studying the galaxy’s fringe have detected some two dozen faint streamers of stars that are the remnants of galaxies past. These ghostly stellar rivers formed when the Milky Way’s more powerful gravity ripped apart smaller galaxies, leaving behind glittering strands of leftovers. At the AAS meeting, the Dark Energy Survey team announced that it had detected 11 more of these streamers, some of which have been given Aboriginal names.

The Galactic Center Is Blowing Hot Air

The Milky Way is blowing massive bubbles of extremely hot gas and energetic particles. Stretching far above and below the galactic plane, these so-called Fermi bubbles balloon straight out of the galaxy’s center, fueled by a wind blowing at two million miles an hour. Unknown until 2010, it’s not entirely clear why the bubbles exist, but scientists think they could be linked to the frenzy of star death and formation in the region around Sagittarius A*.

Gas Clouds Are Fleeing the Galaxy

Observed recently with the Green Bank Telescope, more than a hundred hydrogen gas clouds are zooming away from the galaxy’s core at 738,000 miles an hour. Scientists studying the deserting swarm say the clouds can act as tracers for the powerful processes that produce the giant Fermi bubbles.

Sci-Tech Universe 

Our Universe Shouldn’t Exist, CERN Physicists Conclude – Usman Abrar. 

One of the great mysteries of modern physics is why antimatter did not destroy the universe at the beginning of time. To explain it, physicists suppose there must be some difference between matter and antimatter – apart from electric charge. Whatever that difference is, it’s not in their magnetism, it seems.

Physicists at CERN in Switzerland have made the most precise measurement ever of the magnetic moment of an anti-proton – a number that measures how a particle reacts to magnetic force – and found it to be exactly the same as that of the proton but with opposite sign.

 “All of our observations find a complete symmetry between matter and antimatter, which is why the universe should not actually exist,” says Christian Smorra, a physicist at CERN’s Baryon–Antibaryon Symmetry Experiment (BASE) collaboration. “An asymmetry must exist here somewhere but we simply do not understand where the difference is.”

Antimatter is notoriously unstable – any contact with regular matter and it annihilates in a burst of pure energy that is the most efficient reaction known to physics. That’s why it was chosen as the fuel to power the starship Enterprise in Star Trek. The standard model predicts the Big Bang should have produced equal amounts of matter and antimatter – but that’s a combustive mixture that would have annihilated itself, leaving nothing behind to make galaxies or planets or people.

To explain the mystery, physicists have been playing spot the difference between matter and antimatter – searching for some discrepancy that might explain why matter came to dominate. So far they’ve performed extremely precise measurements for all sort of properties: mass, electric charge and so on, but no difference has yet been found.

Last year, scientists at CERN’s Antihydrogen Laser PHysics Apparatus (ALPHA) experiment probed an atom of anti-hydrogen with light for the first time, again finding no difference when compared with an atom of hydrogen. But one property was known only to middling accuracy compared to the others – the magnetic moment of the antiproton. Ten years ago, Stefan Ulmer and his team at BASE collaboration set themselves the task of trying to measure it.

First they had to develop a way to directly measure the magnetic moment of the regular proton. They did this by trapping individual protons in a magnetic field, and driving quantum jumps in its spin using another magnetic field. This measurement was itself a groundbreaking achievement reported in Nature in 2014.

Next, they had to perform the same measurement on antiprotons – a task made doubly difficult by the fact that antiprotons will immediately annihilate on contact with any matter. To do it, the team used the coldest and longest-lived antimatter ever created. After creating the antiprotons in 2015, the team were able to store them for more than a year inside a special chamber about the size and shape of a can of Pringles.

Since no physical container can hold antimatter, physicists use magnetic and electric fields to contain the material in devices called Penning traps. Usually the antimatter lifetime is limited by imperfections in the traps – little instabilities allow the antimatter to leak through. But by using a combination of two traps, the BASE team made the most perfect antimatter chamber ever – holding the antiprotons for 405 days.

This stable storage allowed them to run their magnetic moment measurement on the antiprotons. The result gave a value for the antiproton magnetic moment of−2.7928473441 μN. (μN is a constant called the nuclear magneton.) Apart from the minus sign, this is identical to the previous measurement for the proton. The new measurement is precise to nine significant digits, the equivalent of measuring the circumference of the Earth to within a few centimeters, and 350 times more precise than any previous measurement.

“This result is the culmination of many years of continuous research and development, and the successful completion of one of the most difficult measurements ever performed in a Penning trap instrument,” says Ulmer.

The universe’s greatest game of spot the difference goes on. The next hotly anticipated experiment is over at ALPHA, where CERN scientists are studying the effect of gravity of antimatter – trying to answer the question of whether antimatter might fall ‘up’.

Sci-Tech Universe 


The Mystery of the Missing Antimatter 


The Big Bang created equal parts matter and antimatter. So what happened to all the antimatter? The answer may finally be within reach, Cathal O’Connell reports.

The universe was a trillionth of a second old, and it was at war.

Out of the big bang’s unimaginable light, two armies emerged and engaged in a frenzied attack. On one side were the particles of matter – electrons and protons. On the other side, antimatter – identical to the matter particles, except with opposite charge. Electrons clashed with their positron counterparts and destroyed one another. Protons duelled antiprotons, to the same violent end. It appeared the two sides were headed toward mutually assured destruction.

Then, a millionth of a second later, the fighting ceased.

In the aftermath lay a wasteland of photons and empty space. But when the dust settled, one side remained standing. For every billion pairs of matter and antimatter particles, a single particle of matter emerged unscathed from the melee. Today, billions of years later, everything we see – galaxies, stars, all of the atoms and molecules that make your body and mine – are descended from those surviving particles.

There’s just one problem: Everything we know about physics says we shouldn’t exist. Matter and antimatter are always created in equal parts, so the two armies should have wiped each other out, leaving the Universe empty, dark and lifeless. That it’s not means there’s something seriously wrong – or embarrassingly incomplete – about our best theory of how the Universe is screwed together.

The triumph of matter is one of the greatest mysteries in all of science. The big question that’s stumped physicists for decades is: what gave us the edge?

Now, using sophisticated new instruments that are as large as their targets are small, scientists are closing in on an unlikely solution. As it turns out, the secret may lie in a quiet corner of particle physics where a newly discovered quirk in the personality of neutrinos is challenging scientists to rethink the origin of the Universe.


Today antimatter is a well accepted ingredient in the recipe book of particle physics. We know that every matter particle has a twin antimatter particle that’s the same mass, but with opposite charge. We even use positrons in medical imaging (see box below ‘Everyday Antimatter’). But until 89 years ago, we had no inkling of antimatter.

It was the winter of 1927, and a young British physicist named Paul Dirac was in the com-mon room at the University of Cambridge, staring at the fire. As taciturn as Dirac was (his colleagues would later define a unit of a “dirac” as one word per hour) he was even more absorbed than usual. He was thinking about a thorny problem.

Physics was in the throes of a revolution. The new theory of quantum mechanics – which recast the subatomic world in terms of particles that could hop between energy levels – was beginning to explain previously mystifying effects. Just two years earlier, Erwin Schrödinger had placed the fledgling theory on a firm footing with his equation describing how electrons orbit an atomic nucleus. This equation allowed physicists to explain the structure of atoms with unprecedented detail. Yet some of those details, such as an electron’s magnetic readings, were off.

The problem was speed. Electrons whizz around the nucleus at about 2,000 kilometres per second and Schrödinger hadn’t accounted for how matter that is approaching the realms of light speed might warp the laws of physics. To address that question he needed Albert Einstein’s theory of special relativity. But nobody had been able to meld quantum mechanics and special relativity into one.

As Dirac stared at that fire, he had a sudden burst of insight – a piece of mathematical wizardry conjured from the flames that allowed him to skirt around the impasse that had stumped so many other physicists. He quickly wrote down a handful of mathematical symbols that combined the ideas of Schrödinger and Einstein in one equation. This new equation spat out a measure of the electron’s magnetism that was bang on. It also corrected some bugs in how Schrödinger’s equation dealt with the structure of the hydrogen atom. Most compelling of all, it provided a meaningful rationale for why electrons seemed to spin like a top – so-called “quantum spin”.

With this one elegant equation, Dirac laid the foundations for the electronics revolution of the 20th century. “If Dirac patented his equation,” Stephen Hawking later said, “he would have become one of the richest men in the world. Every television set or computer would have paid him royalties.”

But as successful as Dirac’s equation was, it posed a problem: it had two solutions. One was for a particle with negative charge, the electron. The other was for a particle that was identical in every way to the electron, except with positive charge. It was a ludicrous notion. As every scientist in 1927 knew, the Universe was made of only two kinds of particles: electrons and protons.

Dirac wrestled with the meaning of his equation’s second solution, first ignoring it out of “pure cowardice”. At last, in 1931, he proposed that a new particle, what he called the anti-electron, must exist. Within a year, the American physicist Carl Anderson announced he had found the tell-tale tracks of such a particle (“positively charged, and with the same mass as an electron”) while studying cosmic rays in a cloud chamber. He called it the positron (from “positive” and “electron”). The name stuck.

It was the first time anyone had predicted a new particle based on mathematical reasoning alone. “That to me is one of the great profound mysteries of science,” says Frank Close, a physicist at Oxford University. “It was a case of mathematics knowing about the Universe before we did.”

Dirac’s discovery won him the Nobel Prize for physics in 1933. In his Nobel lecture, he went on to predict that every particle of matter must have its own mirror-twin of antimatter. He was right: over the next two decades, repeated discoveries of the antiproton, the antineutron and more, bore out the theory.

But the discovery of antimatter raised a maddening question. Scientists soon realised that antimatter must be billions of times more scarce than matter. Why does our Universe seem to contain so little of the stuff?

It turns out there are three possible answers. Dirac himself put forth the first: the missing antimatter is out there, we just haven’t learnt to recognise it yet. Half the stars in the sky could be made of antimatter, he suggested, only they’d look exactly like regular stars, so we’d never be able to tell which was which. “The idea is we are just a little pocket of an even vaster universe in which there are huge galaxies of antimatter out there, but as yet not discovered,” says Close.

Though we may not be able to distinguish a given antimatter galaxy just by looking, there is one tell-tale sign that could help prove Dirac’s idea. Along the boundary between regions of matter and antimatter we’d see signs of their annihilation, a dividing line of gamma rays.

Yet attempts to find that dividing line have come up empty. The Compton Gamma Ray Observatory (CGRO), a satellite launched in 1991, orbited the Earth for 14 years and never detected any such boundary. Helen Quinn, an emeritus physicist at Stanford University, is not surprised. Our maps of the cosmic microwave background radiation – heat leftover from the Big Bang – paint a picture of the early Universe that is amazingly homogenous. The chance of regions of matter and antimatter sifting out from such a well-mixed Universe is “so improbable as to be, in everyday language, impossible”, she says.

The second possible answer is that the Big Bang simply created more matter than antimatter. Our best test of this idea is to recreate a miniature Big Bang by revving particles up to near light speed and ramming them into one another. The huge energy creates millions of particles of matter and antimatter. Since the 1960s, physicists have been recording the products of such collisions, and none have ever shown an imbalance. The result has left particle physicists in no doubt. “When we create fundamental particles … we make equal amounts of matter and antimatter,” says Jeffrey Hangst, a physicist at CERN (the European Organisation for Nuclear Research) in Geneva. “That’s an observed law of nature as far as we know.”

If universes cannot be born lopsided, the third possible solution is that just after the Big Bang, some mysterious process skewed the balance of matter and antimatter. This idea was proposed in 1966 by Russian physicist Andrei Sakharov. (Sakharov was somewhat of an enigma: while best known for developing humanity’s most destructive weapon, the hydrogen bomb, he also won the Nobel Peace Prize for his promotion of human rights in the Soviet Union.)

For Sakharov’s idea to pan out, it meant that antimatter could not be, as Dirac believed, matter’s mirror-twin. This assertion of an imbalance – a broken symmetry – struck at the heart of one of physicists’ basic creeds.

From Einstein onwards, physicists had achieved groundbreaking advances by building their theories on a foundation of symmetry. This culminated in the 1950s and 1960s when physicists built the Standard Model of particle physics – a sort of periodic table of the building blocks of the Universe. Much like the way composers use the rules of a key signature to plug in the notes of a melody, physicists used the rules of symmetry to plug in the missing notes of the Universe.

And now Sakharov was suggesting the Universe was built on a bum note? It was hard to fathom. But on the other side of the world, unbeknownst to Sakharov, other researchers were already finding hints of this broken symmetry.

In New York in 1963, physicists James Cronin and Val Fitch got the shock of their lives. They were measuring the properties of an unstable particle called the K-meson. Sixteen years before, they had been discovered in the showers of particles that rain down when cosmic rays strike the atmosphere. Cronin and Fitch found a way to create a beam of them by firing protons at a beryllium target. The experiment aimed simply to measure the decay rates of the neutral K-meson as it broke up into particles of matter and antimatter.

Mesons are weird particles because they contain both particle and antiparticle components. (The two components are not each other’s counterparts, so they don’t annihilate.) Like double agents playing both sides, they constantly switch identities between matter and antimatter. The laws of symmetry said they should spend exactly half their time in each camp. But what the Cronin-Fitch experiment showed was that the K-meson’s loyalty was skewed –they wore their antimatter uniforms slightly more often than their matter ones.

What’s more, when the K-mesons decayed, they left behind more antimatter than matter. The finding didn’t entirely help Sakharov’s theory. K-mesons were decaying in the wrong direction to explain the abundance of matter in the Universe, and the effect was tiny. “But it did set a precedent,” says Close. If K-mesons could break the rules of symmetry, perhaps other particles could too.

In 1972, Makoto Kobayashi and Toshihide Maskawa at the University of Kyoto made a finding that shed suspicion on another double agent, the B-meson. Their calculations suggested that the decay of B-mesons should also break symmetry.

It took until 2001 for experiments to show that B-mesons did, indeed, violate symmetry. This time in the right direction – favouring matter over antimatter.

Mystery solved? “Actually not,” says Quinn. “Yes, you get a small imbalance of matter and antimatter, but too tiny to make even one star with the leftover matter.”

While other teams were exploring asymmetry in mesons, at CERN, physicists were hunting for clues to our unbalanced Universe in the properties of antimatter itself. In 1995 they produced the first atom of antimatter, an antihydrogen – though it immediately annihilated. By the early 2000s they had figured out how to trap antihydrogen long enough to study it directly (see graphic below). That has allowed CERN scientists to compare matter and antimatter in terms of its mass, how its subatomic particles are bound together, and in their electric and magnetic behaviour.

In 2014, for the first time, a CERN project called ALPHA (Antihydrogen Laser Physics Appa-ratus) measured the electric charge carried by an atom of antihydrogen – finding the charge is zero, just like its hydrogen counterpart. So no anomaly there. ALPHA’s next big experiment is to see how antimatter responds to gravity; some theories say that rather than falling down, antimatter could fall up! “Basically we’re looking everywhere we can,” says Hangst, a particle physicist at the ALPHA experiment.

So far, no new clues have been found; matter and antimatter have turned out identical in every property tested. But there is one more particle, so little understood, that it might harbour the secret behind our matter-dominated Universe – the mysterious neutrino.

Neutrinos are neutral particles produced during radioactive decay and in the centre of stars. They are famed for their ghostlike ability to pass through matter at the speed of light. In 1986 Masataka Fukugita and Tsutomu Yanagida of Tohoku University wondered if these mysterious particles might also hold the answer to the imbalance of the Universe. They came up with an extraordinary theory.

Their starting point was to propose that neutrinos actually travelled slightly slower than the speed of light and had a tiny mass. The reason for their extreme lightness, they suggested, is that neutrinos have a big brother – another, much heavier particle – that offsets their mass. The idea is called the seesaw theory because it reminded the physicists of the way a big child on a seesaw can suspend a smaller child high in the air. If you could not see the big brother on the other side, you’d wonder at the extreme lightness of the child perched high on the seesaw. So if the neutrino is extremely light, they reasoned, its brother must be superheavy.

The Japanese pair figured that these superheavy neutrinos would have been unstable. And that their decay may have been skewed toward matter. Given that they would have been created in huge numbers in the Big Bang, that might have tipped the balance towards matter over antimatter. Fukugita and Yanagida’s revolutionary idea was ignored. Their series of assumptions about neutrinos seemed tenuous, each stacked atop each other like a house of cards. Moreover there was no base to the stack because there was no evidence that the neutrino had mass.

Then, in 2001, the physics community was rocked by the discovery that neutrinos shapeshift between three possible forms as they zoom through the Universe. Think of neutrinos as a group of three close-packed riders in a bicycle peloton who take turns leading, with each leader showing a different face. This peloton behaviour could only be explained if neutrinos did carry mass (see Shape-shifting neutrinos that led to a Nobel Prize), though it must be miniscule even compared with the electron. Around the world, the physics community did a collective double-take.

Since the shapeshifting discovery, many other theories have been proposed to explain how the neutrino could be so incredibly light. But the idea of a secret neutrino big brother is the most popular.

Unfortunately, the mass of the proposed superheavy neutrinos is so large that researchers can’t produce them in a particle accelerator and watch them decay. But the seesaw theory does make two testable predictions about neutrinos. If they both prove true, they provide strong evidence that neutrinos are the hero behind matter’s victory in the war of creation.

The first prediction is that neutrinos are their own antiparticle. Right now we know that as neutrinos zip through space, they always spin anticlockwise while antineutrinos spin clockwise. But a thing that spins clockwise coming toward you spins anticlockwise going away from you. This means neutrinos and antineutrinos could be two sides of the same coin. The smoking gun to prove this theory would be detecting a special radioactive process called neutrinoless double beta decay.

Regular double beta decay happens when two neutrons in the same nucleus decay simultaneously, spitting out two electrons and two neutrinos. Normally the path of the emitted electrons is unbalanced because the neutrinos carry away some of the energy – just as two struck billiard balls have unequal paths because the cue ball carries some of the energy. But if a neutrino can act as its own antiparticle, then occasionally those two neutrinos should annihilate. If this happens within the nucleus, it’s as if no neutrino was emitted at all: that’s neutrinoless double beta decay. The absence of neutrinos would leave a unique signature on the electrons that were released. Instead of being imbalanced, the paths of the electrons will be perfectly balanced. By measuring the paths of emitted electrons, physicists hope to nail the dual nature of neutrinos. “This is what the next generation of experiments are going to try to crack,” says Simon Peeters, a physicist at the University of Sussex. He is part of an international collaboration based at Canada’s Sudbury Neutrino Observatory (SNO) where some of the shapeshifting Nobel prize work was performed. When a new detector, SNO+, is operational next year, the team will watch for two tell-tale flashes of light that indicate a tellurium nucleus has decayed by emitting two electrons simultaneously.

The decay is incredibly rare and difficult to detect. “What you’re looking for is basically a handful of these decays in a tonne of material,” says Peeters.

Far from the ridicule that first greeted Fukugita and Yanagida, there is fierce competition to be the first to nail neutrinos as double agents. The Enriched Xenon Observatory in New Mexico is already probing 200 kilograms of liquid xenon for similar tell-tale flashes. And in Italy, the GERDA (Germanium Detector Array) uses huge crystals of the semiconductor germanium to detect the elusive decay.


Prove that neutrinos are their own antiparticle would be the biggest coup for particle physics since the discovery of the Higgs Boson. Yet this would only be one step toward proving the seesaw theory.

The theory’s second prediction is that neutrinos, like K-mesons and B-mesons, break the rules of symmetry between matter and antimatter. We can’t directly test whether neutrinos decay in a lopsided manner. But we can check whether they break the symmetry rules. A major project called DUNE (Deep Underground Neutrino Experiment) at Fermilab, in Illinois, could soon put this idea to the test by firing the world’s most intense beam of neutrinos and antineutrinos at a detector buried 1,300kilometres away in a South Dakota mine. The long distance gives the neutrinos time to shapeshift. DUNE will probe the beam twice along the way to measure such shifts.

The laws of symmetry dictate that neutrinos and antineutrinos should shapeshift at the same rate. If they don’t, then they are part of a select club of symmetry-breakers (see graphic Profile: Matter vs antimatter below) that might break all sorts of other rules. And if they behave badly, their big brother probably does too – possibly decaying in a lopsided manner, leading to our matter-dominated universe.

The US has already devoted $1 billion to the DUNE project, which should be operational by 2022, and a huge list of international collaborators, including CERN, have signed up too.

Admittedly pursuing the weirdness of neutrinos sounds like a huge gamble to try to crack the mystery of antimatter. But they’re still the best chance we’ve got. “Most particle physicists are betting on them,” says Quinn. What’s at stake is nothing less than our reason for being. But for now, we must wait, as two of the most sophisticated physics experiments on the planet run their course. What’s another few years to a mystery 13.8 billion years in the making?

Everyday antimatter

Antimatter sounds like something right out of science fiction – probably because it is. In Star Trek, it was the fuel for the USS Enterprise. In Dan Brown’s Angels and Demons it was the stuff threatening to blow up Vatican City. Yet antimatter is not as exotic as it sounds. In fact, it’s all around you. A banana, for instance, produces a positron about every 40 minutes (through the decay of a particular potassium isotope). Your body also contains potassium, so even you are generating antimatter. And ever heard of a PET scan? That stands for positron emission tomography. It’s a medical imaging technique where doctors inject you with a small radioactive dye that gives off positrons. The positrons immediately annihilate in your bloodstream, and by detecting the gamma rays they emit, doctors can map your circulatory system – even the inner structure of your brain.


Nasa just made all its research available online for free. 

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