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A Physicist Has Worked Out The Math That Makes ‘Paradox-Free’ Time Travel Plausible

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No one has yet managed to travel through time – at least to our knowledge – but the question of whether or not such a feat would be theoretically possible continues to fascinate scientists.

 

As movies such as The Terminator, Donnie Darko, Back to the Future and many others show, moving around in time creates a lot of problems for the fundamental rules of the Universe: if you go back in time and stop your parents from meeting, for instance, how can you possibly exist in order to go back in time in the first place?

It’s a monumental head-scratcher known as the ‘grandfather paradox’, but in September last year a physics student Germain Tobar, from the University of Queensland in Australia, said he has worked out how to “square the numbers” to make time travel viable without the paradoxes.

“Classical dynamics says if you know the state of a system at a particular time, this can tell us the entire history of the system,” said Tobar back in September 2020.

“However, Einstein’s theory of general relativity predicts the existence of time loops or time travel – where an event can be both in the past and future of itself – theoretically turning the study of dynamics on its head.”

What the calculations show is that space-time can potentially adapt itself to avoid paradoxes.

 

To use a topical example, imagine a time traveller journeying into the past to stop a disease from spreading – if the mission was successful, the time traveller would have no disease to go back in time to defeat.

Tobar’s work suggests that the disease would still escape some other way, through a different route or by a different method, removing the paradox. Whatever the time traveller did, the disease wouldn’t be stopped.

Tobar’s work isn’t easy for non-mathematicians to dig into, but it looks at the influence of deterministic processes (without any randomness) on an arbitrary number of regions in the space-time continuum, and demonstrates how both closed timelike curves (as predicted by Einstein) can fit in with the rules of free will and classical physics.

“The maths checks out – and the results are the stuff of science fiction,” said physicist Fabio Costa from the University of Queensland, who supervised the research.

Fabio Costa (left) and Germain Tobar (right). (Ho Vu)

The new research smooths out the problem with another hypothesis, that time travel is possible but that time travellers would be restricted in what they did, to stop them creating a paradox. In this model, time travellers have the freedom to do whatever they want, but paradoxes are not possible.

While the numbers might work out, actually bending space and time to get into the past remains elusive – the time machines that scientists have devised so far are so high-concept that for they currently only exist as calculations on a page.

 

We might get there one day – Stephen Hawking certainly thought it was possible – and if we do then this new research suggests we would be free to do whatever we wanted to the world in the past: it would readjust itself accordingly.

“Try as you might to create a paradox, the events will always adjust themselves, to avoid any inconsistency,” says Costa. “The range of mathematical processes we discovered show that time travel with free will is logically possible in our universe without any paradox.”

The research has been published in Classical and Quantum Gravity.

A version of this article was first published in September 2020.

 

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Black Holes Could Get So Humongous, Astronomers Came Up With a New Size Category

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There are supermassive black holes. There are ultramassive black holes. How large can these strange objects grow? Well, there could be something even bigger than ultramassive: stupendously large black holes, according to the latest research.

 

Such hypothetical black holes – larger than 100 billion times the mass of the Sun – have been explored in a new paper which names them SLABs, an acronym that stands for “Stupendously LArge Black holeS”.

“We already know that black holes exist over a vast range of masses, with a supermassive black hole of 4 million solar masses residing at the centre of our own galaxy,” explained astronomer Bernard Carr of Queen Mary University London.

“Whilst there isn’t currently evidence for the existence of SLABs, it’s conceivable that they could exist and they might also reside outside galaxies in intergalactic space, with interesting observational consequences.”

Black holes have only a few somewhat broad mass categories. There are stellar-mass black holes; those are black holes that are around the mass of a star, up to around 100 solar masses. The next category up is intermediate mass black holes, and how large they get seems to depend on who you talk to. Some say 1,000 solar masses, some say 100,000, and others say 1 million; whatever the upper limit is, these seem to be pretty rare.

 

Supermassive black holes (SMBHs) are much, much larger, on the order of millions to billions of solar masses. These include the SMBH at the heart of the Milky Way, Sagittarius A*, at 4 million solar masses, and the most photogenic SMBH in the Universe, M87*, at 6.5 billion solar masses.

The chonkiest black holes we’ve detected are ultramassive, more than 10 billion (but less than 100 billion) solar masses. These include an absolute beast clocking in at 40 billion solar masses in the centre of a galaxy named Holmberg 15A.

“However, surprisingly, the idea of SLABs has largely been neglected until now,” Carr said.

“We’ve proposed options for how these SLABs might form, and hope that our work will begin to motivate discussions amongst the community.”

The thing is, scientists don’t quite know how really big black holes form and grow. One possibility is that they form in their host galaxy, then grow bigger and bigger by slurping up a whole lot of stars and gas and dust, and collisions with other black holes when galaxies merge.

This model has an upper limit of around 50 billion solar masses – that’s the limit at which the object’s prodigious mass would require an accretion disc so massive it would fragment under its own gravity. But there’s also a significant problem: Supermassive black holes have been found in the early Universe at masses too high to have grown by this relatively slow process in the time since the Big Bang.

 

Another possibility is something called primordial black holes, first proposed in 1966. The theory goes that the varying density of the early Universe could have produced pockets so dense, they collapsed into black holes. These would not be subject to the size constraints of black holes from collapsed stars, and could be extremely small or, well, stupendously large.

The extremely small ones, if they ever existed, would probably have evaporated due to Hawking radiation by now. But the much, much larger ones could have survived.

So, based on the primordial black hole model, the team calculated exactly how stupendously large these black holes could be, between 100 billion and 1 quintillion (that’s 18 zeroes) solar masses.

The purpose of the paper, the researchers said, was to consider the effect of such black holes on the space around them. We may not be able to see SLABs directly – black holes that aren’t accreting material are invisible, since light cannot escape their gravity – but massive invisible objects can still be detected based on the way space around them behaves.

Gravity, for instance, curves space-time, which causes the light travelling through those regions to also follow a curved path; this is called a gravitational lens, and the effect could be used to detect SLABs in intergalactic space, the team said.

The huge objects also would have implications for the detection of dark matter, the invisible mass that’s injecting way more gravity into the Universe than there should be – based on what we can actually directly detect.

One hypothetical dark matter candidate, weakly interacting massive particles (WIMPs), would accumulate in the region around a SLAB due to the immense gravity, in such concentrations that they would collide with and annihilate each other, creating a gamma-radiation halo.

And primordial black holes are themselves a dark matter candidate, too.

“SLABs themselves could not provide the dark matter,” Carr said. “But if they exist at all, it would have important implications for the early Universe and would make it plausible that lighter primordial black holes might do so.”

Also, we couldn’t resist calculating the size of a 1 quintillion solar mass black hole. The event horizon would end up over 620,000 light-years across. Uh. Stupendous.

The team’s research has been published in the Monthly Notices of the Royal Astronomical Society.

 

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Ingenious ‘Wrinkled’ Graphene Could Be The Most Promising Water Filter Yet

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Graphene continues to dazzle us with its strength and its versatility – exciting new applications are being discovered for it all the time, and now scientists have found a way of manipulating the wonder material so that it can better filter impurities out of water.

 

The two-dimensional material comprised of carbon atoms has been studied as a way of cleaning up water before, but the new method could offer the most promising approach yet. It’s all down to the exploitation of what are known as van der Waals gaps: the tiny spaces that appear between 2D nanomaterials when they’re layered on top of each other.

These nanochannels can be used in a variety of ways, which scientists are now exploring, but the thinness of graphene causes a problem for filtration: liquid has to spend much of its time travelling along the horizontal plane, rather than the vertical one, which would be much quicker.

To solve this problem, the team behind the new study used an elastic substrate to scrunch up the graphene layer into a microscopic series of peaks and valleys. That means liquid can scoot down the side of a peak vertically, rather than trekking across the open plains horizontally (all at the nanoscale, of course).

(Brown University)

“When you start wrinkling the graphene, you’re tilting the sheets and the channels out of plane,” says materials scientist Muchun Liu from the Massachusetts Institute of Technology (MIT).

“If you wrinkle it a lot, the channels end up being aligned almost vertically.”

 

To finish the effect, the graphene and substrate are fixed in an epoxy substance, before the tops of the peaks and the bottoms of the valleys are trimmed off. It gives liquid a quicker route through the graphene while still enabling filtration to happen.

Liu and her colleagues have given the new materials the name VAGMEs (vertically aligned graphene membranes), and further down the line they could find uses far beyond making water safe to drink.

“What we end up with is a membrane with these short and very narrow channels through which only very small molecules can pass,” says chemical engineer Robert Hurt, from Brown University.

“So, for example, water can pass through, but organic contaminants or some metal ions would be too large to go through. So you could filter those out.”

The next step will be to put this into practice and work out a practical filtering system, but the theory is sound. The material passed one of its first tests by allowing water vapor to flow through, while trapping larger hexane molecules.

 

Eventually these VAGMEs could find uses in industrial or household filtering systems, the scientists say – just one of many promising ways that graphene is being put to use in various different scientific fields.

As for the nanochannels that operate between super-thin 2D materials such as graphene, there’s plenty of potential here too, according to the experts. The closer that scientists look at these nanomaterials, the more they discover.

“In the last decade, a whole field has sprung up to study these spaces that form between 2D nanomaterials,” says Hurt.

“You can grow things in there, you can store things in there, and there’s this emerging field of nanofluidics where you’re using those channels to filter out some molecules while letting others go through.”

The research has been published in Nature Communications.

 

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Groundbreaking New Laser System Cuts Through Earth’s Atmosphere Like It’s Nothing

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To artists and romantics, the twinkling of stars is visual poetry; a dance of distant light as it twists and bends through a turbulent ocean of air above our heads.

Not everybody is so enamoured with our atmosphere’s distortions. To many scientists and engineers, a great deal of research and ground-to-satellite communication would be a whole lot easier if the air simply wasn’t there.

 

Losing our planet’s protective bubble of gases isn’t exactly a popular option. But Australian and French researchers have teamed up to design the next best thing – a system that guides light through the tempestuous currents of rippling air with the flick of a mirror.

The result is a laser link capable of holding its own through the atmosphere with unprecedented stability.

While astronomers have a few tricks up their sleeve to correct for the atmosphere’s distortions on incoming light, it’s been a challenge to emit a coherent beam of photons from the ground to a distant receiver so they keep together and on point.

Keeping transmissions on target and coherent – with their phases remaining neatly in line – through hundreds of kilometres of shifting air would allow us to link highly precise measurement tools and communications systems.

Satellites could probe for ores or evaluate water tables with improved precision. High-speed data transfer could require less power, and contain more information.

Lead author Ben Dix-Matthews, an electrical engineer with the International Centre for Radio Astronomy Research in Australia, explained the technology to ScienceAlert.

 

“The active terminal essentially uses a small four-pixel camera, which measures the sideways movement of the received beam,” says Dix-Matthews.

“This position measurement is then used to actively control a steerable mirror that keeps the received beam centred and removes the sideways movement caused by the atmosphere.”

In effect, the system can be used to compensate for the warping effects of the moving air in three dimensions – not just up and down, or left and right, but along the beam’s trajectory, keeping the link centred and its phases in order.

So far it’s only been tested across a relatively short distance of 265 metres (about 870 feet). About 715 metres (just under half a mile) of optical fibre cable was run underground between the transmitter and receiver to carry a beam for comparison.

The results were so stable they could be used to connect the kinds of optical atomic clocks used to test fundamental physics, such as Einstein’s theories of relativity.

With the proof of concept demonstrated, there’s no reason to think a similar technique won’t one day be aiming for the sky, and beyond. Though there are a few hurdles that need to be overcome first.

 

“During this experiment we had to do the initial alignment by hand, using a visible guide laser that was in line with the stabilised infrared beam,” Dix-Matthews told ScienceAlert.

“When making links between optical atomic clocks, it would be good to have a way of doing this coarse alignment more easily.”

Fortunately Dix-Matthews’ French collaborators are working on a device that will speed up the initial coarse alignment process, promising a second generation of laser link technology that won’t require such an involved set-up.

The team also found temperature variations in the equipment affected the phase’s stability, limiting the duration of the signal to around 100 seconds. This hurdle will also be the focus of future improvements.

We might not need to wait long. The researchers are already making headway on upgrades for their system.

“We have started using a high-power laser amplifier that should help us deal with the larger power losses expected over longer distances, such as to space,” says Dix-Matthews.

“We have also completely rebuilt our active terminal to make it more sensitive to low received powers and make it more effective at cancelling out the movement of the received beam.”

With orbiting technology rapidly becoming a major focus for many data providers, potentially filling our skies with satellites, innovations that make linking communications systems across our atmosphere will only become more sought after.

As useful as our atmosphere is for, well, keeping us all alive, there are certainly some downsides to being buried under a restless blanket of warm gas.

This research was published in Nature Communications.

 

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Scientists Think These Ridiculous Bones May Belong to New Largest-Ever Dinosaur

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Scientists have unearthed massive, 98-million-year-old fossils in southwest Argentina they say may have belonged to the largest dinosaur ever discovered.

Human-sized pieces of fossilized bone belonging to the giant sauropod appear to be 10-20 percent larger than those attributed to Patagotitan mayorum, the biggest dinosaur ever identified, according to a statement Wednesday from the National University of La Matanza’s CTYS scientific agency.

 

Sauropods were enormous long-necked, long-tailed, plant-eating dinosaurs – the largest terrestrial creatures to ever have lived.

Among them, Patagotitan mayorum, also from Argentina, weighed in at about 70 tonnes and was 40 meters (131 feet) long, or about the length of four school buses.

(Jose Luis Carballido/CTyS-UNLaM/AFP)

Alejandro Otero of Argentina’s Museo de La Plata is working on piecing together a likeness of the new dinosaur from two-dozen vertebrae and bits of pelvic bone uncovered so far.

He has published a paper on the unidentified dinosaur for the scientific journal Cretaceous Research, according to the university statement.

The quest for more body parts, buried deep in rock, continues. For scientists, the holy grail will be the large femur or humerus bones, which are helpful in estimating a long-extinct creature’s body mass.

The massive fossils were discovered in 2012 in the Neuquen River Valley, but excavation work only began in 2015, according to palaeontologist Jose Luis Carballido of the Museo Egidio Feruglio.

(Jose Luis Carballido/CTyS-UNLaM/AFP)

“We have more than half the tail, a lot of hip bones,” said Carballido, who also worked on the classification of Patagotitan a few years ago.

“It’s obviously still inside the rock, so we have a few more years of digging ahead of us.”

 

The massive skeleton was found in a layer of rock dated to some 98 million years ago during the Upper Cretaceous period, added geologist Alberto Garrido, director of the Museum of Natural Sciences of Zapala.

“We suspect that the specimen may be complete or almost complete,” he said.

“Everything depends on what happens with the excavations. But regardless of whether it is bigger (than Patagotitan) or not, the discovery of an intact dinosaur of such dimensions is a novelty.” 

© Agence France-Presse

 

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A Horrible Condition Turning Starfish Into Goo Has Finally Been Identified

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In 2013, the lives of millions of sea stars were mysteriously extinguished. Limbs that were once strong, probing arms searching for sustenance, shrivelled and tore themselves away from the rest of their bodies and melted into a sickly goo.

 

“There were arms everywhere,” ecologist Drew Harvell told The Atlantic‘s Ed Yong last year. “It looked like a blast zone.”

The dismal remains of these animals, who are usually capable of regenerating their own limbs, were strewn along the entire West Coast of North America, in one of the largest mass wildlife mortality events ever recorded. Over 20 species of sea stars were perishing.

In some areas, sunflower star (Pycnopodia helianthoides) populations dropped by an average of around 90 percent in weeks, a loss that saw this once common and abundant species vanish from most of its range in just a few years.

The culprit causing this sea star wasting (SSW) even got to starfish in captivity, killing individual animals within days.

Leg of Pisaster ochraceus disintegrating from sea star wasting syndrome. (Elizabeth Cerny-Chipman/Oregon State University/CC BY-SA 2.0)

This led scientists to suspect some sort of pathogen, like a virus or bacterium, was infecting these stunning sea creatures. However, subsequent studies exonerated the lead viral suspect.

Meanwhile, more sea star deaths followed around the globe, including half a world away in Port Phillip Bay, Australia.

 

Now, San Francisco State University marine biologist Citlalli Aquino and colleagues have finally unravelled the mystery, showing something much more complicated was going on. 

By comparing the types of bacteria within healthy sea stars and those suffering from the wasting disease, the researchers found bacteria that thrive in low oxygen environments were abundant in the sick animals, as were copiotrophs – bacteria that like high-nutrient environments.

Experiments back in the lab confirmed that depleting water of oxygen caused tissue-melting lesions in 75 percent of sea stars. Adding excess nutrients or phytoplankton to the water also caused the sea star’s health to decline.

Re-analysing tissue samples from the 2013 event, the researchers detected excess nitrogen – a sign these animals suffocated to death. 

“Sea stars diffuse oxygen over their outer surface through little structures called papulae, or skin gills,” explained Cornell University marine microbiologist Ian Hewson. “If there is not enough oxygen surrounding the papulae, the starfish can’t breathe.”

These microorganisms aren’t directly causing disease, but stealing the sea stars’ oxygen supply when increased levels of organic matter are triggering the microbes to bloom. As a result, the sea stars literally drown in their own environment. Then their decaying bodies further increase nutrients for the microbes, creating a horrible feedback loop of sea star death.

 

Aquino and team noted most SSW events are reported in late fall or summer, when phytoplankton that increase levels of nutrients in the water via photosynthesis are more abundant.

Warmer temperatures are known drivers of phytoplankton blooms, and the sea star wasting event in Australia followed the longest and most intense heat wave on record. Sea star wasting events elsewhere have also followed increased sea temperatures.

“Warmer waters can’t have as much oxygen [compared with colder water] just by physics alone,” Hewson told Erin Garcia de Jesus at Science News.

None of this bodes well for our future on a warming planet.

University of Vermont biologist Melissa Pespeni, who was not involved in the study, told Science News this complicated tangle of biological and environmental factors is “a new kind of idea for [disease] transmission.”

Devastating repercussions from the loss of these precious stars of the sea have already echoed out across entire ecosystems. The sunflower star is a voracious predator with up to 24 arms that span as far as 1 metre (3.3 ft), feeling their way across the seafloor for sea urchins, snails, and other invertebrates to devour.

Without the sunflower and other sea stars keeping sea urchins in check, these herbivores are eating their way through giant kelp forests. By 2016, sea urchins had already reduced kelp populations by 80 percent in some areas, decimating these once thriving underwater forests.

“This is a very clear example of a trophic cascade, which is an ecological domino effect triggered by changes at the end of a food chain,” said Simon Fraser University marine ecologist Isabelle Côté, who investigated the environmental aftermath last year. 

“It’s a stark reminder that everything is connected to everything else.”

This research was published in Frontiers in Microbiology.

 

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