Is Time Travel Possible?
The way scientists conceive of time has change tremendously since Newton proposed the first concrete picture of time, and these new models open up the possibility of time travel.
It was Isaac Newton who gave us the first concrete picture of time. To him, it was like an arrow: once fired, it went unerringly towards its target, never wavering, never slowing down, never coming back. It was a common sense view that was shared by everyone back then. So one second on earth is one second on Jupiter or anywhere in the universe; clocks tick at the same rate throughout the universe.
But this view has been challenged by the revolution led by Einstein and quantum physicists, who have forced us to stretch the limits of common sense. To Einstein, time was like a river which could slow down and speed up. According to his Special Theory of Relativity of 1905, you would slow down in time the faster you moved. Hence, a clock orbiting in space would slow down relative to a clock on earth. An observer on earth, watching an astronaut's rocket near the speed of light, would see the astronaut moving in slow motion.
This means that, in principle, an astronaut may take over 4 years to reach the nearest star in a super fast rocket ship, but to him, it might only appear to be 4 minutes. Hence, after a round trip, the earth will have aged more than 8 years, but he will have aged only 8 minutes. As incredible as this sounds, this effect is measured every day with our GPS satellites and our atom smashers.
This also means that our astronauts are time travelers, moving a fraction of a second into the future as they orbit the earth. So going forwards in time is something which we physicists measure every day. Contrary to some popular notions, you cannot use this Special Relativistic effect to go backwards in time. For example, in Superman I, the man of steel circles the earth fast enough that he breaks the light barrier, and the earth starts to spin backwards. So Superman, by traveling faster than light, goes back in time. Similarly, in Star Trek IV, the crew of the Enterprise hijack a Klingon ship, whip around the sun and break the light barrier, and wind upin the 1960s in San Francisco. But this is not possible. According to Special Relativity, you also get heavier and flatter the faster you move. When you hit the speed of light, you are infinitely heavy, are infinitesimally flat, and time itself stops. Since this is impossible, you cannot break the light barrier. (The reason you get heavier is because the energy of motion is being converted into mass, making you heavier. In fact, when you calculate how much energy is being converted to making you heavier, you get precisely E = mc squared. In fact, that is how this equation is derived.)
But in 1915, Einstein discovered his General Theory of Relativity, which gave us a much different view of time. Time was still a river, but it couldmeander its way around the universe, speeding up and slowing down near stars and planets. For example, time beats faster on the moon, and slower on Jupiter, than it does on the earth. However, the new wrinkle on all of this which has gained the attention of physicists around the world is that Einstein's General Theory allows for the river of time to have whirlpools, and even fork into two rivers. In this situation, time travel is possible.
Einstein himself was aware that his equations allowed for time travel. His next-door neighbor at Princeton, Kurt Gödel, perhaps the most important mathematical logician of the past millennium, found a new solution of Einstein's equations in 1949 which allowed for time travel into the past. He found that if the universe rotated, and you traveled around the universe fast enough, you could go back in time, and arrive before you left.
In fact, since then, a series of solutions of Einstein's equations have been discovered which allow for time travel back into the past. Time travel is allowed for:
a) traveling around a spinning universe
b) traveling around a spinning cylinder which is infinitely long
c) traveling around two colliding cosmic strings
d) traveling through a spinning black hole
e) stretching or compressing space via negative matter
f) traveling through a wormhole.
Each of these methods has their own advantages and disadvantages, which I discuss in the next blog post, where I answer the following questions:
a) Can you really build a time machine?
b) What does the quantum theory say about time travel?
c) What happens if you meet yourself as a child? (Or kill your parents before you are born?)
To be continued....
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Here's the first evidence to challenge the "fastest sperm" narrative.
Experts argue the jaws of an ancient European ape reveal a key human ancestor.
- The jaw bones of an 8-million-year-old ape were discovered at Nikiti, Greece, in the '90s.
- Researchers speculate it could be a previously unknown species and one of humanity's earliest evolutionary ancestors.
- These fossils may change how we view the evolution of our species.
Homo sapiens have been on earth for 200,000 years — give or take a few ten-thousand-year stretches. Much of that time is shrouded in the fog of prehistory. What we do know has been pieced together by deciphering the fossil record through the principles of evolutionary theory. Yet new discoveries contain the potential to refashion that knowledge and lead scientists to new, previously unconsidered conclusions.
A set of 8-million-year-old teeth may have done just that. Researchers recently inspected the upper and lower jaw of an ancient European ape. Their conclusions suggest that humanity's forebearers may have arisen in Europe before migrating to Africa, potentially upending a scientific consensus that has stood since Darwin's day.
Rethinking humanity's origin story
The frontispiece of Thomas Huxley's Evidence as to Man's Place in Nature (1863) sketched by natural history artist Benjamin Waterhouse Hawkins. (Photo: Wikimedia Commons)
As reported in New Scientist, the 8- to 9-million-year-old hominin jaw bones were found at Nikiti, northern Greece, in the '90s. Scientists originally pegged the chompers as belonging to a member of Ouranopithecus, an genus of extinct Eurasian ape.
David Begun, an anthropologist at the University of Toronto, and his team recently reexamined the jaw bones. They argue that the original identification was incorrect. Based on the fossil's hominin-like canines and premolar roots, they identify that the ape belongs to a previously unknown proto-hominin.
The researchers hypothesize that these proto-hominins were the evolutionary ancestors of another European great ape Graecopithecus, which the same team tentatively identified as an early hominin in 2017. Graecopithecus lived in south-east Europe 7.2 million years ago. If the premise is correct, these hominins would have migrated to Africa 7 million years ago, after undergoing much of their evolutionary development in Europe.
Begun points out that south-east Europe was once occupied by the ancestors of animals like the giraffe and rhino, too. "It's widely agreed that this was the found fauna of most of what we see in Africa today," he told New Scientists. "If the antelopes and giraffes could get into Africa 7 million years ago, why not the apes?"
He recently outlined this idea at a conference of the American Association of Physical Anthropologists.
It's worth noting that Begun has made similar hypotheses before. Writing for the Journal of Human Evolution in 2002, Begun and Elmar Heizmann of the Natural history Museum of Stuttgart discussed a great ape fossil found in Germany that they argued could be the ancestor (broadly speaking) of all living great apes and humans.
"Found in Germany 20 years ago, this specimen is about 16.5 million years old, some 1.5 million years older than similar species from East Africa," Begun said in a statement then. "It suggests that the great ape and human lineage first appeared in Eurasia and not Africa."
Migrating out of Africa
In the Descent of Man, Charles Darwin proposed that hominins descended out of Africa. Considering the relatively few fossils available at the time, it is a testament to Darwin's astuteness that his hypothesis remains the leading theory.
Since Darwin's time, we have unearthed many more fossils and discovered new evidence in genetics. As such, our African-origin story has undergone many updates and revisions since 1871. Today, it has splintered into two theories: the "out of Africa" theory and the "multi-regional" theory.
The out of Africa theory suggests that the cradle of all humanity was Africa. Homo sapiens evolved exclusively and recently on that continent. At some point in prehistory, our ancestors migrated from Africa to Eurasia and replaced other subspecies of the genus Homo, such as Neanderthals. This is the dominant theory among scientists, and current evidence seems to support it best — though, say that in some circles and be prepared for a late-night debate that goes well past last call.
The multi-regional theory suggests that humans evolved in parallel across various regions. According to this model, the hominins Homo erectus left Africa to settle across Eurasia and (maybe) Australia. These disparate populations eventually evolved into modern humans thanks to a helping dollop of gene flow.
Of course, there are the broad strokes of very nuanced models, and we're leaving a lot of discussion out. There is, for example, a debate as to whether African Homo erectus fossils should be considered alongside Asian ones or should be labeled as a different subspecies, Homo ergaster.
Proponents of the out-of-Africa model aren't sure whether non-African humans descended from a single migration out of Africa or at least two major waves of migration followed by a lot of interbreeding.
Did we head east or south of Eden?
Not all anthropologists agree with Begun and his team's conclusions. As noted by New Scientist, it is possible that the Nikiti ape is not related to hominins at all. It may have evolved similar features independently, developing teeth to eat similar foods or chew in a similar manner as early hominins.
Ultimately, Nikiti ape alone doesn't offer enough evidence to upend the out of Africa model, which is supported by a more robust fossil record and DNA evidence. But additional evidence may be uncovered to lend further credence to Begun's hypothesis or lead us to yet unconsidered ideas about humanity's evolution.
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