What is time?
While most people think of time as a constant, physicist Albert Einstein showed that time is an illusion; it is relative- it can vary for different observers depending on your speed through space. To Einstein, time is the “fourth dimension.” Space is described as a three-dimensional arena, which provides a traveler with coordinates — such as length, width and height -showing location. Time provides another coordinate- direction- although conventionally, it only moves forward. (Conversely, a new theory asserts that time is “real.”)
Most physicists think time is a subjective illusion, but what if time is real?
What is Time Dilation?
Time dilation is a difference in the elapsed time measured by two clocks, either due to them having a velocity relative to each other, or by there being a gravitational potential difference between their locations. After compensating for varying signal delays due to the changing distance between an observer and a moving clock (i.e. Doppler effect), the observer will measure the moving clock as ticking slower than a clock that is at rest in the observer’s own reference frame. A clock that is close to a massive body (and which therefore is at lower gravitational potential) will record less elapsed time than a clock situated further from the said massive body (and which is at a higher gravitational potential).
Einstein’s theory of special relativity says that time slows down or speeds up depending on how fast you move relative to something else. Approaching the speed of light, a person inside a spaceship would age much slower than his twin at home. Also, under Einstein’s theory of general relativity, gravity can bend time.
In a sense, this effect, called time dilation, means astronauts are time travelers, as they return to Earth very, very slightly younger than their identical twins that remain on the planet.
One of the most revolutionary concepts that we learned in the 20th century is that time is not a universal measurement. In Einstein’s theory of relativity, time dilation describes a difference of elapsed time between two events, as measured by observers that are either moving relative to each other, or differently, depending on their proximity to a gravitational mass. Basically, it states that the faster we go, the more the time is affected. But if time is as relative as this suggests, it can seem a little contradictory.
Time travel — moving between different points in time — has been a popular topic for science fiction for decades. Franchises ranging from “Doctor Who” to “Star Trek” to “Back to the Future” have seen humans get in a vehicle of some sort and arrive in the past or future, ready to take on new adventures. Each come with their own time travel theories.
Parallel Universe
In 1954, a young Princeton University doctoral candidate named Hugh Everett III came up with a radical idea: That there exist parallel universes, exactly like our universe. These universes are all related to ours; indeed, they branch off from ours, and our universe is branched off of others. Within these parallel universes, our wars have had different outcomes than the ones we know. Species that are extinct in our universe have evolved and adapted in others. In other universes, we humans may have become extinct. Notions of parallel universes or dimensions that resemble our own have appeared in works of science fiction and have been used as explanations for metaphysics.
The study of quantum physics began in 1900, when the physicist Max Planck first introduced the concept to the scientific world. Planck’s study of radiation yielded some unusual findings that contradicted classical physical laws. These findings suggested that there are other laws at work in the universe, operating on a deeper level than the one we know.
The idea that we live in a ‘multiverse’ made up of an infinite number of parallel universes has long been considered a scientific possibility – although it is still a matter of vigorous debate among physicists. The race is now on to find a way to test the theory, including searching the sky for signs of collisions with other universes.
The Multiverse
The multiverse is a hypothetical group of multiple universes. Together, these universes comprise everything that exists: the entirety of space, time, matter, energy, information, and the physical laws and constants that describe them. The different universes within the multiverse are called “parallel universes”, “other universes”, or “alternate universes”.
The inflationary theory of cosmology, an enduring theory about our universe and how it was formed, explains that just after the Big Bang, the universe went through a period of rapid expansion. This theory has been critical to understanding what’s going on in the cosmos today. But now, this long-held notion—which seems to suggest as-yet-unproven and perhaps unprovable features such as the multiverse—is under increasing attack. Through informed debate among architects of the inflationary theory and its prime competitors, this theory explored our best attempts to understand where we came from.
The Grandfather Paradox
The grandfather paradox is a paradox of time travel in which inconsistencies emerge through changing the past. The name comes from the paradox’s common description: a person travels to the past and kills their own grandfather before the conception of their father or mother, which prevents the time traveller’s existence. Despite its title, the grandfather paradox does not exclusively regard the contradiction of killing one’s own grandfather to prevent one’s birth. Rather, the paradox regards any action that alters the past, since there is a contradiction whenever the past becomes different from the way it was.
The grandfather paradox is a potential logical problem that would arise if a person were to travel to a past time. The name comes from the idea that if a person travels to a time before their grandfather had children, and kills him, it would make their own birth impossible. So, if time travel is possible, it somehow must avoid such a contradiction.
But contradictions such as the grandfather paradox don’t mean that time travel is impossible. The logical consistency of time travel largely depends on the concept of time, and physicists have many different ways of conceptualizing time. For example, if some laws of physics are considered probabilistic, rather than precisely determined, it opens the possibility of multiple outcomes from a trip back in time, some of which may not be contradictory.
So is time travel possible?
While time travel does not appear possible — at least, possible in the sense that the humans would survive it — with the physics that we use today, the field is constantly changing. Advances in quantum theories could perhaps provide some understanding of how to overcome time travel paradoxes.
One possibility, although it would not necessarily lead to time travel, is solving the mystery of how certain particles can communicate instantaneously with each other faster than the speed of light.
What Are Black Holes?
Black holes are volumes of space where gravity is extreme enough to prevent the escape of even the fastest moving particles. Not even light can break free, hence the name ‘black’ hole. A German physicist and astronomer named Karl Schwarzschild proposed the modern version of a black hole in 1915 after coming up with an exact solution to Einstein’s approximations of general relativity.
Schwarzschild realised it was possible for mass to be squeezed into an infinitely small point. This would make spacetime around it bend so that nothing – not even massless photons of light – could escape its curvature.The cusp of the black hole’s slide into oblivion is today referred to as its event horizon, and the distance between this boundary and the infinitely dense core – or singularity – is named after Schwarzschild. Theoretically, all masses have a Schwarzschild radius that can be calculated. If the Sun’s mass was squeezed into an infinitely small point, it would form a black hole with a radius of just under 3 kilometres (about 2 miles). Similarly, Earth’s mass would have a Schwarzschild radius of just a few millimetres, making a black hole no bigger than a marble.
For decades, black holes were exotic peculiarities of general relativity. Physicists have became increasingly confident in their existence as other extreme astronomical objects, such as neutron stars, were discovered. Today it’s believed most galaxies have monstrous black holes at their core.
How do black holes form?
It’s generally accepted that stars with a mass at least three times greater than that of our Sun’s can undergo extreme gravitational collapse once their fuel depletes. With so much mass in a confined volume, the collective force of gravity overcomes the rule that usually keeps the building blocks of atoms from occupying the same space. All this density creates a black hole.
The singularity
In scientific terms, a gravitational singularity (or space-time singularity) is a location where the quantities that are used to measure the gravitational field become infinite in a way that does not depend on the coordinate system. In other words, it is a point in which all physical laws are indistinguishable from one another, where space and time are no longer interrelated realities, but merge indistinguishably and cease to have any independent meaning.
The singularity theorem tells us that an inflationary state is past-timelike-incomplete, and hence, most probably did not last a truly infinite amount of time, but rather arose some distant-but-finite point in the past. There are a huge number of Universes out there — possibly with different laws than our own and possibly not — but there are not enough of them to give us alternate versions of ourselves; the number of possible outcomes grows too rapidly compared to the rate that the number of possible Universes grows.
