Please read Part 1 of this blog here: What time is it? Part 1
Time Dilation and Time Travel:
Einstein showed time is "relative" to the speed of the observer, and time can be “dilated” or "slowed" when something or some one is moving very fast – at relativistic velocities approaching speed of light "C" – through the spatial dimension. Another of Einstein's revolutionary idea was the force we call gravity is nothing but "distortions" in the fabric of space time created by massive objects and energy. Gravity in turn changes how time moves, just like how velocity along spatial dimension changes how time moves. A clock at lower gravity will tick faster than will a clock at higher gravity. In other words, if you live in a multi story apartment, time passes more quickly in the top floor penthouse apartment than it does in your basement apartment! The slowing is of such a minute quantity it would go un noticed and billions of years will have to pass before your clock will gain one extra second before your penthouse neighbour. However at conditions of extreme gravity, time indeed slows down significantly. The poster boys for extreme gravity are the Black Holes. What happens to time at the center of black holes? Well it actually comes to a standstill – just as a photon don’t perceive the passage of time, time also comes to an absolute standstill at the center of a black hole.
Strange as it may be, but what is remarkable is that the so-called time dilation effects have been verified in a number of experiments, which used to depend on large scales of distance or velocity. However with exquisitely sensitive modern clocks, scientists are now able to document the extremely small time dilation that happens in ordinary situation. In a series of experiments described in the September 24, 2010, Science, researchers at the National Institute of Standards and Technology (NIST) registered differences in the passage of time between two high-precision optical atomic clocks when one was elevated by just a third of a meter or when one was set in motion at speeds of less than 10 meters per second!
Time travel is a favorite of ploy of fiction writers. But what does Einstein’s theory of general relativity says about time travel? As noted, the faster one moves in the space dimension, the slower the same person’s movement in the time dimension. So time travel to the future is quite feasible, theoretically at least – and the two ways to do this is either to travel very, very fast, or get inside an intense gravitational field. However to have a meaningful time travel to the future, the “fast” means travelling very close to the speed of light, or to be in a gravitational field so intense that can only be provided by a structure like a black hole.
As an example, one could imagine an astronaut taking off from earth in an imaginary rocket that can travel at 99.995 % the speed of light in year 2015. Let us assume he travels at this speed to a star around 500 light years and then travel back to earth. Due to relativistic time dilation (see my blog How fast does Brahma moves? for details), as far as the astronaut is concerned, the whole trip would have lasted only 10 years! While for earth and earthlings, moving leisurely around the sun (at speeds far less than the speed of light), time would have moved on by 1000 years, and when the astronaut is back, 10 years older, earth year would be AD 3015! Again, though it may appear confusing – both the astronaut and earth has moved the same distance in “space time”. The difference is that while most of the astronaut’s movement was in the “space” dimension of space time, for earthlings, most of the movement occurred in the “time” dimension of space time.
How about going backward in time? This is much harder, even theoretically. In fact Einstein’s special theory of relativity forbade both backward travel as well as travelling faster than speed of light. However Einstein’s general theory of relativity lifted this restriction. The first person to use general relativity to describe a universe that permits time travel into the past was Kurt Godel, one of the towering mathematicians of the 20th century. The story is that Godel presented this model universe (where time can move backwards) as a gift to Einstein on his 70th birthday.
The universe Godel described is mathematically complex but accurate, and remains within the bounds of general relativity. These mathematical models or trajectories where time can "flow" backwards is called “closed time like curves.”
A closed time like curve is any path through space time that loops back on itself. In Godel’s model of a rotating cosmos, such a curve would circle around the entire universe, like a latitude line on Earth’s surface. Though it was mathematically accurate, Einstein did not like a universe where time could flow backwards – since if such a curve exists, it would question some our fundamental notions of causality as could be shown by the classic "Grandfather's paradox"“ (What happens to a time traveler who kills his or her grandfather before the grandfather meets the grandmother? Would the time traveler ever be born?)
Fortunately, there is no evidence universe actually has any “closed time like curves”. Godel might not have devised a realistic model of the universe, but he did prove that closed time like curves are completely consistent with the equations of general relativity. The laws of physics do not rule out traveling to the past.
A Universe that birth itself:
As correctly portrayed in the movie, the entrance into a wormhole would be spherical—a three dimensional entrance into a four dimensional tunnel in space time. However the theory predicts that, even for a traveler going through a wormhole, time flows forward at one second per second. It’s just that the traveller’s version of ‘forward’ might be globally out of sync with the rest of the universe. Although physicists can write equations that describe wormholes and other closed time like curves, all the models have serious problems. For one thing, to get a wormhole in the first place, one need negative energy. Negative energy is when the energy in a volume of space spontaneously fluctuates to less than zero. Without negative energy, a wormhole’s spherical entrance and four-dimensional tunnel would instantaneously implode. But a wormhole held open by negative energy “seems to be hard, probably impossible,” As per Sean Carroll. Moreover as the particles moving through a wormhole would loop back an infinite number of times, leading to an infinite amount of energy. And as energy deforms space time, the entire thing would collapse into a black hole—an infinitely dense point in space time. Unlike black holes, which are a natural consequence of general relativity, wormholes and closed time like curves in general are completely artificial constructs— a way of testing the bounds of the theory.
A recent publication (Can the Universe Create Itself? by Gott and Li) on the origin of universe argue that closed time like curves were not merely possible but essential to explain the origin of the universe. They investigated the possibility of whether the universe could be its own mother— whether a time loop at the beginning of the universe would allow the universe to create itself! Gott and Li’s universe “starts” with a bout of inflation—just as in standard big bang cosmology, where an all-pervasive energy field drove the universe’s initial expansion. Many cosmologists now believe that inflation gave rise to countless other universes besides our own. “Inflation is very hard to stop once it gets started,” Gott says. “It makes an infinitely branching tree. We’re one of the branches. But you have to ask yourself, where did the trunk come from? Our theory suggest that one of the branches just loops around and grows up to be the trunk.” A simple two-dimensional sketch of Gott and Li’s self-starting universe looks like the number “6,” with the space time loop at the bottom and our present-era universe as the top stem. A burst of inflation, Gott and Li theorized, allowed the universe to escape from the time loop and expand into the cosmos we inhabit today.
Gott-Li Model of a Universe Giving birth to itself with Closed Time Like Curves (Note that each funnel like structure represent separate universes)
It is difficult to contemplate the model, but its main appeal, Gott says, is that it eliminates the need for creating a universe out of nothing. (Note here that Stephen Hawking and Lawrence Krauss have proposed models in which the universe does indeed arise out of nothing. According to the laws of quantum mechanics, empty space is not really empty but is filled with “virtual” particles that spontaneously pop into and out of existence. Hawking and his colleagues theorized that the universe burst into being from the same quantum-vacuum stew). But in Gott’s view, the universe is not made out of nothing; it is made out of something—itself.
These wildly speculative ideas may be closer to philosophy than to physics. But for now, quantum mechanics and general relativity—powerful, counter intuitive theories—are all we have to figure out the universe.
Singularities and the End of time:
Just like origins, we are equally fascinated by endings. In our experience, nothing really ends – when we die, our bodies decay and the material in them returns to the earth and the air, allowing for the creation of new life. But will that always be the case? Might there come a point sometime in the future when there is no “after”? Modern physics suggests the answer is yes. Time itself could end. All activity would cease, and there would be no renewal or recovery. The end of time would be the end of endings.
This scary prospect was also an unanticipated prediction of Einstein's general theory of relativity. Albert Einstein showed that time can slow down, or stretch out, or let it rip. Time not only affects what matter does but also responds to what matter is doing. But when time begins or ends we call them singularities. The term actually refers to any boundary of time, be it beginning or end. The best known is the big bang, the instant 13.7 billion years ago when our universe—and, with it, time—burst into existence and began expanding. If the universe ever stops expanding and starts contracting again, it will go into something like the big bang in reverse—the big crunch—and bring time crashing to a halt.
Time needn't perish everywhere. Relativity says it expires inside black holes while carrying on in the universe at large. It took physicists decades to accept that relativity theory would predict something so unsettling as end of time itself. To this day, they aren't quite sure what to make of it. Singularities are arguably the leading reason that physicists seek to create a unified theory of physics, which would merge Einstein's brainchild with quantum mechanics to create a quantum theory of gravity. They do so partly in the hope they might explain singularities away. Still, one need to be careful what you wish for. Time's end is hard to imagine, but time's not ending may be equally paradoxical.
To figure out what goes on will take a more encompassing theory, a quantum theory of gravity. Physicists are still working on such a theory, and they figure that it will incorporate the central insight of quantum mechanics: that matter, like light, has wavelike properties. These properties should smear the putative singularity into a small wad, rather than a point, and thereby banish the divide-by-zero error. If so, time may not, in fact, end.
Physicists argue it both ways. Some think time does end. The trouble with this option is that the known laws of physics operate within time and describe how things move and evolve. Time's end points are would have to be governed not just by a new law of physics but by a new type of law of physics, one that does not have temporal concepts such as motion and change in favor of timeless ones such as geometric elegance. One such notion comes from Brett McInnes of the National University of Singapore who used string theory to explain away the singularities. He suggested that the primordial wad of a universe had the shape of a torus; because of mathematical theorems concerning tori, it had to be perfectly uniform and smooth. Such a geometric law of physics differs from the usual dynamical laws in a crucial sense: it is not symmetrical in time. The end wouldn't just be the beginning played backward.
Other quantum gravity researchers think that time stretches on forever, with neither beginning nor end. In their view, the big bang was simply a dramatic transition in the eternal life of the universe. Perhaps the "pre bangian" universe started to undergo a big crunch and turned around when the density got too high—a big bounce. Artifacts of this prehistory may even have made it through to the present day.
By supposing that time marches on, proponents of this approach avoid the need to speculate about a new type of law of physics. Yet they, too, run into trouble. For instance, the universe gets steadily more disordered with time. If it has been around forever, why is it not in total disarray by now?
The bottom line is that physicists struggle with these questions no less than philosophers have. Faced with this dilemma, some people throw up their hands and conclude that science can never resolve whether time ends. It would seem that the boundaries of time are also the boundaries of reason and empirical observation.
Is time continuous or granular (quantized)?
About 100 years ago, most people thought of matter as continuous. Although since ancient times some philosophers and scientists had speculated that if matter were broken up into small enough bits, it might turn out to be made up of very tiny atoms, few thought the existence of atoms could ever be proved. Today we have imaged individual atoms and have studied the particles that compose them. The granularity of matter is old news.
In recent decades physicists and mathematicians have asked if space is also made of discrete pieces. Is it continuous, as we learn in school, or is it more like a piece of cloth, woven out of individual fibers? If we could probe to size scales that were small enough, would we see “atoms” of space, irreducible pieces of volume that cannot be broken into anything smaller? And what about time: Does nature change continuously, or does the world evolve in a series of very tiny steps, acting more like a digital computer?
The past 25 years have seen great progress on these questions. A theory with the strange name of “loop quantum gravity” predicts that space and time are indeed made of discrete pieces. The picture revealed by calculations carried out within the framework of this theory is both simple and beautiful. The theory has deepened our understanding of puzzling phenomena having to do with black holes and the big bang. Best of all, it is possible that current experiments might be able to detect signals of the atomic structure of space-time—if this structure actually exists—in the near future.
Quanta of time
The theory of quantum mechanics was formulated in the first quarter of the 20th century, a development that was closely connected with the confirmation that matter is made of atoms. The equations of quantum mechanics require that certain quantities, such as the energy of an atom, can come only in specific, discrete units. Quantum theory successfully predicts the properties and behavior of atoms and the elementary particles and forces that compose them. No theory in the history of science has been more successful than quantum theory. It underlies our understanding of chemistry, atomic and subatomic physics, electronics and even biology.
Quantum theory and Einstein's general theory of relativity have each separately been fantastically well confirmed by experiment—but no experiment has explored the regime where both theories predict significant effects. The problem is that quantum effects are most prominent at small size scales, whereas general relativistic effects require large masses, so it takes extraordinary circumstances to combine both conditions.
Allied with this hole in the experimental data is a huge conceptual problem: Einstein's general theory of relativity is thoroughly classical, or nonquantum. For physics as a whole to be logically consistent, there has to be a theory that somehow unites quantum mechanics and general relativity. One of the candidates for this long-sought-after theory is called quantum gravity. Because general relativity deals in the geometry of space-time, a quantum theory of gravity will in addition be a quantum theory of space-time.
The theory of loop quantum gravity predicts that space is like atoms: there is a discrete set of numbers that the volume-measuring experiment can return. In other words, space is not continuous. It comes only in specific quantum units of area and volume. The possible values of volume and area are measured in units of a quantity called the Planck length. This length is related to the strength of gravity, the size of quanta and the speed of light. It measures the scale at which the geometry of space is no longer continuous. The Planck length is very small: 10−33 centimeter. The smallest possible nonzero area is about a square Planck length, or 10−66 cm2. The smallest nonzero volume is approximately a cubic Planck length, 10−99 cm3. Thus, the theory predicts that there are about 1099 "atoms of volume" in every cubic centimeter of space. The quantum of volume is so tiny that there are 10 million trillion more such quanta in a single cubic centimeter of space than there are atoms in the known universe (estimated to be 1080!
In the space-time way of looking at things, a snapshot at a specific time is like a slice cutting across the space-time. But it would be wrong to think of such a slice as moving continuously, like a smooth flow of time. Instead, just as space is defined by discrete geometry, time is defined by the sequence of distinct moves that rearrange the spin network. Time flows not like a river but like the ticking of a clock, with “ticks” that are about as long as the Planck time: 5.4x10−44 seconds. Or, more precisely, time in our universe flows by the ticking of innumerable clocks—in a sense, at every location in the spin foam where a quantum “move” takes place, a clock at that location has ticked once.
The arrow of time:
How do we explain the arrow of time—the asymmetry of past and future. This appear one of the most common sense things to most people, that time should always move forward. However our fundamental laws of physic concerning space time are time invariant, meaning they could work equally well in both time moving forward as well as time moving back ward directions. One fundamental law that could explain the arrow of time – always moving to the future - is the second law of thermodynamics, which states that entropy, loosely defined as the amount of disorder within a system, increases with time. Yet no one can really account for the second law.
The leading explanation, put forward by 19th-century Austrian physicist Ludwig Boltzmann, is probabilistic. The basic idea is that there are more ways for a system to be disordered than to be ordered - this is our every day experience too, look at our children's room! If the system is fairly ordered now, it will probably be more disordered a moment from now. As Boltzmann recognized, the only way to ensure that entropy will increase into the future is if it starts off with a low value in the past. Thus, the second law is not so much a fundamental truth but related to events early in the big bang.
Let me end my blog by quoting what Einstein himself thought of time, best summarized in a letter he wrote on the death of his beloved lifelong friend Michele Besso. (Especially poignant in that this was written only few weeks prior to Einstein’s own death)
"...for us physicists belief in the separation between past, present, and future is only an illusion, although a persistent one."
Dedication: I dedicate this blog to Moossa Koya sir, my Physics teacher from Pre Degree days, for instilling the love of physics in me with gratitude.
Suggested further reading:
Dedication: I dedicate this blog to Moossa Koya sir, my Physics teacher from Pre Degree days, for instilling the love of physics in me with gratitude.
Suggested further reading:
3) From Eternity to Here: Sean Carroll
4) Origins: Cosmos, Earth and Mankind: Reeves, Rosnay, Simonnet and Coppense
5) The First Three Minutes: Steven Weinberg
6) A Universe from Nothing: Lawrence M. Krauss
7) The Meaning Of It All: Richard P. Feynman
8) The Accidental Universe: Alan Lighhtman
9) Time Reborn: Lee Smolin
10) Our Mathematical Universe: Max Tegmark
11) Cosmic Jackpot: Paul Davies
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