Stuck in the Common Now A narrative is being built up il,usion worldline. Callender puts this claim under scrutiny; the devil here is in the details, and the reader will benefit from learning more about these two popular programs in quantum gravity callendeg someone who is well-versed in the field. Craig Callender Thanks to Craig Callender for the feedback on a draft. Email required Address never made public. To the left of the wall, the ball appears in two positions; on the right, it does not appear at all.
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He wrote the graphic text fntmducmg Fine and is working on a book on the philosophy, physics and cognitive sdence of time entitled Time: From the Inside Out He assures us tfiat his Ifefeng interest in time has nothing to do with his last name.
The present moment feels special. It is real. However much you may remember the past or anticipate the future, you live in the present. Of course, the moment during which you read that sentence is no longer happening.
This one is. In other words, it feels as though time flows, in the sense that the present is constantly updating itself We have a deep intuition that the future is L open until it becomes present and that the past is fixed.
As time flows, this struc- ture of fixed past, immediate present and open future gets carried forward in time. This struc- ture is built into our language, thought and behavior. How we live our lives hangs on it Yet as natural as this way of thinking is, you will not find it reflected in science. The equations of physics do not tell us which events are occurring right now-they are like a map without the "you are here" symbol.
The present moment does not exist in them and therefore neither does the flow of time. Fundamentally, ican. It has widened as physicists have gradually stripped time of most of the attributes we com- monly ascribe to it. Now the rift between the time of physics and the time of expe- rience is reaching its logical conclusion, for many in theoretical physics have come to believe that time fundamentally does not even exist. The idea of a timeless reality is initially so startling that it is hard to see how it could be coherent.
Everything we do, we do in time. The world is a series of events strung together by time. Anyone can see that my hair is graying, that objects move, and so on. We see change, and change is the variation of properties with respect to time. Without time, the world would be completely still.
A timeless theory faces the challenge of explaining how we see change if the world is not really changing. Recent research attempts to perform just this feat. Although time may not ex- ist at a fundamental level, it may arise at higher levels— just as a table feels solid even though it is a swarm of particles composed mostly of empty space.
Solidi- ty is a collective, or emergent, property of the particles. Time, too, could be an emer- gent property of whatever the basic in- gredients of the world are.
This concept of emergent time is po- tentially as revolutionary as the develop- ment of the theories of relativity and of quantum mechanics a century ago. As physicists pursue his dream of unifSing relativity with quan- tum mechanics, they believe that time is again central. Many held that a unified the- ory will describe a timeless world. Others were loath to get rid of time. The one thing they agreed on was that without thinking deeply about time, progress on unification may well be impossible.
Time has many jobs to do in physics, but as physics has progressed, these jobs have been out- sourced one by one. All observers in principle agree on the sequence in which events happen. Time therefore provides a complete ordering of all the events in the world. Si- multaneity is absolute— an observer-inde- pendent fact. Furthermore, time must be continuous so that we can define velocity and acceleration. Classical time must also have a notion of duration— what physicists call a met- ric—so that we can tell how far apart in time events are from one another.
To say that Olympic sprinter Usain Bolt can run as fast as 27 miles per hour, we need to have a measure of what an hour is. Like the order of events, duration is observer-inde- pendent. If Alice and Bob leave school at 3 RM. In essence, Newton proposed that the world comes equipped with a master clock. The clock uniquely and objectively carves the world up into instants of time. Newton addi- tionally felt that time flows and that this flow gives us an arrow telling us which direction is the future, although these ex- tra features are not strictly demanded by his laws.
Its many features- order, continuity, duration, simultaneity, flow and the arrow— are logically detach- able, yet they all stick together in the master clock that Newton dubbed "time.
Then came the assaults of the late 19th and early 20th centuries. Instead he proposed that the distinction between past and future is not intrinsic to time but arises from asym- metries in how the matter in the universe is organized. Although physicists stifl de- bate the details of this proposal, Boltz- mann convincingly plucked away one fea- ture of Newtonian time.
Einstein mounted the next assault by doing away with the idea of absolute si- multaneity. The search for a unified theo- ry is forcing physicists to reexamine very basic assumptions, and few things are more basic than time. Some physicists argue that there is no such thing as time.
Others think time ought to be promoted rather than de- moted. In between these two posi- tions is the fascinating idea that time exists but is not fundamental. A static world somehow gives rise to the time we perceive. Philosophers have debated such ideas since before the time of Socrates, but physicists are now making them con- crete.
According to one, time may arise from the way that the universe is parti- tioned; what we perceive as time re- flects the relations among its pieces. Relativity theory holds that spacetime can be sliced up in various ways.
But not all are equally sensible. Each frame leads to the next, according to the familiar laws of physics. An alternative considers slices not from past to future but from left to right. Each slice is part space, part time. To the left of the wall, the ball appears in two positions; on the right, it does not appear at all. If this slicing seems strange, it should: it makes the laws of physics very unwieldy. The true arena of events is ot time or space, but their union: space- :ime. Only in :e cases is it possible to synchronize cks and have them stay synchronized, en in principle.
You cannot generally ink of the world as unfolding, tick by according to a single time parame- ter. In extreme situations, the world might not be carvable into instants of time at all.
It then becomes impossible to say that an event happened before or after another. Either the physics does not listen to these clocks, or, if it does, those clocks apply only to small patches of the universe or to particular observers.
Although physicists today fret that a unified theory will have to elimi- nate time, a good argument can be made that time was already lost by and that we just have not fully come to grips with it yet.
You might be tempted to think that the difference be- tween space and time has nearly van- ished and that the true arena of events in a relativistic universe is a big four-dimen- sional block.
Relativity appears to spatial- ize time: to turn it into merely one more direction within the block. Spacetime is like a loaf of bread that you can slice in different ways, called either "space" or "time" almost arbitrarily.
Yet even in general relativity time re- tains a distinct and important function: namely, that of locally distinguishing be- tween "timelike" and "spacelike" direc- tions. Timelike-related events are those that can be causally related. An object or signal can pass from one event to the other, influencing what happens.
Spacelike-relat- ed events are causally unrelated. No object or signal can get from one to the other. Mathematically, a mere minus sign differ- entiates the two directions, yet this minus sign has huge effects. Observers disagree on the sequence of spacelike events, but they all agree on the order of timelike events. Imagine shcing up space- time from past to future; each slice is the 3-D totality of space at one instant of time.
The sum of all these slices of space- like-related events is 4-D spacetime. Al- ternatively, imagine looking at the world sideways and slicing it up accordingly. From this perspective, each 3-D slice is a strange amalgam of events that are spacelike-related in just two dimen- sions and timelike-related.
The first method is familiar to physi- cists, not to mention moviegoers. The frames of a movie represent slices of spacetime: they show space at successive moments of time. One of the more intriguing alternative ideas is causal set theory, developed by Rafael Sorkin and David Rideout of the Perimeter I nstitute for Theoretical Physics in Ontario.
It supposes that the world is a set of events, called a causet, which grows as new events come into existence according to probabilistic rules. The hope is that the process reproduces the features of spacetime that we perceive, including the flow of time. An outstanding question, though, is whether this process produces worlds that are compatible with relativity theory.
The second method of slicing has no simple analogy. It corresponds to carving up spacetime not from past to future but from east to west. An example of such a slice might be the north wall in your house plus what will happen on that wall in the future. From this slice, you apply the laws of physics to reconstruct what the rest of your house and indeed the rest of the universe looks like.
If that sounds strange, it should. It is not immediately obvious whether the laws of physics let you do that. But as mathematician Walter Craig of McMaster University and philos- opher Steven Weinstein of the University of Waterloo have shovm, you can, at least in some simple situations.
Although both methods of slicing are possible in principle, they are profoundly different. In the normal, past-to-future slicing, the data you need to collect on a slice are fairly easy to obtain.
For in- stance, you measure the velocities of ah particles.
IS TIME AN ILLUSION CRAIG CALLENDER PDF
He was in grad school at Rutgers University, in New Brunswick, New Jersey, studying philosophy, when he attended a lecture about quantum physics by David Mermin , a physicist at Cornell University. There he learned about a theorem, developed by Irish physicist John Bell, that said that two quantum particles can communicate information with each other instantaneously, no matter how far apart they are separated. This seemed so bizarre that Callender tracked down the theorem and studied it in depth. As the fourth century philosopher St. Augustine famously wrote in his Confessions : "What, then, is time?
Classical time must also have craigg notion of duration— what physicists call a met- ric—so that we can tell how far apart in time events are from one another. What Makes Time Special? It will plague humans minds for the next or so years. Researchers must now reverse this train of thought and reconstruct the time of experience from the time of nonfundar mental physics, which itself may need to be reconstructed from a network of cor- relations among pieces of a fundamental static world.
Is Time an Illusion?
He wrote the graphic text fntmducmg Fine and is working on a book on the philosophy, physics and cognitive sdence of time entitled Time: From the Inside Out He assures us tfiat his Ifefeng interest in time has nothing to do with his last name. The present moment feels special. It is real. However much you may remember the past or anticipate the future, you live in the present. Of course, the moment during which you read that sentence is no longer happening. This one is.
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