These are deceptively simple questions that are almost impossible to answer. I hope to give you an idea of where science is at right now as physicists grapple with time.

We experience time moving forward, set with a regular ticking away of passing moments (physicists call this a metric). But is this just our perception of time? Neuroscientists know that time cannot be directly perceived. It must be reconstructed in the brain, and it is variable - duration is subjective, depending on who is experiencing it. Drugs such as LSD, psilocybin and mescaline, hypnosis, and several clinical disorders can affect time perception. Some physicists now believe that time does not even exist.

In previous articles I have mentioned space-time and particles that move backward through time. You may have found these ideas hard to digest. They don't make sense based on our reality. We see change all around us - objects moving, plants growing and decaying, heavenly bodies traversing the sky. Without time, wouldn't everything be still? Not necessarily. Scientists now view time as an emergent property of reality. This means that, just as our perception of a solid table emerges from a swarm of spinning zipping particles, our perception of the one-way flow of time emerges from the basic ingredients of the universe. Many believe time is our perception of entropy, and I will flesh out this idea later.

Throughout my articles I have been harping on the need for a unified theory of everything. The Holy Grail in physics is the successful merging of two fundamental theories - relativity and quantum mechanics. Some physicists now believe that in order to achieve this, we must reconceptualize time. This is not an easy task.

Let's begin with how our understanding of time has evolved.

Before Newton, it was thought that time could not be measured and in fact it could not occur without a change occurring somewhere in the universe. Time was considered the measure of intervals between changes in the universe.

Newtonian Time

In the late 17th century, Isaac Newton viewed time as consistent and irreversible. His laws of motion depend on it. This is a quote about time from his masterpiece, "Principia:"

"Absolute, true and mathematical time, in and of itself and of its own nature, without reference to anything external, flows uniformly and by another name is called duration. Relative, apparent and common time is any sensible and external measure (precise or imprecise) of duration by means of motion; such a measure - for example, an hour, a day, a month, a year - is commonly used instead of true time."

His concept of time held up particularly well when working out the motions of planets and moons. But it was not to last. Accurate measurements of the speed of light threw this concept a curveball.

In the late 1800's, physicists came up against a real problem in physics. If nothing, not even light, can travel faster than light speed, what happens when a light source is shone from the front of a traveling object? Doesn't that light surpass light speed? Albert Einstein, thinking over the relationships between velocity, time and distance, realized that time must be motion-dependent.

Einsteinian Time

Many of us still think of time in terms of Newtonian time. Almost all daily phenomena make sense within this framework. However, for those who are studying objects at very high speeds or within very intense gravitational fields, the Newtonian framework must be abandoned in favour of Einstein's special relativity. Few of us have really thought through the implications of Einsteinian time, but if we want to get a feel for how our universe works, we must get a basic handle on special relativity.

Idea # 1: Time is "Stretchy"

Let's warm up our brains with two videos. The first one describes classic relativity and the second describes and compares special relativity to it. Both are brief, about 8-9 minutes, and they are both easy to follow. (These are two of seven videos on relativity from ScienceTV. If you would like to be a relativity expert watch all of them - they're all fun!)

Classical Relativity

Special Relativity

To understand special relativity, we must first accept that time cannot be separated from the dimensions of space. In other words, time depends on what is happening within these other three dimensions. In order to describe any event, that is, any physical situation or occurrence, we need four measurements - three (usually) for space (up/down, forward/backward, left/right) and one (usually) for time. The "usually"' means that according to some contemporary theories, such as string theory, more than four dimensions may exist, here we are focusing strictly on time however.

Time runs slower, it stretches out, when one object moves faster relative to another object. Clocks on board the space shuttles as they orbited Earth ran slower than our Earth-bound clocks relative to us. The word relative is important. For the shuttle crews the clocks kept time perfectly - they did not experience any slowing of time. This concept, called time dilation, is famously expressed in the movie, "Contact." Recall when Palmer tells Arroway that if she ends up traveling anywhere near light speed to the nearest star, Vega, and back in the space travel device, she will be only four years older but over fifty years will have passed on Earth, everyone she knows and loves will be dead.

Now let's explore the nuts and bolts of special relativity. First of all, when an object travels at close to light speed relative to you, it not only experiences time dilation relative to you, it experiences something called Lorentz contraction. It you click this link you can see the graphs for them. You will notice that each effect does not become significant until you get very close to light-speed. The MIT Game Lab just came out with a free downloadable open-source game called "A Slower Speed of Light." As a player, you can experience your surroundings change as you approach relativistic speeds.

Time dilation and Lorentz contraction show us how time is inextricably wound up in the other three dimensions of space. As I think about this for a bit, it raises some very disconcerting questions, such as how has time been affected by the evolution of the universe? When the universe was very young, many physicists believe that the dimensions of space were tightly curled up and these dimensions themselves unfolded faster than light speed (cosmic inflation). How would time behave under these extreme circumstances? To an outside observer, what would our universe look like as it was born? I leave this is an example of the immense implications of Einstein's theory of special relativity. We know it's true but it is very hard to wrap our heads around it. We live our lives in Newtonian time so it is very hard to reconcile these concepts with reality.

Let's focus now on time dilation. The speed of light works very well to help illustrate how time slows down for an observer. If you click this animation, it will show you very nicely how light within a moving apparatus must travel a longer distance relative to a stationary observer than it does when the observer is moving right along at the same velocity. The light speed in each case is the same.* It all depends on the observer relative to the event. That's the crux of relativity. It shows us what time dilation does to atomic clocks, biological clocks, and in fact all processes and phenomena. It means that two events can occur at the same time in one reference frame and at different times if viewed from another one. Time is not an impartial score keeper outside of our universe - it is a function of our universe, perhaps even an artifact. We have just tasted the unsettling malleability of time.

This short 1.3-minute video illustrates how time is stretched relative to an observer:

*That light speed remains constant is a simple but extremely important point. If you think about it, physicists are saying that if we hold light speed as a constant in the universe we must let go of universal time as a constant. One or the other must go, according to relativity, and mountains of experimental evidence back up the consistency of light speed.

Physicists are using special relativity to great advantage in particle accelerators. Most of the interesting particles they want to observe and study last just infinitesimally small fractions of a second, once they are created in high velocity collisions between particles. Take the muon for example. Muons disintegrate in just 2 millionths of a second before they explode into electrons and neutrinos, not much time for study! But if they are created in an accelerator where they are traveling at about 99.5% the speed of light, they can get them to last ten times longer (from the physicists perspective! The muon itself, if it could feel, would not feel it lived any longer). Thanks to relativity, the muon would also be much more massive and its shape, if one could see it, would appear flattened.

To bring home the point of stretchy time even further, it is not just velocity that dilates time. Gravity does the same thing. Gravitation slows time down. Even an atomic clock placed just a few feet higher than another identical one in the same room runs just a tiny bit faster. The gravitational field in which it finds itself is just a tiny bit less intense so to a person observing the two clocks side by side, that one runs just a bit faster. This effect is significant enough that global positioning satellites have to have their clocks adjusted because their clocks run a bit faster than those on Earth's surface (more so than their relative velocity slows them down).

This means that there is a local proper time for every infinitesimal region of space, where the gravitational field in that space affects light and matter equally. It means that no two synchronized clocks, unless they were somehow perfectly superimposed on each other, can remain synchronized. This is a consequence of Einstein's law of general relativity.

Einstein managed to erode time as a stand-alone phenomenon. Its meaning is severely limited unless it is incorporated into space-time.

Now that we have expanded our understanding of time by weaving it into the three dimensions of space, let's examine its directional nature.

Idea # 2: Time is a One-way Arrow. Or is It?

Entropy

All the three spatial dimensions have two arrows, two directions, such as east/west or up/down. But time seems to have just one arrow - forward. Why?

In 1927, British astronomer Arthur Eddington coined the phrase, "arrow of time." He viewed time as the direction of the progressive increase of randomness in the universe, that it is a property of entropy, in other words. What is entropy? The idea of entropy was introduced by Ludwig Boltzmann in the 1870's. You can think of entropy in this sense as a measure of microscopic randomness. According to the second law of thermodynamics, entropy increases with time. Although entropy cannot measure time, according to this theory it can be used to empirically distinguish between future and past, in a closed system. In an open system, some objects, such the human body, for example, decrease in entropy, at the expense of the environment, which then increases entropy, with the overall system gaining entropy over time. The interesting part about the second law of thermodynamics is that it is, by nature, a statistical law. No macroscopic system has ever been observed to violate the second law of thermodynamics.

Maxwell's demon is an excellent thought experiment that tests our understanding of entropy and the second law. This is how physicist James Maxwell set it up in 1867:

A container is divided into two parts by an insulated wall. It has a door that allows only fast-moving energetic or "hot" molecules of gas to flow through to only one chamber in the container. That chamber gradually heats up while the other chamber cools down. This violates the second law of thermodynamics because no work is expended to decrease the entropy of a closed system. A level of order has been imposed on it.

Fellow physicists cleverly pointed out that the entropy of the door itself was neglected in the experiment. It must acquire information about the molecules and that, by itself, would require energy. The expenditure of energy by the door would increase the overall entropy of the system. Real-life Maxwell demons are now widely used, such as single-atom traps and molecular mechanisms used in nanotechnology. None of them violate the second law of thermodynamics.

If you feel a bit fuzzy about Maxwell's Demon, try out this 13-minute online lecture:

You might be able to come up with many examples that demonstrate time's irreversible one-way arrow which also rely on entropy. For example, ice cubes melt in a cup of hot coffee; they never reassemble themselves out of it.

Time in the Mirror?

The one-way time arrow required by entropy seems to break down when we examine the universe at the quantum level. You might recall me talking about CPT symmetry in my antimatter article. CPT stands for charge, parity and time symmetries. All physical laws are perfectly symmetrical with respect to these three values. Before the 1950's, physicists were confident that no system violates charge or parity symmetry. All phenomena should be perfectly reversible in these two regards, as long as the second law of thermodynamics isn't violated. This means that any particle can be interchanged with its antiparticle (c-parity) and left and right can be swapped (p-symmetry) without violating any laws of physics. All charged particles with spin 1/2 (these include the electrons and quarks that make up matter) have antimatter counterparts of opposite charge and opposite parity. They are like mirrors of each other except that the reflection in a mirror is 2-dimensional - a true p-transformation is a reversal in all three spatial axes. This should be expected of nature.

Quantum Time

However, in the 1950's and 1960's, physicists discovered that both charge and parity symmetry are violated during special kinds of nuclear decay, a phenomenon of the weak fundamental force. They are described in detail here. Symmetry is restored if time is permitted to run both backwards and forwards (t has a minus and a plus value) in the formulae used to describe the decay process. If time runs backwards on a macroscopic scale, cause and effect are lost and the universe becomes a jumbled series of unrelated events. The second law of thermodynamics becomes meaningless. However, what if time can run in both directions at the quantum level? Recall that thermodynamics is a statistical theory. As long as events add up to its predictions, the theory remains intact. An individual event, however, such as a particle moving backward through time, may not violate the law. Physicist Richard Feynman followed the argument that by inverting time, one can invert the charge and parity of a particle so, by extension, antiparticles travel backward through time. All of the fundamental laws of quantum physics allow for time reversibility: a time-reversed process is allowed where parity is switched and particles are switched with antiparticles. The problem is how to reconcile this with the irreversibility of entropy.

Some physicists believe that the answer comes at least in part from reexamining the second law of thermodynamics. What we see as the irreversibility of entropy might be the result of the extremely low entropy conditions of the early universe very shortly after the Big Bang. Energy was so uniform at this point that it has been a theoretical challenge to figure out how stars and galaxies later formed. Entropy itself has been refined over the decades as well. We now consider entropy to be the measure of the total number of different individual microscopic states of an object that, macroscopically, are indistinguishable. An omelet has higher entropy than an egg because there are more ways to rearrange its atoms while keeping it an omelet than there are for an egg. Entropy tends to increase over time because there are more ways to be high entropy than there are for low entropy. The gist of this reasoning is that, if increasing entropy is the result of how this

*particular*universe unfolded, then time may likewise be reversible. Perhaps another universe could have unfolded instead, one with decreasing entropy and in which time runs backward. The question of why entropy was so low in the beginning of our universe remains however. And, as you can see, this remains a partial answer.

Idea #3: Time as Artifact

One of the central goals in modern physics is to reconcile quantum mechanics with relativity. A theory of quantum gravity would go a long way toward bridging the gap and, as a result, this field of research is being hotly pursued. Physicists John Wheeler and Bryce DeWitt attempted to formulate a quantum gravity theory as early as the 1960's. They rewrote Einstein's equations for gravity in the same form as the equations for electromagnetism, with the idea being that a quantum theory of electromagnetism could then be applied to gravity as well. The equation they came up with had a weird result - it had no time variable in it. Some physicists regarded this result as the further denigration of time as an independent phenomenon, that time may in fact be nothing more than an artifact of changing physical phenomena such as the relationships between objects in motion. Time in this sense is simply a way to relate physical systems to one another, much like money is used to measure the transactions of objects of value. This has spurred many physicists to come up with ways to test the validity of time itself, a quest in progress.

Life As a Photon?

If you read my article on the nature of light, you might have felt frustrated at Richard Feynman's apparent acceptance of the notion of photons sensing where they will ultimately go. How can any particle try out infinite paths and require no time at all to do it? There is something fishy about QED and yet there is already a mountain of direct experimental evidence that it works. I can't give you a satisfying answer here but I can suggest that particles such as photons do not experience the universe like we do.

Let's explore this notion further. First I recommend a really cool 1992 essay written by astronomer, John Reed, called, "The life of a Photon." It describes the lifespan, from emission to absorption, of a single photon that happens to reach your retina from a distant star in the Andromeda galaxy while you are staring up at the night sky. The author does not insert special relativity into his description so let's consider that. The photon is traveling at exactly light speed. From our perspective it took two million years to make the trip. From its perspective it took an instant, right? Now consider a photon from the universe's origin, a part of the microwave background, striking a TV antenna. For it, the entire universe just flashed instantly into existence. To an object approaching the speed of light, the universe has a width approaching zero (this is called Lorentz contraction, a consequence of special relativity) and everything in the universe from its beginning to the present exists within one flat 2-dimensional plane, with the latest version of things directly ahead and the most distant past visible around the circumference. You can get an approximation of light speed but this description relies on Lorentz transformation equations and when you plug the speed of light into them you get an undefined answer.

At present we don't really have any way of knowing just what "life" from the perspective of a photon, where time stops, is like. And we have no way to ask photons what they are up to as they go about their spooky business. What these examples point out, however, is that our understanding of time is incomplete. We don't have the theoretical framework yet in which to test the veracity of time.

You might be tempted to throw out time altogether, and some physicists are considering doing just that. Let's consider this viewpoint as we take a look at physicist Sascha Vongehr's article in which he argues that light not only does not exist, it cannot exist. He makes an excellent and fascinating argument that, although about photons, builds upon John Reed's exploration and helps us expand our reference frame to ask what time really means. His argument that light cannot exist is based on the logic that, for a photon, time is squeezed down to zero, and if it does not exist in time how can it exist at all? An analogous argument is made based on Lorentz contraction. Distance, to a photon, is likewise squeezed down to zero, so the photon does not travel anywhere. By these arguments, photons don't really exist. It's an excellent thought experiment to ponder over. Let's use this logic to further explore "life" as a photon. To a photon, the universe has no time and no width. Each photon experiences the universe as a photo-like snapshot, regardless of how far it travels before it is absorbed. Each photon's (disk shaped) snap shot would be unique, reflecting its point of absorption (end or death from our perspective) in the exact middle and its point of origin smeared thinly around the circumference. Two photons with an intersecting time line (briefly co-existing, according to our frame of reference) would have a section of overlapping view in their snap shot. Consider how daunting a task it might be for an intelligent light-speed dwelling being to figure out the dimensions of time and the third spatial dimension. Photons would have to compare their snap shots and wonder why some have overlapping parts and others do not. The center of each photon's snap shot would reflect a unique point in their 2-dimensional world, and perhaps a comparison between the sizes of individual snap shots would hint at the hidden dimension of time. This is the same kind of quandary we find ourselves in when we speculate that we are three-dimensional projections within a four-dimensional (four spatial dimensions + time) universe. This is the basis of the holographic universe theory. I think this logic does a better job of placing time as a dimension on par with spatial dimensions, asking us to further explore the multidimensionality of the universe, than it does trying to prove that time does not exist.

So, as a result, I think it is premature to throw out time yet. Many experts find the notion of timelessness completely unpalatable. For one thing, the absence of time forces us to rethink quantum mechanics. Consider Schrodinger's famous cat for example. Its life-or-death fate depends on which quantum state a particle collapses into once a measurement takes place. If time does not exist the fate of the cat can never be resolved. It could be dead with respect to itself, alive from my point of view and dead once again according to you. To extrapolate from this, the quantum state of every particle in the universe would be both collapsed and uncollapsed simultaneously, depending only on the reference point of who is observing it.

Time is, perhaps surprisingly, treated much like Newtonian time in quantum mechanics. In the equations, it acts like a constant, and the equations work very well, with the caveat that "t" can be negative or positive as I mentioned earlier. This is extremely pesky to those trying to reconcile this with Einsteinian time, where time (and space) treads upon slippery ground. No, it goes even further - it

*is*the slippery ground.

As another possible partial answer, there may be a way to view time that respects both its quantum reversibility and entropy's one-way arrow. In 2008, Physicist Claus Kiefer suggested that while the universe may be timeless, if it were broken into pieces, some of the pieces could serve as clocks for the others. We perceive time because we are one of those pieces. This idea is reminiscent of time as a consequence of relationships between physical systems. And, it humbly brings us back to a point before all this Newtonian/Einsteinian time confusion began.

I think this idea, or any idea about time, needs a fully fleshed out theory of quantum gravity before we can really begin to understand the nature of time within the universe as a whole. And here we have a conundrum: quantum gravity might need a better understanding of time in in order to be worked out. As an intriguing side note, some physicists are working on a theory of quantum gravity based on the holographic principle (and string theory). According to it, gravity is so much weaker than the other three fundamental forces because it originates in a fourth spatial dimension. We experience it merely as a shadow. This holographic idea seems to be gaining traction - for example it can be used to solve a puzzle called the black hole information paradox, but it is not without controversy, as this article attests to one of Stephen Hawking's famous heated arguments over it.

Time is an extreme puzzle to physicists right now. All possibilities are up in the air, and rather than fret about it I urge you to let various ideas sink in over time (pardon the pun), to mull things over for yourself, and to appreciate what an exciting and rare opportunity this is. We are alive when things are about to get very interesting. All it is going to take is another Einstein-like mind to finish what was started 100 years ago.

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