Tag Archives: general relativity

The Physics of Time?

Our conception of time as moving in one direction, from past to present to future, is so commonplace that we accept it as fact. But what if our experience of time is misleading us, and perhaps hiding the true reality of the universe? Can we rely on our senses to accurately perceive something as abstract as time? Is time real, or just an illusion caused by other physical effects? Can science provide any clues into understanding time?

It was once thought that time existed as absolute and unchanging, flowing at a constant rate and moving in one direction. This was true for scientists and the public alike. Isaac Newton considered time in much the same way as space; time and space providing the arena in which the universe unfolds. Newton’s famous laws of gravity and motion assumed absolute space and time. His laws work extremely well for our corner of the universe, that which is accessible to human senses. They are still used today to calculate the gravitational forces of the sun, moon and planets, as well as the motion of spacecrafts and objects close to earth.

There is a catch, however; Newton’s laws are not 100% accurate. Absolute space and time is not an acceptable assumption when dealing with extreme scales of the universe, a reality that was hidden from Newton in his time. The modern laws of physics question our everyday concept of time. In the early 1900s, Einstein devised the theories of special relativity and general relativity, and the idea that space and time could be flexible was born.

Einstein’s Revision of Newtonian Time

More than 300 years ago Isaac Newton wrote that, “He did not need to define time because it is something well known to all.” For obvious reasons our common sense perception of time has been called Newtonian time. The concept of absolute time had gone unchallenged until Einstein came along.

With Einstein’s revision of Newton’s ideas we have to envision a universe where each celestial body and each observer (what concerns us) carries their own clock with them. With relativity, the passage of time is relative to influences of mass and motion. In short, massive objects like stars and planets cause space and time to warp, resulting in gravitational effects and slowing down time. Also, time elapses slower for an object in motion than for an object at rest; the discrepancy in the passage of time gets proportionally larger as the speed increases. Even though it can be said that time runs at different rates (or two observers disagree on the passage of time), each perspective is equally valid. When one observer moves relative to another observer, clocks will not agree.

Flexible time is a property that applies everywhere in universe, however, the effects are minuscule in everyday life. Although the effects of relativity are not visibly apparent to us, observations have confirmed that this is how the universe really works. The scientific evidence is conclusive; time is relative, not absolute. Just as one can move through space, one can also move through time. No longer could space and time be considered as two separate entities; a new term called spacetime was brought into use to better account for the relationship between the two.

A hypothetical situation of an alien in a distant galaxy shows how bizarre relative time can be. If you are stationary here on earth and the alien moves away from you, the alien’s now coincides with a moment in your past. If the alien turns around and moves towards you, then the alien’s now coincides with a moment in your future. Just as extremes in speed and gravity alter the passage of time, extreme distance has a similar effect on what constitutes a given moment of time for two observers. This is the kind of universe that Einstein described.

I cannot think of a better everyday example of flexible time than GPS devices. The clocks in the satellites in orbit need to account for the fact that clocks on the Earth run a little bit slower. This is due to the combined effects of the motion of the satellites and the gravity on earth (the Earth’s gravity having the largest effect). If not for the application of relativity, GPS devices would quickly become inaccurate.

The Laws of Physics, Entropy and The Arrow of Time

Whether we examine small physical systems or the universe as a whole, there is no arrow of time found in the laws of physics. For example, if a scientist knows all the current conditions, he can determine precisely what happened in the past or predict a future outcome. This can be achieved by applying the same laws either backward or forward in time.

Is there anything in science that indicates an arrow of time? There is a concept in physics called entropy, which may give us an arrow of time. Simply stated, entropy is the measure of disorder, and the implication of entropy is that physical systems move towards a direction of increasing disorder. The reason being, that there are many ways in which disorder can come about. Conversely, there are few ways that order can be achieved.

Let’s take the example of the pages of a book (all numbered in order). If we were to randomly mix up the pages (and re-stack them) the chances are extremely high that the pages will end up disordered. In only one configuration will the pages be ordered, while many arrangements will be disordered. In almost all cases it takes a special effort to create order and no effort at all to create disorder.

The puzzle is: how has the universe created stars, galaxies, planets and life on earth? If entropy rules, you would think that the universe would be in chaos forever. To get an answer we may have to go back to the birth of the universe. The Big Bang is believed to have been a highly ordered event (perhaps the most ordered state of the universe). From that point on the universe has evolved into greater disorder. Entropy may give us an arrow of time. From the point of most order (in the past) towards increasing disorder (in the future).

This should make us pause and consider our present conditions on earth. Conditions favorable for life are extremely difficult to come by, and entropy is bound to rule in the end. In the grand scales of the universe, in both time and space, life is a newcomer and rare (as far as we know). Life on earth is destined to be extinguished, at least at some time in the far future.

Our experience shows us that many things only happen in one direction, and usually in the direction of more disorder. For example: A glass can fall to the ground and break, but a glass can’t reassemble by itself. A drop of ink can mix in water, but the ink can’t come back together into a drop. An egg can be broken, but can’t reassemble back into the shell. This is entropy at work, and possibly the scientific reason behind our common-sense experience of an arrow of time.

The River of Time

Clearly, there is a sense that time moves from past to present to future, like a river, which flows in one direction from one moment to another. From the present perspective the past is gone forever and the future is yet to be realized. However, for physicists it is not as clear cut. From Einstein’s perspective, what constitutes a given moment of time is dependent on the observer. Because time is relative to each observer, my now could coincide with a past or future experience of someone else in a far-away galaxy. There is no sense that the whole universe progresses at the same rate. There is no now that everyone can agree on.

How could this be? As long as there are discrepancies in time for different locations and observers, there can be no universal now for all. Equally, there can be no past or future moment that all can agree on. If this is true the implications are unsettling: All moments of the universe exist. From a physicist point of view Brian Greene concludes in The Fabric of the Cosmos:

” … if you agree that your now is no more valid than the now of someone located far away in space who can move freely, then reality encompasses all of the events in spacetime… Just as we envision all of space as really being out there, as really existing, we should also envision all of time as really being out there, as really existing, too.”

Einstein also saw the paradox between physics and experience: “For we convinced physicists, the distinction between past, present, and future is only an illusion, however persistent.”

Does time really flow like a river? Even from a common sense perspective the distinction of past, present and future is relative to the individual. For me, someone who lived many years ago existed in the past. Someone that will live 100 years from now will exist in the future. That’s all from my perceptive or from my point of reference. From the perspective of a historical figure, like Einstein, he lives in the present and I will exist only in the future. With each moment there is no essential difference, no temporal absolute, just the relative perspective of each individual.

Change as The Scorekeeper of  Time

If I haven’t created enough doubt as to your assumed notion of time, I will conclude with one more observation. This has to do with change. Is it possible that the only real aspect of time is change? At least could change be the only way that time is perceived?

We notice time has elapsed because something has changed. It is reinforced by our mind. Our memories tell us that an event was in the past, and our imagination projects that something could happen in the future. In essence, we experience the passage of time or that time flows because of continual change. If there were no change at all, would time even exist? Imagine a universe with every object being still or no objects at all. Every moment would be identical.

A reality with no change is not our experience, nor is it how the universe presently works. However, a particular question about the Big Bang Theory may shed some light: That is, what happened before the bang? Science can’t take us back any further, as the Big Bang represents a theoretical barrier. Perhaps we don’t need to look further. Physicists believe that time and space as we know it were created at the Big Bang. This may be highly speculative, yet it could be that there was no change before the Big Bang; or conditions were so chaotic that there would have been no discernible events. Thus, that would mean that nothing really happened before.

At the other end of the spectrum, one current model of the universe predicts that space will continue to expand at an increasing rate. This expansion will drag every galaxy farther apart with no end in sight. Far, far into the future everything in the universe will become diluted. In the end, if we can call it that, everything will decay, leaving only random particles drifting in space. The universe will be cold, dark and practically empty. We could even conclude there will be no change and time will also come to an end.

Coming up with an explanation for time is challenging. You could even make a case that time does not exist. What we experience as time may be something else altogether. With each perspective of time I have mentioned there is something intriguing, and still something seems to be missing. How could something as familiar as time be explained differently, with each explanation having some merit? That’s how it appears to me.

Newtonian time aligns very well with our daily experience of time. Einstein’s relativity is in agreement with modern observations of the universe. Entropy gives us an arrow of time not found in the laws of physics. The river of time points to everyone’s unique frame of reference. And finally, change gives us a physical component that marks the passage of time.

 

References: Brian R. Greene, The Fabric of the Cosmos (New York: Alfred A. Knopf, 2004), 139.

The Fabric of the Cosmos: The Illusion of Time, Life Sciences, Published on Apr 12, 2016. https://www.youtube.com/watch?v=pPA83Ap0Xsg.