But what if time is like the palindrome above? What if the so-called arrow of time flies both ways, forward and back? What then? What now? What next?
People have debated the nature of time since, well, people invented it. Time is, in many ways, a fabrication of our minds, a superficial construct that helps us explain the universe, plot our course through existence and show up when we're supposed to."The only reason for time is so that everything doesn't happen at once," Albert Einstein once said.
And so it goes, one thing happens, then another - a phenomenon called cause-and-effect. "It's a notion so deeply ingrained that it's hard to think about things any other way," said Daniel Sheehan, a professor of physics at the University of San Diego.
But Sheehan does, as do other physicists who met recently at USD to discuss and debate the concept of "reverse causation," a fantastical notion that suggests effects can precede causes, and the future can influence the past, assuming the past and future actually "exist" in the first place.
(The recent symposium was part of the 87th annual meeting of the Pacific Division of the American Association for the Advancement of Science.)
"I don't think we've reached any kind of consensus coherent enough to be called a state of thinking," said York Dobyns, a physicist at Princeton University who attended the meeting. "There's a tremendous amount of disagreement about reverse causation between people who think the whole subject is just too speculative to deal with and people who have actually grappled with it, either theoretically or experimentally."
This much, however, can be said: While reverse causation (also called backward or retro-causation) may sound like science fiction, it is firmly grounded in classical laws of physics. These laws say time is symmetrical, that it moves - or should be able to move - in all directions with equal ease.
Case in point: electromagnetism, one of the four fundamental forces of nature. (The others are gravity and strong and weak nuclear force.)
In the 19th century, Scottish mathematician and physicist James Clerk Maxwell developed equations explaining how electricity and magnetism work in tandem. It was Maxwell, in fact, who determined that electromagnetic energy, such as light and radio, traveled in waves through empty space at the speed of light.
But Maxwell's equations say nothing about the direction of time. It's irrelevant. The equations work equally well whether electromagnetic waves arrive after or before they are transmitted. In effect, writes Paul Davies, a physicist at the Australian Centre for Astrobiology and author of "About Time," the waves "are indifferent to the distinction between past and future."
Feeling dizzy yet?
Most physicists accept the idea of time symmetry (at least in the context of things like Maxwell's equations). The same cannot be said of reverse causation, which goes farther by suggesting the future can influence the past.
"The tendency is to ignore it, to say it's just a fact of nature that time moves one way," said Michael Ibison, a physicist at the University of Texas at Austin.
If reverse causation is real, it most likely occurs at the largely theoretical and unseen level of quantum mechanics, a place where subatomic particles with names like mesons and quarks interact in ways contrary to both classical physics and common sense.
To wit: Mesons exist simultaneously as both particles and waves until they are observed. But until they are observed, they don't exist.
"Anyone who thinks they can talk about quantum theory without feeling dizzy hasn't yet understood the first word about it," said the late, great Danish physicist Niels Bohr who, incidentally, invented much of the theory.
"People know how to calculate with quantum mechanics, but that's not to say they know what it means," agreed Sheehan. "Quantum mechanics is like poetry. The poem is right there, for everyone to see, but it has many different interpretations."
Sheehan offers a couple of scenarios to ponder:
First, imagine a large boulder at the top of a hill. The boulder begins rolling downhill. Now freeze the action with the boulder midway along its descent. Call this the boulder's present. At this point in time, Sheehan says the boulder is being influenced both by its past (when it was atop the hill) and by its future (when it will come to rest at the bottom of the hill). The boulder's current position midway down the hill cannot happen without the effect of both the past and the future.
"The present is always a negotiation between the past and the future," said Sheehan.
Or think about this: You're invited to a Saturday wedding. On Friday, you go to the barber for a haircut. As you sit in the chair, the future is influencing the present. The wedding hasn't happened. It may not happen at all. And yet its possibility changes what will be the past.
The best evidence for reverse causation - perhaps the only evidence, said Sheehan - comes from parapsychology, which investigates phenomena not explained by the known laws of science, such as telepathy, clairvoyance and psychokinesis (the alleged ability to move matter with the mind).
NUMBERS IN LIMBO
In 1992, a paranormal investigator named Helmut Schmidt set up a radioactive decay counter to generate sequences of random numbers, both positive and negative. The numbers were recorded, but not seen by any person. Several months later, these numbers were shown to a group of students who had been asked to use their "mind power" to skew the sequences in favor of positive numbers. Elaborate precautions were taken to prevent cheating.
According to fundamental physical laws, there should have been an equal number of positive and negative numbers. But Schmidt reported that the students saw more positive numbers; the probability of that happening was less than 1 in a 1,000.
Did the students actually influence the outcome of radioactive decay rates recorded months before? Henry Stapp, a theoretical physicist at University of California Berkeley, thought so.
Stapp was one of the independent monitors of Schmidt's experiments. Two years later, he published a possible explanation for what had happened. In essence, he suggested that human consciousness had interacted with the numbers, effectively altering the past (when the numbers were recorded).
The idea, which Stapp and others have since expanded upon and promoted, is that human consciousness is an unexplained, nonlinear force of nature. Like subatomic particles in quantum mechanics, the numbers in Schmidt's experiment existed in a sort of limbo in which they were positive, negative and neither until the students saw them. At that point, human consciousness and intent (instructions to think positive) induced the numbers to assume a specific condition or quantum state.
The physics of consciousness is controversial, to say the least. And Stapp is first to say much more study and experimentation is necessary, especially by respected scientists in well-regarded scientific journals.
"You'd think people would want to either refute or confirm some of these reports," said Stapp, "but the only people willing to test them are people who already tend to believe them. Most mainstream labs shy away for fear of sullying their reputations, as if they would be dirtying their hands by even imagining some of this is possible."
For Stapp, who now works at the Lawrence Livermore National Laboratories, it's not inconceivable that quantum mechanics plays some role in alleged paranormal phenomena like extrasensory perception and remote viewing, which is the projection of one's consciousness to distant locations.
These abilities may be a consequence of nonlocality, a well-established quantum concept that says entities far-flung in distance or time are still entangled and interact via a faster-than-light, quantum mechanical connection.
Einstein called this phenomenon "spooky action at a distance." He couldn't explain it, didn't like it and regarded it as quantum trickery.
In recent decades, nonlocality has been repeatedly observed, tested and measured in experiments. In one seminal experiment in 1982, physicist Alan Aspect at the University of Paris noted that by changing the polarity of one speeding photon (a particle of light) he could induce another photon from the same source speeding in the opposite direction to change its polarity. The interaction happened faster than light, with sufficient distance between the photons that they shouldn't have "known" what was happening to the other. And yet, inexplicably, there was some sort of link.
In contrast, paranormal phenomena like ESP and remote viewing are not as well-substantiated. Supporting evidence tends to be anecdotal. Purposeful deception and fraud are common.
In the 1970s, the U.S. Army and the CIA spent millions investigating the potential of remote viewing, but that effort apparently went for naught and funding ceased. In 1979, the Princeton Engineering Anomalies Research (PEAR) program began investigating interaction between human consciousness and the physical world. Over the years, PEAR has produced a wealth of data indicating human intent by itself, without any physical connection, can alter the behavior or results of unthinking machines. The PEAR experiments, many similar to Schmidt's 1992 random number generator test, produced only small effects, but they were observable, measurable and repeatable.
PEAR's operations, however, are now in the process of closing down, with researchers moving on to other institutions.
Dobyns, an analytical coordinator for PEAR, said he still thinks "parapsychology and related areas are useful places to look for evidence (of reverse causation)."
But he is not optimistic that many mainstream physicists will ever take up the cause. "They say it's impossible because there's no evidence and there's no evidence because it's impossible."
But physicists like Sheehan say what we do understand about the universe fundamentally depends upon the idea that time is fluid and dynamic. "To say that it's impossible for the future to influence the past is to deny half of the predictions of the laws of physics," he said.
Nobody's predicting a speedy or conclusive resolution to the question of reverse causation. Sheehan says it's the journey that counts, how we get from Point A to B to C - or, perhaps, from C to B to A.