I resolved to have what I termed a little midlife crisis each day of my life in the hope of avoiding a larger one once I reached forty. Be active, not a passive worrywart. Find magic in the moment, joy in making someone smile. Most of all, marvel at the wonder that eons of evolutionary time and all your unique experiences have joined to comprise the symphony that is YOU. Yesterday is all of the days that have come before today. Tomorrow is all of the days that will come after today. Three days are all that we have in which to live our lives.
Tomorrow will be too late. Today is the day of reckoning for each of us. The direction of change does not matter.
Avoiding Paradoxes in Multiverse Time Travel Narratives | Luck | MOSF Journal of Science Fiction
The amount of change drives the effect. After you have rummaged through your old-memories store, write down the specifics of the memory. What exactly happened? If you met up with an older version of yourself, we know with absolute certainty that once you age into that older self, you will be there to meet your younger self. That is because, from your personal point of view, that meet-up happened, and there is no way to make it un-happen, any more than we can change the past without any time travel complications.
There may be more than one consistent set of things that could happen at the various events in space-time, but one and only one set of things actually does occur. Consistent stories happen; inconsistent ones do not. The vexing part is understanding what forces us to play along. The issue that troubles us, when you get down to it, is free will. We have a strong feeling that we cannot be predestined to do something we choose not to do.
That becomes a difficult feeling to sustain if we have already seen ourselves doing it. Of course, there are some kinds of predestination we are willing to accept. If we get thrown out of a window on the top floor of a skyscraper, we expect to hurtle to the ground, no matter how much we would rather fly away and land safely elsewhere. The much more detailed kind of predestination implied by closed timelike curves, where it seems that we simply cannot make certain choices like walking away after meeting a future version of ourselves , is bothersome.
The arrow of time is simply the distinction between the past and the future. We can turn an egg into an omelet, but not an omelet into an egg; we remember yesterday, but not tomorrow; we are born, grow older, and die, never the reverse. A neatly stacked collection of papers has a low entropy, while the same collection scattered across a desktop has a high entropy.
The entropy of any system left to its own devices will either increase with time or stay constant; that is the celebrated second law of thermodynamics. The arrow of time comes down to the fact that entropy increases toward the future and was lower in the past. But in the presence of closed timelike curves, some events are in our past and also in our future. So do we remember such events or not? In general, events along a closed timelike curve cannot be compatible with an uninterrupted increase of entropy along the curve.
Something has to give. To emphasize this point, think about the hypothetical traveler who emerges from the gate, only to enter it from the other side one day later, so that his entire life story is a one-day loop repeated ad infinitum. The traveler would have to ensure that, one day later, every single atom in his body was in precisely the right place to join up smoothly with his past self. He would have to make sure, for example, that his clothes did not accumulate a single extra speck of dust that was not there one day earlier.
This seems incompatible with our experience of how entropy increases. In either case, though, the insistence that we be in the right place at the right time puts a very stringent constraint on our possible future actions. Our concept of free will is intimately related to the idea that the past may be set in stone, but the future is up for grabs.
A closed timelike curve seems to imply predestination: We know what is going to happen to us in the future because we witnessed it in our past. Closed timelike curves, in other words, make the future resemble the past. It is set in stone, not up for grabs at all.
The reason we think the past is fixed once and for all is that there is a boundary condition at the beginning of time. The entropy of the universe started very small at the time of the Big Bang and has been growing ever since. Ordinarily we do not imagine that there is any analogous boundary condition in the future—entropy continues to grow, but we cannot use that information to draw any conclusions. If we use a closed timelike curve to observe something about our future actions, those actions become predestined.
If closed timelike curves exist, ensuring that all events are consistent is just as strange and unnatural to us as a movie played backward, or any other example of evolution that decreases entropy.
The Time Paradox Quotes
So either closed timelike curves cannot exist, or big, macroscopic things cannot travel on truly closed paths through space-time—unless everything we think we know about entropy and the arrow of time is wrong. Life on a closed timelike curve seems pretty drab. Once you start moving along such a curve, you are required to come back to precisely the point at which you started.
An observer standing outside, however, has what is seemingly the opposite problem: What happens along such a curve cannot be uniquely predicted from the prior state of the universe.
We have the strong constraint that evolution along a closed timelike curve must be consistent, but there will always be a large number of consistent evolutions that are possible, and the laws of physics seem powerless to predict which one will actually come to pass. In the usual way of thinking, the laws of physics function like a computer. You give as input the present state, and the laws return as output what the state will be one instant later or earlier, if we wish. By repeating this process many times, we can build up the entire history of the universe, from start to finish.
In that sense, complete knowledge of the present implies complete knowledge of all of history. Closed timelike curves would make such a program impossible, as a simple thought experiment reveals. Hark back to the stranger who appeared out of the gate into yesterday, then jumped back in the other side a day later to form a closed loop.
There would be no way to predict the existence of such a stranger from the state of the universe at an earlier time. The laws of physics purportedly allow us to predict what happens in the future of that moment.
See a Problem?
This ability vanishes as soon as someone builds a time machine and creates a closed timelike curve. Mysterious strangers and other random objects can then appear out of thin air and disappear just as quickly. We can insist all we like that what happens in the presence of closed timelike curves be consistent. But that requirement is not enough to make the events predictable, with the future determined by the laws of physics and the state of the universe at one moment in time. The warping associated with the closed timelike curve could cause our slice to twist back on itself, making it impossible to divide all of space-time into distinct moments.
We would therefore have to abandon the concept of determinism, the idea that the state of the universe at any one time determines the state at all other times. We would also have to abandon free will—because witnessing part of our future history implies some amount of predestination. Do we value determinism so highly that we should reject the possibility of closed timelike curves entirely? Not necessarily. We could imagine a different way in which the laws of physics could be formulated—not as a computer that calculates the next moment from the present moment but as a set of conditions that are imposed on the history of the universe as a whole.
It is not clear what such conditions might be, but we have no way of excluding the idea on the basis of pure thought. All this may sound like vacillation, but it provides an important lesson. Some of our understanding of time is based on logic and the known laws of physics, but some of it is based purely on convenience and reasonable-sounding assumptions. We think that the ability to uniquely determine the future from knowledge of our present state is important, but the real world might end up having other ideas. If physicists discover that closed timelike curves really can exist, we will have to dramatically rethink the way we understand time.