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THE FABRIC of the COSMOS, Brian Greene, 2004
```(annotated and with added bold highlights by Epsilon=One)
```(annotated and with added bold highlights by Epsilon=One)
Chapter 15 - Teleporters and Time Machines
Rethinking the Puzzles
Recall that in Chapter 5 we discussed the flow of time, from the perspective of classical physics, and came upon an image that differs substantially from our intuitive picture. Careful thought led us to envision spacetime as a block of ice with every moment forever frozen in place, as opposed to the familiar image of time as a river sweeping us forward from one moment to the next. These frozen moments are grouped into nows — into events that happen at the same time — in different ways by observers in different states of motion. And to accomodate this flexibility of slicing the spacetime block into different notions of now, we also invoked an equivalent metaphor in which spacetime is viewed as a loaf of bread that can be sliced at different angles.
But regardless of the metaphor, Chapter 5's lesson is that moments — the events making up the spacetime loaf — just are. They are timeless. Each moment — each event or happening — exists, just as each point in space exists. Moments don't momentarily come to life when illuminated by the "spotlight" of an observer's present; that image aligns well with our intuition but fails to stand up to logical analysis. Instead, once illuminated, always illuminated. Moments don't change. Moments are. Being illuminated is simply one of the many unchanging features that constitute a moment. This is particularly evident from the insightful though imaginary perspective of Figure 5.1, in which all events making up the history of the universe are on view; they are all there, static and unchanging. Different observers don't agree on which of the events happen at the same time — they time-slice the spacetime loaf at different angles — but the total loaf and its constituent events are universal, literally.
Quantum mechanics offers certain modifications to this classical perspective on time. For example, we saw in Chapter 12 that on extremely short scales, space and spacetime become unavoidably wavy and bumpy. But (Chapter 7), a full assessment of quantum mechanics and time requires a resolution of the quantum measurement problem. One of the proposals for doing so, the Many Worlds interpretation, is particularly relevant for coping with paradoxes arising from time travel, and we will take that up in the next section. But in this section, let's stay classical and bring the block-of-ice/loaf-of-bread depiction of spacetime to bear on these puzzles.
Take the paradoxical example of your having gone back in time and having prevented your parents from meeting. Intuitively, we all know what that's supposed to mean. Before you time-traveled to the past, your parents had met — say, at the stroke of midnight, December 31, 1965,* at a New Year's party — and, in due course, your mother gave birth to you. Then, many years later, you decided to travel to the past — back to December 31, 1965 — and once there, you changed things; in particular, you kept your parents apart, preventing your own conception and birth. But let's now counter this intuitive description with the more fully reasoned spacetime-loaf depiction of time.
At its core, the intuitive description fails to make sense because it assumes moments can change. The intuitive picture envisions the stroke of midnight, December 31, 1965 (using standard earthling time-slicing), as "initially" being the moment of your parents meeting, but envisions further that your interference "subsequently" changes things so that at the stroke of midnight, December 31, 1965, your parents are miles, if not continents, apart. The problem with this recounting of events, though, is that moments don't change; as we've seen, they just are. The spacetime loaf exists, fixed and unchanging. There is no meaning to a moment's "initially" being one way and "subsequently" being another way.
If you time-traveled back to December 31, 1965, then you were there, you were always there, you will always be there, you were never not there. December 31, 1965, did not happen twice, with your missing the debut but attending the encore. From the timeless perspective of Figure 5.1, you exist — static and unchanging — at various locations in the spacetime loaf. If today you set the dials on your time machine to send you to 11:50 p.m., December 31, 1965, then this latter moment will be among the locations in the spacetime loaf at which you can be found. But your presence on New Year's Eve, 1965, will be an eternal and immutable feature of spacetime.
This realization still leads us to some quirky conclusions, but it avoids paradox. For example, you would appear in the spacetime loaf at 11:50 p.m., December 31, 1965, but before that moment there would be no record of your existence. This is strange, but not paradoxical. If a guy saw you pop in at 11:50 p.m. and asked you, with fear in his eyes, where you carne from, you could calmly answer, "The future." In this scenario, at least so far, we are not caught in a logical impasse. Where things get more interesting, of course, is if you then try to carry out your mission and keep your parents from meeting. What happens? Well, carefully maintaining the "spacetime block" perspective, we inescapably conclude that you can't succeed. No matter what you do on that fateful New Year's Eve, you'll fail. Keeping your parents apart — while seeming to be within the realm of things you can do — actually amounts to logical gobbledygook. Your parents met at the stroke of midnight. You were there. And you will "always" be there. Each moment just is; it doesn't change. Applying the concept of change to a moment makes as much sense as subjecting a rock to psychoanalysis. Your parents met at the stroke of midnight, December 31, 1965, and nothing can change that because their meeting is an immutable, unchangeable event, eternally occupying its spot in space-time.
In fact, now that you think about it, you remember that sometime in your teens, when you asked your dad what it was like to propose to your mother, he told you that he hadn't planned to propose at all. He had barely met your mother before asking the big question. But about ten minutes before midnight at a New Year's party, he got so freaked by seeing a man pop in from nowhere — a man who claimed to be from the future — that when he met your mother he decided to propose, right on the spot.
The point is that the complete and unchanging set of events in space-time necessarily fits together into a coherent, self-consistent whole. The universe makes sense. If you time-travel back to December 31, 1965, you are actually fulfilling your own destiny. In the spacetime loaf, there is someone present at 11:50 p.m. on December 31, 1965, who is not there at any earlier time. From the imaginary, outside perspective of Figure 5.1, we would be able to see this directly; we would also see, undeniably, that the person is you at your current age. For these events, situated decades ago, to make sense, you must time-travel back to 1965. What's more, from our outside perspective we can see your father asking you a question just after 11:50 p.m. on December 31, 1965, looking frightened, rushing away, and meeting your mother at midnight; a little further along the loaf, we can see your parents' wedding, your birth, your ensuing childhood, and, later on, your stepping into the time machine; If time travel to the past were possible, we could no longer explain events at one time solely in terms of events at earlier times (from any given perspective); but the totality of events would necessarily constitute a sensible, coherent, noncontradictory story.
As emphasized in the last section, this doesn't, by any stretch of the imagination, signify that time travel to the past is possible. But it does suggest strongly that the purported paradoxes, such as preventing your own birth, are themselves born of logical flaws. If you time-travel to the past, you can't change it any more than you can change the value of pi. If you travel to the past, you are, will be, and always were part of the past, the very same past that leads to your traveling to it.
From the outside perspective of Figure 5.1, this explanation is both tight and coherent. Surveying the totality of events in the spacetime loaf, we see that they interlock with the rigid economy of a cosmic crossword puzzle. Yet, from your perspective on December 31, 1965, things are still puzzling. I declared above that even though you may be determined to keep your parents from meeting, you can't succeed in the classical approach to this problem. You can watch them meet. You can even facilitate their meeting, perhaps inadvertently as in the story I've told. You can travel back in time repeatedly, so there are many of you present, each intent on preventing your parents' union. But to succeed in preventing your parents from meeting would be to change something with respect to which the concept of change is meaningless.
But, even with the insight of these abstract observations, we can't help asking: What stops you from succeeding? If you are standing at the party at 11:50 p.m. and see your young mother, what stops you from whisking her away? Or, if you see your young father, what stops you from — oh, what the heck, let's just say it — shooting him? Don't you have free will? Here is where, some suspect, quantum mechanics may enter the story.
But regardless of the metaphor, Chapter 5's lesson is that moments — the events making up the spacetime loaf — just are. They are timeless. Each moment — each event or happening — exists, just as each point in space exists. Moments don't momentarily come to life when illuminated by the "spotlight" of an observer's present; that image aligns well with our intuition but fails to stand up to logical analysis. Instead, once illuminated, always illuminated. Moments don't change. Moments are. Being illuminated is simply one of the many unchanging features that constitute a moment. This is particularly evident from the insightful though imaginary perspective of Figure 5.1, in which all events making up the history of the universe are on view; they are all there, static and unchanging. Different observers don't agree on which of the events happen at the same time — they time-slice the spacetime loaf at different angles — but the total loaf and its constituent events are universal, literally.
Quantum mechanics offers certain modifications to this classical perspective on time. For example, we saw in Chapter 12 that on extremely short scales, space and spacetime become unavoidably wavy and bumpy. But (Chapter 7), a full assessment of quantum mechanics and time requires a resolution of the quantum measurement problem. One of the proposals for doing so, the Many Worlds interpretation, is particularly relevant for coping with paradoxes arising from time travel, and we will take that up in the next section. But in this section, let's stay classical and bring the block-of-ice/loaf-of-bread depiction of spacetime to bear on these puzzles.
Take the paradoxical example of your having gone back in time and having prevented your parents from meeting. Intuitively, we all know what that's supposed to mean. Before you time-traveled to the past, your parents had met — say, at the stroke of midnight, December 31, 1965,* at a New Year's party — and, in due course, your mother gave birth to you. Then, many years later, you decided to travel to the past — back to December 31, 1965 — and once there, you changed things; in particular, you kept your parents apart, preventing your own conception and birth. But let's now counter this intuitive description with the more fully reasoned spacetime-loaf depiction of time.
At its core, the intuitive description fails to make sense because it assumes moments can change. The intuitive picture envisions the stroke of midnight, December 31, 1965 (using standard earthling time-slicing), as "initially" being the moment of your parents meeting, but envisions further that your interference "subsequently" changes things so that at the stroke of midnight, December 31, 1965, your parents are miles, if not continents, apart. The problem with this recounting of events, though, is that moments don't change; as we've seen, they just are. The spacetime loaf exists, fixed and unchanging. There is no meaning to a moment's "initially" being one way and "subsequently" being another way.
If you time-traveled back to December 31, 1965, then you were there, you were always there, you will always be there, you were never not there. December 31, 1965, did not happen twice, with your missing the debut but attending the encore. From the timeless perspective of Figure 5.1, you exist — static and unchanging — at various locations in the spacetime loaf. If today you set the dials on your time machine to send you to 11:50 p.m., December 31, 1965, then this latter moment will be among the locations in the spacetime loaf at which you can be found. But your presence on New Year's Eve, 1965, will be an eternal and immutable feature of spacetime.
This realization still leads us to some quirky conclusions, but it avoids paradox. For example, you would appear in the spacetime loaf at 11:50 p.m., December 31, 1965, but before that moment there would be no record of your existence. This is strange, but not paradoxical. If a guy saw you pop in at 11:50 p.m. and asked you, with fear in his eyes, where you carne from, you could calmly answer, "The future." In this scenario, at least so far, we are not caught in a logical impasse. Where things get more interesting, of course, is if you then try to carry out your mission and keep your parents from meeting. What happens? Well, carefully maintaining the "spacetime block" perspective, we inescapably conclude that you can't succeed. No matter what you do on that fateful New Year's Eve, you'll fail. Keeping your parents apart — while seeming to be within the realm of things you can do — actually amounts to logical gobbledygook. Your parents met at the stroke of midnight. You were there. And you will "always" be there. Each moment just is; it doesn't change. Applying the concept of change to a moment makes as much sense as subjecting a rock to psychoanalysis. Your parents met at the stroke of midnight, December 31, 1965, and nothing can change that because their meeting is an immutable, unchangeable event, eternally occupying its spot in space-time.
In fact, now that you think about it, you remember that sometime in your teens, when you asked your dad what it was like to propose to your mother, he told you that he hadn't planned to propose at all. He had barely met your mother before asking the big question. But about ten minutes before midnight at a New Year's party, he got so freaked by seeing a man pop in from nowhere — a man who claimed to be from the future — that when he met your mother he decided to propose, right on the spot.
The point is that the complete and unchanging set of events in space-time necessarily fits together into a coherent, self-consistent whole. The universe makes sense. If you time-travel back to December 31, 1965, you are actually fulfilling your own destiny. In the spacetime loaf, there is someone present at 11:50 p.m. on December 31, 1965, who is not there at any earlier time. From the imaginary, outside perspective of Figure 5.1, we would be able to see this directly; we would also see, undeniably, that the person is you at your current age. For these events, situated decades ago, to make sense, you must time-travel back to 1965. What's more, from our outside perspective we can see your father asking you a question just after 11:50 p.m. on December 31, 1965, looking frightened, rushing away, and meeting your mother at midnight; a little further along the loaf, we can see your parents' wedding, your birth, your ensuing childhood, and, later on, your stepping into the time machine; If time travel to the past were possible, we could no longer explain events at one time solely in terms of events at earlier times (from any given perspective); but the totality of events would necessarily constitute a sensible, coherent, noncontradictory story.
As emphasized in the last section, this doesn't, by any stretch of the imagination, signify that time travel to the past is possible. But it does suggest strongly that the purported paradoxes, such as preventing your own birth, are themselves born of logical flaws. If you time-travel to the past, you can't change it any more than you can change the value of pi. If you travel to the past, you are, will be, and always were part of the past, the very same past that leads to your traveling to it.
From the outside perspective of Figure 5.1, this explanation is both tight and coherent. Surveying the totality of events in the spacetime loaf, we see that they interlock with the rigid economy of a cosmic crossword puzzle. Yet, from your perspective on December 31, 1965, things are still puzzling. I declared above that even though you may be determined to keep your parents from meeting, you can't succeed in the classical approach to this problem. You can watch them meet. You can even facilitate their meeting, perhaps inadvertently as in the story I've told. You can travel back in time repeatedly, so there are many of you present, each intent on preventing your parents' union. But to succeed in preventing your parents from meeting would be to change something with respect to which the concept of change is meaningless.
But, even with the insight of these abstract observations, we can't help asking: What stops you from succeeding? If you are standing at the party at 11:50 p.m. and see your young mother, what stops you from whisking her away? Or, if you see your young father, what stops you from — oh, what the heck, let's just say it — shooting him? Don't you have free will? Here is where, some suspect, quantum mechanics may enter the story.
* Of course, I really should say January 1, 1966, but let's not worry about that.