Physics, Metaphysics, & Other Nonsense Handout 13
H.S. Hestevold Spring 2014
Is Time Travel Logically Possible?
I.A problem to contemplate…
Dr. Wenn invented a time machine. Inviting his assistant, Lucy, to step inside, he explained that when she emerged she would be in the future.
“Really? How exciting,” Lucy replies, “Can I pick a date?”
Dr. Wenn smiles. “Certainly. Anytime from now until, let us say, next Tuesday.”
Lucy is not much impressed, and asks if it would not be possible to go into the next century. Unfortunately, Dr. Wenn explains, there are many problems, not least that he has not yet worked out how to travel back from the future again. Second, the machine can only transport people for short amounts of time. Even to transport someone a week into the future requires, at full power, about seven days’ run time. Lucy is not impressed and climbs back out of the time machine. “Dr. Wenn,” she says, “I don’t call this a time machine it’s just about as exciting as locking myself up in a cupboard for a week.”
Dr. Wenn is most hurt. He thinks his assistant is overlooking the fact that his machine does transport people through time, even if, as she says, it’s at no greater speed than at which they would have gone anyway.[1]
Study Question. If Lucy is right that Dr. Wenn’s invention isn't really a time machine, then what, exactly, is the difference between Dr. Wenn’s contraption and a machine that could move someone into the future? What, exactly, would be true of the latter that is not true of the former?
II.Introduction to the metaphysics of time travel
A.Why time travel matters: the backwards-causation objection to Static Time.
It is logicallyimpossible that you are both human and nonhuman. And, it is thereby also physically impossible that you are both human and non-human: if there is no logically possible world in which you are human and nonhuman, then there is no physically possible world in which you are human and nonhuman (i.e. there is no possible world with the same laws of physics as our world in which you are both human and non-human).
Though it is logically impossible that you are both human and non-human, it may be logicallypossible that you jump twenty feet straight up. Of course, if there is a logically possible world in which you do jump twenty feet straight up, that world may be physically impossible: given what we know about gravity and the physiology of your legs, the actual laws of physics may well imply that, physically, you can’t actually jump twenty feet straight up (even though that laws of physics in some other logically possible world would allow you to do so). Similarly, physicists may tell us that there exists a tiny, 3D particle that is physically indestructible a particle such that, it is physically impossible to divide it in half. Though such a particle may be physically indivisible, it is not logically indivisible: it has, for example a left half and right half; a top half and bottom half, etc. (In principle, we could paint the left half blue and the right half red; or we could pain the top have yellow and the bottom half green.)
Now, what about backwards causation isbackwards causation logically possible or logically impossible? That is, is it or is it not logically possible that an event that happens at this moment causally affects events that occur at an earlier moment? Is it is possible that you could do something at this moment that would bring about a certain state of affairs at an earlier moment? Or, does the nature of time itself imply that such backwards causation is logically impossible?
Now, so what? Why should anyone care about the modal status of backwards causation? Backwards causation matters to those who are interested in the modal status of time travel: if backwards causation is logically impossible, then time travel would also be logically impossible given that time travel involves one’s causally affecting earlier events. For example, suppose that you step into a time machine wearing your favorite pair of Converse All Stars; you set the GO TO dial to ’65 Million Years Ago’, and then press START. When you exit the time machine, you step into mud and find yourself staring eye-to-eye at a triceratops the last living dinosaur. If this were to happen, then your pressing the START button in 2014 brought it about that, say, a fresh Converse shoeprint coexists with a fresh triceratops footprint; and your pressing the START button brought it about that the triceratops' retinas registered images of a human. The upshot: time travel to the past requires backwards causation; so, if backwards causation is logically impossible, then time travel to the past is logically impossible. Put another way, if time travel to the past is logically possible, then backwards causation is logically possible. But so what? Who should care whether backwards causation is possible or impossible? The modal status of backwards causation is itself relevant to the nature of time.
If backward causation is logically impossible, then there would seem to be a fundamental difference between time and space. Causal relations obtain in all spatial directions: you can causally affect (or be causally affected by) something behind or in front of you, to your left or to your right, of above or below you. If, however, backwards causation is impossible, then causal relations cannot obtain in all temporal directions: you can causally affect something later but not something earlier; and you can be causally affected by something earlier but not by something later. If there is such a fundamental difference between space and time, then this difference would weaken the case for Static Time: Static Time is plausible only if time isclosely spacelike. The less that time is spacelike, the less plausible it is that Static Time is the correct account of time.
Below is an interpretation of this modest argument against Static Time:
1. Static Time is plausible only insofar as space and time are closely analogous.
2. Although causation across any direction of space is possible, it is false that causation across any direction of time is possible backwards causation is impossible.
3. If backwards causation is impossible, then space and time are not closely analogous.
4. Therefore, space and time are not closely analogous. (from2,3)
5. Therefore, Static Time isn't plausible. (from 1,4)
III.Two preliminary objections to time travel
A.The temporal order objection: "Time travel involves an inherent contradiction. If you press the time-machine START button in 2014 and travel back in time to the end of the Triassic Period, it would then be true that you exist in 2014 and afterwards exist 65 million years ago. This is absurd: the Triassic Period can't be after 2014!"
Study Question. There may be good reasons to conclude that time travel is logically impossible, but this is not a good reason. What is an appropriate reply to this objection?
B.The suicide objection: "If time travel is logically possible, then it is logically possible that the inventor of a time machine in 2014 travels back in time and commits suicide in 2004 a decade before she invents the time machine. This is impossible. Therefore, time travel is impossible."
Study Question. There may be good reasons to conclude that time travel is logically impossible, but this is not a good reason. What is an appropriate reply to this objection?
IV.If Presentism/Endurance is correct, then is time travel logically possible?
A.The assumptions. Assume that Presentism is correct and that at least some things endure. Suppose as well that persons endure. On this view, then, you are now the very same person who was happy when you graduated from high school; and the one who was happy on graduation day is the same one who is now perplexed about time travel and who will be relieved when the final exam in PHL490 is over.
B.The graduation scenario in Presentist terms. Suppose that you are the first person to invent a time machine. You set the timemachine dial for ‘HIGH SCHOOL GRADUATION’ and now press the START button. You now open the timemachine door and watch yourself walk across the stage to receive your high school diploma. You exit the time machine and walk to the end of the stage, shake hands with yourself, and begin the following conversation:
The time traveler: “Congratulations, graduate! What are your plans?”
The graduate: “Why thank you! I plan to major in philosophy at UA.”
The time traveler: “Philosophy? Don’t major in philosophy! Instead, major in electrical engineering and minor in history. This combination will allow you to become the first person to build a time machine! And, with your education in history, you'll be better able to select interesting time-travel destinations (and to avoid the bad ones!).”
The graduate: “Wow! I could be famous! And instead of reading about Socrates, I might be able to argue with Socrates himself! I’ll do it -- I’ll switch from philosophy to electrical engineering.”
The rest of the story: Inspired by the time traveler, you change your plans: you drop Introduction to Philosophy and register instead as a first-semester electrical engineering student. Years after graduating from UA, you do indeed build the first time machine, and travel back to your high school graduation.
C.Study Questions. What are the obvious implications of Presentism/Endurance for the graduation scenario?
1.Backwards causation: If the graduation scenario is logically possible, must backwards causation also be logically possible?
2.Causal loops: The graduation scenario implies that you would never have built a time machine had you not traveled back in time and inspired yourself to build a time machine. Ignoring the problem raised below in (7), does this causal loop alone imply that the graduation scenario is logically impossible?
3.Changing the past: Instead of inspiring the graduate to build a time machine, is it logically possible that the time traveler murders the graduate instead?
4.A causal question: If Presentism is correct, then what exactly causes the time-traveler to appear at your graduation?
5.Do you remember pressing the START button? When you arrive at the graduation ceremony, you presumably remember having pressed the time machine’s START button. If Presentism is correct, then howis it that you came to have this memory?
6.Where are you? Suppose that you press the START button at 11:30 AM on April 17, 2012. Where are you at 11:31 AM?
7.Confronting yourself: If Presentism/Endurance is correct, would it be possible to shake hands with yourself at your high school graduation?
D.For further thought…If you are committed to Presentism, then is there any alternative model of time travel that could, in some qualified sense, preserve the graduation scenario? Is there any sense in which you could be an enduring time traveler that attends “your earlier graduation”?
V.David Lewis's defense of the logical possibility of time travel
Read: David Lewis, “The Paradoxes of Time Travel” in SUP, 65.L.1-4. (Skip 65.L.5-65.R.2.)
A.Introduction. Lewis will try to convince you that, given certain philosophical presuppositions, time travel is logically possible and that apparent paradoxes of time travel are mere oddities.
B.The time-travel discrepancy in need of resolution: Imagine the time traveler who, say, travels for an hour in a time machine and then arrives several years into the past or future. A theory of time travel should explain the apparent discrepancy between the amount of time traveled (e.g. a journey of one hour's duration) and the separation in time between one's departure and one's arrival in the past or future.
Read: Lewis, “The Paradoxes of Time Travel,” 65.R.3-66.L.2.
C.Lewis's philosophical presuppositions: Perdurance/ST. Lewis writes that "Enduring things are timelike streaks…" and that "Change is qualitative difference between different stages -- different temporal parts -- of some enduring thing…" Lewis is not using the word 'enduring' in the technical sense that we adopted earlier this semester. By 'enduring', Lewis means 'persisting'. So, he is informing the reader that persisting timelike wholes are (4D) streaks across time and that change involves a persisting (4D) whole's having different temporal parts that exhibit different temporal properties. (That is, a persisting skillet changes in the sense that it has a red-hot temporal part at 6PM and a cool-black temporal part at 10 PM.) The bottom line: Lewis adopts a Perdurance/ST view of the world in order to defend the logical possibility of time travel.
Read: Lewis, “The Paradoxes of Time Travel,” 66.L.3.
D.Three types of time travelers. See Lewis, 66.L.3.
Study question: Lewis mentions three different types of time travelers. Be able to explicate each model, making clear how it differs from the other two.
Study question: As you study Lewis’s model, is it consistent with all three types of time travelers? With only one of them?
For further thought… On 3/21/14, Hestevold asked LeClair this question: “Do the models of time travel that you have sketched imply that, if a time traveler travels from time t to time t*, the time traveler exists at every time between t and t*? (That is, do your models allow that a time traveler could “hop” from t to t* while failing to exist at some or all times in between?)” LeClair replied that the models that he discussed do imply the time traveler’s continuous existence between t and t*. How should we think of LeClair’s models for time travel in terms of Lewis’s three models? Do LeClair’s models all involve zig-zagging? For example, does wormhole time travel involve zig-zagging?
Read: Lewis, “The Paradoxes of Time Travel,” 66.L.4-67.L.3.
E.External time. As noted above, there appears to be a discrepancy between the amount of time a timetraveler travels and the amount of time that lies between the time of the timetraveler’s departure and the time (in the past or future) of the timetraveler’s arrival. For example, one might imagine that a timetraveler spends three hours in a time machine in order to get from March, 2014 to the time of the death of the last dinosaur. What could this discrepancy in time involve? Could this require postulating two discrete temporal dimensions? Lewis suggests that the discrepancy can be explained in terms of the difference between external time and personal time. Here is one account of what the elapsing of external time involves:
N-units of external time elapse between two events e and g = There exist two times t and t* such thate begins at t and g ends at t*; and between t and t*, exactly N nonoverlapping timekeeping events occur [e.g. ticks of the second hand; rotations of the Earth; rotations of the Earth around the Sun, etc.]
F.Personal time. Here is an interpretation of Lewis's view regarding personal time:
N-units of someone S's personal time elapse between two events e and g = Between events e and g, exactly N nonoverlapping ordered timekeeping events occur [e.g. ticks of the second hand; rotations of the Earth; rotations of the Earth around the Sun, etc.], and each of these N time-keeping events involves S.[2]
G.Resolving the discrepancy via external time and personal time. How can one explain coherently the time-travel phenomenon that, by traveling an hour in a time-machine, one lands years in the past? Lewis frames the problem to be resolved (and solves it) in 66.R.1. Lewis notes that, if time travel is logically possible, it should make sense to say, "Soon the time traveler will be in the past."
Study question: "Soon the time traveler will be in the past". How does Lewis himself make sense of this claim?
In class, Hestevold will make use of this analysis:
With respect to someone S's personal time, earlier event e occurs after later event g = e occurs at a time that occurs prior to the time at which g occurs; N-units of S's personal time elapse between g and e; and at least one timekeeping event involving S that occurs simultaneously with g causally contributes to the occurrence of at least one time-keeping event involving S that occurs simultaneously with e.
H.Lewis's time-travel model: the general picture.
1.The scenario: Relative to the time traveler's personal time, her arrival 65 MYA at the end of the Triassic Period occurs afterher stepping into the time machine in 2014. Her watch ticks, say, at 10:01:01 when she presses the time machine's START button, and her watch ticks, say, at 11:01:00 when she walks out onto a scene of dying dinosaurs 65 MYA. Yet, relative to external time, her arrival at the end of the Triassic Period occurs beforeher stepping into the time machine in 2010. Assume that the time traveler feels sorry for the last dinosaur that is dying in agony. She pulls a pistol and shoots the last dinosaur to put it out of its misery. An hour after arriving in the Triassic Period, the time traveler returns to the time machine and presses the RETURN TO PRESENT button. The time traveler disappears from the Triassic landscape and reappears in 2014, just a couple hours after she first pressed the START button.
Study Questions: