Topology of Time and Time Travel Essay The attached is the instructions of this paper and the class notes on the topic TIME and TIME TRAVEL There are 3 mai

Topology of Time and Time Travel Essay The attached is the instructions of this paper and the class notes on the topic TIME and TIME TRAVEL
There are 3 main views on the nature of time in philosophy:
(1) Eternalism: There is nothing special about the present. All times exist at all times. The
past and future are just as real and existent as the present.
“All times, past, present and future, are actual times just like all points distributed in space
are actual points in space. One cannot privilege any one moment in the dimension of time as
“more” real than any other moment just like one cannot privilege any point in space as
“more” real than any other point. The universe is thus a spacetime “block,” a view that has
philosophical roots at least as far back as Parmenides. Everything is one; the appearance of
things coming to be and ceasing to be, of time passing or flowing, is simply phenomenal, not
real. Objects from the past and future have equal ontological status with present objects.”
(2) Presentism: the present is the only thing that exists.
(3) Possibilism: The present and the past exists, but the future does not exist yet.
Less deterministic than eternalism. The future doesn’t exist yet, so it is not yet determined.
“Growing block”
Which one is true? I know, a difficult question, but what is your initial impression?
Time in physics:
“Newton argued that space, time and motion were absolute, that is, that the entire universe was a
single, uniform inertial frame and that time passed equably throughout it according to an
eternally fixed, immutable and inexorable rate, without relation to anything external. Natural
time travel in the Newtonian universe is impossible; there are no attributes or topography of
space or time that can be exploited for natural time travel stories.”
The point: for Newton, time flowed like a river and it was always the same time for anything all
throughout existence.
It was like there was a single clock for the entire universe.
Things changed with Einstein.
One core concept of the special theory of relativity is that the speed of light is constant.
However, time is no longer constant throughout existence.
“The invariance of the speed of light according to Special Relativity replaces the invariance of
time and distance in the Newtonian universe. Intervals of space, like length, and intervals of time
(and hence, motion) are no longer absolute quantities.”
With Einstein, “we lose the common sense meaning of simultaneity.”
“For example, the same event happens at two different times if one observer’s inertial frame is
stationary relative to another observer’s inertial frame moving at some velocity. Furthermore, an
observer in the stationary inertial frame may determine two events to have happened
simultaneously, but an observer in the second moving inertial frame would see the same two
events happening at different times.”
Huh? Say that A and B are two events. With Newton, A and B would happen at the same time
for everyone in the universe no matter what. If A and B happened at the same time, they
happened at the same time for everyone.
With Einstein, things depend upon one’s frame of reference. A and B might happen at different
times from one frame of reference, but at the same time from a different frame of reference.
So what?
Well, “the lack of simultaneity across frames of reference means that we might experience the
phenomenon of time dilation. If your frame of reference is moving at some fraction of the speed
of light, your external time passes more slowly than the external time in a frame of reference that
is stationary relative to yours.”
Time slows down as you approach the speed of light.
This leads to one type of time travel being possible…time travel to the future.
“An example of Special Relativity time travel is of an astronaut who travels some distance in the
universe at a velocity near the speed of light. The astronaut’s personal time elapses at the same
rate it always has. He travels to his destination and then returns home to find that external time
has passed there quite differently. Everyone he knew has aged more than he, or perhaps has even
been dead for hundreds or thousands of years.”
However, special relativity suggested that it would be impossible to go backwards in time.
What do you make of this? Do you understand it? Einstein suggested that we can sort of
time travel into the future in a sense? Do you think we can travel backwards in time?
There might be various relationships between the three views of time in philosophy and possible
views of time in physics.
For example, some argue that presentism is inconsistent with Einstein.
“Presentism and the growing-past theory need to account for the Special Theory of Relativity’s
treatment of the present. Relativity implies there is no common global present, but only different
presents for each of us. Relativity theory allows event a to be simultaneous with event b in one
reference frame, while allowing b to be simultaneous with event c in some other reference frame,
even though a and c are not simultaneous in either frame. Nevertheless, if a is real, then
shouldn’t c be real? But neither presentism nor the growing-past theory can allow c to be real.
This argument against presentism and the growing-past theory presupposes the transitivity of coexistence.”
The point: presentism says that only the present exists. But Einstein suggests that there is no such
thing as an objective present. You have a present, someone else might have a different
present…But then it looks like presentism cannot be true.
Good argument against presentism?
A definition of time travel:
“Time travel is commonly defined with David Lewis’ definition: An object time travels if and
only if the difference between its departure and arrival times as measured in the surrounding
world does not equal the duration of the journey undergone by the object. For example, Jane is a
time traveler if she travels away from home in her spaceship for one hour as measured by her
own clock on the ship but travels two hours as measured by the clock back home, assuming both
clocks are functioning properly.”
So we’ve seen that it looks some kinds of time travel is possible…time travel into the future.
But what about time travel into the past?
This brings up the issue of the grandfather paradox.
Grandfather paradox:
Can someone travel back in time and kill their grandfather?
The Grandfather Paradox
“Heloise despises her paternal grandfather. Heloise is homicidal and has been trained in various
lethal combat techniques. Despite her relish at the thought of murdering her grandfather, time has
conspired against her, for her grandfather has been dead for 30 years. As a crime investigator might
say, she has motive and means, but lacks the opportunity; that is, until she fortuitously comes into
the possession of a time machine. Now Heloise has the opportunity to fulfill her desire. She makes
the necessary settings on the machine and plunges back into time 80 years. She emerges from the
machine and begins to stalk her grandfather. He suspects nothing. She waits for the perfect moment
and place to strike so that she can enjoy the full satisfaction of her hatred. At this point, we might
pause to observe: “If Heloise murders her grandfather, she will have prevented him from fathering
any children. That means that Heloise’s own father will not be born. And that means that Heloise
will not be born. But if she never comes into existence, then how is she able to return…?” And so
we have the infamous grandfather paradox…”
1. P is born.
2. Assume backwards time travel is possible.
3. P can kill grandfather. From 2.
4. P is never born. From 3.
5. A contradiction.
6. Backwards time travel is not possible.
This is a reductio argument.
What do we make of this?
Stephen Hawking: “According to his so-called Chronology Protection Conjecture, he claims that
the laws of physics conspire to prevent macroscopic inconsistencies like the grandfather paradox.
A “Chronology Protection Agency” works through events like vacuum fluctuations or virtual
particles to prevent closed trajectories of spacetime curvature in the negative direction (CTCs).”
There are a bunch of other puzzles involving time. We can talk about some of them next week.
But another issue is does time have a beginning?
Aristotle said no because we “can always imagine an earlier time.”
Aquinas said we can’t always be sure that our imagination is accurately telling us the way things
are (which seems completely obvious, but this was a time before modern science etc), and said
that time is finite. (This issue obviously has religious implications)
Newton thought time was infinite in both directions.
What do physicists say today? I guess they are not entirely sure. If the Big Bang theory is true,
time is finite at least going backwards.
“In the most well accepted version of the Big Bang Theory (namely, for large scale activity, the
Friedman model of Einstein’s equations plus early inflation), the past is finite. Time began a little
more than 13.7 billion years ago when the universe had infinitesimal size and a nearly infinite
gravitational field. About 10-35 second after the beginning, there was an inflation or accelerating
expansion that lasted for 10-30 seconds during which the universe expanded by a factor of 1025,
which is considerably faster than today’s speed of light. Once this brief period of inflation ended,
the energy causing the inflation was transformed into a dense gas of expanding hot radiation that
has never stopped expanding. As it expanded, the radiation cooled, allowing individual material
particles to form and eventually to clump into galaxies over billions of years. Nine billion years
after the big bang, dark energy took over and started to accelerate the expansion again. This
expansion of the universe will accelerate forever, turning space into an almost perfect vacuum as
the remaining matter-energy becomes more and more diluted. The implication for both space and
time is that, although they both were finite in the past, they will be potentially infinite in the future.
Actually the age of the universe since the Big Bang depends on where you are in calculating the
age. If you are out in relatively empty space between the galaxies, the age is a few billion years
more than 13.7. These differences in age are due to gravitational time dilation.”
Here is a philosophical argument from Newton-Smith (1980) for the claim that time is infinite:
“And as we have reasons for supposing that macroscopic events have causal origins, we have
reason to suppose that some prior state of the universe led to the product of [the Big Bang]. So the
prospects for ever being warranted in positing a beginning of time are dim.”
(1)
Events have causes.
(2)
The Big Bang was an event.
(3)
The Big bang had a cause. (from (1) and (2))
(4)
Whatever that cause was had a cause too and so on forever (from (1)).
Is this argument any good?
TIME (part two)
Quick review from last time.
There are 3 main views on the nature of time in philosophy:
(4) Eternalism: There is nothing special about the present. All times exist at all times. The
past and future are just as real and existent as the present.
(5) Presentism: the present is the only thing that exists.
(6) Possibilism: The present and the past exists, but the future does not exist yet.
We talked about how the view of time in physics has changed over the centuries. Newton
thought that time was absolute (it is always the same time everywhere in the universe) but
Einstein said that time was relative to one’s frame of reference (time can pass slower for
someone going faster etc.).
It looks like Einstein is inconsistent with presentism (because if Einstein is right, there is no such
thing as one fixed present).
We discussed how time travel to the past might be impossible because of the grandfather
paradox, and how there is an old debate about whether the universe is infinitely old or had a
beginning (e.g., the Big Bang).
Today: we’ll just look at a handful of other issues or argument concerning time before we move
on to a new topic on Wednesday.
There have been some interesting experiments concerning our subjective experience of time in
psychology, cognitive science and neuroscience.
We mentioned a well-known experiment by the neuroscientist Libet in connection with free will
(remember that some think that Libet’s work suggests that we don’t have free will, though this
was and is controversial).
“The most surprising experimental result about psychological time is Benjamin Libet’s
experiments in the 1970s that show, or so it is claimed, that the brain events involved in initiating
our free choices occur about a third of a second before we are aware of our choice. Before Libet’s
work, it was universally agreed that a person is aware of deciding to act freely, then later the body
initiates the action. However, Libet’s own experiments have been difficult to repeat because he
drilled through the skull and inserted electrodes to shock the underlying brain tissue.” (Quotes
from “Time” in the Internet Encyclopedia of Philosophy).
So the way that we consciously experience time is misleading. We think that we make a decision
and then act on it, but this suggests that it is the other way around: our brain makes a decision for
us and starts the action and then we (think that we) consciously decide what to do (even though it
has already been decided).
The brain is actively involved in constructing our experience of time:
“The brain takes an active role in building a mental scenario of what is taking place beyond the
brain. For example, try touching your nose with one hand and your knee with your other hand at
the same time. Even though it takes longer for the signal from your knee to reach your brain than
the signal from your knee, you will have the experience of the two touches being simultaneous
thanks to the brain’s manipulation of the data. Neuroscientists suggest that your brain waits about
80 milliseconds for all the relevant input to come in before you experience a “now.””
We seem to experience time differently than one another (which is weird to me):
“Craig Callender surveys the psycho-physics literature on human experience of the present, and
concludes that, if the duration between two experienced events is less than a quarter of a second,
then humans will say both events happened simultaneously, and this duration is different for
different people but stable within the experience of any single person. Also, “our impression of
subjective present-ness…can be manipulated in a variety of ways” such as by what other sights or
sounds are present at nearby times. (Callender, 2003-4, p. 124)”
And some people wonder if we could speed up our experience of time…
“Neuroscientists and psychologists have investigated whether they can speed up our minds relative
to physical time. If so, we might become mentally more productive, and get more high quality
decision making done per fixed amount of physical time, and learn more per minute. Several
avenues have been explored: using cocaine, amphetamines and other drugs, undergoing extreme
experiences such as jumping backwards off a tall tower with bungee cords attached to one’s ankles,
and trying different forms of meditation. So far, none of these avenues have led to success
productivity-wise.”
What do we make of any of this? Is it the case that the brain plays an important role in
shaping our experience of time? And should we mess with that?
Does it seem like a good idea to mess with the brain’s experience of time to increase
productivity? Maybe in 20 years we’ll be able to buy something in convenience stores to do
it, next to the five-hour energy drinks?
Another question is what is the relationship between us and external time?
There are various possible positions one can take here.
Aristotle asked,
“Whether, if soul (mind) did not exist, time would exist or not, is a question that may fairly be
asked; for if there cannot be someone to count there cannot be anything that can be counted…”
[Physics, chapter 14].”
He didn’t take a stand on it though.
St. Augustine said that time only exists in the mind.
Kant sort of said that too. Kant said that the mind structures experience through time (and
space…and various “categories” like causation). Time is not something we learn through
experience; rather, it is a necessary precondition of us having experience at all. If not for our
subjective experience of time, experience itself would make no sense. There might be an external
time too, but we cannot know “things in the themselves” (i.e., we cannot step outside of our
experience to know things as they are independently of the mind).
“It has been suggested by some philosophers that Einstein’s theory of relativity, when confirmed,
showed us that time depends on the observer, and thus that time is subjective, or dependent on the
mind. This error is probably caused by Einstein’s use of the term “observer.” Einstein’s theory
implies that the duration of an event depends on the observer’s frame of reference or coordinate
system, but what Einstein means by “observer’s frame of reference” is merely a perspective or
coordinate framework from which measurements could be made. The “observer” need not have a
mind. So, Einstein is not making a point about mind-dependence.”
But some physicists have argued that time is unreal too…
“In the mid-twentieth century, Gödel argued for the unreality of time because Einstein’s equations
allow for events to precede themselves. In the twenty-first century some physicists such as Julian
Barbour who are hoping to reconcile general relativity with quantum mechanics suggest that time
does not exist or else that it is not fundamental in nature; see Callender (2010).”
Is it possible that time doesn’t exist (or only exists in our heads)? It time a purely mental
phenomenon?
McTaggart: in 1908, formulated a philosophical argument that says time is not real (and this
argument is confusing).
He says we can order positions in time in two ways.
The A series: moments “can be ordered according to their possession of properties like being two
days future, being one day future, being present, being one day past, etc.” These relations are
constantly changing. What is “two days future” is different every day etc.
And the:
The B series: moments can be ordered by claims like “two days earlier than, one day earlier
than, simultaneous with, etc.” These never change. Two days earlier than today is always the
same day.
“McTaggart says that the A series is essential to time. His reason for this is that change (he says)
is essential to time, and the B series without the A series does not involve genuine change (since
B series positions are forever “fixed,” whereas A series positions are constantly changing).”
So, without the A series, there is no time.
However, the A series is contradictory:
“McTaggart also argues that the A series is inherently contradictory. For (he says) the different A
properties are incompatible with one another. (No time can be both future and past, for example.)
Nevertheless, he insists, each time in the A series must possess all of the different A properties.
(Since a time that is future will be present and past, and so on.)”
So think about March 26th (tomorrow). March 26th is tomorrow. But on Wednesday, March 26th
will be yesterday. The same day can’t be tomorrow and yesterday; that’s a contradiction. So the
A series is impossible (because contradictions are impossible).
So,
(1)
(2)
(3)
There is no time without the A series.
The A series cannot exist (because it is contradictory).
So there is no time.
What do we make of this (again, he’s trying to show that time isn’t real)?
Yet another issue people have a…
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