Transcript Time Travel - Dan Weijers

Time Travel
L1: Introduction to
Time and Time Travel
Apologies
• This lecture owes a lot to youtube.com
– Visuals are very useful when trying to
understand 4-dimensions
Afraid of the Dark?
• Who remembers trying to race against
light so you could get in bed before it got
dark?
• Turned out light was really fast
• If only we had a time machine
Proof of Time Travel
• Theoretically, the indeterminacy of subatomic particles could create very strange
effects on our ‘level’
• … possibly including effects that could
allow time travel
• … and here’s the proof!
Einstein’s Great Idea
• The speed of light is constant(ly very fast)
• If we had very fast spaceships, this would
allow us to observe some crazy things
– For example
– The twin paradox
• But we don’t need a spaceship
– A regular passenger jet will do
– Or even just a tower! (Visual explanation)
Backwards Time Travel
• Going forward in time is (relatively) easy
• Going back is much harder
• But Einstein’s theory of general relativity
seems to allow it
• It depends just how much space-time can
curve
Making a Worm Hole
• Black holes warp space-time a lot because
of their intense mass
• Flying a spaceship through one would be
pretty risky
• And, who knows where it would take you
even if you could make it through
• But, there may be a more convenient
way…
Dr Ronald Mallett
• Mallett thinks that light can warp spacetime too
• He’s currently trying to get funding to see if
his time machine will work
• Previous video
• Coffee analogy
Possibilities for Backwards Time
Travel
• Mallett’s light machine
• A naturally occurring closed time-like loop
in space-time
• A visiting alien race with a time machine
on board their ship
Being Specific about Time Travel
• Actually going to the future or past
– Not just some replica of it
• It’s all relative
• For me to time travel, there would need to
be a difference between the speed time
passes for me and the speed time passes
for someone or everyone else
What is Forward Time Travel?
• Forward time travel: my time goes slower
than yours (like aging slower than
everyone else)
• My time goes slower than yours as my
speed increases (relative to yours) and/or
if I am closer to mass than you (and if that
mass increases)
What is Backwards Time Travel?
• What: to me, it seems like your
time is reversing into our shared
past
• How: space-time (where I am) is
warped so significantly that it
creates a closed time-like loop
or wormhole to our shared past
• The loop allows me to travel
faster than the speed of light
(not absolutely, but relative to
our past as we know it or you
watching me go through it)
A Closed Time-Like Loop
This is not a Physics Course
• Don’t worry if that’s
confusing
– the exam question
will be based on the
logical paradoxes of
time travel
• Any questions on
what time travel is?
– (Not how to do it on
a student budget!)
Take-Home Lessons
• Time is not like what it seems
• If you have an assignment to do, you
should do it on the top floor of the library
instead of the bottom… because you’ll
work faster! (As long as you can get there
and back quickly)
• To stay young-looking compared to your
friends, make sure you take more longhaul flights than them!
Next Time
• Read
• The Case of Jocasta Jones p207
– By Jonathan Harrison
• Get ready to discuss
• Some logical paradoxes of time travel
Time Travel
L2: The (Easy)
Paradoxes of Time
Travel
Time Travel is Possible!!!
• This might be a hoax, but time travel really
is possible
– In fact, many of us have already done it
What is Time Travel Again?
• Actually going to the future or past
– Not just some replica of it
• It’s all relative
• For me to time travel, there would need to
be a difference between the speed time
passes for me and the speed time passes
for someone or everyone else
What is Forward Time Travel?
• Forward time travel: my time goes slower
than yours (like aging slower than
everyone else)
• My time goes slower than yours as my
speed increases (relative to yours) and/or
if I am closer to mass than you (and if that
mass increases)
What is Backwards Time Travel?
• What: to me, it seems like your
time is reversing into our shared
past
• How: space-time (where I am) is
warped so significantly that it
creates a closed time-like loop
or wormhole to our shared past
• The loop allows me to travel
faster than the speed of light
(not absolutely, but relative to
our past as we know it or you
watching me go through it)
A Closed Time-Like Loop
This is not a Physics Course
• Don’t worry if that’s
confusing
– the exam question
will be based on the
logical paradoxes of
time travel
• Any questions on
what time travel is?
– (Not how to do it on
a student budget!)
The Case of Jocasta Jones
• Follow the story and keep your eye out for:
– Weird happenings
– Illegal acts
Jocasta’s 2000
The Case of
Jocasta Jones
Jocasta’s 2001
Jocasta’s 1910?
Jocasta’s 2019
Jocasta’s 1900?
Fathering Yourself is Impossible
• How is it possible for someone to be their
own father??
Fathering Yourself is Backwards
Causation
• To father yourself, you would have to
procreate with your mother, but that
probably requires a few shots… and
backwards causation – doesn’t it?
• Surely it would be causing a change in the
past, which is impossible!
– The past has already happened, so how can
we change it?
Backwards Causation
• We normally think of effects as following
their cause
– Study (usually) causes the effect of a good
grade
– The history of Europe has caused the borders
between nations to be recognised as they are
today
• But, if you father yourself then the effect of
that procreation is also the cause!
– Like getting a good grade causing you to have
already studied!
The ‘No Destination’ Objection
• If you haven’t thought through all of the
implications of 4-D space-time, then you
may still believe:
– That the past no longer exists
– That the future doesn’t exist yet
– That only the present time exists
• If these points are true, then time travel is
impossible because there is no other time to
travel to
Reply to the ‘No Destination’
Objection
• 4-Dimensional space-time
• It only seems like the past and future don’t
exist because we are not aware of them
– ‘We’: the event that is our consciousness at this
current time and place in 4-D space-time
• 4-Dimensional space-time is widely
accepted in physics… and…
• Einstein came up with it
Time Discrepancy Paradoxes
• If you missed the first lecture, you may still
believe:
– That time or the passage of time is a constant
– That time passes the same for everyone and
everything everywhere
– That Newtonian or absolute time exists
• If these points were true, then time travel is
impossible because it involves differences in
how time passes between the time traveller
and everyone else
Reply to the Time Discrepancy
Paradoxes
• 4-Dimensional space-time
• It turns out that time (and the spatial
dimensions) are relative
– Time is not fixed or absolute or constant
• The speed of light is constant, but time is
relative to speed and proximity to mass
– Physicists have proven this… and…
• Einstein came up with it
The In Two Places at Once Paradox
• No one person can wholly be in two places
at once
– Wholly is important here
• If you believe this, (which you should) then
doesn’t that mean time travel is impossible?
– (because it would allow you to go and wish
yourself many happy returns at your last
birthday party)
Reply to the In Two Places at Once
Paradox
• Big spatial objects have different parts that make
up its whole.- e.g. the North Island
• But, despite what Aucklanders think, the North
Island is never wholly located in one part
• All objects are really spatio-temporal objects
• In this moment, I am not the whole of me
– Right now I am just a spatio-temporal part of the whole
that is me throughout space-time
• Just like tectonic plates (different parts of the
earth’s crust) can rub against each other, It’s
possible for me(present part) to shake my(past part) own
hand at my last birthday
Visiting Myself
My last
birthday
party
Me
existing
through
spacetime
Take-Home Lessons
• If you time travel into the past, avoid your
parents at all costs
• Newton was very smart, but he was wrong
– time is not a constant
• The past exists and part of us (and parts
our parents) currently exist in the past
– So, assuming your parents had at least one
cool moment in their lives, they are, in a way,
still partly cool
Next Time (+ Exam Tip!!!!)
• Read
• The Paradoxes of Time Travel p208
– By Ken Perszyk & Nick Smith
• Get ready to attempt to solve the logical
paradoxes of time travel, including the
Grandfather Paradox – which will be very
useful to know for the exam
Time Travel
L3: The (Hard)
Paradoxes of Time
Travel
Time Travel is Possible!!!
• John Titor came from the
future… and provided truth
(available on youtube and
at www.johntitor.com)
• This might be a hoax, but is backwards
time travel really possible?
• Physicists say that it is… but what about
the logical contradictions that would result?
The Paradox of Changing the Past
• If you could go back in time, you could do
more study for the test and get a better mark
• But, you already have a mark! You can’t
change it – its part of history now!
• P1) If time travel is possible, then you could
change the past
• P2) It is never possible to change the past
• C) Therefore, time travel must not be
possible
Replying to the Paradox of
Changing the Past
• Denying P1: Time travel is possible, but it
does not and cannot change the past
– When you time travel, you create a new alternate
universe
• So you can change things, but not things from the
past in our universe
– When you time travel, you can affect the past
because you already did
• You don’t change history… you had already affected
it!
• An accurate history book would already have a record
of you being there!
Telling Myself to Study
When I
should
have been
studying
Me
existing
through
spacetime
Jocasta’s 2000
The Case of
Jocasta Jones
Jocasta’s 2001
Jocasta’s 1910?
Jocasta’s 2019
Jocasta’s 1900?
Jocasta’s Son/Husband’s Lifeline
Jocasta is born
1980
1900
2000
2019
1910
Jocasta
The guy
The guy dies
The guy is born
The Grandfather Paradox
• If you could go back in time, you could kill your grandfather
before he could produce any offspring
• But, if you did that, then there would have been no you to
make a time machine and go and kill him!
• Which means that you can’t have killed him…
The Grandfather Paradox (Cont.)
• P1) If time travel is possible, then you could
kill your grandfather (before he had kids)
• P2) It is impossible for you to have existed if
you succeed in killing your grandfather
(before he had kids)
• C) Therefore, time travel must not be
possible
Replying to the Grandfather Paradox 1
• Denying P1: Time travel is possible, but you
do not and cannot kill your grandfather
– When you time travel, you create a new alternate
universe
• So you can kill some old guy… but it’s not your
grandfather (not from your past)
– But does time travelling to an alternate universe
really count?
• Is it time travel or inter-universal travel?
• For time travel to be meaningful does it have to be to
our own past?
Replying to the Grandfather Paradox 2
• Denying P1: Time travel is possible, but you
do not and cannot kill your grandfather
– When you time travel, you can affect the past,
but only because you already did
• This works in the Jocasta Jones case, but
• This justification doesn’t work this time
• E.g. If you had already been there and killed your
grandfather… then you would never have existed
Replying to the Grandfather Paradox 3
• Denying P1: Time travel is possible, but you
do not and cannot kill your grandfather
– When you time travel, you can attempt to kill
your grandfather, but you will never succeed
Replying to the Grandfather Paradox 3
• Denying P1: Time travel is possible, but you
do not and cannot kill your grandfather
– When you time travel, you can attempt to kill
your grandfather, but you will never succeed
– It doesn’t have to be magic or Time Guardians
that prevents the impossible from happening
– Because it’s impossible to kill your grandfather…
it’s necessarily true that you didn’t kill him
– (however that came about)
OK Fine, but where are the Time
Travellers, then?
• P1) If time travel ever becomes possible in
the future, then time travellers would most
likely have visited our past already
• P2) But there is no evidence of time
travellers in our past
• P3) It is very unlikely that the time travellers
could have left no evidence
• C) Therefore, it’s very unlikely that time
travel ever becomes possible
Replying to the No Evidence of Time
Travellers Problem 1
• Accept the argument
– It’s true that there are no time travellers… but
only because:
•
•
•
•
Global warming, or
Nuclear war, or
Super-viruses, or
The robotic uprising of the (very) late nineties…
– … killed off all of the humans before we got
around to making time machines!
Replying to the No Evidence of Time
Travellers Problem 2
• Denying P2: There is evidence of time travellers!
– John Titor www.johntitor.com
– That guy who went under his sink (Hakan Nordkvist)
• www.youtube.com/watch?v=E2sp-clMk8s&feature=related
– That mullet guy
Replying to the No Evidence of Time
Travellers Problem 3
• Denying P1: Only when time travel becomes
possible (by a human creating and turning on
a time machine – a closed time-like loop)
would we expect to start seeing things
appear from the future (recall what Dr. Mallett
said)
– So, we won’t expect to see anything until the first
closed time-like loop is set up
Replying to the No Evidence of Time
Travellers Problem 3
• But what about naturally occurring closed
time-like loops?
– If these exist in the future, then they could be
used to travel to our past
• Sure… they could… but they would have to
conveniently connect the right two locationmoments in space-time
• This is just like aliens possibly existing
– They may just be too far away in space-time for
us to ever meet
Take-Home Lessons
• Time travel in movies is usually illogical, but
• Time travel is theoretically and logically possible
in real life
• But if you have homicidal intentions towards
your ancestors…
then make sure that you look out for banana
skins
How to Find out More
• Want to know more about time, time travel and
how they impact on other topics from this course
(like free will)?
• Enroll in the metaphysics course next year (or
the year after): PHIL225/325
• Keep up with the latest science news
– C.E.R.N’s L.H.C. (Large Hadron Collider)
• http://lhc.web.cern.ch/lhc/
– Dr. Ronald Mallet (real time machine maker)
• http://www.physics.uconn.edu/~mallett/main/main.htm
Next Week