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What do the inner and outer cylinders represent in Godel's model of time travel?
What represents the motion of a person traveling through space-time in Godel's model?
What happens when the traveller reaches the edge of the inner cylinder?
What does moving anti-clockwise around the outer cylinder represent?
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As a traveller moves from point A to point B in the outer region, what temporal effect occurs?
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What is the primary factor affecting the warping of space-time according to Einstein's theory?
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Which of the following concepts suggests the potential for time travel?
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In Einstein's model, what happens to an object's existence in relation to time?
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What does the curvature of space-time imply about time's progression?
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What critical question does the discussion of time travel lead to?
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What is the consequence of killing your grandfather as a boy according to the grandfather paradox?
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What can time travelers do to the past according to the grandfather paradox?
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Why is it impossible to kill grandfather from the perspective of the paradox?
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What is the primary issue with the traditional view of time travel as described?
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What is a requirement for a time traveler to kill someone in the past successfully?
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Which concept allows for preventing changes to the original timeline during time travel?
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What remains an unanswered question raised by the grandfather paradox?
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What is the focus of the discussion on time travel as stated?
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What is the grandfather paradox primarily concerned with?
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What does the new timeline concept permit in the scenario of time travel?
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What does Lewis's theory suggest about the identity of a person over time?
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What is a consequence of applying Leibniz’s Law to time travel identity?
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How might one resolve the contradiction of meeting one's younger self while considering them identical?
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What metaphor does Lewis use to explain his concept of identity over time?
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What is the suggested issue with considering You25 and You15 as the same person?
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What do causal loops in time travel suggest about the origin of certain events or items?
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Which view argues that causal loops are merely strange but not impossible?
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In the second example of a causal loop involving a time machine, where does the knowledge of how to create the time machine originate?
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What is one of the objections to the possibility of time travel related to causal loops?
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According to one of the views discussed, what could prevent time travel from resulting in causal loops?
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Study Notes
Einstein’s theory of space-time
- Einstein believed that the speed of light is constant for all observers
- Einstein's theory of general relativity states that space-time can be affected by the mass of objects within it.
- Massive objects warp the topography of space-time
- The warping of space-time can create black holes and worm holes, making time travel possible.
Godel’s Time Travel Model
- Godel demonstrated how time travel is compatible with Einstein's general theory of relativity.
- Godel describes time travel by using two cylinders that represent two space-time regions wrapped around one another.
- The traveller starts in the inner region of space-time and finishes at an earlier time in the inner region.
- The traveller moves from the inner to the outer cylinder and back again
Why Time Travel is Logically Impossible
- Some argue that time travel is impossible because it is logically contradictory.
- “Second time around” refers to the idea that the traveller can witness an event twice, once without their presence and then again while they are there.
- It is contradictory to say that an event happens twice.
- Time travellers cannot change the past – if a traveller were to alter a past event, it would be a contradiction.
Summary
- Massive objects warp the topography of space-time, according to Einstein’s theory of general relativity.
- Godel's model shows how time travel could be achieved, but it is logically impossible, because events can only happen once, even for a time traveller.
Introduction
- Time travel does not mean past events happen again.
- It does not allow for changing the past.
Parallel Timelines
- One conception of time travel allows for creating a new timeline.
- Travelling to the past creates a new timeline, branching off from our own.
- This does not allow for changing the past in our timeline but only allows for creating a different timeline.
The Grandfather Paradox
- The most famous paradox about time travel is the grandfather paradox.
- Imagine traveling back in time to kill your grandfather before he became evil.
- You cannot kill your grandfather as a boy because it would mean that you, the time traveler, would not have been born.
The Grandfather Paradox: Contradictions
- This creates contradictions: 1) you are born and travel back in time, 2) you are never born because your grandfather was killed, 3) your grandfather died peacefully in old age, 4) your grandfather was killed.
- Time travelers can kill people in the past but cannot kill anyone who did not die in the past.
- This means that you cannot change the past but can influence the past to be what it already is.
Grandfather Paradox Again
- The grandfather paradox raises the question of what prevents you from killing your grandfather.
- You might be able to kill the boy, having the gun, but you cannot kill him because he lived to be an old man.
- No mysterious forces are required, according to Lewis. Ordinary events such as a banana peel, a jammed gun, or losing nerve will prevent you from killing your grandfather.
Improbable Time Travel
- Horwich argues that Lewis' view makes time travel highly improbable.
- It states that time travel will create a string of unlikely coincidences to prevent changing the past, which makes time travel improbable.
Mysterious Ring-fencing
- Others argue that Lewis' view involves an unexplainable ring-fencing of the past, which prevents changing it.
- It involves a mysterious force for keeping the past as it already is, which is a concept that is not easily explained.
Causal Loops
- Time travel could allow causal loops where effects occur before their causes.
- A common example is a time traveler bringing an object from the future, which then influences events that led to their existence.
- Such loops create a paradox because the object seems to originate from "nowhere."
- An alternative scenario involves someone learning of time travel in the future, returning to the past to teach their younger self, leading to the question of how the knowledge originally came about.
- While causal loops might appear impossible, some philosophers argue they are merely "strange" not "impossible."
- These loops can be compared to the inexplicable origin of the Big Bang; if we can accept its existence despite lack of explanation, we might do the same for causal loops.
Time Travel and Identity
- Time travel introduces challenges to our understanding of personal identity.
- The paradox arises when a traveler meets their younger self, potentially possessing different characteristics (e.g., wisdom vs. foolishness).
- Identity relies on Leibniz's Law, which states that if two things are identical (e.g., two versions of the same person), they must share all properties. However, this seems contradicted by the time traveler having different properties at different points in their life.
Solutions to the Identity Problem
-
Lewis's Temporal Parts Theory:
- Suggests that individuals are "space-time worms" spread across time, composed of temporal parts.
- This allows for different temporal parts of the same person to have distinct properties without violating Leibniz's Law.
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Relativized Properties:
- Proposes that properties are relative to locations in space-time, not absolute.
- If a traveler is wise in the future but foolish in the past, they possess wisdom at place A (future) and foolishness at place B (past).
- This allows for identical individuals to have seemingly contradictory properties.
- Both solutions offer explanations for how identity can maintain within a time travel framework.
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Description
Explore the fascinating theories of time travel as proposed by Einstein and Godel. Understand the concepts of space-time warping, the effects of mass, and the logical contradictions surrounding time travel. This quiz delves into their groundbreaking ideas and implications for the universe.
