1. Time
Week 05

2. In today’s lecture... The nature of time Newton, Kant and Einstein
Time travel
Time travel paradoxes

3. The nature of time
Does time exist in the external world, like trees and buildings? Or does time exist only in our minds?

4. The nature of time
What exactly is time? Can we explain the meaning of time in terms of concepts such as ‘motion’, ‘change’, ‘events’, ‘the past’, ‘the present’ and ‘the future’ ?

5. The nature of time
If there is no change and motion, we are unable to measure time, and we cannot even have awareness of time passing; time would stand still.
Without events, motion and change, it is not possible to have awareness of time.

6. The nature of time
It is difficult to think of time without change. In fact, our awareness of time passing is nothing but our awareness of changes occurring.
Change happens, for example, when future events become present, present events become past, and past events become more past.

7. The nature of time
However, the concepts of past, present and future only exist in people’s minds. If there were no living beings with consciousness, nothing would be past, present or future.

8. The nature of time
For example, it is possible that billions of years later, all the energy in the universe necessary to sustain life and motion would have been used up (i.e. a condition known in physics as ‘heat death’ 熱寂).
Does it make sense to say that time will continue to exist in a universe without conscious life and motion?

9. Newton, Kant and Einstein
Newton’s most important contribution to science was his mathematical definition of how motion changes with time.
Isaac Newton 牛頓 ( )

10. Newton, Kant and Einstein
For Newton, time is connected to motion. We need to have a constant reference (恒常的參照) to measure the movement of objects.
However, Newton believes that time exists independently of motion and change – it is the background against which change and motion occurs.

11. Newton, Kant and Einstein
Time, in Newton’s view, exists objectively as an infinitely large container of all events (motion and change) in the universe. It can exist with or without the events.

12. Newton, Kant and Einstein
For Newton, there is only one ‘absolute’ time in the universe, which is the same for everyone everywhere. The pace (速度) of the passage of time never varies (轉變).
For Newton, time exists even when there is no change, no motion, no events and no conscious beings.

13. Newton, Kant and Einstein
The idea of a single absolute time can be symbolized by an imaginary clock at the center of the universe to which all of us can set our watches. It clicks at a constant rate throughout the whole universe.

14. Newton, Kant and Einstein
In Newton’s own words: “Absolute, true, and mathematical time, from its own nature, passes equably without relation to anything external, and thus without reference to any change or way of measuring of time (e.g., the hour, day, month, or year).”

15. Newton, Kant and Einstein
The German philosopher Immanuel Kant argues that time does not exist independently of the mind.
Immanuel Kant 康德
( )

16. Newton, Kant and Einstein
Kant understands correctly that Newton’s idea of ‘absolute time’ is not derived from scientific observation, nor can this idea be tested using the scientific method.
Kant argues that time is the form of our ‘inner sense’ (or ‘sensibility’ 感官). As such, all of our experiences of the external world are structured in time.

17. Newton, Kant and Einstein
To put it in modern terms, Kant’s argument is that all humans are programmed to perceive (感覺、感受) or experience the external world in time. (Time is like a program installed in our minds.)
In Kant’s view, the world of our experience is partly the outcome of our mental structure (思想架構).

18. Newton, Kant and Einstein
Time, according to Kant, is presupposed (預設) in all human experience. It is a property of the mind rather than a feature of mind-independent, external reality.
Time, as such, is ‘subjective’ because it has been built into our minds – it is a necessary subjective condition for all of our experience of the world.

19. Newton, Kant and Einstein
The implication (含意) of Kant’s view is that time does not meaningfully exist if there are no minds, conscious beings or subjects of experience.
Do you think Kant confuses (混淆) time itself with people’s experience (or awareness) of time?

20. Newton, Kant and Einstein
Both Newton and Kant argue that time is one and the same for everyone. However, Newton believes that time exists objectively as a mind-independent reality, whereas Kant believes that time is subjective and therefore does not exist independently of the human mind.
Who is right, Newton or Kant?

21. Newton, Kant and Einstein
Kant’s view assumes that all of us (human beings) have more or less the same mental structure. (The same program has been installed in all of us.)
Is it possible that some individuals’ mental structure is so unique (獨特) that their sense of time is completely different from normal people?

22. Newton, Kant and Einstein
Einstein revolutionizes physics with his ‘special theory of relativity’ (狹義相對論) by showing how time changes with motion.
Albert Einstein 愛因斯坦
( )

23. Newton, Kant and Einstein
Einstein notices from the results of scientific experiments that the speed of light is independent of motion.
In other words, light always travels at the same speed: 186,000 miles per second. It doesn’t matter if the source of the light is moving or if the observer is moving.

24. Newton, Kant and Einstein
Because the speed of light is always the same, Einstein theorizes that time has to vary (改變) to keep the speed of light constant (恒常的).
Einstein rejects Newton’s idea of ‘absolute time’ by arguing that time is relative to the speed one is traveling at – the faster you move, the slower time passes.

25. Newton, Kant and Einstein
Einstein’s theory of relativity has a striking implication: It means that time travel is possible: the faster you go, the slower time passes.
Einstein’s theory predicts, for example, that if a clock is traveling rapidly in a spacecraft (太空船) at a rate close to the speed of light, it will tick very slowly.

26. Newton, Kant and Einstein
This phenomenon (現象) – the ‘slowing down’ of a clock traveling at a very high speed – is known as ‘time dilation’ (時間膨脹), which is noticeable only at very high speeds.
The effect of time dilation has been measured and confirmed using atomic clocks, aircrafts, satellites and rockets.

27. Newton, Kant and Einstein
Consider the following paradox (悖論):
My twin brother and I were both 20 years old in One day, I went on a high-speed space journey and traveled for one year. When I come back, I find that it is already 2060 on Earth and my brother is 71 years old! But I am still 21!

28. Newton, Kant and Einstein
The so-called ‘twin paradox’ is not really a paradox, i.e. it does not entail any contradiction. As mentioned earlier, it is an effect predicted by Einstein’s theory of relativity (相對論), a phenomenon known as ‘time dilation’, which has been verified (證實) experimentally using precise measurements of clocks flown in aircrafts and rockets.

29. Time travel
Just as people 200 years ago thought that flying on a plane was impossible, some people today still think that time travel is impossible.
However, after Einstein published his theory of relativity showing that time can be slowed down, time travel has become a serious subject of scientific investigation.

30. Time travel
Do you notice that we often come across the idea of time travel, for example, in Hollywood movies and children’s comics?

31. Time travel
During a time-travel journey, the time experienced by a time traveler is not the same as the time undergone by other people.
The traveler’s journey, as judged by the traveler, takes a different amount of time than the same journey as judged by people who do not take the journey.

32. Time travel
According to the theory of relativity, people traveling at different speeds (or velocities) experience time at different rates.
Scientifically speaking, time travel to the future (cases similar to the ‘twin paradox’) is possible – time slows down when a person or an object travels at a very high speed.

33. Time travel
From a scientific point of view, the known laws of physics do not rule out the possibility of time travel – forward time travel (traveling forward in time to the future) is at least theoretically possible.
How about backward time travel? Is traveling back in time possible?

34. Time travel
Time travel into the future is consistent with the laws of nature. But it is debatable (有爭議的) whether traveling backward in time is possible.
While all physicists today admit that time travel to the future is possible, many still believe that time travel to the past will never be attainable (可實現的).

35. Time travel
“If backward time travel was possible, we should have seen many time travelers by now, but nobody has seen any time travelers so far. Therefore, traveling back in time to the past is not possible.”
Is this a sound argument? Why or why not?

36. Time travel
Physicist Stephen Hawking (霍金) argues that backward time travel is not possible because we have not seen any time travelers from the future.
Stephen Hawking: “If time travel is possible, where are the time travelers?”

37. Time travel
In response to Hawking, physicist and astronomer Carl Sagan puts forward some counterarguments:
[1] A time travel trip to the past could be extremely expensive because it requires an enormous amount of energy. So even if time travel is possible, it may not be economical to do so.

38. Time travel
[2] There is the possibility that the time travelers are here and we do see them, but we call them something else, e.g. UFOs or ghosts.

39. Time travel
[3] There is the possibility that time travel is perfectly possible, but it requires a great advance in our technology, and human civilization may have destroyed itself before the technology is invented.

40. Time travel paradoxes
Now we are going to watch a short video. And then we will consider some paradoxes associated with backward time travel.

41. Time travel paradoxes
From the standpoint of philosophy, we need to consider whether time travel is logically possible, i.e. whether the idea of time travel entails contradiction (矛盾).

42. Time travel paradoxes
Many philosophers believe that backward time travel is impossible because it will give rise to paradoxes involving causality (因果關係).
The classic example is the ‘grandfather paradox’: Can a person go back in time and kill his own grandfather?

43. Time travel paradoxes
Suppose a time traveler goes back in time and kills his grandfather when the latter was only a child. If his grandfather died in childhood, then the time traveler’s father and the time traveler himself could not have existed. But if the time traveler was never born, how could he traveled back in time in the first place?

44. Time travel paradoxes
It is inevitable (不能避免) that when someone travels back in time, it will cause some changes to the past. But if the time traveler changes some aspects of the past, the course of history will be different. Does it imply that backward time travel is logically impossible?

45. Time travel paradoxes
“If it was possible to travel back in time to kill Hitler before the outbreak of World War II, WW2 would not have occurred.”
Do you see any problem with this line of reasoning?

46. Time travel paradoxes
Some philosophers suggest that even if time travel was possible, the time traveler would not be able to interact with people, objects and events of the past.
If the time traveler was able to do so, he or she would be able to change the past. But it is impossible because the very idea of ‘changing the past’ is self-contradictory (自相矛盾的).

47. Time travel paradoxes
In the film Somewhere in Time, a young man meets an old woman who gives him a watch. He then travels back in time to the past to meet the woman when she was still young. He fell in love with her and gave her the watch. Later, the man died but the woman keeps the watch as she grows old – until one day she meets a young man and gives him the watch…

48. Time travel paradoxes
The problem is: where did the watch come from?

49. Time travel paradoxes
A similar paradox involves the transfer of information: One day, you receive a letter from a stranger with the instructions on how to build a time machine. You follow the instructions, build the time machine and then use it to travel back in time. Then, you write a letter to your younger self, giving instructions on how to build a time machine…

50. Time travel paradoxes
The problem is: Where did the knowledge of building a time machine come from in the first place?

51. Time travel paradoxes
Finally, time travel also leads to puzzles about personal identity:
Suppose Jenny travels back in time to visit her younger self. Are old Jenny and her younger self one and the same person? Or are they two different persons?

52. Time travel paradoxes
To an external observer, Jenny and her younger self are two different persons separated by a physical distance of, say, a few feet.
However, old Jenny and her younger self share one and the same consciousness (意識) linked together by old Jenny’s memory.

53. Time travel paradoxes
If personal identity depends on memory and consciousness, can we say that old Jenny and her younger self are one and the same person?

54. Read! Filmandphilosophy.weebly.com
Read the article ‘The Impossibility of Time Travel’ by Franz Kiekeben. The link to the article can be found on the course website at:
Filmandphilosophy.weebly.com

55. Next week