Time Travel I PDF

TIME TRAVEL I PHI101 INTRODUCTION TO PHILOSOPHY DR SIMON LANGFORD  OUTLINE Einstein’s view of space-time Godel’s time travel Is time travel contradictory?  INTRODUCTION The subject of time travel was of interest to both scientists a...

TIME TRAVEL I PHI101 INTRODUCTION TO PHILOSOPHY DR SIMON LANGFORD  OUTLINE Einstein’s view of space-time Godel’s time travel Is time travel contradictory?  INTRODUCTION The subject of time travel was of interest to both scientists and philosophers during the 20th century. There are two kinds of questions we might ask about time travel: 1. Is it physically possible—is it compatible with the laws of nature? 2. Is it logically possible—does it even make sense or is it contradictory?  TIME AND TIME TRAVEL Let’s start with the science of space-time Between 1905 and 1916, Einstein had been developing a new approach to gravity, space and time based on the assumption that the speed of light is constant for all observers. ( the ball and the train example ) In 1916, he published his theory of general relativity. SPACE-TIME AND TIME TRAVEL On Einstein’s picture, space-time is affected by the physical objects within it. Massive objects warp the topography of space-time. SPACE-TIME AND TIME TRAVEL This warping of space-time gives rise to the possibility of black holes and worm holes. It also allows for the possibility of time travel.  SPACE-TIME AND TIME TRAVEL To see how time travel might be possible, let’s start with a simple graph showing an object’s existence in space and time. A The line shows where the object is at different times. It begins its existence at place A. It stays Time still and then wobbles away from place A before returning back and staying still again. A It ceases to exist at a later time. Space  SPACE-TIME AND TIME TRAVEL On Einstein’s approach to space- time, space-time can be curved round. Just imagine taking a flat piece of paper and curving it round to make a cylinder. This new graph The object startsshows in onean object’s place and doesn’t move until it wobbles a bit existence and in thestill then stays same wayBut again. as on on the this graph, the object ceases to exist at lastangraph. earlier time than when it began to exist (because time is curved)!  SPACE-TIME AND TIME TRAVEL In 1949, Godel gave a more complicated picture of how time travel is compatible with Einstein’s general theory of relativity. Godel’s model allows you to travel easily to any place or time in the universe—past, present or future—in a way that is fully compatible with Einstein’s laws of nature.  TIME AND TIME TRAVEL There are two cylinders—an inner cylinder and an outer cylinder. These cylinders represent two space- time regions, one wrapped around another. The two regions are connected, so you can travel between them easily just by crossing from one into the other.  TIME AND TIME TRAVEL The swirling line represents a person travelling through space- time. The traveller starts in the inner region of space-time and finishes at an earlier time in the inner region. Let’s unpack this slowly to see how it works.  TIME AND TIME TRAVEL B Below we see a slice of the inner cylinder representing the inner region of space at a particular time. The outer cylinder has been removed for now. A Suppose you want to get from place A to place B. That takes time, which will take you up the cylinder.  B TIME AND TIME TRAVEL Going up, each layer of the cylinder represents the inner region of space at a later and later time. A Time If you reach the edge of the cylinder and keep going, you pass into the outer cylinder of space- time.  A TIME AND TIME TRAVEL This rectangular slice represents space in the outer region at a particular B time. Suppose you want to get from place A to place B in this outer region of space. That will take time, which means moving around the outer cylinder anti- clockwise.  A Time B TIME AND TIME TRAVEL Going anti-clockwise, each slice represents space in the outer region at successive times. So, as you move from A down to B, you also move around the outer cylinder anti- clockwise (forwards in time). Notice that as you move down in the outer region of space, you’re moving backwards in time relative to the inner region.  A Time B TIME AND TIME TRAVEL Point A is high up and close to later times of the inner region. Point B is low down and close to earlier times of the inner region. Time  TIME AND TIME TRAVEL Now, imagine you begin your journey at place A of the inner region. Then you move to place B, A travelling forwards in time (up the cylinder). Next, you cross to the outer region at place C.  D TIME AND TIME TRAVELC From C, you travel down through space to place D of the outer region, moving forwards in time (round anticlockwise).  TIME AND TIME TRAVEL E Finally, you cross back to the inner region at point E. You have now arrived back in the inner region at an earlier time than when you left the inner region. You have travelled backwards in time!  TIME AND TIME TRAVEL That’s roughly what’s happening in the Godel model, but in a slightly more complicated way. In this model, the object returns to the inner region at a time before it began to exist.  TIME AND TIME TRAVEL [B]y making a round trip on a rocket ship it is possible in these worlds to travel into any region of the past, present, and future and back again, exactly as it is possible in other worlds to travel to distant parts of space.  TIME AND TIME TRAVEL For this reason, some scientists think that time travel is physically possible given Einstein’s conception of space-time. I.e., it’s compatible with the laws of nature described by Einstein’s theory. But: some arguments suggest that time travel is still impossible because it is contradictory and so doesn’t make any sense!  NO SECOND TIME AROUND Imagine you travel back in time to see Julius Caesar getting stabbed. We might think about this as follows. First, in 44BCE, Caesar was stabbed, but you weren’t there to see it.  NO SECOND TIME AROUND But in 2030, you travel back in time to 44BCE. This time, when Caesar gets killed, you are there to see it. But now we have a contradiction: In 44BCE, you were not there to see Caesar killed. In 44BCE, you were there to see Caesar killed.  NO SECOND TIME AROUND To avoid contradiction, we need to understand time travel differently with no second time around of past events. Suppose it’s true that in 2030 you travel back to 44BCE and see Caesar killed. In that case, it was always true that you were there in 44BCE to see Caesar killed. ( events happens one time, if we suppose that they happen 2 times then there must be other timlines otherwise its contracditionary ) There was never a version of Caesar’s death that happened without you being there. Events only happen once.  NO SECOND TIME AROUND Either you were present in 44BCE when Caesar was killed or you were not. If you were not present, that means you will never travel back to 44BCE and see Caesar being killed. But what if in 2030 you will travel back to 44BCE to see Caesar killed? Well, if that’s true, then you were there in 44BCE to see Caesar killed after all. I.e., it must already be true now that you were there in 44BCE because your future self will travel there.  NO CHANGING THE PAST What about changing the past? Suppose you really like Caesar and want to go back to 44BCE to save him from being killed. In 2030, you travel back in time to 44BCE. You arrive at the Roman Forum in time to warn Caesar that there’s a plot to kill him. Caesar calls his guards, and his enemies don’t get the chance to kill him.  NO CHANGING THE PAST Now we have a contradiction again: In 44BCE Caesar was killed. In 44BCE Caesar was not killed. Those statements cannot both be true. If Caesar was killed in 44BCE, then it’s impossible for you to save him. Events only happen once; there is no second time around in which you can change things. The idea that you can change the past runs into logical contradictions. So, it’s impossible.  SUMMARY Einstein’s view that space-time can be stretched and twisted by massive objects Godel’s models of Einstein’s space-time in which it is possible to travel to the past Time travel cannot involve past events happening a second time around (it’s contradictory) Time travelers cannot change the past (it’s contradictory)  QUESTIONS What is the difference between being physically possible and being logically possible? Explain how time travel is physically possible given Einstein’s view of space-time. Explain the “second time around” view of time travel and say why it is impossible.  NEXT CLASS We will explore further how time travel should and should not be understood. We’ll look at more problems with the idea of time travel.