# The Special Theory of Relativity explained to children

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1 The Special Theory of Relativity explained to children (from 7 to 107 years old) Charles-Michel Marle Université Pierre et Marie Curie, Paris, France Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 1/3

2 Prologue Maybe one day, one of my grand-children, at the age of 15 or 16, will ask me: Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 2/3

3 Prologue Maybe one day, one of my grand-children, at the age of 15 or 16, will ask me: Grand-father, could you explain what is Relativity theory? My Physics teacher lectured about it, talking of rolling trains and of lightnings hitting the railroad, and I understood almost nothing! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 2/3

4 Prologue Maybe one day, one of my grand-children, at the age of 15 or 16, will ask me: Grand-father, could you explain what is Relativity theory? My Physics teacher lectured about it, talking of rolling trains and of lightnings hitting the railroad, and I understood almost nothing! This is the discussion I would like to have with her (or him). Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 2/3

5 Prologue (2) Do you know the theorem: the diagonals of a parallelogram meet at their middle point? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 3/3

6 Prologue (2) Do you know the theorem: the diagonals of a parallelogram meet at their middle point? Yes, I do! And I believe that I know a proof! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 3/3

7 Prologue (2) Do you know the theorem: the diagonals of a parallelogram meet at their middle point? Yes, I do! And I believe that I know a proof! Good! You know all the stuff needed to understand the basic idea of Relativity theory! However, we must first think about Time and Space. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 3/3

8 Prologue (2) Do you know the theorem: the diagonals of a parallelogram meet at their middle point? Yes, I do! And I believe that I know a proof! Good! You know all the stuff needed to understand the basic idea of Relativity theory! However, we must first think about Time and Space. Time and space seem to me very intuitive, and yet difficult to understand in deep... Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 3/3

9 Prologue (3) Many people feel the same. The true nature of Time and Space is mysterious. Without a deep knowledge of that true nature, we cannot do better than to use mental pictures of Space and Time. We should be ready to change these mental pictures if some experimental result shows that we were wrong. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 4/3

10 Prologue (3) Many people feel the same. The true nature of Time and Space is mysterious. Without a deep knowledge of that true nature, we cannot do better than to use mental pictures of Space and Time. We should be ready to change these mental pictures if some experimental result shows that we were wrong. Let me now indicate how the mental pictures of Time and Space used by scientists have evolved, mainly from Newton to Einstein. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 4/3

11 The absolute Time of Newton The great scientist Isaac Newton ( ) used, as mental picture of Time, a straight line T. Each particular time corresponds to a particular element of that straight line. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 5/3

12 The absolute Time of Newton The great scientist Isaac Newton ( ) used, as mental picture of Time, a straight line T. Each particular time corresponds to a particular element of that straight line. Newton considered that for each event happening in the universe, there was a corresponding well defined time (element of the straight line T ), the time at which that event happens. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 5/3

13 The absolute Time of Newton The great scientist Isaac Newton ( ) used, as mental picture of Time, a straight line T. Each particular time corresponds to a particular element of that straight line. Newton considered that for each event happening in the universe, there was a corresponding well defined time (element of the straight line T ), the time at which that event happens. Newton considered as possible to compare two time intervals, even when they were many centuries apart. In modern mathematical language, that property determines an affine structure on T. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 5/3

14 The absolute Time of Newton (2) For the mathematician, that property means that we can apply transforms to T by sliding it along itself, without contraction nor dilation, and that these transforms (called translations) do not change its properties. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 6/3

15 The absolute Time of Newton (2) For the mathematician, that property means that we can apply transforms to T by sliding it along itself, without contraction nor dilation, and that these transforms (called translations) do not change its properties. For the physicist, it means that the physical laws remain the same at all times. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 6/3

16 The absolute Time of Newton (2) For the mathematician, that property means that we can apply transforms to T by sliding it along itself, without contraction nor dilation, and that these transforms (called translations) do not change its properties. For the physicist, it means that the physical laws remain the same at all times. Other important property of Time: it always flows from past to future. To take it into account, we endow T with an orientation; it means that we consider the two directions (from past to future and from future to past) as not equivalent. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 6/3

17 The absolute Space of Newton OK, I roughly agree with that mental picture, although it does not account for the main property of Time: it flows continuously and we cannot stop it! And what about Space? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 7/3

18 The absolute Space of Newton OK, I roughly agree with that mental picture, although it does not account for the main property of Time: it flows continuously and we cannot stop it! And what about Space? Newton identified Space with the three dimensional space of geometers, denoted by E : the space in which the theorems developed in Euclidean geometry can be used. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 7/3

19 The absolute Space of Newton OK, I roughly agree with that mental picture, although it does not account for the main property of Time: it flows continuously and we cannot stop it! And what about Space? Newton identified Space with the three dimensional space of geometers, denoted by E : the space in which the theorems developed in Euclidean geometry can be used. For Newton, every object A of the physical world occupies, at each time t (element of T ) for which that object exists, a position A t in Space E. The motion of of A is described by its successive positions A t when t varies in T. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 7/3

20 Newton s Space-Time Let me introduce now a new concept, that of Space-Time, due to the German mathematician Hermann Minkowski ( ). Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 8/3

21 Newton s Space-Time Let me introduce now a new concept, that of Space-Time, due to the German mathematician Hermann Minkowski ( ). Newton s Space-Time is simply the product set E T, whose elements are pairs (called events) (x,t), made by a point x of E and a time t of T. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 8/3

22 Newton s Space-Time Let me introduce now a new concept, that of Space-Time, due to the German mathematician Hermann Minkowski ( ). Newton s Space-Time is simply the product set E T, whose elements are pairs (called events) (x,t), made by a point x of E and a time t of T. The Time T t 1 t 0 T x0 E t1 Newton s Space-Time E T E t0 (x 0, t 0 ) The Space E x 0 x 1 (x 1, t 1 ) T x1 Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 8/3

23 Newton s Space-Time (2) Space-Time is very convenient to describe motions. For example, the motion of a material particle A is described by a line in E T, made by the events (A t,t), for all t in the interval of time during which A exists. That line is called the world line of A. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 9/3

24 Newton s Space-Time (2) Space-Time is very convenient to describe motions. For example, the motion of a material particle A is described by a line in E T, made by the events (A t,t), for all t in the interval of time during which A exists. That line is called the world line of A. Newton s Space-Time E T World line of c World line of a The Time T t a(t) World line of b E t T b a t b t c t The Space E b c(t) Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 9/3

25 Absolute rest and motion For Newton, rest and motion are absolute concepts: a physical object is at rest if its position in Space does not change with time; otherwise, it is in motion. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 10/3

26 Absolute rest and motion For Newton, rest and motion are absolute concepts: a physical object is at rest if its position in Space does not change with time; otherwise, it is in motion. This seems very natural. Why should we change this view? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 10/3

27 Absolute rest and motion For Newton, rest and motion are absolute concepts: a physical object is at rest if its position in Space does not change with time; otherwise, it is in motion. This seems very natural. Why should we change this view? Because nothing is at rest in the Universe! The Earth rotates around its axis and around the Sun, which rotates around the center of our Galaxy. And there are billions of galaxies in the Universe, each one moving with respect to any other one. For these reasons, Newton s concept of an absolute Space was criticized very early, notably by his contemporary, the great mathematician and philosopher Gottfried Wilhelm Leibniz ( ). Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 10/3

28 Reference frames But without knowing what is at rest in the Universe, how Newton managed to study the motions of the planets? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 11/3

29 Reference frames But without knowing what is at rest in the Universe, how Newton managed to study the motions of the planets? He used a reference frame. It means that he used a body R which remained approximately rigid during the motion he wanted to study, and he made as if that body was at rest. Then he could study the relative motion of any moving body A with respect to R. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 11/3

30 Galilean (or inertial) frames All reference frames are not equivalent. A Galilean frame a, also called an inertial frame, is a reference frame in which the principe of inertia holds true. a in memory of Galileo Galilei, ( ), the founder of modern experimental Physics. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 12/3

31 Galilean (or inertial) frames All reference frames are not equivalent. A Galilean frame a, also called an inertial frame, is a reference frame in which the principe of inertia holds true. That principle, formulated at first for absolute motions in Newton s absolute space E, says that the (absolute) motion of a free particle takes place on a straight line, at a constant velocity. a in memory of Galileo Galilei, ( ), the founder of modern experimental Physics. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 12/3

32 Galilean (or inertial) frames All reference frames are not equivalent. A Galilean frame a, also called an inertial frame, is a reference frame in which the principe of inertia holds true. That principle, formulated at first for absolute motions in Newton s absolute space E, says that the (absolute) motion of a free particle takes place on a straight line, at a constant velocity. That principe remains true for the relative motion of a free particle with respect to all reference frames determined by rigid bodies whose absolute motion is a translation at a constant velocity. These reference frames are the Galilean frames. No measurement founded on mechanical properties can distinguish the absolute frame from another Galilean frame. a in memory of Galileo Galilei, ( ), the founder of modern experimental Physics. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 12/3

33 Leibniz Space-Time Following Leibniz, several scientists doubted about the existence of an absolute Space, and proposed to call Galilean frame a frame in which the principle of inertia holds true. We can now formulate these views by changing our mental picture of Space-Time. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 13/3

34 Leibniz Space-Time Following Leibniz, several scientists doubted about the existence of an absolute Space, and proposed to call Galilean frame a frame in which the principle of inertia holds true. We can now formulate these views by changing our mental picture of Space-Time. We will consider that at each time t T, there exists a Space at time t, denoted by E t, whose properties are those of the three-dimensional Euclidean space of geometers. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 13/3

35 Leibniz Space-Time Following Leibniz, several scientists doubted about the existence of an absolute Space, and proposed to call Galilean frame a frame in which the principle of inertia holds true. We can now formulate these views by changing our mental picture of Space-Time. We will consider that at each time t T, there exists a Space at time t, denoted by E t, whose properties are those of the three-dimensional Euclidean space of geometers. Leibniz Space-Time, which will be denoted by U (for Universe), is the disjoint union of all the Spaces at time t E t, for all t T. So, according to Leibniz views, we still have a Space-Time, but no more an absolute space! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 13/3

36 Leibniz Space-Time (2) Absolute Time T Absolute Time T Absolute fixed Space E No more absolute fixed Space! Newton s Space-Time Leibniz Space-Time Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 14/3

37 Leibniz Space-Time (2) Absolute Time T Absolute Time T Absolute fixed Space E No more absolute fixed Space! Newton s Space-Time Leibniz Space-Time But how do you put together the Spaces at various times E t to make Leibniz Space-Time U? Are they stacked in an arbitrary way? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 14/3

38 Leibniz Space-Time (2) Absolute Time T Absolute Time T Absolute fixed Space E No more absolute fixed Space! Newton s Space-Time Leibniz Space-Time But how do you put together the Spaces at various times E t to make Leibniz Space-Time U? Are they stacked in an arbitrary way? Of course no! The way in which they are stacked is not arbitrary, it is determined by the principle of inertia. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 14/3

39 Leibniz Space-Time (3) The principle of inertia can be formulated in a way which does not use reference frames, by saying: Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 15/3

40 Leibniz Space-Time (3) The principle of inertia can be formulated in a way which does not use reference frames, by saying: The world line of any free particle is a straight line. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 15/3

41 Leibniz Space-Time (3) The principle of inertia can be formulated in a way which does not use reference frames, by saying: The world line of any free particle is a straight line. So formulated, the principle of inertia determines the affine structure of U. A physical law, the principle of inertia, is so embedded in the geometry of Leibniz Space-Time U! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 15/3

42 Leibniz Space-Time (4) By using a reference frame, one can split Leibniz Space-Time into a product of two factors: a space E R, fixed with respect to that frame, and the absolute Time T. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 16/3

43 Leibniz Space-Time (4) By using a reference frame, one can split Leibniz Space-Time into a product of two factors: a space E R, fixed with respect to that frame, and the absolute Time T. Absolute Time T Space E R of the rigid body R Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 16/3

44 Leibniz Space-Time (4) By using a reference frame, one can split Leibniz Space-Time into a product of two factors: a space E R, fixed with respect to that frame, and the absolute Time T. Absolute Time T Space E R of the rigid body R But of course, the space E R depends on the choice of the reference frame R. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 16/3

45 Light and Electromagnetism Einstein was led to drop Leibniz Space-Time when trying to reconcile the theories used in two different parts of Physics: Mechanics on one hand, Electromagnetism and Optics on the other hand. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 17/3

46 Light and Electromagnetism Einstein was led to drop Leibniz Space-Time when trying to reconcile the theories used in two different parts of Physics: Mechanics on one hand, Electromagnetism and Optics on the other hand. According to the theory built by the great Scotch physicist James Clerk Maxwell ( ), electromagnetic phenomena propagate in vacuum as waves, with the same velocity in all directions, independently of the motion of the source of these phenomena. Maxwell soon understood that light was an elecromagnetic wave, and lots of experimental results confirmed his views. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 17/3

47 The luminiferous ether In Leibniz Space-Time (as well as in Newton s Space-Time) relative velocities behave additively. In that setting, it is with respect to at most one particular reference frame that light can propagate with the same velocity in all directions. Physicists introduced a new hypothesis: electromagnetic waves were considered as vibrations of an hypothetic, very subtle, but highly rigid medium called the luminiferous ether. They thought that it was with respect to the ether s reference frame that light propagates at the same velocity in all directions. This amounts to come back to Newton s absolute Space identified with the ether. There were even physicists who introduced additional complications, by assuming that the ether, partially drawn by the motion of moving bodies, could deform with time! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 18/3

48 Michelson and Morley experiments But if the luminiferous ether really exists, accurate measurements of the velocity of light in all directions should allow the determination of the Earth s relative velocity with respect to the ether! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 19/3

49 Michelson and Morley experiments But if the luminiferous ether really exists, accurate measurements of the velocity of light in all directions should allow the determination of the Earth s relative velocity with respect to the ether! Good remark! These measurements were made several times, notably by Albert Abraham Michelson ( ) and Edward Williams Morley ( ), between 1880 et No relative velocity of the Earth with respect to the luminiferous ether could be detected. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 19/3

50 Michelson and Morley experiments But if the luminiferous ether really exists, accurate measurements of the velocity of light in all directions should allow the determination of the Earth s relative velocity with respect to the ether! Good remark! These measurements were made several times, notably by Albert Abraham Michelson ( ) and Edward Williams Morley ( ), between 1880 et No relative velocity of the Earth with respect to the luminiferous ether could be detected. These results remained not understood until 1905, despite many attempts. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 19/3

51 No more absolute Time! Einstein was the first a to understand that Michelson and Morley experiments could be explained by a deep change of the properties attributed to Space and Time. In 1905, his idea appeared as truly revolutionary. But now it may appear as rather natural, if we think along the following lines: a The great French mathematician Jules Henri Poincaré ( ) has, almost simultaneously and independently, presented very similar ideas, without explicitely recommending to drop the concept of an absolute Time. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 20/3

52 No more absolute Time! Einstein was the first a to understand that Michelson and Morley experiments could be explained by a deep change of the properties attributed to Space and Time. In 1905, his idea appeared as truly revolutionary. But now it may appear as rather natural, if we think along the following lines: When we dropped Newton s Space-Time in favour of Leibniz Space-Time, we recognized that there is no absolute Space, but that Space depends on the choice of a reference frame. a The great French mathematician Jules Henri Poincaré ( ) has, almost simultaneously and independently, presented very similar ideas, without explicitely recommending to drop the concept of an absolute Time. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 20/3

53 No more absolute Time! Einstein was the first a to understand that Michelson and Morley experiments could be explained by a deep change of the properties attributed to Space and Time. In 1905, his idea appeared as truly revolutionary. But now it may appear as rather natural, if we think along the following lines: When we dropped Newton s Space-Time in favour of Leibniz Space-Time, we recognized that there is no absolute Space, but that Space depends on the choice of a reference frame. Maybe Time too is no more absolute than Space, and depends on the choice of a reference frame! a The great French mathematician Jules Henri Poincaré ( ) has, almost simultaneously and independently, presented very similar ideas, without explicitely recommending to drop the concept of an absolute Time. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 20/3

54 Minkowski s Space-Time But if we drop absolute Time, which properties are left to our Space-Time? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 21/3

55 Minkowski s Space-Time But if we drop absolute Time, which properties are left to our Space-Time? In 1905, Einstein implicitly considered that Space-Time still was a 4-dimensional affine space, which will be called Minkowski s Space-Time and will be denoted by M. Translations of M leave its properties unchanged. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 21/3

56 Minkowski s Space-Time But if we drop absolute Time, which properties are left to our Space-Time? In 1905, Einstein implicitly considered that Space-Time still was a 4-dimensional affine space, which will be called Minkowski s Space-Time and will be denoted by M. Translations of M leave its properties unchanged. He considered that the principe of inertia still holds true in M, when expressed as follows, without use of reference frames: the world line of any free particle is a straight line. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 21/3

57 Minkowski s Space-Time But if we drop absolute Time, which properties are left to our Space-Time? In 1905, Einstein implicitly considered that Space-Time still was a 4-dimensional affine space, which will be called Minkowski s Space-Time and will be denoted by M. Translations of M leave its properties unchanged. He considered that the principe of inertia still holds true in M, when expressed as follows, without use of reference frames: the world line of any free particle is a straight line. He also kept the notion of a Galilean frame. In M, a Galilean frame is determined by a direction of straight line. The bodies which are at rest with respect to that frame are those whose all points have their world lines parallel to that direction. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 21/3

58 Light cones Let us call light lines the straight lines in M which are possible world lines of light signals. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 22/3

59 Light cones Let us call light lines the straight lines in M which are possible world lines of light signals. Given an event A M, the light lines through A make a 3-dimensional cone, the light cone with apex A; the two layers of that cone are called the past half-cone and the future half-cone with apex A. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 22/3

60 ight cones, time-like and space-like lines A Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 23/3

61 ight cones, time-like and space-like lines A Time-like straight lines are straight lines which lie inside the light cone with any one of their elements as apex; Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 23/3

62 ight cones, time-like and space-like lines A Time-like straight lines are straight lines which lie inside the light cone with any one of their elements as apex; and space-like straight lines are straight lines which lie outside the light cone with any of their element as apex. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 23/3

63 ight cones, time-like and space-like lines A Time-like straight lines are straight lines which lie inside the light cone with any one of their elements as apex; and space-like straight lines are straight lines which lie outside the light cone with any of their element as apex. Only directions of time-like straight lines determine a Galilean frame. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 23/3

64 Einstein s two principles Under these hypotheses, the properties of Space-Time follow from two principles: Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 24/3

65 Einstein s two principles Under these hypotheses, the properties of Space-Time follow from two principles: Principle of relativity: all physical laws have the same expression in all Galilean frames; Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 24/3

66 Einstein s two principles Under these hypotheses, the properties of Space-Time follow from two principles: Principle of relativity: all physical laws have the same expression in all Galilean frames; Principle of constancy of the velocity of light: the modulus of the velocity of light is an universal constant, which depends neither on the Galilean frame with respect to which it is calculated, nor on the motion of the source of that light Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 24/3

67 Einstein s two principles Under these hypotheses, the properties of Space-Time follow from two principles: Principle of relativity: all physical laws have the same expression in all Galilean frames; Principle of constancy of the velocity of light: the modulus of the velocity of light is an universal constant, which depends neither on the Galilean frame with respect to which it is calculated, nor on the motion of the source of that light You said that a direction of straight line was enough to determine a Galilean frame. But how is that possible, since we no more have an absolute Time? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 24/3

68 Isochronous subspaces Let A be a time-like straight line. We want to determine the isochronous subspaces for the Galilean frame determined by the direction of A. They must be such that the length covered by a light signal, calculated in that reference frame, during a given time interval, also evaluated in that reference frame, is the same in any two opposite directions. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 25/3

69 Isochronous subspaces Let A be a time-like straight line. We want to determine the isochronous subspaces for the Galilean frame determined by the direction of A. They must be such that the length covered by a light signal, calculated in that reference frame, during a given time interval, also evaluated in that reference frame, is the same in any two opposite directions. In a schematic 2-dimensional Space-Time, the direction of isochronous subspaces is easily obtained as shown on the following picture, by using the theorem which says that the diagonals of a parallelogram meet at their middle point! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 25/3

70 Isochronous subspaces (2) L g A g A A d L d A 2 E A2 A d 1 A 1 A g 1 E A1 A E A Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 26/3

71 Isochronous subspaces (3) And what for the true 4-dimensional Minkowski Space-Time M? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 27/3

72 Isochronous subspaces (3) And what for the true 4-dimensional Minkowski Space-Time M? It is the same, we use the intersection of the future light half-cone of A with the past light half-cone of another point A 2 of A, as shown on the next picture. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 27/3

73 Isochronous subspaces (3) And what for the true 4-dimensional Minkowski Space-Time M? It is the same, we use the intersection of the future light half-cone of A with the past light half-cone of another point A 2 of A, as shown on the next picture. L g A L d A 2 A d 1 E A1 A g 1 A 1 S A Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 27/3

74 Change of Galilean frame What happens if you change your Galilean frame? Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 28/3

75 Change of Galilean frame What happens if you change your Galilean frame? Of course, as for Galilean frames in Leibniz Space-Time, the direction of the (straight) world lines of points at rest with respect to the Galilean frame is changed. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 28/3

76 Change of Galilean frame What happens if you change your Galilean frame? Of course, as for Galilean frames in Leibniz Space-Time, the direction of the (straight) world lines of points at rest with respect to the Galilean frame is changed. Moreover, contrary to what happened in Leibniz Space-Time, the direction isochronous subspaces is also changed! Therefore, the chronological order of two events can be different when it is appreciated in two different Galilean frames! Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 28/3

77 L g Change of Galilean frame (2) B B 2 A L d B g 1 A 2 E B B 1 A d 1 A g 1 A 1 E A B d 1 A = B E A E B Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 29/3

78 Comparison of time intervals Up to now, we have compared the lengths of two straight line segments in M only when they were supported by parallel straight lines. That was allowed by the affine structure of M. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 30/3

79 Comparison of time intervals Up to now, we have compared the lengths of two straight line segments in M only when they were supported by parallel straight lines. That was allowed by the affine structure of M. We need more, because by using identical clocks, time intervals measured in two different Galilean frames can be compared. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 30/3

80 Comparison of time intervals Up to now, we have compared the lengths of two straight line segments in M only when they were supported by parallel straight lines. That was allowed by the affine structure of M. We need more, because by using identical clocks, time intervals measured in two different Galilean frames can be compared. Two line segments A A 1 and A B 1 supported by two different time-like straight lines A and B are equal (when each one is measured in its own Galilean frame) if and only if the events A 1 and B 1 lie on the same arc of hyperbola which has the light lines L d and L g as asymptotes. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 30/3

81 L g Comparison of time intervals (2) B B 2 A L d E B1 B g 1 A 2 E B B 1 A d 1 A g 1 B A A 1 E A B d 1 A = B E A E B E B1 Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 31/3

82 Comparison of time intervals (3) That follows from the equality, which says that the apparent dilation of times is the same for each Galilean frame with respect to the other, A A 1 A A = A B 1 A B. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 32/3

83 Comparison of lengths The comparison of lengths on two non-parallel space-like straight lines follows from the comparison of time intervals: the natural measure of a segment on a space-like straight line is the time taken by a light signal to cover that length, measured in a Galilean frame in which that segment lays in an isochronous subspace. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 33/3

84 Comparison of lengths The comparison of lengths on two non-parallel space-like straight lines follows from the comparison of time intervals: the natural measure of a segment on a space-like straight line is the time taken by a light signal to cover that length, measured in a Galilean frame in which that segment lays in an isochronous subspace. Two line segments A A d and A B d supported by two space-like straight lines E A et E B, are of equal length if and only if A d and B d lie on the same hyperboloid with the light cone of A as asymptotic cone. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 33/3

85 L B B 2 Comparison of lengths (2) A E B B g 1 A 2 L d B 1 A d 1 A 1 B g A g 1 E A B d 1 A = B A d A g E A B d 1 Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 34/3 g B d

86 Conclusion The comparison of time intervals and lengths presented above, founded on very simple geometric arguments, allows a very natural introduction of the pseudo-euclidean metric of Minkowski s Space-Time. The construction of isochronous subspaces in two different Galilean frames leads to the formulas for Lorentz transformations with a minimum of calculations. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 35/3

87 Conclusion The comparison of time intervals and lengths presented above, founded on very simple geometric arguments, allows a very natural introduction of the pseudo-euclidean metric of Minkowski s Space-Time. The construction of isochronous subspaces in two different Galilean frames leads to the formulas for Lorentz transformations with a minimum of calculations. The pictures above allow a very easy explanation of the apparent contraction of lengths and dilation of times associated to a change of Galilean frame. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 35/3

88 Conclusion (2) Given two events, with one in the future of the other, the straight line is the longest time-like path (when measured in proper time) which joins these two events. That property leads to a very simple explanation, without complicated calculations, of the (improperly called) paradox of Langevin s twins. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 36/3

89 Conclusion (2) Given two events, with one in the future of the other, the straight line is the longest time-like path (when measured in proper time) which joins these two events. That property leads to a very simple explanation, without complicated calculations, of the (improperly called) paradox of Langevin s twins. By explaining that the affine structure of Space-Time should be questioned, by speaking about the gravity force and the equivalence principle, a smooth transition towards General Relativity, suitable from children 8 to 108 years old, seems possible. Albert Einstein Century International Conference, Paris, July 2005 The Special Theory of Relativity explained to children p. 36/3

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