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4. Current and emerging understanding about time and space has been dependent upon earlier models of the transmission of light Outline the features of.

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Presentation on theme: "4. Current and emerging understanding about time and space has been dependent upon earlier models of the transmission of light Outline the features of."— Presentation transcript:

1 4. Current and emerging understanding about time and space has been dependent upon earlier models of the transmission of light Outline the features of the aether model for the transmission of light The idea was that light travels as a wave so it needs a medium - the ‘aether’ The luminiferous aether: filled all of space, low density, transparent permeated all matter, but was completely permeable great elasticity to support and propogate light waves

2 4. Current and emerging understanding about time and space has been dependent upon earlier models of the transmission of light Light travels as a wave so it needs a medium - the ‘aether’ The luminiferous aether: filled all of space, low density, transparent permeated all matter, but was completely permeable great elasticity to support and propogate light waves Outline the features of the aether model for the transmission of light

3 Describe and evaluate the Michelson-Morley attempt to measure the relative velocity of the Earth through the aether It was thought that light waves could not travel through a vacuum, but needed an aether as the medium. Michelson and Morley set out to detect the aether. They set up an apparatus that emitted in phase light waves that were split in two by a half-silvered mirror. The waves then travelled along two perpendicular paths and returned to a telescope where they observed the interference pattern. Under the aether model the interference pattern should be dimmer than light waves in phase. As the turntable is rotated the brightness of the observed interfering light pattern should change. It didn’t and the conclusion was that no observable difference in the speed of light through the aether could be determined. This method of comparing light waves involves the very sensitive effect of interference so it should accurately indicate any difference in velocity. Michelson and Morley further enhanced the validity by repeating the experiments many times and by investigating variables such as differing times of the day and year - all with the same result. (M.E. 2004)

4 Discuss the role of the Michelson-Morley experiments in making determinations about competing theories The null results in the Michelson-Morley experiments were a major factor in the acceptance by Physicists that the aether does not exist. The scientific community had accepted the aether theory and suggestions were even made that planets could drag the aether along with them and that objects contract in the direction of the aether wind. These suggestions did not survive closer scrutiny and the null result obtained by Michelson and Morley (and repeated by others since) provided the evidence to dispel the aether theory. This experiment also helped people accept Einstein’s idea that the aether was not necessary.

5 Describe and evaluate the Michelson-Morley attempt to measure the relative velocity of the Earth through the aether Discuss the role of the Michelson-Morley experiments in making determinations about competing theories It was not thought that waves could be transmitted through a vacuum. It was proposed that throughout the universe permeated an aether, through which light waves could travel. Michelson and Morley set out to measure the speed of light waves through the aether. They set up an apparatus that emitted in phase light waves that were split in two by a half- sivered mirror. The waves then travelled along two perpendicular paths and returned to a detector where they observed the interference pattern. Under the aether model the interference pattern should not have resembled that of light waves in phase. It did and the conclusion was that no observable difference in the speed of light through the aether could be determined. This method of comparing light waves involves the very sensitive effect of interference so it should accurately indicate any difference in velocity. Michelson and Morley further enhanced the validity by repeating the experiments many times and by investigating variables such as differing times of the day and year - all with the same result. The null results in the Michelson-Morley experiments were a major factor in the acceptance by Physicists that the aether does not exist. The scientific community had accepted the aether theory and suggestions were even made that planets could drag the aether along with them and that objects contract in the direction of the aether wind. These suggestions did not survive closer scrutiny and the null result obtained by Michelson and Morley (and repeated by others since) provided the evidence to dispel the aether theory. This experiment also helped people accept Einstein’s idea that the aether was not necessary.

6 Michelson and Morley helped to dispel the aether model for the transmission of light. Explain what the aether model was and how they helped to dispel it. Gather and process information to interpret the results of the Michelson-Morley experiment Jacaranda Experiment 5.1 Tennis balls and fan from Zealey Lasers and mirrors Using the telescope on an interferometer to view the returning light rays, an interference pattern is evident. The pattern observed indicated no shift of phase so the light rays had travelled at the same speed, showing no effect or existence of any ‘aether wind’.

7 Michelson and Morley helped to dispel the aether model for the transmission of light. Explain what the aether model was and how they helped to dispel it. Jacaranda Experiment 5.1 Tennis balls and fan from Zealey Lasers and mirrors Interpret the results of the Michelson- Morley experiment Using the telescope on an interferometer to view the returning light rays, an interference pattern is evident. The pattern observed indicated no shift of phase so the light rays had travelled at the same speed, showing no effect or existence of any ‘aether wind’.

8 Outline the nature of inertial frames of reference Perform an investigation to help distinguish between non-inertial and inertial frames of reference Jacaranda Experiment 5.2 using data-logger and motion sensor The plug and retort stand - stationary, constant v, accelerating, decelerating, rotating platform

9 All steady motion is relative and cannot be detected without reference to an outside point Discuss the principle of relativity Einstein's special theory of relativity deals with how we observe events, particularly how objects and events are observed from different frames of reference. This principle applies only for inertial frames of reference and states that, from within such a reference frame, you cannot perform any experiment or observation to detect motion The luminiferous aether is superfluous

10 Discuss the principle of relativity (cont.) (1)(The relativity principle): The laws of Physics have the same form in all inertial reference frames (1) makes perfect sense: an inertial reference frame is one which is stationary or moving at a constant velocity, so we expect all our laws of Physics to hold when we are stationary or at constant v. e.g. drop a ball in a stationary bus or bus moving at constant v and it will fall straight down. We expect things to behave differently when we are in an accelerating reference frame - e.g. a dropped ball will not fall straight down if the bus you are in is accelerating or turning. (2) is a bit more difficult to accept, because we would think that if light comes from a moving object then it would have more or less velocity depending on which way the source was moving. Well, it doesn't! - the speed of light is constant regardless of the motion of the source. (2) (Constancy of the speed of light): Light propogates through empty space with a definite speed, c, independent of the speed of the observer The luminiferous aether is superfluous

11 Galileo put forward the idea that all steady motion is relative and cannot be detected without reference to an outside point. Einstein's special theory of relativity deals with how we observe events, particularly how objects and events are observed from different frames of reference. (1)(The relativity principle): The laws of Physics have the same form in all inertial reference frames (1) makes perfect sense: an inertial reference frame is one which is stationary or moving at a constant velocity, so we expect all our laws of Physics to hold when we are stationary or at constant v. e'g. drop a ball in a stationary bus or bus moving at constant v and it will fall straight down. We expect things to behave differently when we are in an accelerating reference frame - e.g. a dropped ball will not fall straight down if the bus you are in is accelerating or turning. (2) is a bit more difficult to accept, because we would think that if light comes from a moving object then it would have more or less velocity depending on which way the source was moving. Well, it doesn't! - the speed of light is constant regardless of the motion of the source. (2) (Constancy of the speed of light): Light propogates through empty space with a definite speed c independent of the speed of the observer This principle applies only for inertial frames of reference and states that, from within such a reference frame, you cannot perform any experiment or observation to detect motion. The inference from this is that you could not perform an experiment to detect motion through the ether. The luminiferous aether is superfluous Discuss the principle of relativity

12 Galileo put forward the idea that all steady motion is relative and cannot be detected without reference to an outside point. Einstein's special theory of relativity deals with how we observe events, particularly how objects and events are observed from different frames of reference. (1)(The relativity principle): The laws of Physics have the same form in all inertial reference frames (1) makes perfect sense: an inertial reference frame is one which is stationary or moving at a constant velocity, so we expect all our laws of Physics to hold when we are stationary or at constant v. e'g. drop a ball in a stationary bus or bus moving at constant v and it will fall straight down. We expect things to behave differently when we are in an accelerating reference frame - e.g. a dropped ball will not fall straight down if the bus you are in is accelerating or turning. (2) is a bit more difficult to accept, because we would think that if light comes from a moving object then it would have more or less velocity depending on which way the source was moving. Well, it doesn't! - the speed of light is constant regardless of the motion of the source. (2) (Constancy of the speed of light): Light propogates through empty space with a definite speed c independent of the speed of the observer This principle applies only for inertial frames of reference and states that, from within such a reference frame, you cannot perform any experiment or observation to detect motion. The inference from this is that you could not perform an experiment to detect motion through the ether. The luminiferous aether is superfluous Discuss the principle of relativity

13 Analyse and interpret some of Einstein’s thought experiments involving mirrors and trains and discuss the relationship between thought and reality The Relativity of Simultaneity Two events which are simultaneous to one observer are not necessarily simultaneous to another observer. e.g. A stationary train is passed by a very fast moving train. You are standing in the middle of the stationary train. Martin stands in the middle of the very fast moving train. At the exact moment that Martin's train is in line with your train, one bolt of lightning hits the front of your train and another hits the back of your train. You see both bolts at the same time (simultaneous). Martin sees the bolt he is travelling towards slightly before the one he is travelling away from. So simultaneity is relative, not absolute, suggesting that time is also not an absolute quantity. Explain qualitatively and quantitatively the consequence of special relativity in relation to: - the relativity of simultaneity - the equivalence between mass and energy - length contraction - time dilation - mass dilation Einstein used thought experiments involving mirrors and trains to illustrate relativity concepts. This allowed the reader to envision and understand ideas that were not able to be investigated experimentally. Thought is thus used to model and analyse a reality which is not achievable with the technology of the day.

14 Simultaneity Two events which are simultaneous to one observer are not necessarily simultaneous to another observer. e.g. A stationary train is passed by a very fast moving train. You are standing in the middle of the stationary train. Martin stands in the middle of the very fast moving train. At the exact moment that Martin's train is in line with your train, one bolt of lightning hits the front of your train and another hits the back of your train. You see both bolts at the same time (simultaneous). Because the speed of light is constant and unaffected by his moving frame of reference, Martin sees the bolt he is travelling towards slightly before the one he is travelling away from. So simultaneity is relative, not absolute, suggesting that time is also not an absolute quantity. Explain qualitatively and quantitatively the consequence of special relativity in relation to the relativity of simultaneity Einstein used thought experiments involving mirrors and trains to illustrate relativity concepts. This allowed the reader to envision and understand ideas were not able to be investigated experimentally. Thought is thus used to model and analyse a reality which is not achievable with the technology of the day.

15 Martin sees the beam travel from Rebecca's starting point in space to where the mirror is in space (when the spaceship has moved along a bit) back to where Rebecca has moved to in space (when the spaceship has moved along even more). The time that this takes is longer because it was a longer distance at the speed of light. So time is relative. Rebecca sees the beam travel a short distance to mirror and back. The time this takes is short, because it was a short distance at the speed of light. Remember speed = dist/time so time = dist/speed Martin on earth observes the beam travelling to the mirror and back. e.g. Rebecca on the spaceship flashes a light beam to a mirror on the roof and back. The constant speed of light means that for a spacecraft travelling near the speed of light, time passes more slowly when observed from outside the spaceship. t v = t 0 /  (1 – v 2 /c 2 )

16 Now, since the speed of light is constant and time is relative, length must also change. In fact as speed of an object increases, it appears to contract along the direction of motion. Rebecca's spacecraft appears to be not as long horizontally. There is no vertical motion so it is not shorter vertically. For Rebecca on her spacecraft, she measure less time to travel from one point to another than Martin observes. If the speed of light is constant, Rebecca measures less distance from one point to another! L v = L 0  (1- v 2 /c 2 )

17 The mass of an object appears to change when measured by someone in a different frame of reference. The relativistic mass does not increase much from the rest mass until light speed is approached, when the mass quickly becomes infinite. Further input of energy results in further mass increase, to the extent that enough energy can never be input to cause velocity to go past the speed of light. The force required to accelerate this amount of mass requires enormous amounts of energy - which creates even more mass! The relativistic mass can be calculated using this equation: m v = m 0 /  (1 – v 2 /c 2 )

18 "It followed from the special theory of relativity that mass and energy are both but different manifestations of the same thing -- a somewhat unfamiliar conception for the average mind. Furthermore, the equation E = mc 2, in which energy is put equal to mass, multiplied by the square of the velocity of light, showed that very small amounts of mass may be converted into a very large amount of energy and vice versa. The mass and energy were in fact equivalent, according to the formula mentioned before. This was demonstrated by Cockcroft and Walton in 1932, experimentally." E = mc 2 Objects travelling near the speed of light acquire extra mass as energy is input, so Einstein inferred that the mass contained the extra energy. So relativity gives the energy of a moving body as E = K.E. + mc 2 and for a stationary body (no kinetic energy) E = mc 2 This equation clearly shows that mass has an energy equivalent which can be calculated and that energy has a mass equivalent.

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22 Analyse information to discuss the relationship between theory and the evidence supporting it, using Einstein’s predictions based on relativity that were made many years before evidence was available to support it Discuss the concept that length standards are defined in terms of time in contrast to the original meter standard Originally 1x10 -7 times length of Earth’s quadrant passing through Paris then two marks on a bar. Now uses constancy of c and accuracy of second to define: Does a theory need evidence to support it? How long after Einstein’s theories were atomic clocks able to verify them?

23 A theory can be postulated based on reason without supporting evidence and may exist until such time as it is proved or disproved. If a theory is proved it may become the accepted scientific teaching or law. If it is disproved it may be discarded and a new theory may be postulated. Einstein’s predictions as to time dilation, for example, were not supported by evidence for many years until a maser clock could be flown around the world and compared to a clock which remained on Earth. Using the relationship that speed=distance/time and the constant speed of light in a vacuum, any length can be precisely defined as the distance light travels in a vacuum during a given time period. The meter was originally 1x10 -7 times the length of the Earth’s quadrant passing through Paris, then it was two marks on a bar. Now the definition uses the constancy of c and accuracy of second to define: Discuss the relationship between theory and the evidence supporting it,using Einstein’s predictions based on relativity that were made many years before evidence was available to support it Discuss the concept that length standards are defined in terms of time in contrast to the original meter standard

24 Describe the significance of Einstein’s assumption of the constancy of the speed of light Identify that if c is constant then space and time become relative Einstein’s assumption of the constancy of the speed of light is significant because it changes our perception of space and time. Newtonian physics attributes the achievement of different distances observed in different reference frames to different relative velocities. But a constant speed of light means that different distances must now be attributed to different times (TIME DILATION). Similarly, different times in different frames of reference must now be attributed to different distances travelled (LENGTH CONTRACTION).

25 Describe the significance of Einstein’s assumption of the constancy of the speed of light Identify that if c is constant then space and time become relative Ordinarily at low speed if we observe a change in the distance that an object travels in a certain time, it is because the relative velocity is different. e.g. a bouncing ball on a high speed plane has a different relative velocity to someone on the plane and someone on earth watching it. But light has no different relative velocities- it is constant! -- so time changes instead! The light observed on the plane travels a short distance so time is short (passed more slowly). The light observed from the ground travelled a large distance so time was longer (passed more quickly). Conversely, if we travel a distance in a shorter time, it's usually because we travel faster, but c is constant so d is less! Time and space are not constant, but dependent on the motion of the observer. There is a continuum, where if one changes, the other is affected. The speed of light is the constant. Four-dimensional spacetime To the observer, it seems that when time dilates (gets bigger) (passes more slowly) length gets shorter, so time and space are intimately connected - space gets exchanged for time and vice-versa. So any object is specified by four quantities, 3 to describe where in space and one to describe when in time. Although space and time are not the same, they are not independent of one another.

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27 Solve problems and analyse information using: E = mc 2 L v = L 0  (1- v 2 /c 2 ) t v = t 0 /  (1 – v 2 /c 2 ) m v = m 0 /  (1 – v 2 /c 2 ) Discuss the implications of mass increase, time dilation and length contraction for space travel Where L 0 = the length of an object measured from its rest frame L v = the length of an object measured from a different frame of reference v = relative speed of the two frames of reference c = speed of light t 0 = time taken in the rest frame of reference = proper time t v = time taken as seen from the frame of reference in relative motion to the rest frame m 0 = the mass of an object measured from its rest frame m v = the mass of an object measured from a different frame of reference E = Energy m = change in mass Relativistic mass does not increase dramatically above the rest mass until near light speed, when the mass quickly heads towards infinity. Space travel is thus unable to pass light speed because a great energy input is required to cause acceleration and this energy input converts to even more mass.

28 Solve problems and analyse information using: E = mc 2 L v = L 0  (1- v 2 /c 2 ) t v = t 0 /  (1 – v 2 /c 2 ) m v = m 0 /  (1 – v 2 /c 2 ) Where L 0 = the length of an object measured from its rest frame L v = the length of an object measured from a different frame of reference v = relative speed of the two frames of reference c = speed of light t 0 = time taken in the rest frame of reference = proper time t v = time taken as seen from the frame of reference in relative motion to the rest frame m 0 = the mass of an object measured from its rest frame m v = the mass of an object measured from a different frame of reference E = Energy m = change in mass Describe each of the quantities in the formulae above Discuss the implications of mass increase, time dilation and length contraction for space travel

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31 (b)1 mark L v = L 0 (1-v 2 /c 2 ) 0.5 L v = 11.9(1 – c 2 /c 2 ) 0.5 L v = 7.2 light years Solve problems and analyse information using: E = mc 2 L v = L 0  (1- v 2 /c 2 ) t v = t 0 /  (1 – v 2 /c 2 ) m v = m 0 /  (1 – v 2 /c 2 ) Where L 0 = the length of an object measured from its rest frame L v = the length of an object measured from a different frame of reference v = relative speed of the two frames of reference c = speed of light t 0 = time taken in the rest frame of reference = proper time t v = time taken as seen from the frame of reference in relative motion to the rest frame m 0 = the mass of an object measured from its rest frame m v = the mass of an object measured from a different frame of reference E = Energy m = change in mass

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