Caroline Chisholm College Physics 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 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

Caroline Chisholm College Describe and evaluate the Michelson-Morley attempt to measure the relative velocity of the Earth through the aether Caroline Chisholm College Physics Discuss the role of the Michelson-Morley experiments in making determinations about competing theories

Caroline Chisholm College Question 5 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. Caroline Chisholm College Physics 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

Outline the nature of inertial frames of reference Caroline Chisholm College Physics Outline the nature of inertial frames of reference An object will continue in it's state of rest or uniform motion in a straight line unless acted upon by an unbalanced force Perform an investigation to help distinguish between non-inertial and inertial frames of reference Jacaranda Experiment 5.2 using data-logger and motion sensor

Discuss the principle of relativity Caroline Chisholm College Physics Discuss the principle of relativity Galileo: All steady motion is relative and cannot be detected without reference to an outside point 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 An experiment that would detect movement through the aether would violate 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. Einstein's postulates: (1)(The relativity principle): The laws of Physics have the same form in all inertial reference frames (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 (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.

Caroline Chisholm College Physics 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. 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 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.

Time Dilation Caroline Chisholm College Physics Analyse and interpret some of Einstein’s thought experiments involving mirrors and trains and discuss the relationship between thought and reality Caroline Chisholm College Physics Time Dilation 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. e.g. Rebecca on the spaceship flashes a light beam to a mirror on the roof and back. Martin on earth observes the beam travelling to the mirror and back. Remember speed = dist/time so time = dist/speed 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. 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.

Caroline Chisholm College Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Length contraction Caroline Chisholm College Physics 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. In the time it takes to register the rear of Rebecca's spacecraft, it will have moved a distance, d, so it 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!

Caroline Chisholm College Physics

Caroline Chisholm College Physics

Caroline Chisholm College Physics 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 Does a theory need evidence to support it? How long after Einstein’s theories were atomic clocks able to verify them? Discuss the concept that length standards are defined in terms of time with reference to the original meter 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: 'the length of the path travelled by light in a vacuum during the time interval of 1 / 299 792 458 of a second'

Caroline Chisholm College Identify the usefulness of discussing space/time, rather than simple space Account for the need, when considering space/time, to define events using four dimensions 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 Caroline Chisholm College Physics speed = dist/time 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. 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.

Caroline Chisholm College Physics

Travel to space and age less Travel at high speeds takes less time Caroline Chisholm College Physics Solve problems and analyse information using: Lv = L0(1- v2/c2) and tv = t0 / (1 – v2/c2) Discuss the implications of time dilation and length contraction for space travel Travel to space and age less Travel at high speeds takes less time Distances become shorter Where Possibility of reaching our neighbours in space c = speed of light v = relative speed of the two frames of reference What is the twins PARADOX? L0 = the length of an object measured from its rest frame Lv = the length of an object measured from a different frame of reference t0 = time taken in the rest frame of reference = proper time tv = time taken as seen from the frame of reference in relative motion to the rest frame

Caroline Chisholm College Physics

Caroline Chisholm College Physics

Caroline Chisholm College Physics Solve problems and analyse information using: Lv = L0(1- v2/c2) and tv = t0 / (1 – v2/c2) c = speed of light v = relative speed of the two frames of reference L0 = the length of an object measured from its rest frame Lv = the length of an object measured from a different frame of reference t0 = time taken in the rest frame of reference = proper time tv = time taken as seen from the frame of reference in relative motion to the rest frame (b) 1 mark Lv = L0(1-v2/c2)0.5 Lv = 11.9(1 – 0.82c2/c2)0.5 Lv = 7.2 light years

The End Extended space travel requires speeds near the speed of light Caroline Chisholm College Physics Extended space travel requires speeds near the speed of light Energy costs would be prohibitive - acceleration is the most energy costly phase and mass accumulation at near light speeds would require massive energy input for marginal increases in speed Gather, process, analyse information and use available evidence to discuss the relative energy costs associated with space travel The End