Ultimately, a radical modification of these latter concepts, and consequently of Newton’s equations themselves, was found to be necessary. It was Albert Einstein who, by combining the experimental results and physical arguments of others with his own unique insights, first formulated the new principles in terms of which space, time, matter and energy were to be understood. These principles, and their consequences constitute the Special Theory of Relativity. Later, Einstein was able to further develop this theory, leading to what is known as the General Theory of Relativity.
Amongst other things, this latter theory is essentially a theory of gravitation. The General Theory will not be dealt with in this course. Relativity (both the Special and General) theories, quantum mechanics, and thermodynamics are the three major theories on which modern physics is based. What is unique about these three theories, as distinct from say the theory of electromagnetism, is their generality. Embodied in these theories are general principles which all more specialized or more specific theories are required to satisfy.
Consequently these theories lead to general conclusions which apply to all physical yester, and hence are of enormous power, as well as of fundamental significance. The role of relativity appears to be that of specifying the properties of space and time, the arena in which all physical processes take place. It is perhaps a little unfortunate that the word ‘relativity immediately conjures up thoughts about the work of Einstein. The idea that a principle of relativity applies to the properties of the physical world is very old: it certainly predates Newton and Galileo, but probably not as far back as Aristotle.
What the principle of relativity essentially states is the allowing: The laws of physics take the same form in all frames of reference moving with constant velocity with respect to one another. This is a statement that can be given a precise mathematical meaning: the laws of physics are expressed in terms of equations, and the tort t at n these equations take in deterrent reticence trades moving with constant velocity with respect to one another can be calculated by use of transformation equations – the so-called Galilean transformation in the case of Newtonian relativity.
The principle of relativity then requires that the transformed equations have exactly the same form in all frames of reference, in other words that the physical laws are the same in all frames of reference. This statement contains concepts which we have not developed, so perhaps it is best at this stage to illustrate its content by a couple of examples. First consider an example from ‘everyday experience’ – a train Cambridge moving smoothly at a constant speed on a straight and level track – this is a frame of reference’.
Suppose that in this Cambridge is a pool table. If you were a passenger on this Cambridge and you decided to play a game of pool, one f the first things that you would notice is that in playing any shot, you would have to make no allowance whatsoever for the motion of the train. Any Judgment of how to play a shot as learned by playing the game back home, or in the local pool hall, 3 would apply equally well on the train, irrespective of how fast the train was moving.
If we consider that what is taking place here is the innate application of Newton’s Laws to describe the motion and collision of the pool balls, we see that no adjustment has to be made to these laws when playing the game on the moving train. This argument an be turned around. Suppose the train windows are covered, and the Cambridge is well insulated so that there is no immediate evidence to the senses as to whether or not the train is in motion. It might nevertheless still be possible to determine if the train is in motion by carrying out an experiment, such as playing a game of pool.
But, as described above, a game of pool proceeds in exactly the same way as if it were being played back home – no change in shot-making is required. There is no indication from this experiment as to whether or not the train is in motion. There is o way of knowing whether, on pulling back the curtains, you are likely to see the countryside hurtling by, or to find the train sitting at a station. In other words, what the principle of relativity meaner is that it is not possible to determine whether or not the train Cambridge is moving. This idea can be extended to encompass other laws of physics.
To this end, imagine a collection of spaceships with engines shut off all drifting through space. Each space ship constitutes a frame of reference’, an idea that will be better defined later. On each of these ships a series of experiments is reformed: a measurement of the half life of uranium, a measurement of the outcome of the collision of two billiard balls, an experiment in thermodynamics, e. G. A measurement of the specific heat of a substance, a measurement of the speed of light radiated from a nearby star: any conceivable experiment.
If the results of these experiments are later compared, what is found is that in all cases (within experimental error) the results are identical. In other words, the various laws of physics being tested nerve yield exactly the same results tort all the spaceships, in accordance with the principle of relativity. Thus, quite generally the principle of relativity meaner that it is not possible, by considering any physical process whatsoever, to determine whether or not one or the other of the spaceships is ‘in motion’.
The results of all the experiments are the same on all the space ships, so there is nothing that definitely singles out one space ship over any other as being the one that is stationary. It is true that from the point of view of an observer on any one of the space ships that it is the others that are in motion. But the same statement can be made by an observer in any space ship. All that we can say for certain is that the space ships are in relative motion, and not claim that one of them is truly stationary, while the others are all truly moving.
This principle of relativity was accepted (in somewhat simpler form I. E. With respect to the mechanical behavior of bodies) by Newton and his successors, even though Newton postulated that underlying it all was ‘absolute space’ which defined the state of absolute rest. He introduced the notion in order to cope with the difficulty of specifying with respect to what an accelerated object is being accelerated. To see what is being implied here, imagine space completely empty of all matter except for two masses Joined by a spring.
Now suppose that the arrangement is rotated, that is, they undergo acceleration. Naively, in accordance with our experience, we would expect that the masses would pull apart. But why should they? How do the masses ‘know that they are being rotated? There are no ‘signposts’ in an otherwise empty universe that would indicate that rotation is taking place. By proposing that there existed an absolute space, Newton was able to claim that the masses are being accelerated with respect to this absolute pace, and hence that they would separate in the way expected for masses in circular motion.
But this was a supposition made more for the convenience it offered in putting together his Laws of motion, than anything else. It was an assumption that could not be substantiated, as 4 Newton was well aware – he certainly felt misgivings about the concept! Other scientists were more accepting of the idea, however, with Maxwell theory of electromagnetism for a time seeming to provide some sort of confirmation of the concept. One of the predictions of Maxwell theory was that light was an electromagnetic wave that traveled with a speed c 3 x 108 ms-l . But relative to what?
Maxwell theory did not specify any particular frame of reference for which light would have this speed. A convenient resolution to this problem was provided by an already existing assumption concerning the way light propagated through space. That light was a form of wave motion was well known – Young interference experiments had shown this – but the Newtonian world view required that a wave could not propagate through empty space: there must be present a medium of some sort that vibrated as the waves passed, much as a string vibrates as a wave travels long it.
The proposal was therefore made that space was filled with a substance known as the ether whose purpose was to be the medium that vibrated as the light waves propagated through it. It was but a small step to then propose that this ether was stationary with respect to Newton’s absolute space, thereby solving the problem of what the frame of reference was in which light had the speed c.
Furthermore, in keeping with the usual ideas of relative motion, the thinking then was then that if you were to travel relative to the ether towards a beam of light, you would measure its peed to be greater than c, and less than c if you traveled away from the beam. It then came as an enormous surprise when it was found experimentally that this was not, in fact, the case. This discovery was made by Michelson and Morley, who fully accepted the ether theory, and who, quite reasonably, thought it would be a nice idea to try to measure how fast the earth was moving through the ether.
They found to their enormous surprise that the result was always zero irrespective of the position of the earth in its orbit around the sun or, to put it another way, they measured the peed of light always to be the same value c whether the light beam was moving in the same direction or the opposite direction to the motion of the earth in its orbit. In our spaceship picture, this is equivalent to all the spaceships obtaining the same value for the speed of light radiated by the nearby star irrespective of their motion relative to the star.
This result is completely in conflict with the rule for relative velocities, which in turn is based on the principle of relativity as enunciated by Newton and Galileo. Thus the independence of the speed of light on the motion of he observer seems to take on the form of an immutable law of nature, and yet it is apparently inconsistent with the principle of relativity. Something was seriously amiss, and it was Einstein who showed how to get around the problem, and in doing so he was forced to conclude that space and time had properties undreamt of in the Newtonian world picture.
All these ideas, and a lot more besides, have to be presented in a much more rigorous form. The independence of results of the hypothetical experiments described above on the state of motion of the experimenters can be understood at a fundamental level in terms of the thematic forms taken by the laws of nature. All laws of nature appear to have expression in mathematical form, so what the principle of relativity can be understood as saying is that the equations describing a law of nature take the same mathematical form in all inertial frames of reference.
It is this latter perspective on relativity that is developed here, and an important starting point is the notion of a frame of reference. FRAMES OF REFERENCE 5 Frames to Reticence Newton’s laws are, of course, the laws which determine how matter moves through space as a function of time. So, in order to give these laws a precise meaning we have o specify how we measure the position of some material object, a particle say, and the time at which it is at that position. We do this by introducing the notion of a 2. A Framework of Rulers and Clocks First of all we can specify the positions of the particle in space by determining its coordinates relative to a set of mutually perpendicular axes X, Y , Z. In practice this could be done by choosing our origin of coordinates to be some convenient point and imagining that rigid rulers – which we can also imagine to be as long as necessary – are laid out from this origin along these three mutually perpendicular directions. The position of the particle can then be read off from these rulers, thereby giving the three position coordinates (x, y, z) of the particle .
We are free to set up our collection of rulers anywhere we like e. G. The origin could be some fixed point on the surface of the earth and the rulers could be arranged to measure x and y positions horizontally, and z position vertically. Alternatively we could imagine that the origin is a point on a rocket traveling through space, or that it coincides with the position of a subatomic particle, with the associated rulers being carried along with the moving octet or particles . By this meaner we can specify where the particle is.
In order to specify when it is at a particular point in space we can stretch our imagination further and imagine that in addition to having rulers to measure position, we also have at each point in space a clock, and that these clocks have all been synchronized in some way. The idea is that with these clocks we can tell when a particle is at a particular position in space simply by reading off the time indicated by the clock at that position. According to our ‘common sense’ notion of time, it would appear efficient to have only one set of clocks filling all of space.
Thus, no matter which set of moving rulers we use to specify the position of a particle, we always use the clocks belonging to this single vast set to tell us when a particle is at a particular position. In other words, there is only one time’ for all the position measuring set of rulers. This time is the same time independent of how the rulers are moving through space. This is the idea of universal or absolute time due to Newton. However, as Einstein was first to point out, this idea of absolute time is untenable, and that the measurement of mime intervals (e. G. He time interval between two events such as two supernovae occurring at different positions in space) will in fact differ for observers in motion relative to each other. In order to prepare ourselves for this possibility, we shall suppose that for each possible set of rulers – including those fixed relative to the ground, or those moving with a subatomic particle and so on, there are a different set of clocks. Thus the position measuring rulers carry their own set of clocks around with them. The clocks belonging to each set of rulers are of course synchronized with respect to each other.
Later on we shall see now this synchronization can be achieved. Probably a better construction is to suppose that space is filled with a scaffolding of rods arranged in a three dimensional grid. 2 If, for instance, the rocket has its engines turned on, we would be dealing with an accelerated frame of reference in which case more care is required in defining how position (and time) can be measured in such a frame. Since we will ultimately be concerning ourselves with non-accelerated observers, we will not concern ourselves with these problems.
A proper analysis belongs to General Relativity. 1 6 The idea now is that relative to a particular set of rulers we are able to specify where a particle is, and by looking at the clock (belonging to that set of rulers) at the position of the particle, we can specify when the particle is at that position. Each possible collection of rulers and associated clocks constitutes what is known as a frame of reference or a reference frame. Z X Figure 1: Path off particle as measured in a frame of reference. The clocks indicate the times at which the particle passed the various points along the way.
In many texts reference is often made to an observer in a frame of reference whose bob apparently is to make various time and space measurements within this frame of reference. Unfortunately, this conjures up images of a person armed with a stopwatch and a pair of binoculars sitting at the origin of coordinates and peering out into space watching particles (or planets) collide, stars explode and so on. This is not the sense in which the term observer is to be interpreted. It is important to realism that measurements of time are made using clocks which are positioned at the spatial point at which an event occurs.
Any centrally positioned observer would have o take account of the time of flight of a signal to his or her observation point in order to calculate the actual time of occurrence of the event. One of the reasons for introducing this imaginary ocean of clocks is to avoid such a complication. Whenever the term observer arises it should be interpreted as meaning the reference frame itself, except in instances in which it is explicitly the case that the observations of an isolated individual are under consideration.
If, as measured by one particular set of rulers and clocks (I. E. Frame of reference) a particle is observed to be at a position at time t (as indicated by the clock at (x, y, z)), we can summarize this information by saying that the particle was observed to be at the point (x, y, z, t) in space-time. The motion of the particle relative to this frame of reference would be reflected in the particle being at different positions (x, y, z) at different times t.
For instance in the simplest non-trivial case we may tint that the particle is moving at constant speed v in the direction of the positive X axis, I. E. X = Vt. However, if the motion of the same particle is measured relative to a frame of reference attached to say a butterfly lettering erratically through the air, the positions (x , y , z ) at different times t (given by a series of space time points ) would indicate the particle moving on an erratic path relative to this new frame of reference.
Finally, we could consider the frame of reference whose spatial origin coincides with the particle itself. In this last case, the position of the particle does not change since it remains at the spatial origin of its frame of reference. However, the clock associated with this origin keeps on ticking so that the particle’s coordinates in space-time are (O, O, O, t) with t the time indicated on he clock at the origin, being the only quantity that changes.
If a particle remains stationary relative THE GALILEAN TRANSFORMATION 7 to a particular frame of reference, then that frame of reference is known as the rest frame for the particle. Of course we can use frames of reference to specify the where and when of things other than the position of a particle at a certain time. For instance, the point in space-time at which an explosion occurs, or where and when two particles collide etc. , can also be specified by the four numbers (x, y, z, t) relative to a particular frame of reference.
In fact any event occurring in space and time can be specified by four such numbers whether it is an explosion, a collision or the passage of a particle through the position (x, y, z) at the time t. For this reason, the four numbers (x, y, z, t) together are often referred to as an event. 2. 2 Having established how we are going to measure the coordinates of a particle in space and time, we can now turn to considering how we can use these ideas to make a statement about the physical properties of space and time.
To this end let us suppose that we have somehow placed a particle in the depths of space far removed room all other matter. It is reasonable to suppose that a particle so placed is acted on by no forces whatsoever 3 . The question then arises: What kind of motion is this particle undergoing? In order to determine this we have to measure its position as a function of time, and to do this we have to provide a reference frame. We could imagine all sorts of reference frames, for instance one attached to a rocket traveling in some complicated path.
Under such circumstances, the path of the particle as measured relative to such a reference frame would be very complex. However, it is at his point that an assertion can be made, namely that for certain frames of reference, the particle will be traveling in a particularly simple tuition – a straight line at constant speed. This is something that has not and possibly could not be confirmed experimentally, but it is nevertheless accepted as a true statement about the properties of the motion of particles in the absences of forces.
In other words we can adopt as a law of nature, the following statement: There exist frames of reference relative to which a particle acted on by no forces moves in a straight line at constant peed. This essentially a claim that we are making about the properties of capacities. It is also simply a statement of Newton’s First Law of Motion. A frame of reference which has this property is called an inertial frame of reference, or Just an inertial frame.
Gravity is a peculiar force in that if a reference frame is freely falling under the effects of gravity, then any particle also freely falling will be observed to be moving in a straight line at constant speed relative to this freely falling frame. Thus freely falling frames constitute inertial frames of reference, at least locally. The Galilean Transformation The above argument does not tell us whether there is one or many inertial frames of reference, nor, if there is more than one, does it tell us how we are to relate the coordinates It is not necessary to define what we mean by force at this point.
It is sufficient to presume that if the particle is far removed from all other matter, then its behavior will in no way be influenced by other matter, and will instead be in response to any inherent properties of space (and time) in its vicinity. 3 8 of an event as observed from the point-of-view of one inertial reference frame to the ordinates of the same event as observed in some other. In establishing the latter, we can show that there is in fact an infinite number of inertial reference frames.
Moreover, the transformation equations that we derive are then the mathematical basis on which it can be shown that Newton’s Laws are consistent with the principle of relativity. To derive these transformation equations, consider an inertial frame of reference S and a second reference frame S moving with a velocity vs. relative to S. Z SAVVY 4-5″ vs. s x Figure 2: A frame of reference S is moving with a velocity vs. relative to the inertial frame S. An event occurs with spatial coordinates (x, y, z) at time t in S and at (x , y , z ) at time t in S.
Let us suppose that the clocks in S and S are set such that when the origins of the two reference frames O and O coincide, all the clocks in both frames of reference read zero I. E. T = t = O. According to ‘common sense’, if the clocks in S and S are synchronized at t = t = O, then they will always read the same, I. E. T = t always. This, once again, is the absolute time concept introduced in Section 2. 1 . Suppose now that an event of some kind, e. G. An explosion, occurs at a point (x , y , z , t) according to S . Then, by examining Fig. 2), according to S, it occurs at the point x = x + vs. t , and at the time z=Z (1) These equations together are known as the Galilean Transformation, and they tell us how the coordinates of an event in one inertial frame S are related to the coordinates of the same event as measured in another frame S moving with a constant velocity relative to S. Now suppose that in inertial frame S, a particle is acted on by no forces and hence is moving along the straight line path given by: r = or + UT (2) where u is the velocity of the particle as measured in S.