# Thoughts on relativity

Thoughts on relativity

Based mainly on David Bohm and Roger Penrose

While quantum theory has been censored out of much of popular thought and education, relativity has suffered the opposite fate of being popularised in a ‘wow! You wouldn’t guess that style’, with an arguable lack of balance as to what is most important in the theory.

Mass-energy equivalence:
It could be argued that mass-energy equivalence is of greater significance than more crowd-stopping ideas such as different time orderings. A particle not in motion has rest mass. It has been shown that all of the rest mass of a particle can be converted into energy. For instance in some particle collisions, both particles are annilihated as a shower of high-energy photons, which may be ultimately converted into heat (oscillation of other quanta) as a result of collisions with these other quanta. The matter of the universe is made up of such particles, so all of its matter or mass is capable of being converted into energy, and it thus transpires that matter is an ‘insubstantial pageant’. Energy is always conserved, morphing from one form to another and from matter into radiation or heat. Energy in the form of essentially transient quanta could be taken to be just an oscillation or disturbance of spacetime.

Spacetime:
When we look at relativity’s treatment of spacetime, it is possible that we find something altogether more concrete than mass-energy. Einstein’s special relativity can be interpreted geometrically with a diagram known as  a light cone. This diagram starts with an observer at point O, at time, t=0. If the observer is taken to be at rest his progress can be represented as a vertical straight line, where the vertical axis represents time and and the horizontal axis represents space. An observer or a particle is interpreted as a series of events, each one different in terms of time and potentially also space. The idea of a process of different events replaces the earlier concept of permanent objects or permanent particles, which is seen to be invalid as aresult of mass-energy equivalence. The world is analysed in terms of events and processes rather than objects or particles.

Time is here treated in the same way as space, with each point or frame of reference having its own time as well as its own coordinates in space. The idea of a moment in time pervading the whole universe is no more relevant than the idea of the whole space of the universe being at one coordinate. In Newtonian physics, position could change according to three spatial coordinates, and an object could maintain the same volume if a change in height was compensated by a change in breadth. In respect of time, however, an absolute time pervaded the whole universe. But in relativity, time plays a similar role to the spatial dimensions, with space and time capable of being transformed into one another. The faster movement in space transforms into slower movement in time. This is the import of the famous twin paradox. The stay at home twin has made all his movement in time, while the space-travelling twin has had much of his movement converted from time to space.

Light cones – inside and outside:
Another perhaps understated aspect of relativity is the difference between the inside and outside of an observer’s light cone. In this the observer is at O on a horizontal axis. This horizontal axis is a divider between past and future. The future path of a stationary observer comprises the vertical axis representing movement in time but not in space. Movement at an angle to the left or right represents spatial movement. Rays of light are depicted at a 45 degree angle to the vertical axis. Massive objects moving more slowly than light are depicted as somewhere between the vertical axis of the stationary observer and the 45 degree angle of a light wave. The important point is that while moving in both space and time, it is impossible to go outside the 45 degree light cone because nothing can mover faster than light.

All cause and effect sequences relevant to a specific observer or particles have to take place within their light cone. In this respect, relativity is quite different from Newtonian physics where there is no distinction between different segments of space. In relativity, points within the light cone are described as time-like separated, with some element of time always passing in moving from one point to another, while those outside the light cone are space-like separated, and cannot be reached by signals from the observer. The area outside the light cone is referred to as ‘the absolutely elsewhere’, and is distinct from the area inside the light cone for causal purposes.

Much is made of the apparent differences in time-ordering of events as seen by different observers. This arises from the difference between the view of a stationary observer and the view of a moving observer. The path of a stationary observer is represented by a vertical line indicating only change in time, while the horizontal axis marks the boundary between past and future. The question arises as to what happens at the 45 degree angle which is the speed of light and the boundary of the light cone. Relativity tells us that time stops at this point. The movement of a light wave is all in space and none at all in time, the exact opposite of a stationary observer. In fact the 45 degree line is just a tilting of the horizontal line, and still represents the boundary between past and future, except there is no difference between the two for a light wave.

This brings us to the state of a massive object that is moving at a speed less than that of light. The line dividing past and future for this will be tilted somewhere above the horizontal of the stationary observer, but less than the 45 degrees of the light wave. The angle of tilt will be the same as the angle by which the moving object diverges from the vertical axis of the stationary observer. This means that the stationary observer and the moving observer have different dividing lines between past and the future. While for the stationary observer one event is perceived to be in the past and another in the future, the time-ordering can be opposite for the moving observer. Much is made of this, but the curious thing is that it is not really important because the events whose time order can be flipped are outside one another’s light cones. This means that the apparent difference in time-order have no influence on any causal process. The important thing here is the distinction between being inside or outside the light cone.

Disagreements on time-ordering appears to be the basis of the ‘block universe’ theory in which relativity is supposed to support the argument that all time periods of the universe always exist and are predetermined in the same way that all of space exists. The main problem for the theory looks to be that disagreements on time ordering do not relate to anything causal. With past light cones from which causal influences do arise, there is no disagreement on time-ordering. It should be no surprise that this rather perverse theory has something like consensus support amongst modern philosophers, drawn like moth to the determinist flame.

More important than time-ordering may be the concept of separation between points or frames of reference. Between every spatial point within a light cone there is a separation measurable in some mixture of time and space. The repreentations of objects etc. in our brains bear no resemblance whatever to the quanta, which comprise all the matter and energy that exists in the external world, but the representation of spacetime separation corresponds to something external, albeit in a distorted way. The suggestion here might be that spacetime is the underlying reality, in which  the quanta are disturbances or oscillations.

The present moment:
Despite the popularity of the block universe theory, relativity looks to fit quite well with the concept of the present moment. The theory has not only a future light cone, but also a past light cone from which both light and slower moving influences converge on frames of reference at the divide between past and future. This looks like a privileged point, at which causes from the past become effects, and new causes are generated for the future. In terms of relativityy’s frames of reference, there is no potential for a conscious observer to be anywhere except at one of these privileged convergences at the divide between past and future or in other words the present moment.

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