If only …

This piece examines the consequences of the idea that there are many possible versions of the future, and laws of motion in physics must reflect this 

It is simply the idea that events can turn out in more than one way – that the details of the future are not just beyond our knowledge, they are undetermined in advance. There can be few people who genuinely approach life with the only alternative view, that there is one possible future and all events are inevitable.  

This idea has an uneasy place in philosophical circles, where it is debated amongst the issues of determinism and causality. Determinism is often presented as the alternative to free will; the idea considered here is definitely not the existence of free will, but the much simpler idea that there is an element of randomness in the way events happen. Aside from our human instincts, there is persuasive evidence in its favour from biology – for example in the randomness of genetic variations – and from physics. We will meet some of the examples from physics below. 

So if we assume that there are many possible versions of the future, what does this mean in practice? 

(i)                  The best possible description of the future must consist of a variety of different events, each with its own probability. 

The most obvious consequence of the future not being fixed is that we cannot make definite predictions about it. The best we can hope to do is to narrow down the range of possible futures, and if we are clever to assign probabilities to them in some way. This is really just restating the central point, which is that the future is not just unknown, but genuinely undetermined. 

(ii)                Records of past events should be expected to contain inexplicable developments. 

If the future does not unfold inexorably, this will also be apparent from sequences of events in the past. In exactly the same way that the present must contain genuine surprises as one of the possible future events happens at random, the past must contain examples of events that do not follow inexorably from those that preceded them. In physics, the timing of the emission of an alpha particle from a radioactive nucleus (an example of quantum tunnelling), or the amount of deflection of a particle as it passes through a narrow slit (particle diffraction), are exactly this sort of inexplicable event. Why then, or why that much? There is apparently nothing that we can learn from preceding events that will answer these questions.

If we believe that future events are not absolutely fixed in advance, we should expect that the details of some past events are simply beyond explanation by any theory, however advanced, and beyond calculation using any “hidden variables”. 

(iii)       The future is not “real”. 

Many papers, one of the best being Mermin (1985), explain how experiments that test Bell’s theorem rule out the possibility of there being both realism and locality in the physical world. Locality means that one event cannot influence another if a signal between them would have to travel faster than the speed of light – this will not concern us here. 

Realism is defined in a recent paper as the view that “all measurement outcomes depend on pre-existing properties of objects that are independent of the measurement” (Gröblacher et al, 2007). Such definitions of realism are meant to assert that there is a world “out there” which isn’t just conjured into existence whenever we choose to ”measure” or “observe” something. But if the future is not fixed, the future is indeed not “pre-existing” and “real” in this sense, so we should not expect the realism assumption to predict the correct results in experiments that are subtle enough to test it. With many possible futures, by definition, events in our world are conjured into existence in the moment of the present, somehow or other. This is remarkable, but it is a far less disturbing idea than assigning significance to “observation” or “measurement” – indeed it is an idea very close to our personal experience. The results of the experiments that test Bell’s theorem can be interpreted as evidence not that the world is seriously weird, but simply that there are many possible versions of the future.  

It is sometimes argued that physics, specifically special relativity, has stopped “the past” and “the future” having any objective meaning. Since 1905 we have known that how we move in space affects how we move in time, so that not only is there no universal “now”, but different observers can disagree about the order of events.  

For our purposes, however, we don’t need “the future” to apply to everywhere at once – we just need the future to be defined at any particular place and time, and that is not a problem. Indeed special relativity clarifies this, separating events (and potential events) around us into ones in our past, ones in our future, and ones that might be in our past or in our future, but which can’t have any influence on us thanks to the unbeatable speed of light.  

(iv)       Any law of motion must consist of a way to calculate the probability of being at specific locations at specific future times. 

We can say even at this stage what a law of motion should be like. More to the point, we can say what it should not be like. The true law of motion for a moving object cannot take the form of an equation that can be solved to give its location at all future times. We could call this the classical extreme, and it may be approximately true in many situations, but we know that it cannot be the whole story. We should instead expect a law of motion to be a rule for calculating the probability for an object to be at a specific location at a specific time, given our knowledge of its recent locations and times.   This reasoning brings us a long way into the territory usually called quantum physics, including most (if not all) of its weird features, from one simple and intuitive idea.  

References  Gröblacher, S., Paterek, T., Kaltenbaek, R., Brukner, C., Zukowski, M., Aspelmeyer M. & Zeilinger, A.: An experimental test of non-local realism. Nature 446, 871-875 (2007)  

Mermin, N. D.: Is the moon there when nobody looks? Reality and the quantum theory.  Physics Today  (April) 38-47 (1985)

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