# The Leggett-Garg correlation

From the above correlations, using definition (25.3) we find

Now the parameters of an experiment are under the control of the experimentalist. Therefore, we shall assume that the parameters *a _{1}, b_{1}, a_{2},* and

*b*

*can be chosen to be whatever we wish, subject to the unitarity constraints | a*

_{2}_{1}1

^{2}+| b

_{1}|

^{2}=

| a_{2}1^{2} + | b_{2}1^{2} = 1, and the limits of what is possible in the laboratory. Assuming there is no practical barrier, consider the reparametrization

where *Q _{1}, Q_{2}, ф*

*and ф*

_{1}_{2}are real. Now take ф

_{1}= ф

_{2}. Plotting

*K*as

*Q*

*and*

_{1}*Q*

*range from*

_{2}*—n*to

*n*each gives the following figure:

Fig. 25.1 *Values ofK greater than* 1 *demonstrate a quantum mechanical violation of the Leggett-Garg inequality.*

It is clear that there are values of the parameters where *K* exceeds the classical limit +1. In fact, the maximum value of *K* over the region shown is 1.5 This is an entirely non-classical result. How can we explain it?

# Understanding the Leggett-Garg prediction

**There is a deep issue being demonstrated in the above analysis, to do with the way humans think about reality and time. It is intimately bound up with the motif running throughout this book, viz., the contextuality of truth.**

**Let us say right at this point we are ***not*** going to ‘explain’ the quantum breakdown of the LG inequality. Merely saying that there is some bizarre quantum superposition effect going on explains nothing. What we can do is to say what is wrong with the classical calculation that set up the LG inequality in the first place. It’s not quantum that is wrong, but our preconditioned classical way of thinking about the universe and time.**

**In fact, we are fortunate that our faulty conditioning works 99.99999 . . . per cent of the time. Indeed, it took very sophisticated technology for quantum phenomena to be detected: they tend to hide themselves very successfully.**

**So how about the present scenario. Can we see where our classical thinking went wrong?**

**The answer is given by John Archibold Wheeler’s philosophy of observation: ***if you haven’t actually done it, don’t assume anything about it.*

**Consider the LG correlation ***K***. It is calculated from three separate correlations. Now classically, these correlations can be regarded as coming from the same experiment. First, a bit evolves from time t _{0} to time **

*t*

_{j}**.**

*Snap!*

**—we observe its state at that time, compare it with its initial state, and hence work out the temporal correlation C**

_{01}, cost free. Then, without further ado, we let it evolve to time t_{2}where again,*snap!*

**we observe its final state and work out not just C**

_{12}but also C_{02}. We have, after all, obtained all the information required to do those calculations.**But, according to Wheeler, the correlation C _{02} **

*does not involve any observation at time*

**tj. The evolution operator**

*U*

_{20}**is applied on the strict understanding that no attempt is made to extract information between times t**

_{0}and t_{2}. And that is the essential point. Classically, we have been conditioned to think it must be the case that the ‘observed’ state of the SUO at time t_{j}should play a role in C_{02}. But how can it? We did not observe it when the system ran from t_{0}to t_{2}.**Essentially, there are ***three separate ‘sub-experiments’*** involved in the determination of ***K;*** evolution from t _{0} to t_{j}, evolution from t_{j} to t_{2}, and evolution to t_{0} to t_{2}. Classically, we can believe there is just one experiment. Quantum mechanically, we have to recognize context and factor it in.**

**What appears to be going on with the C _{02} correlation is that our lack of information about the ‘actual’ classical state of the SUO at time t_{j} in the C_{02} subexperiment allows some sort of quantum interference between the two possible classical states **

*up*

**and**

*down*

**to occur at that time. This reminds us of the doubleslit experiment, where an interference pattern on a screen is observed provided no attempt is made to determine through which slit the particle had gone.**

**The above is not an explanation, because we cannot account for the mysterious rules of quantum mechanics, such as the Born outcome probability rule, and so much more. But perhaps that is just as well. How could humans ever deceive themselves into thinking they could explain and understand the universe, time, and hence themselves?**