Does God exist? This is one of the oldest questions in philosophy and is still much debated. The debate on the God particle is much more recent but searching for it has cost a large fortune and inspired people’s careers. But before we can answer the questions implied in the title, we have to decide what we mean when we say something exists. The approach here follows that of my previous essay that defines knowledge in terms of models that make successful predictions.
Let us start with a simple question: What does it mean when we say a tree exists? The evidence for the existence of trees falls into two categories: direct and indirect. Every autumn, I rake the leaves in my backyard. From this I deduce that the neighbour has a tree. This is indirect evidence. I develop a model that the leaves in my backyard come from a tree in the neighbour’s yard. This model is tested by checking the prediction that the leaves are coming from the direction of the neighbour’s yard. Observations have confirmed this prediction. Can I then conclude that a tree exists? Probably, but it would be useful to have direct evidence. To obtain this, I look into my neighbour’s yard. Yup, there is a tree. But not so fast–what my eye perceives is a series of impressions of light. The brain then uses that input to construct a model of reality and that model includes the tree. The tree we see is so obvious that we frequently forget that it is the result of model construction, subconscious model construction, but model construction none-the-less. The model is tested when I walk into the tree and hurt myself.
Now consider a slightly more sophisticated example: atoms. The idea of atoms, in some form or other, dates back to ancient India and Greece but the modern idea of atoms dates to John Dalton (1766 – 1844). He used the concept of atoms to explain why elements always interact in the ratios of small whole numbers. This is indirect evidence for the existence of atoms and was enough to convince the chemists but not the physicists of that time. Some like Ernst Mach (1838 – 1916) refused to believe in what they could not see up until the beginning of the last century. But then Albert Einstein’s (1879 – 1955) famous 1905 paper on Brownian motion (the motion of small particles suspended in a liquid) convinced even the most recalcitrant physicists that atoms exist. Einstein showed that Brownian motion could be easily understood as the result of the motion of discrete atoms. This was still indirect evidence but convincing to almost everyone. Atoms were only directly seen after the invention of the scanning electron microscope and even then there was model dependence in interpreting the scanning electron microscope results. As with the tree, we claim that atoms exist because, as a shown by Dalton, Einstein and others, they form an essential part of models that have strong track record of successful predictions.
Now on to the God particle. What a name! The God particle has little in common with God but the name does sound good in the title of this essay. Then again, calling it the Higgs boson is not without problems as people other than Peter Higgs (1920 – ) have claimed to have been the first to predict its existence. Back to the main point, why do we say the God particle exists? First there is the indirect evidence. The standard model of particle physics has an enviable record of successful predictions. Indeed, many (most?) particle physicists would be happier if it had had some incorrect predictions. We could replicate most of the successful predictions of the standard model without the God particle but only at the expense of making the model much more complicated. Like the recalcitrant physicists of old who rejected the atom, the indirect evidence for the God particle was not good enough for most modern-day particle physicists. Although few actually doubted its existence, like doubting Thomas, they had to see it for themselves. Thus, the Large Hadron Collider (LHC) and its detectors were built and direct evidence was found. Or was it? Would lines on a computer screen have convinced the logical positivists like Ernst Mach? Probably not, but the standard model predicted bumps in the cross-sections and the bumps were found. Given the accumulated evidence and its starring role in the standard model of particle physics, we confidently proclaim that the God particle, like the tree and the atom, exists. But remember, that even for the tree our arguments were model dependent.
Having discussed the God particle what about God? I would apply the same criteria to His/Her/Its existence as for the tree, the atom, or the God particle. As in those cases, the evidence can be direct or indirect. Indirect evidence for God’s existence would be, for example, the argument from design attributed to William Paley (1743 – 1805). This argument makes an analogy between the design in nature and the design of a watch. The question is then is this a good analogy? If we adopt the approach of science this reduces to the question: Can the analogy be used to make correct predictions for observations? If it can, the analogy is useful, otherwise it should be discarded. There is also the possibility of direct evidence: Has God or His messengers ever been seen or heard? But as the previous examples show, nothing is ever really seen directly but depends on model construction. As optical illusions illustrate, what is seen is not always what is there. Even doubting Thomas may have been too ready to accept what he had seen. As with the tree, the atom or the God particle, the question comes back to: Does God form an essential part of a model with a track record of successful predictions?
So does God exist? I have outlined the method for answering this question and given examples of the method for trees, atoms and the God particle. Following the accepted pedagogical practice in nuclear physics, I leave the task of answering the question of God’s existence as an exercise for you, the reader.
 Yes, 1905 was the last century. I am getting old.
 He had more than one famous 1905 paper.
 Why do we claim Peter Higgs exists? But, I digress.