What is real?
Take for example the first successful theory of gravity, Ptolemy's system of epicycles. It was used before Newton to describe the motion of the planets, by assuming that the earth was static in the center of the world and the planets and the sun revolved around it. It gave relatively accurate results for astronomical observations. Was it real? I guess not. It just helped the calculations, correctly, to be carried out.
Then Newton's theory came about, with the sun in the center of the solar system and the planets revolving around it in ellipses. It also gives predictions for the motion of the heavenly bodies, much better than Ptolemy's theory did. Do we consider it real? Of course we do. We are somehow convinced that the sun is really in the center and that the earth is spinning and not the other way around, and also it feels real and we are comfortable with these notions.
Then take as a second example the ether. When Maxwell derived his electromagnetic equations, people naturally assumed that these EM waves must be traveling in a medium, the ether. The equations worked pretty well with the ether, although none had ever "seen" it, yet people considered it real. Then experiments came a few years later and said that ether was not necessary and that the waves can propagate on vacuum. Here goes the beloved ether. Now, is this real? We tend to believe so, that indeed there is nothing there so that the EM waves travel through.
Is energy real? When the concept of energy was first introduced, none had ever seen or smelled or tasted energy. It was just a bunch of numbers that told us something about things. Nobody back then believed that there was an actual existence that was real and meant something. It was merely a mathematical convenience. And where are we now, in the year 2004ad? We know how to measure energy, we now how to predict where it goes, how it is transferred, how to manipulate it, and we use it in our everyday lives (the calories that we eat or the power of a light bulb). Yet none ever in the history of mankind has been able to "see" energy, or to understand what it is, or draw a picture of it. Under this sense, energy cannot be real, although it is very convenient for us to take advantage of it.
My point is this: scientists propose theories, all of which tell us something (usually in mathematics) about the world. However some of them are not "real", although they might be giving accurate results (up to a point maybe). The question is, where do you draw a line between a theory that is a mathematical convenience and a theory that actually describes reality?
That question might not have been so tough before the theory of Quantum Mechanics arrived in the first half of the 20th century. First of all, there are at least 4 different equivalent mathematical descriptions of the theory, each of which tells us how the atoms and the particles behave and move around. Some of them are easier to grasp and understand, while others seem distant and unfamiliar. Yet they all yield the exact same results, since they are the different mirror images of the same reality. But then, which of the theories can be called real? Is the matrix formulation, the quantum numbers, the wave picture, or Feynman's many stories description more real?
In the Quantum version of electromagnetism, QED, although Feynman's idea that a particle moving from one place to another follows ALL possible routes at the same time seems absurd and unrealistic, this is the theory that scientists use in order to make calculations, because it is easier than the other theories, although 100% equivalent to them! Worse, in modern particle physics, the only description we have and works almost perfectly in terms of accurate predictions states that there are virtual particles, particles that do not actually exist and pop out nowhere, and the exchange of trillions of particles describes the forces of nature. No one has ever seen a photon or a boson, we only perceive them through machines that tells us that something was there. Even worse, take the quarks, the elementary particles that comprise the protons. We will never be able to see a single quark yet by assuming their existence we can explain some secondary effects they produce accurately by looking through our excellent machines. Again, the math works out well but can we tell that the quarks are real, and in what sense since none of us will ever hold a quark in his hand?
Quantum Theory is so strange. In the electrons orbiting a nucleus, it tells us that the electrons are not real unless we detect them. That means, when we detect an electron we know that we have detected and electron, but if we ask where the electron is then we do not have a definite answer. It might be anywhere around the nucleus, and because of that we assume that the electron is at the same time in many places! We can no longer imagine an electron as a definite dot sitting somewhere outside the nucleus, the answers we get are wrong. If we assume it's everywhere, we get correct results. The dot-electron now isn't real, but the ghost-electron is, even if we cannot perceive it?
Here are the cases for a certain theory:
1. The predictions are correct, we can perceive it, yet the theory is false (Ptolemy and many more).
2. The predictions are correct, we can perceive it, and the theory is probably correct (Newton, to some extent).
3. The predictions are wrong, we can perceive it, and the theory is (obviously) false (Bohr's planetary orbiting picture of the atom).
4. The predictions are correct, we cannot perceive it, and the theory is probably correct (Quantum Mechanics).
In cases 1, 2 and 3 something be perceive might be right or wrong, yet we consider the correct case (2) to be real. Is #4 real? Is something real even if we cannot perceive it? Einstein's answer to this question was no: something we cannot perceive cannot be real, and he believed that Quantum Mechanics will eventually yield definite, realistic, dot-like Newtonian answers. Yet it was proved that it is not possible.
What is your answer?