In classical physics, things like energy, momentum and angular momentum are scene as measurable quantities that are bound into a restricted and quantifiable system with discrete values. This means that they can be measured objectively and observed even though Einstein's theory of relativity taught the world that the position of the observer is important to factor in, as well. In quantum physics, on the other hand, there are limits to the precision with which these things can be measured, known as uncertainty. In fact, objects can have multiple characteristics at the same time; for example, being both a particle and a wave. Only when an observation is made, according to quantum theory, can a measurement of one or other characteristic be established. This is a paradoxical thought, famously made into a metaphor by Schrödinger who imagined a dead cat placed in a box to explain the idea.