Most people today have heard of quantum physics. It is a cornerstone of our modern understanding of the physical world. But despite having heard of quantum physics, many aren’t really aware of what the theory is all about. Have you ever wondered what quantum physics is? How it was discovered? Why it was discovered?
Quantum physics is our understanding of light and matter and the way they interact. Its discovery came about simply as a matter of doing physics—of looking at things, analysing what was seen, and trying to understand the physical origins of the properties of light that were discovered. In that sense, the reason why the quantum world was discovered is really just that humans are curious beings who see things and want an explanation. In the same sense, the process that led to the discovery of quantum physics probably seems a lot like what astronomers do.
If someone had told you before you started this course that one of the topics you’d be studying a few weeks in is quantum physics, you might have wondered why an astronomy course would ever even mention the term. After all, if you know anything at all about quantum physics you’ll know it’s the theory that describes the very small—electrons, protons, quarks, etc.—things that exist at the subatomic level. The quantum world is incomprehensibly small, whereas the very first thing you explored in this course was just how incomprehensibly large the astronomical world really is.
However, knowing that quantum physics is what we use to describe the ways that light and matter interact, and (from Module 4) that astronomy is all about observing light that once interacted with distant matter, you should already have an idea of just how essential quantum physics is to astronomy. By utilising our knowledge of the ways that light and matter interact, astronomers gain a wealth of knowledge about the physical Universe simply by looking at it.
In Module 4, you saw that light is a radiating, oscillating electromagnetic field. Depending on its wavelength (or its frequency, or energy), a beam of light might have a particular colour or it might not be visible at all. A source of white light, for instance, emits a whole spectrum of beams of electromagnetic radiation covering a range of wavelengths, and this spectrum can be seen by shining a signal of white light through a prism.
As we’ll see in this module, most light sources do not emit a specific wavelength of light, but light from a range in the electromagnetic spectrum. An object’s colour generally depends on the intensity of light that it emits at different wavelengths. If the most intense signal coming from an object is blue, that object will appear blue.
A graph of the intensity of light emitted over all wavelengths is referred to as the object’s spectrum, and it turns out that by studying an object’s spectrum we can tell a great deal about many physical characteristics, such as its chemical composition, surface temperature, and velocity. It turns out that we can tell a lot about a distant star or galaxy—things so far away that all we can currently do is look at them—simply by analysing their spectra.
Eventually in this module, you will study the manner in which stars produce light, and the way that light interacts with atoms in their atmospheres, with the planets in orbit around them, or with surrounding gas clouds to produce various spectral features. Understanding how all this occurs is the key to knowing how astronomers are able to look out at distant objects and know what they are. But before coming to that, we must explore how matter leaves its fingerprints on the light that it interacts with.
We begin in the next section with some definitions of the basic components of an atom, which we’ll draw upon throughout the module. From there, we’ll start exploring the different types of spectra that eventually led physicists to our modern understanding of the structure of an atom, and the story of how we got there.
cosmiCave.org 