Isotropy
Imagine being in a giant sphere filled with translucent beads. You look to the left, and you see a prism of distorted light, refracting as far as you can see into a uniform color and consistency. When you look to the right, it’s the same—no matter how far you peer, and no matter what direction you look in, everything looks the same.
This condition is called isotropy. It comes from the Greek words isos and trópos, which roughly mean “equal turn.” To say that something is isotropic is to say that everything is uniform in all directions.
Our universe is isotropic. No matter how many billions of light years away you look, and no matter the direction, space and its relationship to matter are identical—that is, once you get to a large enough scale. There’s no way to know if you’re peering out at the distant cosmos from Nigeria or from Bolivia, except for the stuff that’s close enough for us to have mapped.
Well… it’s very, very isotropic. But even at very big scales, there are regions of anisotropy—places where it’s not quite the same everywhere. Scientists were keen to measure how much of this there was out there, so they devised the Wilson Microwave Anisotropy Probe (WMAP for short—abbreviation or acronym?) in order to measure the anisotropies (tiny temperature variations) of the entire universe.
They took a look at the CMBR (Cosmic Microwave Background Radiation) and saw that the temperature of the universe is very uniform at a large scale. How uniform? We’re talking within millionths of a degree.
This principle is taken very seriously by astrophysicists, who have embedded isotropy into the cosmological principle, which states that the universe is both isotropic and homogeneous when you look at it on a large enough scale. The idea is important in order to reinforce Big Bang cosmology.
While isotropy is everything is the field of cosmology, the concept is useful—even vital—in other fields.
In materials science, you really need to have confidence in a material behaving the same way in all directions. Heat tolerance, conductivity, and material strength all need to be taken into account. How consistent they are throughout the material can have a direct relationship with how reliable the material is.
Similarly, the idea is useful in engineering, where assuming a material is uniform in all directions simplifies unimaginably difficult calculations.
Radiation can be isotropic, too. It can be focused like a LASER, but it can also radiate outward in all directions like our Sun. This understanding matters when we design everything from wifi to bluetooth, and when we study astronomical objects like stars.
The force of gravity is isotropic, too. It warps space equally in all directions, radiating outward in identical measure no matter where you look.
What are some other examples of isotropy you can think of? Where does this matter the most? Did you think about a Chuck-E-Cheese ball pit during the opening paragraph?
Isotropy, and its counterpart, anisotropy, is also important is earthquake science. The speed of seismic waves through Earth’s interior may not be isotropic in all directions. Specifically in the upper mantle, shearing of tectonic plates on the viscous mantle shears rocks and forms anisotropic layering that preserves spreading history of the crust. Analysis of the fast and slow directions of seismic waves provides details about how the Earth has evolved overtime. Thanks for the read, Andrew!
In antenna design work, the basic reference antenna against which you compared others is the isotropic radiator, a.k.a. the point source. The sun is a point source. A spotlight is a highly directional light source. Antennas do the same with radio waves.
Antennas can steer energy in certain directions and not in other directions. This is referred to as an antenna pattern in 3-space. This pattern of an antenna is often quantified versus the point source, and given in dBi or decibels above an isotropic radiator. It's a measure I use all the time in antenna system work.