Editor's note: Portions of this were previously published on the author's website.
Is outer space a void? What is a void? Is it the same as a vacuum?
The definition of void, according to Webster’s Ninth New Collegiate Dictionary (1988) is “empty space.” If you go by the Merriam-Webster online dictionary, the meaning is more precise: "containing nothing," with the example "void space." The difference between the two definitions is that the first assumes it must involve space; the second, although its example cites space, doesn’t necessarily invoke the concept of a framework of three-dimensional space.
Void often is used to mean outer space. In a NASA press release titled "Walking in the Void" and dated Sept. 3, 2004, the agency talks about space walks its astronauts have taken. A more recent usage of the word is in a NASA Technology Transfer publication from earlier this year titled "Robotics: Into the Void" (see spinoff.nasa.gov). But NASA uses a colloquial meaning, not the precise definition of void as “containing nothing.”
Space isn't empty. The scientific consensus is that one atom, on average, occupies every cubic centimeter of space. In "Astronomy Today" (Prentice and Hall, 1993), authors Eric Chaisson and Steve McMillan agree with the average, adding that the density can be as high as 1,000 atoms per cubic centimeter and as thin as 0.1 atom per cubic centimeter (as noted online at The Physics Factbook – Hypertextbook at hypertextbook.com).
Let’s pretend to examine a cubic centimeter that happens to contain no atoms. Every object we see radiates light, photons that travel in the form of waves and particles without mass. Visible light is part of the electromagnetic spectrum, which includes radio waves, microwaves, infrared radiation, ultraviolet light, X-rays and gamma rays, according to another NASA site at imagine.gsfc.nasa.gov. The lonely centimeter is filled with pulsations of all sorts of electromagnetic fields. It isn’t a void.
Now imagine a box in space, one made of such impossibly impervious material that not even neutrinos — which can zip straight through planets — could penetrate. It happens to enclose an area without any atoms. Still, inside the box forces remain that can’t be blocked, such as gravity. When black holes collide, the resulting gravitational waves ripple through the fabric of space. The events may have happened billions of years ago and billions of light-years away, yet the waves distort everything they reach, all of space itself, including the supposedly empty box. So the box doesn’t enclose a void.
Even in the spaces between subatomic particles, attraction, repulsion and other forces are busily at work.
A possible exception may be the singularity at the center of a black hole. Since it’s impossible to know the heart of a black hole, it just might be a void. If so, voids are numerous throughout our physical universe in these special conditions.
A true void may have existed at the time of the mysterious Big Bang. This is an event that nearly all astrophysicists accept (not the few steady-state holdouts; also, we’re not talking here about complex but fascinating theories about multiverses, other dimensions and more unusual notions of cosmology). According to standard physics, the Big Bang, the moment when our universe began, can be dated to 13.8 billion years ago. It was the start of both space and time.
Into what did the Big Bang expand? Was it a void? Can such a concept exist outside of space and time?
Many physicists, probably most, would argue that the universe itself began then and that nothing could be beyond the universe — certainly not something called a void. But this reviewer isn’t certain.
At the Big Bang, everything we can measure came into existence as an incredibly tiny, unbelievably hot plasma mass that expanded with a rapidity that, scientists believe, could have exceeded the speed of light. No physical object can travel faster than light, but this is the expansion of space itself. The expansion of the universe continued to its present state, and continues from here.
MIT Technology Review notes that static from the cosmic microwave background has traveled 45 billion years, making the universe at least 90 billion light-years across. The reality is, the universe is much bigger than that minimum, it indicates.
The Review notes that a 2011 study printed in the Monthly Notices of the Royal Astronomical Society attempts to estimate the extent of the universe. The study, "Applications of Bayesian model averaging to the curvature and size of the Universe," is authored by Mihran Vardanyan of Oxford University, England; Roberto Trotta, Imperial College London, and Joseph Silk, Oxford. According to the Review, the researchers came up with a conservative ("more tightly constrained than many other models") calculation that the universe is "at least 250 times bigger than the Hubble volume." The Hubble volume is defined as 28 billion light-years across.
This makes the extent of the physical universe no less than 28 billion light-years times 250 across — about 7 trillion light-years. That is, 3.5 trillion light-years in radius.
Out 3.5 trillion light-years away, the universe is expanding. However, light and other radiation from those farthest stars must be pushing outward from there at the speed of light, 186,000 miles per second. How far has electromagnetic radiation extended?
The answer is, at best, extremely hard to calculate. But wherever that is, beyond the faintest gleaming of the most distant pioneer stars, this writer contends, is “without form, and void.”