That title is another way of saying: “There’s no such thing as an empty box.” Let’s try to make one with a “thought experiment.” Einstein did these; they do not have to be practical, just imaginable. He came up with the Theory of Relativity when thinking about overtaking a light beam.
Think of a tiny metal box, about one millimeter (1/25th inch) on a side. Close it in air and it will contain about a trillion molecules of nitrogen and oxygen. Imagine a perfect vacuum pump extracting every last molecule.
Is the box then empty? No atoms left, but it is full of electromagnetic fields and radiation.
On Earth there will be a gravitational field in the box, so let’s take it far into space and let it float freely. Magnetic fields can be easily shielded, and external electric fields stay outside.
What about electromagnetic waves? It will be dark inside, but all surfaces radiate black-body radiation depending on their temperature. Your body emits infrared radiation with a peak wavelength near ten micrometers, making you visible in the dark with an infrared camera. The radiation from the walls of our box peaks at about 20 micrometers, the width of a fine human hair. To exclude that radiation, we can make the box small enough to fit inside a hair and at near absolute zero temperature (minus 460 degrees Fahrenheit). In outer space it will naturally be that cold.
Our tiny box is now as empty as we can make it. A cosmic ray particle will cross it occasionally. But we cannot prevent thousands of neutrinos from the sun passing through even this hair-size box every second. Neutrinos can pass through a light year of lead! But they zip through at nearly the speed of light, so in any given microsecond our box may have none.
Congratulations, your tiny cold metal box floating in space with a microsecond exposure time is empty. Champagne! Then you remember dark energy and the Higgs field. OK, let’s say they are the true vacuum, the lowest possible energy state. Cheers!
If you made the exposure time much shorter, a billionth of a trillionth of a second, interesting things start to happen. Electron-positron pairs pop into existence. Shorter still, and heavier particles appear, according to Heisenberg’s energy-time uncertainty principle. The vacuum is teeming with activity if you look hard enough, using particle accelerators. The harder you look, the more you see.
Editors’s Note / In a somewhat related note, PN Columnist Mike Albrow has developed a “happy-making, positive-thinking memory game” called “WISE WORDS.” The card game for most ages aims to bring joy and laughter to all who play.
In addition to being a retired particle physicist at Fermilab, Mike’s a writer, lecturer, actor and singer. All his interests, including sports and more, are featured in Wise Words. He’s looking for folks who’d like to support his endeavor via Kickstarter ($16 and up!) by November 20, 2019. For information, visit www.wisewordsthegame.com. Be sure to click on the small prompts posted for navigating pages at the top of the Wise Word website. Mike will appreciate your feedback and interest for proof that the harder you look, the more you see.
