Ages 8–12 · about 4 minutes

Empty Space Is Not Empty

If you took everything out of a box, what would be left inside?

Do this first · Try this at home

Which waves fit?

Start with your hands. The reading makes far more sense afterwards.

You need: Paper, a pencil and a ruler

Tick each step as you do it

Draw two dots 10 centimetres apart. Join them with a curved line shaped like one hill.
Draw another pair of dots 5 centimetres apart. Fit the same one-hill pattern between them.
Draw a straight guide line between the first pair of dots. Draw a wave with one hill above that line and one dip below it, crossing the line halfway. Compare how many curved sections fit between the dots.

0 of 3 steps done

What should I notice?

Notice: The endpoints help decide which wave patterns fit. This drawing models a wave with fixed ends; it does not measure a Casimir force.

Now read on to find out why it happens.

The big idea

Two nearby conducting surfaces change the electromagnetic field and can attract each other.

vacuum: A region without ordinary matter, such as air or dust.
field: A physical quantity with a value at each point in space. Light is a wave in the electromagnetic field.
zero-point energy: The energy that remains in a quantum system's lowest-energy state.

Take the air and dust out of a box. You have removed matter, but the electromagnetic field still has a lowest-energy state. Quantum fluctuations remain in that state. This is what the vacuum's ‘jitter’ means, not a sound you could hear.

Two wooden toy sailing boats in a bathtub full of little waves, leaning toward each other while the water between them stays calm. — The boats are a wave analogy. In a Casimir experiment, conducting surfaces change electromagnetic field modes.

Which waves fit?

A guitar string held at both ends can make some standing-wave patterns and not others. Change its length and you change the patterns.

Conducting surfaces also change the electromagnetic field's allowed patterns, called modes. In 1948, Hendrik Casimir compared the field energy when two ideal parallel plates were close together and when they were far apart. He calculated an attraction.

The gap matters. In that ideal plate calculation, halving the gap makes the attractive force per unit area sixteen times larger. Real surfaces need corrections for their materials and shape.

Measuring a tiny force

In 1997, Steve Lamoreaux used a curved conducting surface facing a flat one. A sensitive twisting balance measured the balancing force needed to keep its arm from turning.

In 1998, Umar Mohideen and Anushree Roy used an atomic force microscope. They measured the bending of a tiny flexible arm carrying a metal-coated ball near a flat surface.

Both experiments checked the separation and electrical attraction, which also affect the signal. Comparing the remaining force with the calculation is the measurement—not watching toy boats move.

Changing the surroundings

In 2026, Wang and colleagues reported that a material called NbSe2, placed in a resonator shaped like a ring with a gap, became superconducting—able to carry electric current without resistance—at a higher temperature. Changing the field's surroundings changed a material property.

Moving surfaces changes the field's energy; moving them also means doing work.

What would you change to find out whether a tiny force came from the surfaces, leftover electric charge, or the measuring instrument?