Key Takeaways

Plain English Takeaway

Scientists showed that by choosing the right materials and putting them in a liquid, they can make a force that usually pulls objects together instead push them apart.

Study Aim

The main goal of this paper is to directly measure whether the Casimir–Lifshitz force, a quantum force that usually attracts objects, can become repulsive (push objects apart) when the right combination of materials is used in a fluid. The authors want to test if theory matches real-world measurements for this effect. Simply put: The study wants to see if a force that usually pulls things together can be made to push them apart by using special materials in a liquid.

Study Design

The researchers used an atomic force microscope to measure forces between a gold-coated polystyrene sphere and either a gold or silica plate, all submerged in the liquid bromobenzene. They carefully cleaned all surfaces and checked for unwanted electric charges. By switching the plate material from gold to silica, they observed how the force changed. They calibrated their measurements to separate the quantum force from other effects, like fluid movement, and repeated the experiment with different spheres and plates to confirm the results. Simply put: The team measured how hard a tiny gold ball and different plates pushed or pulled on each other in a special liquid, making sure nothing else affected the results.

Findings

The study reveals that when both the sphere and plate are gold, the Casimir–Lifshitz force is attractive, pulling them together. However, when the plate is changed to silica, the force becomes repulsive, pushing the objects apart. This switch matches theoretical predictions based on the materials' optical properties. The measured repulsive force is weaker than the attractive one. The results confirm that, with the right materials in a fluid, long-range quantum forces can be made repulsive. This could help create devices that avoid sticking and have very low friction. Some differences between theory and experiment are likely due to uncertainties in the materials' properties and surface roughness at small distances. Simply put: The experiment proved that the force can be made to push instead of pull, which could help make tiny machines work better.

Abstract

No abstract available

Referenced In

StarTalk Show Notes

21 days ago

StarTalk

Season 17, Episode 31: The Casimir Effect (and Wormhole Maintenance)

Hey StarTalkers! Season 17, episode 31 was another grab-bag Cosmic Queries edition, with Neil and Chuck going through a bunch of reader questions about everything “from wine to wormholes.” The second-to-last question was a really wild one:

Cosmic Queries – From Wine to Wormholes - StarTalk Radio

(from 48:00)

Neil and Chuck are floored, and I was too. So what is this person even talking about? Is Neil right about the Casimir effect only being attractive?

The Casimir Effect Explained

Hendrik Casimir’s original paper explains the effect fully in mathematical terms, but you can get a good idea of what’s going on without the math.

Casimir was studying van der Waals forces – an attractive force between neutral molecules – and was struck by a question. What if there were two mirrors facing each other in a vacuum instead of molecules?

In the same way quantum fluctuations create virtual particles in seemingly empty space, electromagnetic vacuum fluctuations would happen between these mirrors (and outside of them, too).

In free space, the whole spectrum of wavelengths can exist. But if the mirrors were close enough together, specific waves would be amplified while others were cancelled out. This would leave only waves where a whole number of half-wavelengths fit in the gap (see below).

These fields contain energy, but there is an imbalance. Outside, we have all wavelengths, but inside, only selected ones. This creates a difference in radiation pressure, pushing the plates microscopically closer together. This is the Casimir effect.

Neil’s Slight Mistake: Casimir Forces Can Separate Plates

Neil makes a slight error when he said that it’s a purely attractive force. Under some circumstances, with carefully-chosen materials, you can also make repulsive Casimir forces. It’s not normal but it is possible.

Where the Question Comes From

The mind-bending question likely has its origins in a 1988 paper co-authored by Kip Thorne, which has an interesting section on “wormhole maintenance.” In it, the authors wonder how a highly advanced civilization might be able to keep a wormhole they created open, and suggest that they may use perfectly conducting spherical plates to produce a repulsive Casimir effect to kind of pry it open.

Is it possible? Even the authors of the paper didn’t know. Given the challenge in even testing Casimir’s idea on Earth, it seems practical concerns would stop them even if theory doesn’t.

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Created: May 27, 2026