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Optical Levitation: Laser Radiation Pressure & Optical Tweezers

Optical levitation demonstrates that pure light can counteract gravity. By harnessing the infinitesimal momentum carried by photons, tightly focused laser beams can trap, manipulate, and levitate micro- and nanoscale particles in liquids, air, and ultra-high vacuum.

Levitation vs Antigravity Principle

Levitation is not automatically antigravity. An object can levitate when an upward force balances its weight. Magnetic, acoustic, aerodynamic, and optical forces can all produce levitation. These effects oppose gravity, but they do not remove or cancel the gravitational field itself.

Photon Radiation Force Equation

Radiation Force for Absorbed/Reflected Light:
F_rad = (1 + R) · P / c

Where P is laser power in Watts, c is the speed of light (3 × 10⁸ m/s), and R is reflectivity (0 for absorbing, 1 for ideal mirror).

1. The Two Optical Forces in Levitation

Force Component	Physical Origin	Direction	Role in Levitation
Scattering Force	Direct momentum transfer along beam axis (p = E/c)	In direction of light propagation	Pushes the particle upward against gravity
Gradient Force	Induced dipole in electric field gradient (Lorentz force)	Toward region of maximum light intensity	Pulls particle into beam focus for 3D lateral stability

2. Scale and Force Magnitude

Because the speed of light c is so large, a 1 Watt laser beam exerts an upward radiation force of only 3.34 to 6.68 nanoNewtons (nN).

While this force cannot lift a human, a 1-micrometer silica sphere has a mass of only ~1 picogram (10⁻¹⁵ kg) and a gravitational weight of ~0.01 picoNewtons (pN). A few milliwatts of laser power easily overcomes gravity by millions of times!

Frequently Asked Questions

What is optical levitation?

Optical levitation is the physical suspension and trapping of micro- and nano-scale dielectric particles using the momentum carried by focused laser beams (photon radiation pressure and optical gradient forces).

How can light exert physical force?

Photons carry momentum p = E / c = h / λ. When a photon is reflected, absorbed, or refracted by a particle, its momentum changes. By Newton’s Third Law, this imparts an equal and opposite mechanical force F = dp/dt on the particle.

Who discovered optical levitation and optical tweezers?

Arthur Ashkin at Bell Laboratories pioneered optical levitation in 1970 and invented single-beam gradient optical tweezers in 1986, for which he was awarded the Nobel Prize in Physics in 2018.