Let's examine an experiment: The photoelectric effect refers to the phenomenon where electrons escape from a metal surface when exposed to light of specific frequencies, forming a photoelectric current (photocurrent). The core experimental setup includes: a vacuum phototube (cathode K and anode A), a light source with adjustable frequency/intensity (mercury lamp or LED), a adjustable DC power supply (forward/reverse), a sensitive ammeter, and a voltmeter.

The fundamental principle is Einstein's photoelectric equation hv=Wo+Ekmax. Experimental steps: First, connect the circuit, shield the phototube from light, and preheat the apparatus; Second, irradiate K with a specific frequency, apply reverse voltage, adjust until the photocurrent reaches zero, and record the cutoff voltage Uc; Third, switch to different light frequencies and repeat measurements for multiple sets (U, Uc); Fourth, plot the Uc-U graph, derive the slope K=h/e, and calculate the h value. Key experimental principles: First, there exists a cutoff frequency Vo: No photocurrent occurs below V<Vo, independent of light intensity/time; Second, maximum initial kinetic energy depends solely on V: Higher V values yield greater EKmax, unrelated to light intensity; Third, saturation current increases proportionally with light intensity: When V≥Vo, stronger light intensifies current; Fourth, photoelectrons are emitted instantaneously upon light exposure, with no energy accumulation time. The photoelectric effect is a transient process, with a delay time not exceeding 10^n (n=-9) seconds.

The Key Development Stages of the Photoelectric Effect: The first stage began in 1887 when Heinrich Hertz observed that ultraviolet light irradiating metal plates enhanced spark discharge during his experiments verifying electromagnetic wave existence. Although he did not delve into the underlying mechanisms, this marked the first documented record of the photoelectric effect. The second stage emerged in 1899 when British physicist Joseph John Thomson confirmed the existence of electrons through cathode ray experiments, hypothesizing that light induces electron emission from metal surfaces. This groundbreaking work established the fundamental particle concept for subsequent research. The third stage occurred in 1902 when Max Planck conducted systematic experiments investigating photoelectron kinetic energy under varying light frequencies and intensities. His findings revealed that the maximum kinetic energy of photoelectrons correlates with light frequency rather than light intensity, directly challenging the classical physics expectation of "energy increasing with light intensity." The fourth stage began in 1905 when Albert Einstein proposed the "photon" concept in his revolutionary paper "A Thought-Provoking View on the Production and Transformation of Light." He demonstrated that light energy exists in discrete "quantum" units, with each photon's energy proportional to its frequency (E=hv), explaining how light frequency determines photoelectron kinetic energy while light intensity controls electron emission quantity.

These observations demonstrate that when light above a specific frequency strikes the surface of metals or other materials, electrons within the material absorb the light's energy. If this energy exceeds the "work function" required for electron escape, the electrons overcome the material's binding force and detach from the surface, resulting in the phenomenon of "photons." Furthermore, a detailed analysis reveals that for electrons to escape the material's binding force, a photon energy greater than the material's binding force must be present to trigger the photoelectron phenomenon. This constitutes the fifth source of the force.

How can we derive the formula for converting photon energy E=hv into photon force? Work, the universal measure of energy conversion, is expressed as W=∆E, where ∆E represents the change in energy across the system. Here, ∆E=hv. The formula W=FScosθ defines the force, with F being the magnitude of the force, S the displacement, and θ the angle between the force and displacement directions. According to the instantaneous electron transition mechanism, Scosθ=ct, where c is the speed of light and t is the instantaneous delay time of the photoelectric effect (10ⁿ seconds, n=-9). This means that when photons strike electrons on a material surface, the electrons undergo a subtle and brief displacement before escaping. Substituting these values yields the photon force formula F=hv/ct, which clearly demonstrates that photon force is also dependent on the light's frequency v, with the frequency playing a pivotal role.
Some might argue that photon force is essentially electromagnetic force, since light is the manifestation of electromagnetic waves. However, it's crucial to recognize the fundamental distinction between particles and waves. Photon force refers to the energy produced by photons, which are massless, whereas electrons and magnetic particles possess mass. To equate photon force with electromagnetic force would contradict the massless nature of photons. The debate over the dual nature of particles and waves has persisted for centuries in the history of natural science. Although we now acknowledge the coexistence of wave-particle duality, the essential difference between particles and waves remains. To further clarify the distinction between photon force and electromagnetic force, I have specifically listed the key developmental stages of the photoelectric effect.

Why is the fifth force—photon force—proposed? Because the existence of energy and momentum necessitates the existence of force, just as the presence of electric energy and electric motors leads to electricity; the existence of nuclear energy and nuclear-powered submarines leads to nuclear force; similarly, the presence of light energy and the emission of electrons from materials when illuminated by light inevitably results in the photon force.

The photoelectric effect has been widely used in modern physics, optoelectronics, medicine and semiconductor technology, such as solar cell, photomultiplier tube, photoelectric imaging, photoelectric sensor and laser technology.

The photon force, as the most primordial force, has objectively, universally, and persistently existed throughout the universe's evolution since its inception, continuing to this day and beyond. Its discovery will play an increasingly prominent role in both microscopic and macroscopic realms. This is because all things are in constant motion and change, all experimental observations rely on light, and all discoveries and inventions are inseparable from light. Yet to this day, many mysteries of nature remain unsolved—numerous objective natural phenomena remain unexplained, and many natural laws lack scientific interpretation. Questions like how black holes form at galactic centers, where their energy originates, why planets orbit stars, whether universal gravitation exists, and how the four fundamental forces are unified still await answers.

2026/2/28