Human limitations in perceiving time have driven scientists to constantly explore observation techniques beyond the naked eye. As early as the 1920s, MIT engineer Harold "Doc" Edgerton set out to find a solution to the unpredictable behavior of electric motors due to fluctuations in the electrical grid. Because motors spun too fast, neither the human eye nor the slow exposure times of cameras at the time could capture their motion details. Edgerton noticed that every time he triggered a power surge, his equipment gave off a bright flash of light. When that flash hit the motor, the moving parts appeared to stand perfectly still as if frozen in time. This insight led to the birth of strobe photography (Strobe Photography: A photographic technique that uses brief, intense flashes of light to "freeze" moments of high-speed motion).

Edgerton's strobe light worked by using a high voltage power source to load electrons onto a capacitor. When a trigger sent a high voltage pulse through a wire wrapped around a glass tube, the electric field from that pulse would rip electrons off the gas atoms inside the chamber (such as argon or xenon), ionizing the gas and turning it into a conductor. In that instant, the charge stored in the capacitor would surge through, heating the gas to around 10,000 Kelvin, producing a very bright, very brief flash of light lasting just 10 microseconds. Then the electrons would recombine with the gas atoms, stopping the current, and the circuit would go dark again. This brief but intense flash could precisely capture moments of high-speed motion.

By the early 1930s, Edgerton was eager to test his strobe outside the lab, traveling to factories to show workers how to "freeze" motors in time, allowing them to take sharp pictures of gears in motion. He was not just an engineer but also an artist with a unique eye for photography. He applied strobe photography to everyday scenes, such as tennis balls pancaked against a racket and hummingbirds frozen in time. Through influential magazines like "Life Magazine" and "National Geographic," he communicated what was happening at timescales we couldn't see.

The critical challenge for strobe photography was precisely timing the flash. Edgerton ingeniously used sound as a trigger mechanism, for example, by popping a balloon or using the sonic boom (Sonic Boom: A powerful shock wave produced when an object moves at supersonic speeds) from a bullet to trigger the flash, capturing the moment a bullet passed through a playing card. This technique even played a significant role in military reconnaissance. In 1939, US Major George Goddard approached Edgerton, seeking to develop a strobe powerful enough to illuminate the ground from a plane a mile or so up in the sky for night reconnaissance. Edgerton successfully developed a flash lamp that released about 60,000 joules in a single millisecond, with a peak power of roughly 60 megawatts, comparable to the output of a large solar farm. This flash lamp was quickly utilized in World War II, allowing the Allies to take pictures of Normandy the night before D-Day, confirming that German troops were unprepared for the attack.

Compared to a 2020 research-grade slow-mo camera shooting at 20,000 FPS, Edgerton's strobe photos from the 1930s still demonstrated superior sharpness. This highlights the fundamental trade-off between spatial resolution (Spatial Resolution: The number of pixels in an image) and temporal resolution (Temporal Resolution: The ability to capture a single frame or a progression of frames). Hardware limitations often mean you have to trade one resolution for the other: either a high pixel count with a lower frame rate or a high frame rate with fewer pixels. Edgerton's strobe technique pushed temporal resolution to its limit, achieving unparalleled sharpness with a single frame.

On the path to extreme temporal resolution, scientists explored another extreme method: the single-pixel trillion-FPS camera (Single-Pixel Trillion-FPS Camera: An imaging system that achieves ultra-high temporal resolution of a trillion frames per second by using only a single-pixel sensor, combined with multiple measurements and scanning techniques). While this camera can only "see" one pixel at a time, its temporal resolution can reach a trillion frames per second.

The working principle of this camera is similar to LIDAR (LIDAR: Light Detection and Ranging, a remote sensing technology that determines distance and shape by emitting laser pulses and measuring the time it takes for the pulses to return) in our phones. It uses a sensor sensitive enough to register even a single photon, counting incoming photons around a trillion times a second, with each "frame" lasting roughly one picosecond. In that short time, light itself travels only 0.3 millimeters.

To take ultra-slow motion videos of light traveling, researchers set up a scaled-down room with different shapes like a cone, a sphere, and a mirror. They shoot out a really short laser pulse that hits just one point in the scene, and that laser pulse has a ton of photons in it, which scatter everywhere. The single-pixel camera initially points at the top left corner. When a pack of photons from the laser pulse hits a random spot on the wall, reflects to the corner, and finally bounces into the camera, the sensor picks up their faint signal at a trillion frames per second. Because the exposure time is so short, the photon return from each measurement is very faint. Therefore, researchers take a bunch of measurements and group them all together to get actual usable information about how far away the light traveled in a scene.

Then, the camera is moved slightly, and the experiment is repeated. By scanning a grid of points across the whole scene hundreds of times and recording the signal from each new position, a complete image of light propagation can be constructed. The most important thing for this technique to work is that the scene has to play out pretty much exactly the same every time, because if it didn't, then every pixel would tell a different story. Thankfully, the laser pulse in our scene scatters pretty predictably. By scanning more points along this grid, higher spatial resolution can be achieved; for example, if you want 4K, you simply scan a 4K grid of pixels, though it will take more time.

Using this technique, scientists successfully visualized the dynamic process of light traveling through a bottle, forming wavefronts, and bouncing off the cap. They can even take this a step further by rotating the scene and recording multiple points of view, using algorithms to create "fly-throughs," allowing one to see the light propagate from any direction. In some visualizations, because the camera is moving faster than the light propagation, the wavefront appears stationary, creating an astonishing "physics-breaking" visual effect. These incredible videos were created at the University of Toronto and MIT, and enthusiasts have even successfully built one of these speed-of-light cameras in their garage.

Pushing the principles of strobe photography and trillion-FPS cameras to their limits, scientists aim to capture even more microscopic and rapid phenomena—the motion of electrons. Although the quantum nature of electrons prevents them from being "photographed" or "videotaped," attosecond X-ray pulse (Attosecond X-ray Pulse: X-ray pulses with durations in the attosecond (10^-18 seconds) range, used to probe ultrafast electron dynamics) technology allows us to observe the dynamic changes of electron clouds within molecules with unprecedented precision.

This groundbreaking research is conducted at the SLAC National Accelerator Laboratory (SLAC National Accelerator Laboratory: A US national laboratory housing a 3.2-kilometer-long linear electron accelerator, dedicated to particle physics and X-ray science research). SLAC houses a 3.2-kilometer-long, perfectly straight electron accelerator that accelerates electrons to over 99.9999992% the speed of light. Studying electron motion is crucial because electrons essentially create the fields in which everything else happens; molecular bonds break and form because electrons essentially give them a push to do so. Therefore, being able to look into their motion is the most fundamental way of studying materials and matter.

To achieve a "nanoscopic strobe" for electron motion, scientists utilize X-ray Free-Electron Lasers (X-ray Free-Electron Laser: An advanced light source that produces extremely bright, ultrashort X-ray pulses by accelerating high-speed electron beams through magnetic fields). First, relativistic electron pulses are fed through a set of devices called undulators (Undulator: A series of magnets with alternating poles that cause a high-speed electron beam to oscillate periodically, thereby emitting synchrotron radiation). These undulators are stacks of magnets spaced only a few millimeters apart with alternating poles. Due to the Lorentz force (Lorentz Force: The force exerted by a magnetic field on a moving electric charge, perpendicular to both the charge's velocity and the magnetic field lines), the electron beam "wiggles" periodically. This wiggling motion causes the electrons to emit electromagnetic radiation. Due to relativistic effects, for electrons traveling near the speed of light, the macroscopic undulator period is extremely compressed, resulting in electromagnetic radiation with wavelengths much smaller than millimeters, entering the X-ray domain. Furthermore, an observer at the far end of the accelerator will see an additional blueshift (Blueshift: The phenomenon where the wavelength of electromagnetic waves shortens due to the light source moving towards the observer), producing X-rays as small as 50 picometers in wavelength.

Initially, these X-rays are created randomly along the undulator, producing an incoherent light pattern. But soon after, the electric fields from the X-rays start to interact with the electrons, speeding some of them up and slowing others down, causing faster electrons to catch up with slower ones. This leads to them getting bunched up into periodic structures, called microbunching. These sheets of electrons then emit light as unified fronts, so the resulting X-rays come out coherently as a laser pulse, dramatically increasing their intensity. These pulses are incredibly tightly packed, being only a few femtoseconds long, and can get as short as a couple hundred attoseconds (Attosecond: 10^-18 seconds, one quintillionth of a second). The attosecond is to the second what the second is to the age of the universe, an absurdly quick pulse. On an attosecond scale, you can see electrons zip around essentially atoms and molecules.

In experiments, after the undulators, the X-ray pulses are sent to experimental stations at the end of the tunnel, focusing into what is called an interaction point (Interaction Point: The precise location in an experiment where the X-ray beam interacts with the target molecules). Researchers fill this interaction point with the molecules whose electrons they want to study, shining the X-ray pulse on them. When it hits the molecule, it will ionize the molecule, predominantly from these inner shells, the very core parts. The thing is, core-level electrons (Core Level Electron: Electrons in the inner shells of an atom, typically having higher binding energies) from different elements have different ionization energies (e.g., nitrogen needs around 400 electron volts, whereas oxygen needs around 550). By tuning the X-ray energy to match these ionization energies, you get to choose which of these atoms within the molecule the X-ray is going to ionize. Any excess energy left after an X-ray has ejected an electron will be taken by that electron as kinetic energy (Kinetic Energy: The energy an object possesses due to its motion).

By measuring the kinetic energy of the electrons you eject, you can infer what the electron density (Electron Density: The measure of the probability of an electron being present at a particular point in space) was around that electron. If you have a high electron density around a particular atom, the core-level electrons will be bound less tightly to the nucleus, so its ionization energy will actually be slightly lower. Whereas if you have a lower electron density than average around an atom, those core-level electrons will be bound more tightly with a higher ionization energy. Therefore, by measuring the kinetic energy of the electrons you eject, you can use the difference between the input X-ray energy and the output kinetic energy to infer what that electron density was.

To create a "molecular movie," researchers first use a traditional infrared laser (Infrared Laser: A laser with wavelengths in the infrared range, often used to excite molecular vibrations or initiate chemical reactions) pulse to create some non-equilibrium state in the molecule, driving some dynamics. Then, the attosecond X-ray pulse probes it after a time delay t. By measuring the kinetic energy of the electron, you can study how the electron density of the molecule reacted to the trigger laser, getting an attosecond snapshot. Then, by taking another sample of the same molecule, shooting it with a laser again, but this time increasing the probe delay slightly to t + delta t, you can get a sequence of snapshots of how that electron density evolves over time. The big assumption here is that the molecular dynamics must be repeatable. If the scene is repeatable, then like the trillion FPS camera, you can use this technique to create a molecular movie, with frames only a few hundred attoseconds apart, technically running at over a quadrillion frames per second.

For example, a simulation of the dynamics of a small molecular system called para-aminophenol (para-aminophenol: A small molecular system) after the removal of an electron showed that when an X-ray pulse removes an electron, it initiates some charge distribution that starts to move across the molecule (red color represents an increase in electron density, and blue represents a decrease). Although the video is a simulation, it has been validated by experiments done at SLAC. When predictions and measurements diverge, especially after a few femtoseconds, it's the most exciting time in science, because you just found something you didn't know ahead of time. Being able to actually see these electron densities move around is spectacular and fundamental to understanding nature.