In an experiment, physicists have for the first time observed how electrons behave at the ultimate spacetime limit. The study reveals that while Heisenberg’s uncertainty principle does not formally restrict space and time, measuring both attributes simultaneously triggers an unavoidable trade-off that alters the electron’s physical shape.
In the quantum realm, the rules of classical physics collapse. According to Heisenberg’s uncertainty principle, certain pairs of physical properties, such as a particle’s position and momentum, cannot be measured simultaneously with infinite precision. However, this fundamental blurriness does not apply universally to all variables. There is no formal uncertainty relation between time and space. In theory, this implies that the temporal structure of an electron’s wave function can be analyzed without distorting its spatial dimensions.
Yet, translating theory into reality has proven notoriously difficult. When electrons undergo ultrafast changes—such as during chemical reactions or quantum tunneling—space and time become coupled in non-trivial ways. Observing this dynamic interaction requires a temporal resolution in the attosecond range (one billionth of a billionth of a second), a threshold that has long remained out of reach.
Lasers and Tunneling: The Ultimate High-Speed Camera
To break through this barrier, a research team led by Simon Maier from the University of Regensburg designed a novel experiment. They isolated a single copper atom on a silver surface inside a scanning tunneling microscope. The microscope’s ultra-fine metallic tip, just one atom thick, hovered a mere few atomic diameters above the sample. The team then bombarded this junction with two distinct, non-overlapping, ultrashort near-infrared laser pulses.
These laser bursts provided the necessary energy for the surface electrons to breach the energy barrier and tunnel between the tip and the atom. By systematically shifting the delay between the two laser pulses, the physicists could track precisely how the electrons responded. The pulses effectively functioned as a high-speed camera, allowing the scientists to map both the temporal progression and the spatial expansion of the electron wave packet during the tunneling process.
The Trade-Off: Delays and Spatial Expansion
The measurements exposed two distinct phenomena when electrons are driven by light on these extreme timescales. First, the electrons do not react instantaneously to the initial laser pulse; instead, they exhibit a miniscule delay of approximately 500 attoseconds. Second, this delay is directly tied to a physical transformation. The more energy injected into the system to pin down the exact timing, the larger the volume of the electron wave packet becomes.
This reveals a fascinating correlation: even though Heisenberg’s uncertainty principle does not forbid a simultaneous precise measurement of space and time, the physical act of measurement forces a compromise. Pinning down an electron’s exact temporal behavior requires higher energy, which inevitably causes its wave packet to expand in space.
From Fundamental Physics to Next-Gen Computing
This successful observation confirms the existence of a suspected spacetime limit for electronic behavior. The findings offer unprecedented insights into fundamental electromagnetic interactions and ultrafast chemical processes, where the dynamic reshaping of electron wave functions plays a key role in the making and breaking of chemical bonds.
Beyond theoretical physics, the discovery holds immense potential for the future of information technology. By understanding the intrinsic speed limits and structural changes of moving electrons, researchers hope to pave the way for quantum information processing and electronics that operate at the absolute speed limit of matter—potentially outperforming current CMOS technologies by a factor of hundreds of thousands.
References
- Maier, S., Gloeckl, K., Spachtholz, R., Huber, R. et al. (2026). Tracking electron dynamics at the intrinsic spacetime limit. Nature Photonics. doi:10.1038/s41566-026-01932-0
