It's possible to make real-world depictions of an electron's wavefunctions, even though the math involves imaginary numbers. Quantum tunneling is the process by which particles—electrons and so forth—can exit a system even though they appear to be trapped. Tunneling is responsible for a number of phenomena, including the radioactive decay of some nuclei and certain ionization processes. All of these begin with a confined particle and end with it freed to go somewhere else. Measuring the rate of tunneling is a challenge, as responses in these quantum systems may occur on the order of 10-16 seconds (hundreds of attoseconds). However, knowing this information is essential for precision ultrafast physics experiments.