# Can particles have imaginary momentum / energy less than their rest mass?

In the Feynman lectures on Physics, he talks about quantum tunneling and, basically, there's a non-zero amplitude to be found in the barrier, where the particle's energy is below the rest mass, i.e. kinetic energy is negative, and because $E_k = p^2 / (2m)$, that implies $p$ is imaginary. Can this actually be observed? Can you collapse the wave function by putting a detector in the barrier and get a count? What properties does matter have in this state?

• If you have found the particle in the barrier, then you must have just measured its position. This means that the particle is now in a state which is localised in position and consequently it does not have a well defined momentum – By Symmetry Feb 6 '17 at 16:58
• How much does this differ from the case of bound state? A particle's energy is below it's rest mass and you just need some energy to kick it to another state. – jaromrax Feb 6 '17 at 17:16
• @Symmetry, but you could have a well-enough defined momentum for the question to still be completely valid. Your detectors' position measurement can have a very large uncertainty for a large-enough barrier. – user1247 Jun 1 '17 at 16:31