How can time be at the same time

Can an electron be in two places at the same time?

Max Planck researchers in Berlin use electrons from nitrogen molecules to demonstrate that the wave-particle character appears at the same time

In a kind of molecular double-slit experiment, scientists from the Fritz Haber Institute (FHI) of the Max Planck Society, in collaboration with researchers from the California Institute of Technology in Pasadena / USA, have demonstrated for the first time on electrons that these have properties of waves and particles at the same time and that they have, so to speak Switched back and forth between the two states at the push of a button. In addition, the researchers succeeded in demonstrating that a disturbance of the mirror symmetry of these molecules through the incorporation of two isotopes of different weights, in this case N14 and N15, leads to a partial loss of coherence, since the electrons are partially at one of the two, now distinguishable atoms to start locating. These research results could be of importance for the construction and control of "artificial molecules", which consist of semiconductor quantum dots and are considered as components of quantum computers (nature, September 29, 2005).

A hundred years ago, the dual character of nature postulated in natural philosophy began to be gradually recognized on the level of elementary physical processes. Albert Einstein was the first to draw this conclusion from Planck's quantum hypothesis in 1905. He assigned particle character to the photon, which is clearly known as the electromagnetic wave. This is the quintessence of his work on the photo effect. Later it was mainly deBroglie who recognized in 1926 that all the building blocks of nature known to us as particles - electrons, protons, etc. - behave like waves under certain conditions.

Nature in its entirety is therefore dual; not a single one of its components is just particle or wave. In order to understand this fact, Niels Bohr introduced the correspondence principle in 1923, which simply means: Every component of nature has both particle and wave character and it only depends on the observer which character he sees at the moment. In other words: It depends on the experiment which property - particle or wave - you are currently measuring. This principle has gone down in the history of physics as the principle of complementarity.

Albert Einstein was suspicious of this dependence of natural properties on the observer throughout his life. He believed there had to be a reality independent of the observer. But quantum physics has simply accepted the fact that there seems to be no independent reality as a given over the years without questioning it further, as all experiments have confirmed it again and again and with increasing accuracy.

The best example is the Young’s double slit experiment. In this double slit experiment, coherent light is allowed to fall onto a diaphragm with two slits. An interference pattern of light and dark stripes is then shown on an observation screen behind the diaphragm. The experiment can not only be carried out with light, but also with particles such as B. electrons are carried out. If you send individual electrons one after the other through the open Young’s double slit, a strip-shaped interference pattern appears on the photo plate behind it, which does not contain any information about the path that the electron has taken. If one of the two slits is closed, however, a blurred image of the respective open slit appears on the photographic plate, from which the path of the electron can be read off directly. However, a combination of stripe pattern and situation map is not possible in this double slit experiment; this requires a molecular double slit experiment, which is not based on the spatial impulse uncertainty but on the mirror symmetry.

It was not for nothing that the experiment was voted the most beautiful experiment of all time in a survey by the English physical society in the journal Physics World in 2002. Although each electron seems to pass individually through one of the two slits, a wave-like interference pattern builds up at the end, as if the electron had split when passing through the double slit and then reunited. But if you keep a gap closed or you observe which gap the electron passes through, it behaves like a completely normal particle that is only in a certain place at a certain time, but not both at the same time. So, depending on how the experiment is carried out, the electron is either at location A or at location B or at both at the same time.

Bohr's principle of complementarity, which explains this ambiguity, requires at least that one can observe only one of the two manifestations at a given time in a given experiment - either wave or particle, but not both at the same time. Despite all the ambiguity of quantum physics, this remnant of unambiguity remains in every experiment. Either a system is in a state of the wave-like "both-and" or the particle-like "either / or" state with regard to its localization. In principle, this is a consequence of Heisenberg's uncertainty principle, which states that one can only determine one variable from a complementary pair of variables (e.g. position and momentum) at the same time as precisely as desired. The information about the other size is lost in inverse proportion.

Recently, a class of experiments has shown that these different manifestations of matter can be converted into one another, that is, one can switch from one form to the other and, under certain conditions, back again. This class of experiments is called quantum markers and quantum erasers. In the last few years you have shown in atoms and photons, and recently also in electrons, that there is a juxtaposition of "as well as" and "either-or" for all forms of matter, i.e. a gray area of ​​complementarity. Accordingly, there are experimentally verifiable situations in which matter appears both as a wave and as a particle at the same time.

Such situations are described with a duality relation, which is an extended complementarity principle of quantum physics, which one could also call the coexistence principle. It says that the normally mutually exclusive manifestations of matter, such as local and non-local, coherent and non-coherent, can be detected simultaneously in a certain transition area, i.e. are present by measurement. One speaks of partial localization and partial coherence or of partial visibility and partial distinctiveness; Quantities that are linked to one another via the duality relation.

The principle of complementarity and thus the complementary dualism of nature is expanded in this transitional area by a principle of coexistence, i.e. a parallel dualism. This shows that nature has a more ambivalent character than previously assumed. Examples of this are atomic interferometry, where this behavior was first found in 1998 for atoms, i.e. composite particles.

In the current issue of Nature, the Berlin Max Planck researchers, together with researchers from the California Institute of Technology in Pasadena / USA, report on molecular double-slit experiments with electrons, i.e. non-composite elementary particles. These are based on the fact that molecules with identical and thus mirror-symmetrical atoms behave like a microscopic double slit built up by nature. This includes nitrogen, where every electron - including the highly localized inner electrons - is on both atoms at the same time. If such a molecule is ionized with soft X-ray radiation, this property leads to a coherent, i.e., wave-like, strictly coupled emission of an electron from both atomic sides, just as in the double-slit experiment with single electrons.

For the first time, the researchers were able to directly demonstrate the coherent character of the electron emission of such molecules, analogous to the double-slit experiment. To do this, they released the innermost and thus most localized electrons of nitrogen from the molecule using soft X-rays and then tracked their movement in the reference system of the molecule using a coincident measurement with the ionic molecule fragments. In addition, the researchers succeeded in the long-doubted proof that a disturbance of the mirror symmetry of these molecules through the incorporation of two isotopes of different weight, in our case N14 and N15, leads to a partial loss of coherence, since the electrons are partially attached to one of the two, now distinguishable atoms, begin to localize. This corresponds to a partial marking of one of the two columns in a Young’s double-column experiment. One speaks of partial "which way" information, because the marking provides information about which path the electron has taken.

The experiments were carried out by employees of the "Atomic Physics" working group of the FHI at the synchrotron radiation laboratories BESSY in Berlin and HASYLAB at DESY in Hamburg. The measurements by means of a multi-detector arrangement for combined electron and ion detection took place behind so-called undulator beam tubes, which deliver soft X-rays with high intensity and spectral resolution.