What is a real vacuum
Physicists assume that the so-called vacuum fluctuations have had a decisive influence on the distribution of matter in the universe. These are virtual pairs of particles that appear out of nowhere within fractions of a second and then immediately disappear again. Alfred Leitenstorfer from the University of Konstanz described the first direct measurement of these particles from nowhere in our podcast.
Heisenberg's uncertainty principle states that certain pairs of variables in a system cannot be determined with any precision - for example, location and speed, or energy and time. This statement of quantum physics has consequences for the empty space, because accordingly it is not that empty. Instead, particles spontaneously arise in it, which immediately disintegrate again: the so-called vacuum fluctuations. The blurring of energy and time ensures that these particles do not violate any physical laws, such as the conservation of energy, if they only disappear again quickly enough. This also applies to light, which consists of oscillating electric and magnetic fields.
Alfred Leitenstorfer from the University of Konstanz
Alfred Leitenstorfer: “At the same time, the consequence of this is that both variables cannot disappear at the same time. Even in the basic state of the electromagnetic field - i.e. in absolute darkness, in absolutely free space - there is a finite fluctuation bandwidth. This ground state is afflicted with vacuum fluctuations, both in the electric and in the magnetic field. "
As a result, electromagnetic fields are always present even in absolutely empty space, but only on extremely short time scales of a quadrillionth of a second or even shorter. Vacuum fluctuations are therefore not noticeable in everyday life. But indirectly they leave their mark - for example in certain lamps.
“A fluorescent tube works with the help of a gas made up of atoms through which a stream of electrons is sent. The electrons collide with the atoms in this gas, adding energy to them. That is why they bring the atoms out of their basic state into a quantum-mechanically excited state. "
At first, quantum physics puzzled scientists here: Because, according to you, the atoms should remain in their excited state - light, on the other hand, is only emitted when the atoms return to their basic state from this excited state and emit a photon. Only a little later did the researchers realize that the interactions of the atom with the vacuum fluctuations actually ensure that the excited state is unstable - and that the atom therefore returns to its ground state after a short time. Result: The fluorescent tube lights up. In addition, vacuum fluctuations played a role much earlier - shortly after the universe was formed 13.8 billion years ago.
“We see that the universe is structured on very large scales, in galaxies for example. Cosmic inflation theory traces this structure back to the vacuum fluctuations that existed during the Big Bang. According to this, the vacuum fluctuations shortly after the Big Bang impressed a structure on the universe through their statistical distribution of the fields. "
According to the inflation theory, the universe expanded so quickly shortly after the Big Bang that the vacuum fluctuations also expanded. They are said to have caused density fluctuations in space, from which the structures of our present-day universe ultimately formed - including the gigantic empty spaces and galaxies and super clusters.
Experiment to measure vacuum fluctuations
In terrestrial laboratories, on the other hand, the vacuum fluctuations could help explain the so-called Casimir effect. A force acts - seemingly out of nowhere - on two closely spaced, electrically conductive plates. Responsible for this are the vacuum fluctuations of the light, in which so-called virtual photons, i.e. light particles, arise and immediately disappear again
“Vacuum fluctuations play a role in the Casimir effect in such a way that two bodies can be brought so close together that the wavelength of these virtual photons no longer fit between them. This then creates an effective force, because outside of these two bodies all virtual photons are still present and raining down on these circuit boards. So on the one hand there are more photons than on the other. "
The result: Overall, an external force acts on the circuit boards, which presses them together. However, the Casimir effect can also be explained purely classically in an alternative approach, i.e. without the effects of quantum physics and vacuum fluctuations. What researchers have lacked so far has been the direct measurement of vacuum fluctuations. It was even assumed that such an observation was not even possible.
“This possibly applies to all detection methods now in existence for particles that have a rest mass. But in our case we are doing optics, we want to measure photons. And this can be sampled directly if I can take a measurement on a time scale that is shorter than the time scale on which these virtual photons arise and decay again. "
In order to measure the vacuum fluctuations directly, Alfred Leitenstorfer's team designed a sophisticated experiment. The core of this experiment is a special laser that emits ultrashort light pulses: a single light pulse lasts only six femtoseconds, i.e. six quadrillionths of a second. These light pulses are thus shorter than the period of oscillation of the fields in the observed infrared range, which arise and disappear again due to the vacuum fluctuations - and can therefore observe them. In addition, the researchers used a so-called electro-optical crystal, the special properties of which are influenced by the vacuum fluctuations:
Illustration of the measurement of vacuum fluctuations
“Electro-optical materials have the property that they change their refractive index proportionally to the applied electric field. The electromagnetic field penetrates this crystal in a way that it is minimally modified. "
Due to the changed refractive index due to the vacuum fluctuations, the crystal changes the polarization of light, i.e. its direction of oscillation. This also applies to the laser pulse. In his case, the direction of oscillation is initially exactly fixed before the pulse crosses the crystal.
“Before the laser pulse hits the crystal, it is exactly linearly polarized. The geometry of the crystal is chosen so that when the laser pulse runs through this crystal and no external field is applied to the crystal, this linear polarization is exactly maintained. But if there is a finite electric field amplitude, it is elliptically polarized in exactly the same way after passing through the crystal. "
Leitenstorfer and his colleagues found out: After crossing the crystal, the laser pulses were actually somewhat elliptically polarized. The electric field of the vacuum fluctuations had influenced the optical properties of the crystal in such a way that the direction of oscillation of the light changed statistically exactly as the scientists had expected for the interaction with the fluctuating vacuum field. This is the first time they have directly measured the vacuum fluctuations of the electromagnetic field.
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