Quantum physics: Controlling the speed of particles that appear to move faster than light
In the quantum mechanical tunneling effect, particles appear to violate the most important principle in the theory of relativity. Two physicists believe that the measurements have not been done correctly yet. They now propose a new technique to better determine the speed of quantum objects.
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The tunneling effect allows quantum particles to break through the energy barrier. But do they also break the fabric of space and time?
According to Einstein’s theory of relativity, nothing can travel faster than light in empty space. But in a certain quantum effect, particles that “break through” a barrier seem to violate this principle. Many sophisticated laboratory experiments have indicated this for years. Now Patrick Schach and Enno Giese from Technical University Darmstadt Propose an alternative experimental approach. Their measurement method aims to show whether quantum objects really take a forbidden shortcut.
According to Schach and Geis, the time it takes for a particle to undergo the tunneling effect has not yet been adequately measured. In the tunnel effect, atoms, electrons and other objects break through obstacles even though their energy is insufficient according to the classical view – as if the ball had simply rolled up a hill without being given sufficient momentum. It seems as if quantum mechanics is showing the way through a secret tunnel, hence the name of the effect.
For decades, various research groups have been studying in more detail how quickly this tunneling occurs. In the course of several difficult experiments, it became clear that particles spend less time in the obstacle than they need to travel the same distance in free space. In the extreme case, when it comes to light particles, this could paradoxically mean: photons rush through the wall at a speed faster than their own speed of light.
Quantum mechanics is rich in absurd processes. For example, the entanglement effect allows for communication that also appears to violate the speed limit set by the theory of relativity, much to the annoyance of Albert Einstein. Today it is clear: we have to accept some anomalies that, upon closer examination, are consistent with the theory of relativity. Accordingly, there are already ideas on how to solve the time tunnel paradox. So Particles can travel faster than light, but they no longer transmit any useful information. This means that violations of their rules will have no effect in practice.
What does time actually mean in quantum mechanics?
The fundamental problem in measuring the tunnel effect is the following question: How should one determine the amount of time that has passed for the particle? Unlike mass, for example, time is not something that an object possesses in itself. Time is a property that only has meaning in comparison to other things – it describes change. So what changes can be recorded during tunneling?
Pioneering concepts for measuring tunnel time approach the search for an answer differently. The highly successful approach, pioneered by Ephraim Steinberg of the University of Toronto over three decades, is based on the so-called Larmour Initiative. The atom changes its quantum mechanical sense of spin as it passes through a barrier in which a magnetic field acts. The deflection force indicates how long the atom remained in the obstacle. Ursula Keller of the Swiss Federal Institute of Technology in Zurich developed an alternative measurement principle using the “Attoclock”. There, electrons pass through a rotating field. Behind the obstacle they fly in a direction that depends on the current direction of the field. This is also how travel time should be determined.
These methods combine different physical principles and therefore depend on several practical influencing factors and theoretical assumptions. This makes results difficult to evaluate and interpret, even after decades of troubleshooting and improving. That is why Schach and Giese, using the measurement method they proposed, tried not to make the time specification dependent on external fields and other properties in the experimental setup. Instead, they wanted to implant a clock into the tunneling particles themselves.
How to build a clock in atoms
To do this, the two physicists used the principle on which modern atomic clocks work. It was developed by later Nobel laureate Norman Ramsey, and is based on comparing the vibrations of two molecules. In order to measure the tunneling time, the components of the two atomic clouds can first be excited to oscillate using a laser pulse. While one of the atomic beams tunnels through the barrier, the other flies the same distance through empty space and thus serves as a reference. If you then fit both columns, you can determine very accurately how much they will shift from each other. We then know how the time experienced by the atoms varies.
According to Schach and Giez’s calculations, the tunneling particle should be delayed slightly, just as one would intuitively expect—which is inconsistent with previous results. However, they have not yet implemented their concept practically in the laboratory. Such a practical test could help clarify whether the tunnel effect actually allows faster-than-light movements or whether this explanation is due to influencing factors unknown in previous complex experiments. Accurate measurements and increasingly experimental setups have led to tremendous progress in atomic clocks in recent years. They can now rescue the theory of relativity on the quantum scale, at least with regard to the tunneling effect. Then there are still enough strange things in quantum physics.
Scientific Progress 10.1126/sciadv.adl6078, 2024
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