From the light of a 7.5 billion light-years distant quasar galaxy of ammonia molecules, which are located in a 6 billion light-years distant galaxy, individual frequencies absorbed. Which these are depends on the ratio of the masses of proton and electron. With the help of the Effelsberg 100-meter radio telescope, researchers were able to show that 6 billion years ago, this mass ratio was the same with high accuracy as it is today. (Image: N.Junkes / A.Biggs / NASA / ESA / STScI)
Could it be that a distant day planets stop circling their suns, or that atoms fall apart and all the matter in the universe simply dissolves? Yes, it could be, say at least modern physical theories. According to these theories, it is possible that physical laws of nature will change over time. But the data is not clear. While some research groups in recent years had found evidence of changing values ​​in some fundamental natural sizes, Christian Henkel from the Max Planck Institute for Radio Astronomy in Bonn and his colleagues using the 100 -meter radio telescope Effelsberg could now prove that one of these fundamental proportions, namely the ratio of proton to electron mass, six billion years ago in our universe had exactly the same value as today. The fundamental laws of physics consist of mathematical equations, which usually contain constants of nature. These constants, among other things, depend on the strengths of the basic physical forces - such as those of the electromagnetic force and the strong nuclear force, which are both responsible for the cohesion of the atomic nuclei. If the strength ratio between these two forces changed, then the properties of the matter surrounding us, and of which we ourselves are made, would change dramatically, or atoms would cease altogether.

But why do these two forces have the strength that enables the existence of matter as we know it? Or why, for example, does the speed of light have the value 299, 792.458 kilometers per second? Nobody can answer these questions at the moment. We simply do not know it - which some physicists consider catastrophic inadequacy of current physical theories.

For this reason, physics is working on new theories, such as string theory, in which the basic building blocks of matter are tiny threads that produce the known physical elementary particles through various vibrational states. In string theory, there are up to seven additional dimensions in addition to the three space and one time dimensions. We do not perceive these extra dimensions because they are "rolled up" in a tiny space - into spheres, cylinders, or much more complex shapes that mathematicians can compute for seven dimensions, but which we can hardly imagine.

Just as in Einstein's General Theory of Relativity the curvature of four-dimensional space-time produces the gravitational force, so the geometry of these seven dimensions is responsible for the remaining basic physical forces and the values ​​of the associated natural constants. And just as our Universe is currently expanding, so could the geometry and size of the additional dimensions change - with the result that the corresponding natural constants change. display

Whether this is actually the case, however, still has to be found out. One thing is clear: when natural constants change at all, they do so only extremely slowly. After all, we would have noticed clear changes since our world would then look drastically different. That is why astronomers are the ones who are looking for altered values ​​of natural constants. For the light that reaches us, for example, from a galaxy six billion light-years away, has been traveling for six billion years, and has thus been generated by physical processes that obey the laws of nature that were valid six billion years ago. Thus, in principle, it should be possible to read from the properties of this light whether the laws of nature six billion years ago were the same as today.

That this is not only possible in principle, but actually possible, and moreover with a very high degree of accuracy, has now been proven by Henkel and his colleagues. They wanted to find out if the ratio of proton to electron mass six billion years ago was the same as it is today. According to measurements in terrestrial laboratories, a proton is exactly 1836.15 times as heavy as an electron. Indirectly, a change in this numerical ratio would indicate that the strength of the strong interaction has changed. Because this force binds the three quarks that make up the proton. The binding energy, in turn, determines the largest part of the proton mass after Einstein's equivalence of energy and mass. Since the electron mass would not be affected by a change in the strong interaction, a change in the ratio of proton to electron mass can lead to a change in the strength of the strong interaction getting closed.

Astronomers measure light spectra of ammonia molecules in a galaxy six billion light-years away. These molecules were irradiated by an even further light quasar galaxy and absorbed certain frequencies from this light. The point is that the exact position of the absorbed light frequencies depends sensitively on the ratio of proton to electron mass. If this ratio at the time when the ammonia molecules absorbed the light, that is, six billion years ago, would have been different than today, then the position of the absorbed light frequencies would have been changed.

But such a change Henkel and his colleagues have not found. Taking into account the possible accuracy of their measurements, the researchers can rule out that the ratio of proton to electron mass has increased by more than a few thousandths of a thousand in the last six billion years Has changed. Due to some special properties of the ammonia molecule, the accuracy of this result is about ten times higher than that of other scientists who found a change in the ratio of proton to electron mass.

Nevertheless, it can not be definitively ruled out that these or other natural constants do not change. For another group had found, for example, that the ratio of the two masses about 20 billionths of a billionth ago was 20 thousandths of a tenth of a thousandth of their own than today. Since this is five billion years earlier than the period studied by Henkel's group, the two results are not mutually exclusive. For the theoreticians working on string theory, however, they provide valuable clues as to what the far-fetched final version of string theory should look like.

M: Murphy et. al. : Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe, Science 320, 1611 The Physikalisch-Technische Bundesanstalt has published a magazine in the Magazinma magazine series on the subject of "natural constants" dedicated to st be . The individual articles can be downloaded for free here. Axel Tilleman

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