'One of physics' greatest mysteries': We've studied distant suns in the most precise astronomical test of electromagnetism yet

‘One of physics’ greatest mysteries’: We’ve studied distant suns in the most precise astronomical test of electromagnetism yet

There is an inconvenient and annoying problem with our understanding of the laws of nature that physicists have been trying to explain for decades. It’s about electromagnetism, the law of the interaction of atoms and light, which explains everything from why you don’t fall through the ground to why the sky is blue.

Our theory of electromagnetism is arguably the best physical theory humans have ever made – but it has no answer for why electromagnetism is as strong as it is. Only experiments can tell you the strength of electromagnetism, which is measured by a number called α (aka alpha, or the fine structure constant).

American physicist Richard Feynman, who helped come up with the theory, called it “one of the greatest mysteries in physics” and urged physicists to “stick that number up on your wall and get rid of it.” worry about it”.

In research just published in Science, we decided to test whether α is the same at different places in our galaxy by studying stars that are nearly identical twins of our Sun. If α is different in different places, it could help us find the ultimate theory, not just of electromagnetism, but of all the laws of nature together – the “theory of everything”.

We want to break our favorite theory

Physicists really want one thing: a situation where our current understanding of physics breaks down. New physics. A signal that cannot be explained by current theories. A signpost for the theory of everything.

To find it, they could wait deep underground in a gold mine for dark matter particles to collide with a special crystal. Or they could carefully maintain the best atomic clocks in the world for years to see if they tell a slightly different time. Or smash protons at (near) the speed of light in the Large Hadron Collider’s 27 km ring.

The problem is, it’s hard to know where to look. Our current theories cannot guide us.

Of course, we look in laboratories on Earth, where it is easier to search thoroughly and precisely. But it’s a bit like the drunk was only looking for his lost keys under a lamp post when in fact he could have lost them on the other side of the road, somewhere in a dark corner.

The Sun’s Rainbow: Sunlight is spread here in separate rows, each covering only a small range of colors, to reveal the many dark absorption lines of atoms in the Sun’s atmosphere.
NA Sharp / KPNO / NOIRLab / NSO / NSF / AURA, CC BY

The stars are terrible, but sometimes terribly similar

We decided to look beyond Earth, beyond our solar system, to see if stars that are nearly identical twins of our Sun produce the same rainbow of colors. Atoms in the atmospheres of stars absorb some of the light battling outward from the nuclear furnaces in their cores.

Only certain colors are absorbed, leaving dark lines in the rainbow. These absorbed colors are determined by α – so measuring the dark lines very carefully allows us to measure α as well.

A close-up image showing the bubbling atmosphere of the Sun.
Hotter and cooler gases bubbling up in the turbulent atmospheres of stars make it difficult to compare absorption lines in stars with those observed in laboratory experiments.
ONS/AURA/NSF, CC BY

The problem is that the atmospheres of stars move – boiling, spinning, looping, spinning – and that moves the lines. The offsets ruin any comparison with the same lines in laboratories on Earth, and therefore any chance of measuring α. Stars, it seems, are terrible places to test electromagnetism.

But we wondered: if you find stars that are very similar – twins to each other – maybe their dark, absorbed colors are also similar. So instead of comparing stars to laboratories on Earth, we compared twins of our Sun to each other.

A new test with solar twins

Our team of students, postdocs and senior researchers, at Swinburne University of Technology and the University of New South Wales, measured the spacing between pairs of absorption lines in our Sun and 16 “solar twins” – stars almost indistinguishable from our Sun.

The rainbows of these stars were observed on the 3.6-meter telescope of the European Southern Observatory (ESO) in Chile. Although it is not the largest telescope in the world, the light it collects is transmitted to probably the best controlled and best understood spectrograph: HARPS. This separates the light into its colors, revealing the detailed pattern of dark lines.



Read more: Explained: seeing the universe through spectroscopic eyes


HARPS spends much of its time observing Sun-like stars to search for planets. Practically, this provided a treasure trove of exactly the data we needed.

A long exposure photo showing stars circling in the night sky behind the silhouette of a domed telescope on a hill.
ESO’s 3.6-meter telescope in Chile spends much of its time observing Sun-like stars to search for planets using its extremely precise spectrograph, HARPS.
Iztok Bončina / ESO, CC BY

From these exquisite spectra, we showed that α was the same in all 17 solar twins with astonishing precision: only 50 parts per billion. It’s like comparing your height to the circumference of the Earth. It is the most precise astronomical test of α ever carried out.

Unfortunately, our new measurements didn’t break our favorite theory. But the stars we studied are all relatively close, just 160 light-years away.

And after?

We recently identified new solar twins much further away, about halfway to the center of our galaxy, the Milky Way.

In this region, there should be a much higher concentration of dark matter – an elusive substance that astronomers say lurks throughout the galaxy and beyond. Like α, we know very little about dark matter, and some theoretical physicists suggest that the inner parts of our galaxy might just be the dark corner in which we should look for connections between these two “damn mysteries of physics”. .

If we can observe these much more distant suns with the largest optical telescopes, we may find the keys to the universe.



Read more: Why do astronomers believe in dark matter?


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