Einstein’s theory of relativity, crucial to GPS, found in distant stars

What do Albert Einstein, the Global Positioning System (GPS), and a pair of stars 200,000 trillion miles from Earth have in common?

The answer to this is the influence of Einstein’s general theory of relativity, called “gravitational redshift”, where light is turned red due to gravity. Using NASA’s Lunar X-ray Observatory, astronomers have discovered the phenomenon of two stars orbiting each other in galaxies about 29,000 light years (200,000 trillion miles) from Earth. While these stars are very distant, gravitational read shifts have a tangible effect on modern life, as scientists and engineers must consider them in order to enable accurate conditions for GPS.

While scientists have found uncontrollable evidence of gravitational redshifts in our solar system, it is challenging to observe them in more distant objects in space. The new lunar results provide evidence for gravitational redshift effects on playing in a new cosmic setting.

An interesting system called 4U 1916-053 has two stars in remarkably close orbit. One is the main part of the star that has stripped its outer layers, leaving a star that is thicker than the sun. Another neutron is the star, which is also made of a weaker object when a large star breaks up in a supernova explosion. The neutron star (gray) is shown in the center of a hot gas disk pulled away from its mate (white star on the left) in its artist’s impression.

Both of these compact stars are about 215,000 miles apart, roughly the distance between Earth and the Moon. When the moon orbits our planet once a month, in 4U 1916-053 the Ga ense companion star whips around a neutron star and completes a full orbit in just 50 minutes.

4 In a new work from 1916-053, the team analyzed X-ray spectra – the amount of X-rays at different wavelengths from the moon. They found the typical signature of X-ray light absorption by iron and silicon in spectra. In three different observations with the moon, the data show a sharp decrease in the detected amount of X-rays near wavelengths where iron or silicon atoms absorb X-rays. The main graphic includes one of the iron-left and right-side dips showing absorption. Additional graphic shows the spectrum with absorption by silicon. The data in both spectra are shown in gray and the computer model in red.

However, the wavelengths of these characteristic signatures of iron and silicon were shifted to a longer or red wavelength compared to the laboratory values ​​found here on Earth (blue, ical is also shown along the line for each absorption signature). The researchers found that the shift of the absorption features was the same in each of the three lunar observations, and that it was large enough to be explained by motion far from us. Instead, they concluded that it was due to a gravitational reed shift.

How does this relate to general relativity and GPS? As predicted by Einstein’s theory, clocks under the force of gravity run at a slower rate than clocks seen from distant regions experiencing weak gravity. This means that clocks on Earth run at a slower rate than orbiting satellites. GPS requires precise accuracy, this effect needs to be taken into account or there will be a small difference in time that will add up quickly, calculating inaccurate positions.

All types of light, including X-rays, are also affected by gravity. An analogy is of a person driving an escalator coming down. As they do this, the person escalates the static or loses more space than going up. The force of gravity has the same effect on light, where the energy gives a lower frequency of reduction. Because light in a vacuum always travels at the same speed, the loss of energy and low frequency means that light shifts at long wavelengths, including signatures of iron and silicon.

This is the first strong evidence for signatures transferred over a longer wavelength by gravity in a pair of stars with a neutron star or black hole. Strong evidence for the gravitational redshift in absorption has previously been seen from the surface of white dwarfs, the wavelength shift is usually only 15% of that for 4U 1916-053.

Scientists say that it is possible that the gaseous atmosphere absorbs the X-rays, producing these results by cutting a disk near a neutron star (shown in blue). (This atmosphere is incompatible with the rise of red gas in the outer part of the disk that blocks light from the inner part of the disk once per orbit.) Will the team be able to calculate how far this atmosphere is from the size of the shift in the spectra? The neutron is far from the star, using normal relativity and assuming the standard set for the neutron star. They found that the atmosphere is located 1,500 miles from a neutron star, about half the distance from Los Angeles to New York, and only 0.7% of the distance from a neutron star. It probably is. Neutrons extend several hundred miles from a star.

Two of the three spectra also have evidence for absorption signatures that correspond to only 0.04% of the distance from the neutron star to the companion, even in the radar wavelength. However, these signatures are detected with less confidence than neutron stars.

Scientists have been given more awards for lunar observation time in the coming year to study this system in more detail.

A paper describing these results was published in the August 10, 2020 issue Astrophysical Journal.