Gravity waves – caused by collision of two neutron stars
WHATS IN THIS BLOG:
• What are neutron stars?
• Scientists record a collision between two neutron stars.
• The collision gives a flash of light, so we know where it originates.
• The collision creates gold and silver.
I have excerpted some words below from an article in Time Magazine: “What we learned from a never-before-seen cosmic collision,” 30 October 2017.
MY PREVIOUS BLOG WAS ABOUT THE DISCOVERY OF GRAVITY WAVES, WHICH CAME FROM A COLLISION OF TWO BLACK HOLES. To read that article, click here. Then hit back-arrow to return to this blog article.
Now the scientists have gone one better. They’ve detected gravitational waves from a collision between two neutron stars. This event was detected on 17 August 2017.
First: to get gravity waves, you have to collide two very dense objects. Meaning two black holes or two neutron stars. These are the densest objects in the universe. When two such objects collide, it’s like dropping a stone in a pond which causes ripples (gravity waves) to spread across the pond (the universe).
But wait….. what exactly are neutron stars, and please remind me what black holes are?
BLACK HOLES: In a black hole, matter is so dense and the gravity force so strong that nothing can escape. Not even light, which is why you cannot see it, and why it’s called a black hole.
A black hole may be 30 times the mass of the sun, but only 20 miles in diameter, which is mind-boggling.
When two back holes collide, the only thing that can escape is gravity waves, and so the only way of knowing what happened is to detect the gravity wavs.
NEUTRON STARS: A neutron star is an ordinary star, such as our sun, but squeezed down to just 12 miles across. This happens after a star explodes late in its life, called a supernova. Half of the star explodes outward, while the other half implodes inward, and forms a neutron star. A single teaspoon of a neutron star weighs a billion tons, more than the weight of Mt Everest.
WHEN TWO NEUTRON STARS COLLIDE they produce gravitational waves, but also deadly gamma rays (one form of radiation used in medicine), X-rays, ultraviolet radiation, and visible light. This means on earth we might be able to see the visible light, and locate its source in the sky.
The astronomers did see the flash of light, and measured the gamma-ray burst (lasting just a few seconds), which came from a distant galaxy. Actually, data from 70 observatories around the world was collected and analyzed.
The collision happened 130 million years ago (it took that long for the light to travel to the earth).
But how do you measure gravity waves? This is an eye-popping achievement in itself, which is described in my previous blog and summarized at the end of this blog.
GAMMA RAY-BURSTS were discovered a few years before 1972 when I worked at Los Alamos National Laboratories. My office was next to another guy named Ian, and he was trying to understand these enigmatic gamma-ray bursts from unknown sources.
These bursts were first recorded by instruments on Vela satellites placed in orbit to find out if the Russians were secretly exploding atomic bombs in the atmosphere of earth. Many such gamma-ray bursts have been recorded since then, and they are the highest-energy explosions known in the universe. But the actual mechanism that produces the short gamma-ray bursts, was unknown. Until now.
Gamma-ray bursts last less than two seconds, and sometimes only a tenth of a second. The discovery described in this blog, 45 years after 1972, verifies that the source of some gamma-ray bursts is a collision between two neutron stars. Fantastic!
PS: I worked on data from the Vela satellites in 1972. But not gamma-ray bursts. I studied bursts of energetic protons and electrons spewed out by a solar flare. It was my first job in the USA.
GOLD IS CREATED FROM NEUTRON STAR COLLISIONS. Lighter elements are created in the interior of stars, but the source of the heavier elements was a mystery. The explosion reported here produced huge amounts of heavy metals such as silver and gold and platinum and then scattered them through space after the initial fireball.
THE UNIVERSE IS EXPANDING REALLY FAST. We already knew the universe was expanding. The gravity waves from a known source made it possible, for the first time, to measure how fast it is expanding. The result is 43 miles per second per megaparsec. In layman’s terms this is “really fast.”
Sadly, I don’t know what the implications are for how the universe will end: will it expand forever, or will it eventually stop and reverse and collapse into itself?
GRAVITY WAVES: how are they measured? It’s very difficult. Scientists built two tunnels in an L-shape. Each arm was 2.5 miles long. If a gravity wave passes through this detector, the force of gravity would be stronger in one tunnel and compress its length. And weaker in the second tunnel and expand its length. But the change in length would only be the diameter of a proton or less, which is unbelievably small.
Despite this difficulty, they were able to use top-class lasers and mirrors to measure the length of each tunnel to incredible accuracy. Initially, the measurements were so sensitive the detector saw movement due to cars entering the site, and due to earthquakes. The scientists eventually did a costly upgrade in 2014 which removed these local sources.
A massive collision on 17 August 2017 between two neutron stars produced detectable gravity waves, gamma-ray burst, X-rays, and light.
We know where the source is and how far away.
This is an incredible discovery — jointly by thousands of scientists, some in different countries.
It’s a triumph of human measurement, and a quantum leap in understanding some elusive aspects of our magnificent universe.
It’s a remarkable confirmation of Einstein’s theory of general relativity, which came out 100 years ago in 2016. If you haven’t seen the TV series on Einstein, it was captivating for me (in both the science and his personal life story).
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