“The process of scientific discovery is, in effect, a continual flight from wonder.”
~ Albert Einstein
“We are not at the end but at the beginning of a new physics. But whatever we find, there will always be new horizons continually awaiting us.”
~ Michio Kaku, Astrophysicist
Science or Myth?
Cosmic Ray Answers Your Questions About the Universe
[Editor’s note: The following is the latest of Dr. Raymond Frey’s special column for Combustus in which the American astrophysicist takes on questions sent in by readers on everything from, “Is time travel really possible?” to “What is dark energy, anyway?”]
Discovery is amazing. Observing or understanding something about Nature that is new to humankind is personally exhilarating. Why? It could be ego. But I suspect it is also in our DNA ~ evolution has rewarded explorers and exploration of various kinds. I have chosen a particular kind of exploration. Some appreciate the complexity of Nature. I do too. But I have always felt drawn by the notion that there are fundamentals ~ fundamental ideas, concepts, particles, forces. This is reductionism. But there is something to it. Well, actually a lot. As one observes closer and closer to the Universe’s beginnings, matter becomes closer and closer to being composed of a few elementary particles (quarks, electrons, neutrinos, etc.). The forces become simpler in their application. The mathematics needed to describe what is happening becomes simpler and at the same time more elegant. I was drawn to study elementary particles and astrophysics by a desire to be on the ship of discovery. We have learned so much, and we see there is so much more. Across the sea; over the horizon. Science is not everything, not by a long shot. But it is a great, long, strange trip…
The last time we met here I shared with you that you can now receive text messages on your phone about new gravitational wave (GW) detections. Well last week, on August 14, we learned of a detection that, upon investigation, seems really unusual.
It seems we’ve detected a possible collision between a black hole and a much smaller object – either a black hole or a neutron star.
I was in the process of writing up a “blast” for Combustus on dark matter and dark energy, but since I am now getting items on my news feed like this:
I thought I would take this opportunity to provide a short “blast” to help explain how we know about colliding bodies based upon our measurements of gravitational waves.
How to Make a Strong Gravitational Wave Signal
It may seem hard to fathom, but any accelerating object produces gravitational waves (GWs). Even what you are doing right now ~ tapping your phone to pull up this article ~ causes gravitational waves, albeit a very, very weak GW signal. Albert Einstein’s theory of General Relativity tells us that we get the strongest gravitational wave signals from massive objects moving fast in a strong gravitational field.
Consider now a neutron star. These result from massive stars which have collapsed, capturing approximately the mass of our sun in an object only 20 kilometers (12 miles) in diameter.
The gravitational field of a neutron star is approximately 100 billion times stronger than what exists at the surface of our earth.
But now look what happens when another neutron star orbits the first one:
This second orbiting neutron star radiates away energy in the form of gravitational waves (GWs), thus making its orbit shrink, until eventually the two stars merge and collide, resulting in a huge cosmic fireball, or “kilonova.”
Before merging, these neutron stars were moving at half the speed of light. With this much mass moving so fast in an enormous gravitational field, we get such an immense gravitational wave production that we can “see” it clearly with our fancy GW detectors, LIGO. The huge energy release in these collisions more than makes up for the fact that they may be 100 million light years away.
(In my next Combustus blast, “Where Does Gold Come From?” I will share with you some cool things scientists have learned from the neutron-star-to-neutron-star collision we picked up data from in 2017.)
Mergers Described in Time-Frequency Pixels
The image below shows our LIGO raw data from August 17, 2017 capturing the merger of two neutron stars. The track seen starting at the lower left and ending near the upper right is the characteristic signature of gravitational wave (GW) emission from merging neutron stars.
The horizontal direction is time: We first start seeing a discernible GW signal about 32 seconds before the merger. The vertical direction is frequency. For our neutron stars, frequency is a measure of how quickly they are orbiting each other. 100 hertz (Hz) means that they are completing 50 orbits in one second. (Imagine that!) Their orbits are near-perfect circles, with the center of the orbits being the halfway point between them.
The warmer colors in this image depict an increase in energy detected.
Tracing the Neutron Stars’ Plunge
At 32 seconds before the merger, the neutron star track is becoming visible in the LIGO data, completing 18 orbits per second. At this point, the neutron stars are about 300 km (186 miles) apart.
The neutron stars are already moving at about 10% the speed of light, and their immense mutual gravitational tug is making them radiate away gravitational wave energy so intense that our LIGO detectors can pick it up over a distance of 140 million light years away!
As the stars orbit, they lose energy into gravitaitonal waves (GWs), which pushes their orbits closer and smaller. The orbiting excellerates. The speed becomes so fast that at about 500 Hz, we lose sight of all this action, since our detectors are not as sensitive at these higher frequencies.
Eventually, at about one thousand orbits per second and moving at 44% of light speed, the neutron stars collide, merge, and form a black hole.
Because the neutron stars are small, it takes only 32 seconds for them to complete their death spiral.
This trajectory – frequency rising with time – is often referred to as a “chirp,” since many birds also sing phrases where pitch, or frequency of the sound wave, increases with time.
But Now A Very Different Signature of a Merger
Now consider the 2nd image below. The main features you probably notice immediately are that the gravitational wave (GW) signal track is much shorter: the visible track is less than 0.10 seconds long.
Also, the energy (the warmer pixel colors) peaks at about 0.42 seconds, then dies away. The chirp (frequency rising with time) is evident, so this also represents a merging system which is radiating away huge amounts of energy in the form of gravitational waves.
The fact that the track ends quickly tells us that the two objects are touching much sooner. That is, they are larger than neutron stars.
And when they are close, they radiate more strongly, which gives us the dense region of warm colors.
This is, in fact, a detection we made in 2015 of the merger of two black holes. (It was our first such detection. Now, in 2019, we are getting about one new capture every week.)
Black holes have an event horizon which encircles the star. Light or anything else within the event horizon cannot escape. When the two black holes merge and collige into a single, larger, black hole, we get these warm colors of intense energy radiating.
Measuring the Masses
So here’s where the juicy math comes in: Since we know that the size of a black hole increases in proportion to its mass, and that the more massive a black hole is, the more gravitational wave energy is generated, we were able to make some exciting calculations. Applying mathematical modeling based on General Relativity principles to the measurements we gathered of the strength of the gravitational waves (GWs), we were able to determine that the two black holes each had a mass of roughly 30 times the mass of our sun!
By the way, you might wonder about the relative size of black holes and neutron stars. As I mentioned earlier, a neutron star with the mass of one sun has a diameter of about 20 km (12 miles), while a black hole of the same mass has a diameter (using the event horizon) of only about 12 km (7 miles). Thus, the black holes in our 2015 event were separated by about 360 km (224 miles) when they “touched” each other. At this point of separation, the orbital period is about 64 seconds.
The August 14 Event
So now we can finally get back to what was detected last week. Indeed, what?
We know we can measure the mass of the objects in these binary mergers roughly by the length of time of the signal track in images like those above. And by analyzing the exact shape of the tracks, we can also determine the individual masses and how fast the bodies are spinning.
Now, I’m not allowed to show you the August 14 image yet. (We do need to keep some data for a while to write our papers!) But one of the two objects is certainly a black hole, and the other is much less massive, on the border where it might be either a small black hole or a neutron star.
(While black holes can theoretically have any mass, neutron stars are limited to a narrow range of about one to three times the mass of the sun.)
When a smaller object plunges below the event horizon of a black hole, we often say it is “swallowed” by the black hole, since it effectively disappears.
If the smaller object observed by our LIGO last week is indeed a neutron star, this would be the first detection of a black hole – neutron star system.
In this case, the neutron star can be ripped apart by the gravity of the black hole before it is swallowed, sending energetic matter flying out which can glow and be seen by astronomers with telescopes.
Groups from around the world are currently looking for this. We told them where to look in the sky, based on our best reckoning.
(Quick aside: This stretching a body towards and away from the center of the mass of another body due to a difference in strength of a gravitational field is sometimes referred to as the gravitational “tidal force.” In a far less extreme version of this, the moon’s gravity distorts the earth and its oceans, producing our tides.)
Now if the black hole is too massive, it might swallow the neutron star before it is ripped apart. The ripping apart would happen within the black hole horizon, invisible to us, but interesting to imagine.
If the black hole is the smaller object, then likewise it would not produce a glowing object, since all of its matter is safely inside the event horizon.
We should know more in the next days whether August 14 marked the first detection of a black hole – neutron star system, or a strangely asymmetric black hole – black hole system. But now you might know a bit more of the back story.
Also enjoy my Combustus interview with Dr. Frey, “Sifting Through Stardust: Conversations with Astrophysicist Raymond Frey.”
And Dr. Frey’s column remembering the late Stephen Hawking.
Also, if you missed it, check out Dr. Frey’s column on How Citizen Science is Increasing Our Understanding Of the Universe.
Readers who would like their questions considered for a future Astroblast column are invited to inbox us their questions on the Combustus Facebook fan page.