Here we are, people–it’s finally time to talk about dark matter!
I’ve teased this subject in my last few posts. We’ve discussed how the idea originated–with observations of the rotation of galaxies. We’ve touched on the question of whether dark matter could explain supermassive black holes.
What we have yet to do is take a proper deep dive into what the heck this stuff actually is. But there’s an even more important question…
How do we know it’s actually real?
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Any of you recognize this?
To those who don’t, it probably looks like a pretty unimpressive, blurry ring. In fact, this is the first ever image of a black hole, taken with an interferometer the size of the Earth.
If you’re a science geek, you’ve no doubt seen tons of artists’ conceptions of black holes on the internet. Most use a great deal of artistic license. Some of my favorite “images” of black holes used to be the ones that look like ripples in the fabric of space. Imagine my disappointment when I realized that’s not the case at all.
Black holes are singularities—infinitely dense places of zero radius with at least 3 M☉ (solar masses) of star stuff—surrounded by an event horizon, inside of which gravity is so strong that even light cannot escape. That’s why it’s called a black hole.
But they are not “holes” in the usual sense. They are not giant space potholes that you can easily stumble into, and you certainly don’t fall into them the same way you would a pothole.
So…what are black holes really like?
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Q: What is gravity? (asked by Simon)
Wow…great question. This is a question the greatest scientific minds have asked and tried to answer for centuries. It’s a question not even Stephen Hawking, the scientific genius of the century, has fully answered.
There are a few parts to the gravity question, and they have each been addressed one by one over time:
Isaac Newton stood on the shoulders of the giants before him—Aristotle, Ptolemy, Copernicus, and Kepler—and figured out how gravity works. But he was at a loss to explain what exactly this mysterious force was.
Einstein built on Newton’s work and came up with a theory for what gravity is—that is, distortions in space-time.
We have yet to understand why gravity works. Why is space-time warped? Why do objects distort it as if it were the material of a trampoline? What exactly is the nature of space?
But, lucky for me, the question above specifically asks what gravity is. And that, I can explain.
The best way to do that is to turn one of gravity’s oldest tricks, one that has perplexed scientists and philosophers for thousands of years: What makes the planets move?
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When you hear about “space-time,” it’s just a way to say that space is related to time. And the curvature of space-time, as Albert Einstein predicted, is the way space and time alike literally bend around a mass such as the Earth or the sun.
That’s what’s diagramed above. This is a three-dimensional concept diagram of the way space sort of “clings” to an object. Notice the way it sort of tightens up when you get close to Earth? And because time is part of this whole equation…time sort of tightens up, too.
I assume that explains the “twin paradox,” as it’s called. That’s where the space-traveling twin returns home to Earth younger than their Earth bound twin.
Why? Seems to me it’s because time was tighter and passed faster on Earth, while it spread out and passed a bit slower for the traveler. (Don’t quote me on that, I just guessed that from this diagram.)
Einstein figured all this out. But scientists need evidence. Trusting Einstein’s genius wasn’t enough for them. How did they accept relativity as fact?
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Albert Einstein may have been the genius among physicists, but like all others before his time, he stood on the shoulders of giants.
Einstein did not propose that the sun was the center of the solar system; that idea was already widely accepted when he came around. He didn’t discover elliptical orbits; that distinction belongs with Johannes Kepler.
But Kepler never could figure out why planets orbit the sun in ellipses instead of circles. Even Isaac Newton, who at last identified gravity as the reason we stick to Earth’s surface, couldn’t explain what gravity was—only how it worked.
Einstein provided that explanation with his general theory of relativity.
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Cosmos at Your Doorstep 