Dr Becky Smethurst08 October 2022

The Secret World Of Black Holes - Dr Becky Smethurst - #536

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10Insights

Insights & moments

The myth-busts, hot takes, explainers, and tools worth keeping.

Myth Buster· 2

Myth Buster00:30

Why Black Holes Aren't Actually Holes

Dr. Becky Smethurst explains that black holes are not literal holes in space, but extremely dense 3D objects formed when massive stars collapse. The name 'black hole' causes widespread misconceptions, such as the idea that there's something 'on the other side' of them.

Black holes are not holes — they are spherical, ultra-dense remnants of collapsed stars.
The term 'black hole' leads people to imagine a tunnel or portal, which is incorrect.
They are better understood as prisons for light due to their intense gravity.
A more accurate name might have been 'dark star,' though even that isn't perfect.

“Black holes aren't holes — they are 3D objects that were once stars that have just been crushed down until they are so dense that…”
Dr. Becky Smethurst · 00:30

#black holes#astrophysics#misconceptions

Myth Buster04:30

Black Holes Are Some of the Brightest Objects in the Universe

Despite their name, black holes are not always dark. As gas spirals into them, it heats up and emits intense radiation, making them visible in X-rays, UV, and even visible light. Supermassive black holes can outshine entire galaxies.

Accreting gas around black holes heats up and glows in X-rays and visible light.
Stellar-mass black holes light up the sky when viewed with X-ray telescopes.
Supermassive black holes power quasars — some of the brightest objects in the universe.
The 'black' in black hole refers to the event horizon, not the surrounding luminous material.

“Black holes are some of the brightest objects in the entire universe. They lie up like Christmas trees.”
Dr. Becky Smethurst · 04:30

#black holes#astrophysics#quasars

Explainer· 3

Explainer27:00

What Happens When Two Black Holes Merge?

When two black holes merge, they create ripples in spacetime called gravitational waves. These waves are detectable from Earth for stellar-mass black holes, but supermassive black hole mergers require future space-based observatories like LISA to observe.

Merging black holes spiral together at incredible speeds, warping spacetime.
They emit gravitational waves — ripples that travel at the speed of light.
LIGO detects waves from stellar-mass black hole mergers, but not from supermassive ones.
The LISA mission, planned for the 2030s, aims to detect supermassive black hole mergers.

“When two black holes are coming together and merging, they're spiraling around each other at incredible speeds and curving and uncurving space as they go…”
Dr. Becky Smethurst · 27:30

#gravitational waves#black hole merger#LISA

Explainer30:00

How Black Holes Evaporate via Hawking Radiation

Stephen Hawking theorized that black holes slowly lose mass through quantum effects near the event horizon, emitting radiation over immense timescales. This process, called Hawking radiation, remains unobserved but is consistent with theoretical physics.

Hawking radiation arises from quantum fluctuations near the event horizon.
It allows black holes to slowly lose mass and eventually evaporate.
For supermassive black holes, this would take up to 10^100 years.
No direct observation of Hawking radiation has been made yet.

“Hawking radiation is still a hypothetical concept. We should be able to observe it... but we've never spotted anything that looks like Hawking radiation before.”
Dr. Becky Smethurst · 34:00

#Hawking radiation#quantum physics#black hole evaporation

Explainer36:30

What Is the Schwarzschild Radius?

The Schwarzschild radius defines the event horizon of a black hole — the point beyond which nothing can escape. It depends only on the object's mass and is derived from Einstein's theory of general relativity.

The Schwarzschild radius is the size of the event horizon.
It is calculated using the mass of the object and the speed of light.
Karl Schwarzschild derived it while serving on the Eastern Front in WWI.
You can calculate your own Schwarzschild radius — it's tiny.

“The Schwarzschild radius is the event horizon essentially — that sphere around the black hole where you don't get any information from.”
Dr. Becky Smethurst · 36:30

#event horizon#general relativity#Schwarzschild radius

Story· 1

Story02:00

How 'Black Hole' Got Its Name

The term 'black hole' traces back to the 'Black Hole of Calcutta,' a notorious prison where British soldiers were held. Physicist Robert Dicke used the term metaphorically to describe gravitationally collapsed objects, comparing the crushing of matter to the suffering in the prison.

The 'Black Hole of Calcutta' was a small, overcrowded prison cell in 18th-century India.
Physicist Robert Dicke likened collapsing stars to the crushing conditions in the prison.
The term caught on for its brevity and dramatic flair, despite being misleading.
Contrary to popular belief, the prison was not named after the astronomical object — it's the other way around.

“There was this tale from history where British soldiers were imprisoned in this prison cell which was the size of three double beds and there…”
Dr. Becky Smethurst · 02:00

“He started to use this phrase... you know, something was lost — 'it's gone to the black hole of Calcutta.'”
Dr. Becky Smethurst · 02:30

#history of science#black holes#naming

Q&A· 2

Q&A07:30

Do Black Holes Spin?

Yes, black holes spin — a legacy of the rotation of the stars or neutron stars that formed them. As more matter falls in, it adds angular momentum, causing the black hole to spin faster.

Black holes inherit spin from their progenitor stars or neutron stars.
Accreting matter adds angular momentum, increasing the spin.
Spinning black holes are common in the universe.
The Event Horizon Telescope image of Sagittarius A* appeared blurred partly due to its rotation.

“Yes, black holes do spin because they start life as either stars or neutron stars... you really can't shake off that spin that stars have.”
Dr. Becky Smethurst · 07:30

#black holes#rotation#angular momentum

Q&A40:00

Does Gravity Travel at the Speed of Light?

Yes, gravity travels at the speed of light. This was confirmed when LIGO detected gravitational waves and light from the same neutron star merger arriving simultaneously after traveling billions of years.

Gravity propagates at the speed of light, as predicted by general relativity.
The 2017 neutron star merger observation confirmed this.
Gravitational waves and light arrived on Earth at the same time.
This rules out instantaneous or faster-than-light gravitational effects.

“Gravity travels at the speed of light as well. That's something Einstein's theory general relativity explained as well.”
Dr. Becky Smethurst · 40:30

#gravity#speed of light#gravitational waves

Tool· 1

Tool48:30

The Event Horizon Telescope's Black Hole Images

The Event Horizon Telescope captured the first-ever direct images of black holes, including M87* and Sagittarius A*. These images show the shadow of the event horizon surrounded by glowing accretion material.

First image: M87* in 2019, a supermassive black hole in another galaxy.
Second image: Sagittarius A* in 2022, our Milky Way's central black hole.
Both appear as orange 'donuts' due to false-color imaging of hot plasma.
These images confirmed theoretical models of event horizons.

“When I saw that image for the first time, I remember getting just goosebumps because I never thought that that could ever be the case…”
Dr. Becky Smethurst · 49:30

#Event Horizon Telescope#black hole image#astronomy

Takeaway· 1

Takeaway15:00

The Missing Link: Intermediate-Mass Black Holes

Astronomers have observed many stellar-mass and supermassive black holes, but few in between. These 'intermediate-mass' black holes are considered the missing piece in understanding how supermassive black holes grow.

Stellar-mass black holes are up to ~100 solar masses; supermassive ones are millions to billions.
Intermediate-mass black holes (100–1 million solar masses) are rarely observed.
They may be the 'missing link' in black hole evolution.
Finding them could explain how supermassive black holes formed so early in the universe.

#black holes#intermediate-mass#astrophysics