What If Our Understanding of Gravity Is Wrong?

November 10, 2021

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PBS Space Time - S2021 - E32

What if there is no such thing as dark matter? What if our understanding of gravity is just wrong? New work is taking another shot at that Einstein guy. Let’s see if we’ve finally scored a hit with Modified Newtonian Dynamics aka MOND.

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PBS Space Time: Season 2021 - 35 Episode s

2021x1 - What Happens During a Quantum Jump?
January 12, 2021
Since the very beginning of quantum mechanics, a debate has raged about how to interpret its bizarre predictions. And at the heart and origin of that debate is the quantum jump or quantum leap - the seemingly miraculous and instantaneous transitions of quantum systems that have always defied observation or prediction. At least, until now.
2021x2 - Can We Break the Universe?
January 19, 2021
Today we’re going to delve into a couple of the most famous paradoxes of special relativity: the Twin Paradox, The Ladder Paradox (aka the Barn-Pole Paradox), and a paradox suggested by our very own viewers, which asks whether a spaceship could wrap around the universe & destroy itself. We’ll explore these paradoxes and see why, against our intuition, the universe really does work in this seemingly nonsensical way. But the point of this episode is to go much further - we’re going to try to break the universe by pushing these paradoxes beyond the limit.
2021x3 - Is Dark Matter Made of Particles?
January 26, 2021
Dark Matter Particles: Gateway to The Dark Universe
2021x4 - How Does Gravity Warp the Flow of Time?
February 10, 2021
There’s a deep connection between gravity and time - gravitational fields seem to slow the pace of time in what we call gravitational time dilation. And today we’ll explore the origin of this effect. And ultimately, we’ll use what we learn to understand how curvature in time - this gradient of time dilation - can be thought of as the true source of the force of gravity.
2021x5 - Gravitational Wave Background Discovered?
February 17, 2021
It was pretty impressive when LIGO detected gravitational waves from colliding black holes. Well we’ve just taken that to the next level with a galaxy-spanning gravitational wave detector that may have detected a foundational element of space itself - the gravitational wave background.
2021x6 - Does Time Cause Gravity?
February 24, 2021
We know that gravity must cause clocks to run slow on the basis of logical consistency. And we know that gravity DOES cause clocks to run slow based on many brilliant experiments. But I never explained WHY or HOW gravity causes the flow of time to slow down. And I’m not going to explain it now - because in a sense it’s not true. Gravity does NOT warp the flow of time. It’s the other way around - the warping of time causes gravity.
2021x7 - How Does Gravity Affect Light?
March 9, 2021
We know that gravity exerts its pull on light, and we have an explanation for why. Actually, we have multiple explanations that all predict the same thing. And at first glance, these explanations seem to describe completely different causes. So what is the true connection between light and gravity, or is truth, in fact, entirely relative?
2021x8 - The NEW Crisis in Cosmology
March 16, 2021
I have good news and bad news. Bad news first: two years ago we reported on the Crisis in Cosmology. Since then, it’s only gotten worse. And actually, the good news is also that the crisis in cosmology has actually gotten worse, which means we may be onto something! The most exciting thing for any scientist is when something they thought they knew turns out to be wrong. So it’s no wonder that many cosmologists are starting to get excited by what has become known as the Hubble tension, or the crisis in cosmology. The “crisis” is the fact that we have two extremely careful, increasingly precise measurements of how fast the universe is expanding which should agree with each other, and yet they don’t.
2021x9 - Zeno's Paradox & The Quantum Zeno Effect
March 23, 2021
“A moving arrow is at rest.” This is obviously a nonsensical contradiction. But Zeno, a Greek philosopher famous for his metaphysical trolling, devised a paradox whose conclusion is just this. Here’s how it goes: if you look at an arrow flying through the air at any instantaneous snapshot in time, the arrow doesn’t travel any distance. If time is composed of an infinite number of these snapshots, and the arrow doesn’t move in any of them, then the arrow is at rest during the entirety of its flight. The moving arrow is at rest.
2021x10 - Why the Muon g-2 Results Are So Exciting!
April 7, 2021
When a theory makes a prediction that disagrees with an experimental test, sometimes it means we should throw the theory away. But what if that theory has otherwise produced the most successful predictions in all of physics? Then, that little glitch may be pointing the way to layers of physics deeper than we've yet imagined. Well, FermiLabs Muon G-2 experiment has been chasing the most promising glitch of all, and they've just announced their results
2021x11 - What If Dark Matter Is Just Black Holes?
April 13, 2021
It may be that for every star in the universe there are billions of microscopic black holes streaming through the solar system, the planet, even our bodies every second. Sounds horrible - but hey, at least we’d have explained dark matter.
2021x12 - The NEW Warp Drive Possibilities
April 21, 2021
That Einstein guy was a real bummer for our hopes of a star-hopping, science-fiction-y future. His whole “nothing travels faster than light” rule seems to ensure that exploration of even the local part of our galaxy will be an excruciating slow. But Einstein also gave us a glimmer of hope. He showed us that space and time can be warped - and so the warp drive was conceived. Just recently, a couple of papers contend that these are not pure science fiction.
2021x13 - How To Know If It's Aliens
May 11, 2021
There’s one rule on Space Time: It’s never Aliens. But every rule has an exception and this rule is no exception because: It’s never aliens, until it is. So is it aliens yet? And on today’s Space Time we’re going to examine all the best case scenarios for life beyond Earth.
2021x14 - Breaking The Heisenberg Uncertainty Principle
May 19, 2021
Quantum mechanics forbids us from measuring the universe beyond a certain level of precision. But that doesn’t stop us from trying. And in some cases succeeding, by squeezing the Heisenberg uncertainty principle to its breaking point.
2021x15 - What If (Tiny) Black Holes Are Everywhere?
May 25, 2021
It’s fair to say that black holes may be the scariest objects in the universe. Happily for us, the nearest is probably many light-years away. Unless of course, Planck relics are a thing - in which case they might be literally everywhere.
2021x16 - Are We Running Out of Space Above Earth?
June 9, 2021
While recent news about the Chinese Long March 5 Rocket made a lot of people very nervous because a 22-ton rocket was going to fall out of the sky, this sort of thing happens all the time. Boosters, dead satellites, and sometimes even old space stations get dropped out of the sky fairly often. While the litter seems a little inconsiderate, this is probably far safer than the alternative. The accumulation of space junk poses a huge risk to all human operations in space especially if we cross the threshold into the chain reaction of exponentially growing collisions known as the Kessler Syndrome.
2021x17 - Can Space Be Infinitely Divided?
June 16, 2021
How many times can I half the distance between my hands? Assume perfect coordination and the ability to localize my palms to the quantum level. 15 halvings gets them to within a cell’s width. 33 to within a single atom, 50 and they’re a proton’s width apart. Half the distance 115 times and they’re a single Planck-length apart - 1.6x10^-35 meters. Surely we can keep going - .8, .4, .2 x10^-35 m? Bizarrely, those distances might not even exist in any meaningful way.
2021x18 - How Quantum Entanglement Creates Entropy
June 23, 2021
Entropy is surely one of the most perplexing concepts in physics. It’s variously described as a measure of a system’s disorder - or as the amount of useful work that you can get from it - or as the information hidden by the system. Despite the seeming ambiguity in its definition, many physicists hold entropy to be behind one of the most fundamental laws of physics.
2021x19 - Electrons DO NOT Spin
July 7, 2021
Quantum mechanics has a lot of weird stuff - but there’s thing that everyone agrees that no one understands. I’m talking about quantum spin. Let’s find out how chasing this elusive little behavior of the electron led us to some of the deepest insights into the nature of the quantum world.
2021x20 - Where Are The Worlds In Many Worlds?
July 13, 2021
Many Worlds interpretation of quantum mechanics proposes that every time a quantum event gets decided, the universe splits so that every possible outcome really does occur. But where exactly are those worlds, and can we ever see them?
2021x21 - How Magnetism Shapes The Universe
July 21, 2021
How far can you follow a compass needle? As far as the north magnetic pole where the needle starts spinning wildly? Compass needles align with magnetic field lines; and on the precise spot of magnetic north, those field lines are vertical. So just tilt your compass 90 degrees and you can continue your journey: either down to the molten iron dynamo surrounding Earth's core; or up. But up to where? The answer: to everywhere. And today that's exactly where we're going to go.
2021x22 - How An Extreme New Star Could Change All Cosmology
August 3, 2021
A new white dwarf has been discovered (poetically named: ZTF J1901+1458) that’s doing some stuff that no white dwarf should ever be able to do. In fact, it has multiple properties that are so extreme that it almost certainly did NOT form in the way that we thought all white dwarfs formed. This one peculiar point of faint light may change our understanding of not just white dwarfs, but of all cosmology.
2021x23 - How to Communicate Across the Quantum Multiverse
August 10, 2021
In the Many Worlds interpretation of quantum mechanics, the universal wavefunction is the reality, encompassing all possible histories and futures and all exist. But we are only sensitive to a slice of the wavefunction corresponding to our “world”, and due to the superposition principle our world can happily do its thing unperturbed by other parts of the wavefunction - other “ripples,” or worlds. And while it may seem like it would be physically impossible to have any connection between worlds, it may turn out to be entirely possible to communicate between them.
2021x24 - How Vacuum Decay Would Destroy The Universe
August 18, 2021
The universe is going to end. But of all the possible ends of the universe vacuum decay would have to be the most thorough - because it could totally rewrite the laws of physics. Today I hope to help you understand exactly how terrified you should be.
2021x25 - First Detection of Light from Behind a Black Hole
September 7, 2021
How do you see the unseeable - how do you explore the inescapable? Our cleverest astronomers have figured out ways to catch light that skims the very edge of black holes. Let’s find out what they learned.
2021x26 - Neutron Stars: The Most Extreme Objects in the Universe
September 15, 2021
We’ve traveled to lots of weird places on this show - from the interiors of black holes to the time before the big bang. But today I want to take you on a journey to what has got to be the weirdest place in the modern universe - a place where matter exists in states I bet you’ve never heard of. Today we take a journey to the center of the neutron star.
2021x27 - How Electron Spin Makes Matter Possible
September 21, 2021
Today I’m going to explain why you’re not falling through your chair right now using one simple fact, and one object. The fact is that all electrons are the same as each other, and the object is a structurally critical item of my clothing. There’s a chance this episode could get very weird.
2021x28 - Why Magnetic Monopoles SHOULD Exist
October 5, 2021
What happens if you cut a bar magnetic in half? We get two magnets, each with their own North and South poles. But what happens if you keep on cutting, into fourths and eighths and sixteenths and so on? Will we ever get to a single pole? I’ll spoil the answer for you: we don’t know! But the idea of magnetic monopoles remains one of physics’ most tantalizing maybes.
2021x29 - New Results in Quantum Tunneling vs. The Speed of Light
October 13, 2021
Paradoxically, the most promising prospects for moving matter around faster than light may be to put a metaphorical brick wall in its way. New efforts in quantum tunneling - both theory and experiment - show that superluminal motion may be possible, while still managing to avoid the paradox of superluminal signaling.Paradoxically, the most promising prospects for moving matter around faster than light may be to put a metaphorical brick wall in its way. New efforts in quantum tunneling - both theory and experiment - show that superluminal motion may be possible, while still managing to avoid the paradox of superluminal signaling.
2021x30 - Will Constructor Theory REWRITE Physics?
October 20, 2021
The people behind the greatest leaps in physics - Einstein, Newton, Heisenberg, all had the uncanny ability to see the fundamentals - see the deepest, underlying facts about the world, and from simple statements about reality they built up their incredible theories. Well what if we all had a recipe book for doing exactly this. Well, one might be just around the corner and it’s called Constructor Theory.
2021x31 - Is ACTION The Most Fundamental Property in Physics?
November 2, 2021
It’s about time we discussed an obscure concept in physics that may be more fundamental than energy and entropy and perhaps time itself. That’s right - the time has come for Action.
2021x32 - What If Our Understanding of Gravity Is Wrong?
November 10, 2021
What if there is no such thing as dark matter? What if our understanding of gravity is just wrong? New work is taking another shot at that Einstein guy. Let’s see if we’ve finally scored a hit with Modified Newtonian Dynamics aka MOND.
2021x33 - Are Black Holes Actually Fuzzballs?
November 17, 2021
Black holes are a paradox. They are paradoxical because they simultaneously must exist but can’t, and so they break physics as we know it. Many physicists will tell you that the best way to fix broken physics is with string. String theory, in fact. And in the black holes of string theory - fuzzballs - are perhaps even weirder than the regular type.
2021x34 - What Happens If A Black Hole Hits Earth?
December 20, 2021
The possibility that a black hole could actually impact Earth may seem straight out of science fiction, but the reality is that microscopic primordial black holes could actually hit Earth. If one did, it wouldn't just impact like an asteroid, it'd pass straight through the entire Earth and exit the other side. Perhaps craziest of all, this may have already happened!
2021x35 - How to Find ALIEN Dyson Spheres
December 29, 2021
On our search for alien lifeforms we scan for primitive biosignatures, and wait and hope for their errant signals to happen by the Earth. But that may not be the best way. Any energy-hungry civilization more advanced than our own may leave an indisputable technological mark on the galaxy. And yes, we’re very actively searching for those also. Time to update you on the hunt for galactic empires.

PBS Space Time: All seasons

S0: Specials
8 Episode s
S1: Season 1
1 Episode
S2015: Season 2015
43 Episode s
S2016: Season 2016
48 Episode s
S2017: Season 2017
45 Episode s
S2018: Season 2018
44 Episode s
S2019: Season 2019
40 Episode s
S2020: Season 2020
41 Episode s
S2021: Season 2021
35 Episode s
S2022: Season 2022
34 Episode s
S2023: Season 2023
33 Episode s
S2024: Season 2024
17 Episode s

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2021x1 - What Happens During a Quantum Jump?

2021x2 - Can We Break the Universe?

2021x3 - Is Dark Matter Made of Particles?

2021x4 - How Does Gravity Warp the Flow of Time?

2021x5 - Gravitational Wave Background Discovered?

2021x6 - Does Time Cause Gravity?

2021x7 - How Does Gravity Affect Light?

2021x8 - The NEW Crisis in Cosmology

2021x9 - Zeno's Paradox & The Quantum Zeno Effect

2021x10 - Why the Muon g-2 Results Are So Exciting!

2021x11 - What If Dark Matter Is Just Black Holes?

2021x12 - The NEW Warp Drive Possibilities

2021x13 - How To Know If It's Aliens

2021x14 - Breaking The Heisenberg Uncertainty Principle

2021x15 - What If (Tiny) Black Holes Are Everywhere?

2021x16 - Are We Running Out of Space Above Earth?

2021x17 - Can Space Be Infinitely Divided?

2021x18 - How Quantum Entanglement Creates Entropy

2021x19 - Electrons DO NOT Spin

2021x20 - Where Are The Worlds In Many Worlds?

2021x21 - How Magnetism Shapes The Universe

2021x22 - How An Extreme New Star Could Change All Cosmology

2021x23 - How to Communicate Across the Quantum Multiverse

2021x24 - How Vacuum Decay Would Destroy The Universe

2021x25 - First Detection of Light from Behind a Black Hole

2021x26 - Neutron Stars: The Most Extreme Objects in the Universe

2021x27 - How Electron Spin Makes Matter Possible

2021x28 - Why Magnetic Monopoles SHOULD Exist

2021x29 - New Results in Quantum Tunneling vs. The Speed of Light

2021x30 - Will Constructor Theory REWRITE Physics?

2021x31 - Is ACTION The Most Fundamental Property in Physics?

2021x32 - What If Our Understanding of Gravity Is Wrong?

2021x33 - Are Black Holes Actually Fuzzballs?

2021x34 - What Happens If A Black Hole Hits Earth?

2021x35 - How to Find ALIEN Dyson Spheres

S0: Specials

S1: Season 1

S2015: Season 2015

S2016: Season 2016

S2017: Season 2017

S2018: Season 2018

S2019: Season 2019

S2020: Season 2020

S2021: Season 2021

S2022: Season 2022

S2023: Season 2023

S2024: Season 2024