On a small planet orbiting a typical yellow star, humans evolved with an insatiable curiosity about the universe. Despite having brains adapted for a hunter-gatherer lifestyle, they invented tools like telescopes and particle colliders to explore the cosmos and the fundamental building blocks of matter. Over time, they developed theories to explain how the universe operates.
Currently, we have two major theories that describe the universe: quantum field theory and general relativity. Quantum field theory explains the behavior and interactions of fundamental particles, covering forces like electromagnetism and the strong and weak nuclear forces. On the other hand, general relativity describes how massive objects influence each other through the curvature of space-time, which we perceive as gravity.
Both theories have been tested with remarkable precision, yet they don’t seamlessly integrate. Quantum field theory doesn’t account for gravity, and general relativity doesn’t address the quantum realm. This disconnect becomes particularly problematic in extreme conditions, such as black holes and the Big Bang, where both quantum mechanics and general relativity are relevant.
To understand these phenomena, we need a theory of quantum gravity—a unified theory that combines quantum mechanics and general relativity. The leading candidates are string theory and loop quantum gravity, but neither has been experimentally validated. String theory suggests that fundamental particles are vibrational modes of one-dimensional strings in an 11-dimensional space, while loop quantum gravity attempts to describe the quantum nature of space-time itself.
Progress in developing a theory of quantum gravity has been slow because gravity is much weaker than other forces. For instance, the gravitational attraction between two electrons is negligible compared to their electromagnetic repulsion. This makes it difficult to create experiments where gravity’s quantum effects are significant.
High-energy environments, like those near black holes or during the Big Bang, are the only places where quantum gravity might be observable. However, replicating such conditions in a lab is currently beyond our technological capabilities. Instead, scientists study natural phenomena, such as gravitational waves from black hole collisions and the cosmic microwave background, to gather clues about quantum gravity.
Discovering a theory of quantum gravity could revolutionize our understanding of the universe. It might solve mysteries about black holes, such as whether they contain other universes or serve as gateways to wormholes. It could also shed light on the origins of the universe and the nature of quantum information.
Historically, breakthroughs in fundamental physics have led to transformative technologies. For example, quantum mechanics paved the way for modern computers. A theory of quantum gravity could similarly open up new possibilities that we can’t yet imagine.
While the journey to a theory of quantum gravity is challenging, it holds the promise of deepening our understanding of the universe. As we continue to explore and test the boundaries of our current theories, we remain hopeful for the discoveries that lie ahead.
For those interested in delving deeper into these topics, educational platforms like Brilliant offer courses in gravitational physics and quantum objects, providing a hands-on approach to learning these complex subjects.
Engage in a structured debate with your classmates on the merits and challenges of string theory versus loop quantum gravity. Prepare arguments for both sides and discuss the potential implications of each theory if proven correct.
Use computer software to simulate the curvature of space-time around massive objects. Analyze how these simulations align with general relativity and discuss any discrepancies or insights related to quantum gravity.
Prepare a presentation on the experimental challenges of observing quantum gravity effects. Focus on why high-energy environments are necessary and explore current technological limitations and future possibilities.
Conduct a case study analysis of black holes, focusing on how a unified theory of quantum gravity could change our understanding of these phenomena. Discuss potential breakthroughs and their implications for physics.
Write a short story or essay imagining a future where a theory of quantum gravity has been discovered. Explore the scientific, technological, and societal changes that might result from this breakthrough.
Here’s a sanitized version of the provided YouTube transcript:
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On the third planet orbiting an average yellow dwarf star, a curious creature evolved: humans. So curious, in fact, that they couldn’t stop themselves from trying to figure out how everything worked. Although their simple brains were evolved for a hunter-gathering life, they were not content with what they could see and feel. So, they invented telescopes to peer into the depths of space and particle colliders to smash matter into its constituent parts. Eventually, they discovered how everything in the universe actually works.
However, they haven’t fully managed to do so yet. Here’s the problem: we have two fundamental theories that describe the universe, but we don’t know how to join them together. On one end, we describe how fundamental particles work and interact with each other using quantum field theory, which explains electromagnetism and the strong and weak nuclear forces. On the other end, general relativity explains how objects influence each other due to the curvature they create in space-time, known as gravity.
These two theories are the most complete descriptions we have of how the universe works, and both have been tested and verified to incredible precision. However, they don’t fit together. Gravity isn’t described in quantum field theory, and general relativity says nothing about the quantum world.
You might think that if each theory is doing its job, then there’s no problem. But there is a problem. In between these theories lie two of the biggest mysteries in physics: black holes and the Big Bang. In these extreme events, quantum mechanics and general relativity intersect, and we won’t be able to understand them until we develop a theory of quantum gravity, the fundamental theory of everything.
We have made some attempts. The most promising are string theory (or M-Theory) and loop quantum gravity, but neither has experimental evidence to support them. For example, string theory predicts new fundamental particles called supersymmetric particles, which we haven’t observed. So, for now, these theories remain hypothetical.
In practice, we don’t only use quantum field theory and general relativity to model everything in physics. Often, they would be too cumbersome, so we actually use a suite of other theories that are good approximations for specific situations. But quantum field theory and general relativity are special because they are the most fundamental theories.
So why has it been so long since we’ve made progress on quantum gravity? General relativity was published in 1915, and quantum field theory was completed in the late 1970s. The simple reason is that the force of gravity is much weaker than the other forces, making it very difficult—perhaps impossible—to create an experiment where something significantly feels all of the forces at the same time.
Gravity only becomes strong when dealing with large amounts of matter. For example, it takes the whole Earth to keep you on the ground. For large objects like humans, planets, or stars, quantum effects aren’t noticeable.
Quantum physics is important at the scale of atoms and subatomic particles, which are so small and light that the force they feel due to gravity is negligible compared to other forces. For instance, two electrons a centimeter apart would feel a repulsion from their electric charges but attract each other due to gravity. However, the force from the electric charge is 10^24 times stronger than the gravitational attraction, making any gravitational effect negligible.
The only test bed for quantum gravity is where a large amount of matter is squeezed into very small volumes, which are always high-energy situations, like black holes or the Big Bang. Unfortunately, these are not things we can create experimentally with our current technology. To reach the right energies, we would need a particle accelerator the size of the solar system with detectors the size of Jupiter. Creating enough energy to test quantum gravity would likely result in forming a black hole.
Currently, our best bet for testing theories of quantum gravity is to look at natural experiments: black holes and the Big Bang. We do this by using gravitational wave astronomy and measurements of the cosmic microwave background.
The cosmic microwave background shows us the first light in the universe, released about 380,000 years after the Big Bang, covering the entire sky. This light contains fluctuations that are imprints of quantum fluctuations from the Big Bang when the universe was smaller than an atomic nucleus. This is a clear instance where gravity and the quantum world intersect, and researchers are studying the cosmic microwave background in detail to find patterns in the data. However, these patterns are subtle, and we still don’t know if our current measurements have the resolution needed to see a signature.
We need to apply cautious optimism to our gravitational wave experiments. If there is a theory of quantum gravity, there should be a gravity particle called a graviton, which is a quantization of gravitational waves. Unfortunately, they are too small to be detected by our current gravitational wave detectors, LIGO and VIRGO. While these detectors have successfully detected gravitational waves, seeing a graviton would require detecting minuscule changes in those waves.
Detecting a graviton would be incredibly challenging, and they may never be observable because we would need to measure distances smaller than the Planck length, which is impossible according to quantum mechanics. Instead, astrophysicists are closely examining gravitational waves from black hole collisions, hoping to find small deviations from general relativity. If such deviations exist, they could provide clues for understanding quantum gravity.
Until we obtain new evidence of a departure from our existing theories, we won’t get closer to a grand theory of everything.
Now, let’s look at our best contenders for a theory of quantum gravity: string theory and loop quantum gravity. However, I won’t spend too much time on them because they are highly theoretical and complex.
Quantum field theory posits that there is a field for each fundamental particle, with particles being excitations of those fields. In general relativity, gravity is the curvature of space-time.
String theory treats space-time as another quantum field, attempting to unify gravity with the other forces in one framework, while loop quantum gravity focuses on understanding the quantum nature of space-time.
String theory hypothesizes that fundamental particles and their properties arise from different vibrational modes of one-dimensional strings existing in an 11-dimensional space, with one of these modes corresponding to the graviton. This theory has had some theoretical successes but has faced criticism for not accurately describing the real world.
Loop quantum gravity starts with general relativity but aims to model the quantum nature of space-time at very short distances, implying a minimum possible distance, akin to a space-time pixel.
These are the most popular proposals for quantum gravity, but there are others. It’s important to note that neither theory has experimental observations predicting something not already covered by quantum field theory or general relativity.
What’s the point of discovering a correct theory of quantum gravity? What change would it bring to the world?
Understanding black holes would be fascinating. Currently, general relativity breaks down when space-time becomes infinitely curved. A theory of quantum gravity could potentially solve many mysteries of black holes: Do they contain other universes? Are they gateways to wormholes? What happens to the quantum information of objects that fall into them, which quantum physics tells us cannot be destroyed? It might also provide insights into what existed before the Big Bang and where the universe came from.
While this is all intriguing, how would it affect us directly?
The truth is, we won’t know until we have a new theory. However, looking at the history of science, every paradigm shift in fundamental physics has led to new technologies. For example, understanding quantum mechanics led to the invention of computers and the information age. To reach a theory of quantum gravity, we will likely have to abandon at least one of our fundamental laws of physics. Thus, when we finally have a theory of quantum gravity, it will undoubtedly open up possibilities we can’t even imagine right now.
Most importantly, for me, it would be exciting to understand how the universe actually works.
This part of the video is sponsored by Brilliant, a website offering many online courses designed to help you learn science and mathematics. In my videos, I can’t delve into the details of the subjects I’m discussing, so Brilliant is a great option if you want to explore and practice the underlying physics and mathematics. A couple of relevant courses are Gravitational Physics, where you can practice the fundamentals of Newtonian gravity, and Quantum Objects, where you can explore the quantum realm. Answering questions is the best way to learn, as it forces you to apply your knowledge, revealing what you do and don’t know, which is beneficial when learning. A lot of content is free to try, and the first 200 people to sign up for the paid subscription will receive a 20% discount. Just go to brilliant.org/dos, which lets them know you came from here. The link is in the description below, and thanks to Brilliant for their support.
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Thank you for watching!
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This version removes any formatting artifacts and maintains the content while ensuring clarity and coherence.
Quantum – A discrete quantity of energy proportional in magnitude to the frequency of the radiation it represents, fundamental to quantum mechanics. – In quantum physics, particles can exist in multiple states at once until they are observed.
Gravity – A natural phenomenon by which all things with mass or energy are brought toward one another, including planets, stars, galaxies, and even light. – The theory of general relativity provides a comprehensive explanation of gravity as the curvature of space-time.
Theory – A supposition or a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained. – The Big Bang theory describes the origin of the universe as an expansion from a singularity.
Relativity – A theory, developed by Albert Einstein, which describes the laws of physics in the presence of gravitational fields and the relative motion of observers. – Einstein’s theory of relativity revolutionized our understanding of time and space.
Particles – Minute portions of matter, which are the fundamental constituents of the universe, such as electrons, protons, and neutrons. – In particle physics, the behavior of subatomic particles is studied to understand the fundamental forces of nature.
Black – Referring to black holes, regions of space where the gravitational pull is so strong that nothing, not even light, can escape from it. – The event horizon of a black hole marks the boundary beyond which nothing can return.
Holes – Referring to black holes, which are regions in space-time exhibiting gravitational acceleration so strong that nothing can escape from it. – Scientists use the properties of black holes to test the limits of general relativity.
Space-time – The four-dimensional continuum of space and time in which events occur, as described in the theory of relativity. – The warping of space-time around massive objects is what we perceive as gravity.
Mechanics – The branch of physics dealing with the motion of objects and the forces that affect them, including classical mechanics and quantum mechanics. – Quantum mechanics provides a mathematical framework for understanding the behavior of particles at the atomic and subatomic levels.
Waves – Disturbances that transfer energy through space and matter, characterized by their wavelength, frequency, and amplitude. – Electromagnetic waves, such as light, can travel through the vacuum of space without a medium.
