Physicists have traced three of the four forces of nature the electromagnetic force and the strong and weak nuclear forces to their origins in quantum particles. But the fourth fundamental force, gravity, is different.
Our current framework for understanding gravity, devised a century ago by Albert Einstein, tells us that apples fall from trees and planets orbit stars because they move along curves in the space-time continuum. These curves are gravity. According to Einstein, gravity is a feature of the space-time medium; the other forces of nature play out on that stage.
But near the center of a black hole or in the first moments of the universe, Einsteins equations break. Physicists need a truer picture of gravity to accurately describe these extremes. This truer theory must make the same predictions Einsteins equations make everywhere else.
Physicists think that in this truer theory, gravity must have a quantum form, like the other forces of nature. Researchers have sought the quantum theory of gravity since the 1930s. Theyve found candidate ideas notably string theory, which says gravity and all other phenomena arise from minuscule vibrating strings but so far these possibilities remain conjectural and incompletely understood. A working quantum theory of gravity is perhaps the loftiest goal in physics today.
What is it that makes gravity unique? Whats different about the fourth force that prevents researchers from finding its underlying quantum description? We asked four different quantum gravity researchers. We got four different answers.
Claudia de Rham, a theoretical physicist at Imperial College London, has worked on theories of massive gravity, which posit that the quantized units of gravity are massive particles:
Einsteins general theory of relativity correctly describes the behavior of gravity over close to 30 orders of magnitude, from submillimeter scales all the way up to cosmological distances. No other force of nature has been described with such precision and over such a variety of scales. With such a level of impeccable agreement with experiments and observations, general relativity could seem to provide the ultimate description of gravity. Yet general relativity is remarkable in that it predicts its very own fall.
General relativity yields the predictions of black holes and the Big Bang at the origin of our universe. Yet the singularities in these places, mysterious points where the curvature of space-time seems to become infinite, act as flags that signal the breakdown of general relativity. As one approaches the singularity at the center of a black hole, or the Big Bang singularity, the predictions inferred from general relativity stop providing the correct answers. A more fundamental, underlying description of space and time ought to take over. If we uncover this new layer of physics, we may be able to achieve a new understanding of space and time themselves.
If gravity were any other force of nature, we could hope to probe it more deeply by engineering experiments capable of reaching ever-greater energies and smaller distances. But gravity is no ordinary force. Try to push it into unveiling its secrets past a certain point, and the experimental apparatus itself will collapse into a black hole.
Daniel Harlow, a quantum gravity theorist at the Massachusetts Institute of Technology, is known for applying quantum information theory to the study of gravity and black holes:
Black holes are the reason its difficult to combine gravity with quantum mechanics. Black holes can only be a consequence of gravity because gravity is the only force that is felt by all kinds of matter.If there were any type of particle that did not feel gravity, we could use that particle to send out a message from the inside of the black hole, so it wouldnt actually be black.
The fact that all matter feels gravity introduces a constraint on the kinds of experiments that are possible: Whatever apparatus you construct, no matter what its made of, it cant be too heavy, or it will necessarily gravitationally collapse into a black hole.This constraint is not relevant in everyday situations, but it becomes essential if you try to construct an experiment to measure the quantum mechanical properties of gravity.
Our understanding of the other forces of nature is built on the principle of locality, which says that the variables that describe whats going on at each point in space such as the strength of the electric field there can all change independently. Moreover, these variables, which we call degrees of freedom, can only directly influence their immediate neighbors. Locality is important to the way we currently describe particles and their interactions because it preserves causal relationships: If the degrees of freedom here in Cambridge, Massachusetts, depended on the degrees of freedom in San Francisco, we may be able to use this dependence to achieve instantaneous communication between the two cities or even to send information backward in time, leading to possible violations of causality.
The hypothesis of locality has been tested very well in ordinary settings, and it may seem natural to assume that it extends to the very short distances that are relevant for quantum gravity (these distances are small because gravity is so much weaker than the other forces).To confirm that locality persists at those distance scales, we need to build an apparatus capable of testing the independence of degrees of freedom separated by such small distances. A simple calculation shows, however, that an apparatus thats heavy enough to avoid large quantum fluctuations in its position, which would ruin the experiment, will also necessarily be heavy enough to collapse into a black hole!Therefore, experiments confirming locality at this scale are not possible. And quantum gravity therefore has no need to respect locality at such length scales.
Indeed, our understanding of black holes so far suggests that any theory of quantum gravity should have substantially fewer degrees of freedom than we would expect based on experience with the other forces. This idea is codified in the holographic principle, which says, roughly speaking, that the number of degrees of freedom in a spatial region is proportional to its surface area instead of its volume.
Juan Maldacena, a quantum gravity theorist at the Institute for Advanced Study in Princeton, New Jersey, is best known for discovering a hologram-like relationship between gravity and quantum mechanics:
Particles can display many interesting and surprising phenomena. We can have spontaneous particle creation, entanglement between the states of particles that are far apart, and particles in a superposition of existence in multiple locations.
In quantum gravity, space-time itself behaves in novel ways. Instead of the creation of particles, we have the creation of universes. Entanglement is thought to create connections between distant regions of space-time. We have superpositions of universes with different space-time geometries.
Furthermore, from the perspective of particle physics, the vacuum of space is a complex object. We can picture many entities called fieldssuperimposed on top of one another and extending throughout space. The value of each field is constantly fluctuating at short distances.Out of thesefluctuating fieldsand their interactions, the vacuum state emerges. Particles are disturbances in this vacuum state. We can picture them as small defects in the structure of the vacuum.
When we consider gravity, we find that the expansion of the universe appears to produce more of this vacuum stuff out of nothing. When space-time is created, it just happens to be in the state that corresponds to the vacuum without any defects. How the vacuum appears in precisely the right arrangement is one of the main questions we need to answer to obtain a consistent quantum description of black holes and cosmology. In both of these cases there is a kind of stretching of space-time that results in the creation of more of the vacuum substance.
Sera Cremonini, a theoretical physicist at Lehigh University, works on string theory, quantum gravity and cosmology:
There are many reasons why gravity is special. Let me focus on one aspect, the idea that the quantum version of Einsteins general relativity is nonrenormalizable. This has implications for the behavior of gravity at high energies.
In quantum theories, infinite terms appear when you try to calculate how very energetic particles scatter off each other and interact. In theories that are renormalizable which include the theories describing all the forces of nature other than gravity we can remove these infinities in a rigorous way by appropriately adding other quantities that effectively cancel them, so-called counterterms. This renormalization process leads to physically sensible answers that agree with experiments to a very high degree of accuracy.
The problem with a quantum version of general relativity is that the calculations that would describe interactions of very energetic gravitons the quantized units of gravity would have infinitely many infinite terms. You would need to add infinitely many counterterms in a never-ending process. Renormalization would fail. Because of this, a quantum version of Einsteins general relativity is not a good description of gravity at very high energies. It must be missing some of gravitys key features and ingredients.
However, we can still have a perfectly good approximate description of gravity at lower energies using the standard quantum techniques that work for the other interactions in nature. The crucial point is that this approximate description of gravity will break down at some energy scale or equivalently, below some length.
Above this energy scale, or below the associated length scale, we expect to find new degrees of freedom and new symmetries. To capture these features accurately we need a new theoretical framework. This is precisely where string theory or some suitable generalization comes in: According to string theory, at very short distances, we would see that gravitons and other particles are extended objects, called strings. Studying this possibility can teach us valuable lessons about the quantum behavior of gravity.
See the rest here:
- Quantum and classical computers handle time differently. What does that mean for AI? - The Next Web - September 18th, 2020
- The Fate of Schrdinger's Cat Probably Isn't in The Hands of Gravity, Experiment Finds - ScienceAlert - September 18th, 2020
- Hybrid lightmatter particles offer tantalising new way to control chemistry - Chemistry World - September 18th, 2020
- Scientists Have Shown There's No 'Butterfly Effect' in the Quantum World - VICE - August 19th, 2020
- How Physics Erases The Beginning Of The Universe - Forbes - August 19th, 2020
- Does the Butterfly Effect Exist? Maybe, But Not in the Quantum Realm - Discover Magazine - August 19th, 2020
- Dismantling disciplinary boundaries and decolonizing young India: Decoding the National Educational Policy (20 - The Times of India Blog - August 19th, 2020
- The spread of 'stranger than we can think' - Yahoo Lifestyle - August 19th, 2020
- Raytheon Technologies invests in new transformational STEM high school - PRNewswire - August 19th, 2020
- The Wheel of Time and the Storytelling Problem in the Concept of a Binary - tor.com - August 19th, 2020
- Physicists witness time crystals interacting for the first time ever - New Atlas - August 19th, 2020
- Quantum mechanics is immune to the butterfly effect - The Economist - August 17th, 2020
- Major quantum computational breakthrough is shaking up physics and maths - The Conversation UK - August 17th, 2020
- Physicists watch quantum particles tunnel through solid barriers. Here's what they found. - Space.com - August 17th, 2020
- The science of marketing: taking inspiration from quantum physics - The Drum - August 17th, 2020
- Here's why we need to build a quantum security coalition - World Economic Forum - August 17th, 2020
- The Spread of 'Stranger Than We Can Think' - SFGate - August 17th, 2020
- Nuh Gedik and Pablo Jarillo-Herrero are 2020 Moore Experimental Investigators in Quantum Materials - MIT News - August 17th, 2020
- Students in the news | Announcements - Indiana Gazette - August 17th, 2020
- Indian American Engineer Develops Parachute That Helped Curiosity Land on Mars - India West - August 17th, 2020
- How Quantum Mechanics will Change the Tech Industry - Unite.AI - July 21st, 2020
- Money & Markets: After the virus, make sure you've read the inflationary playbook - E&T Magazine - July 21st, 2020
- Bruce Lee: Inside the mind of the martial arts icon - CNN - July 21st, 2020
- Read Before Pontificating on Quantum Technology - War on the Rocks - July 13th, 2020
- The universe's clock might have bigger ticks than we imagine - Livescience.com - July 13th, 2020
- Testing Einstein's theory of relativity | OUPblog - OUPblog - July 13th, 2020
- Scientists Say This Is the Smallest Unit of Time That Could Exist - lintelligencer - July 13th, 2020
- Study: The Period of the Universe's Clock - lintelligencer - July 13th, 2020
- Book review: From Infinity to Man: The Fundamental Ideas of Kabbalah - The Jerusalem Post - July 8th, 2020
- Book review: Travels with Sushi in the Land of the Mind - The Jerusalem Post - July 8th, 2020
- WATCH: Follow along as this drag queen connects the dots between quantum physics and queer identity - Queerty - July 8th, 2020
- Raytheon Technologies to release second quarter results on July 28, 2020 - PRNewswire - July 8th, 2020
- A Brighter Tomorrow > News > USC Dornsife - USC Dornsife College of Letters, Arts and Sciences - July 8th, 2020
- The logic of the impossible: Moses our rabbi - The Jerusalem Post - July 8th, 2020
- Professor tackles one more mystery about quantum mechanics and times flow - GeekWire - July 5th, 2020
- Quantum fluctuations can jiggle objects on the human scale - MIT News - July 5th, 2020
- Want to Know the Speed of a Complex Nuclear Reaction? - Popular Mechanics - July 5th, 2020
- Try to consciously change the world it might just work - Sentinel & Enterprise - July 5th, 2020
- The Death of Fashion Shows? Not So Fast. | Tim's Take | BoF - The Business of Fashion - July 5th, 2020
- U of T and Hebrew University of Jerusalem launch research and innovation partnership - News@UofT - July 5th, 2020
- Max Planck Created Quantum Theory and Laid a New Foundation for Physics - Interesting Engineering - June 21st, 2020
- Do we need a 'Quantum Generation'? | TheHill - The Hill - June 21st, 2020
- 'Everything was centered around Sara, he was lost': Abhishek Kapoor on Sushant Singh Rajput after 'Kedarnath' - DNA India - June 21st, 2020
- RHOBH: What's with Denise Richards Husband Aaron Phypers? - Screen Rant - June 21st, 2020
- Restructuring cybersecurity with the power of quantum - TechRadar - June 21st, 2020
- In the atmosphere of Mars, a green glow offers scientists hints for future visits - NBCNews.com - June 21st, 2020
- Nano-motor of just 16 atoms runs at the boundary of quantum physics - New Atlas - June 20th, 2020
- Physics - The Period of the Universe's Clock - Physics - June 20th, 2020
- Toronto-based Association Quantum appoints Northern Hive PR - Business Up North - June 20th, 2020
- Physicists have proposed a new theory for Bose-Einstein condensates - Tech Explorist - June 20th, 2020
- Intricate Beauty, Quasiperiodic Structures, and the Cascade to Criticality - SciTechDaily - June 20th, 2020
- AI And The Parallel Universe - AI Daily - June 20th, 2020
- The stories a muon could tell - Symmetry magazine - June 20th, 2020
- Physicists Have Reversed Time on The Smallest Scale Using a Quantum Computer - ScienceAlert - June 13th, 2020
- Duckworth on Education: The Feynman Technique - EMSWorld - June 13th, 2020
- Sussex Uni physicist creates the fifth state of matter whilst working from home - The Tab - June 13th, 2020
- Beware of 'Theories of Everything' - Scientific American - June 13th, 2020
- Francesca Vidotto: The Quantum Properties of Space-Time - JSTOR Daily - June 1st, 2020
- What Is the Many-Worlds Theory of Quantum Mechanics? - The Wire - June 1st, 2020
- MIT Student Probing Reality Through Physics, Philosophy and Writing - SciTechDaily - June 1st, 2020
- An Indian Origin Physicist Created the Fifth State of Matter from Her Living Room - News18 - June 1st, 2020
- Science and the humanities in the time of pandemic: better together - The Irish Times - June 1st, 2020
- Quantum Physicist Invents Code to Achieve the Impossible - Interesting Engineering - May 24th, 2020
- What does the Tenet title mean? Quantum mechanics and Einsteins theory - Explica - May 24th, 2020
- Covid 19 Pandemic: Quantum Computing Technologies Market 2020, Share, Growth, Trends And Forecast To 2025 - 3rd Watch News - May 24th, 2020
- Scientists Create a Cluster of 15 Trillion Entangled Atoms for the First Time Ever - Dual Dove - May 24th, 2020
- Teaching the next generation of quantum scientists | Harvard John A. Paulson School of Engineering and Applied Sciences - Harvard School of... - May 23rd, 2020
- Nasa discovers parallel universe where time runs backwards? Know the truth - Business Standard - May 23rd, 2020
- Company Hopes to Have Carbon Nanotube COVID-19 Detector Available in June - SciTechDaily - May 23rd, 2020
- The world is not as real as we think. - Patheos - May 23rd, 2020
- Physicists Just Built The First Working Prototype Of A 'Quantum Radar' - ScienceAlert - May 19th, 2020
- Quantum Brakes to Learn About the Forces Within Molecules - SciTechDaily - May 19th, 2020
- Armin Strom Discusses Resonance With PhD Of Quantum Physics And Watch Collector In An Easy-To-Understand Way (Video) - Quill & Pad - May 19th, 2020
- Embedded in the community: Outstanding physics student is a third-generation ASU student - ASU Now - May 19th, 2020
- 50 Years of Physical Review B: Solid Hits in Condensed Matter Research - Physics - May 19th, 2020
- Exploring the quantum field, from the sun's core to the Big Bang - MIT News - May 14th, 2020
- Registration Open for Inaugural IEEE International Conference on Quantum Computing and Engineering (QCE20) - thepress.net - May 14th, 2020
- 3 Simple Reasons Why Wolfram's New 'Fundamental Theory' Is Not Yet Science - Forbes - May 14th, 2020
- The Era of Anomalies - Physics - May 14th, 2020
- Exploring new tools in string theory - Space.com - May 14th, 2020