- 3226 - GRAVITY - how can we explain it? Quantum mechanics theory of gravity runs into serious problems. Gravitation is currently explained through the theory of general relativity, which makes very different assumptions about the universe at the macroscopic scale than those made by quantum mechanics at the microscopic scale.
- We all know what gravity is. It is the force that makes us fall to the ground. If the ground did not stop us we believe we would fall to he center of the Earth. We also know that gravity decreases with distance. The rocket launches this month are examples of people going weightless when the fly high above the Earth, away from the “ center of gravity”.
-
- Why do objects fall toward the ground? Why is gravity pulling on us and anything else with mass? The Greek philosopher Aristotle gave one of the earliest and most comprehensive attempts at a scientific explanation of this behavior by putting forth the idea that objects moved toward their "natural place."
-
- This natural place for the element of Earth was in the center of the Earth which was the center of the universe in Aristotle's geocentric model of the universe. Surrounding the Earth was a concentric sphere that was the natural realm of water, surrounded by the natural realm of air, and then the natural realm of fire above that.
-
- Thus, Earth sinks in water, water sinks in the air, and flames rise above air. Everything gravitates toward its natural place in Aristotle's model, and it comes across as fairly consistent with our intuitive understanding and basic observations about how the world works.
-
- Aristotle further believed that objects fall at a speed that is proportional to their weight. In other words, if you took a wooden object and a metal object of the same size and dropped them both, the heavier metal object would fall at a proportionally faster speed.
-
- Aristotle's philosophy about motion toward a substance's natural place was held for about 2,000 years, until the time of Galileo Galilei. Galileo conducted experiments rolling objects of different weights down inclined planes, not dropping them off the Tower of Pisa, despite the popular stories to this effect, and found that they fell with the same acceleration rate regardless of their weight.
-
- In addition to this empirical evidence, Galileo also constructed a theoretical “thought experiment” to support this conclusion.
-
- Some thought experiments are analyzable as rigorous arguments, often of the form “reduction ad absurdum“, in which one takes one's opponents' premises and derives a formal contradiction (an absurd result), showing that they can't all be right.
-
- This proof attributed to Galileo was that heavy things don't fall faster than lighter things (when friction is negligible). If they did, he argued, then since heavy stone A would fall faster than light stone B, if we tied B to A, stone B would act as a drag, slowing A down. But A tied to B is heavier than A alone, so the two together should also fall faster than A by itself. We have concluded that tying B to A would make something that fell both faster and slower than A by itself, which is a contradiction.
-
- The major contribution developed by Sir Isaac Newton was to recognize that this falling motion observed on Earth was the same behavior of motion that the Moon and other objects experience, which holds them in place within relation to each other.
-
- This insight from Newton was built upon the work of Galileo, but also by embracing the heliocentric model and Copernican principle, which had been developed by Nicholas Copernicus prior to Galileo's work.
-
- Newton's development of the law of universal gravitation, more often called the law of gravity, brought these two concepts together in the form of a mathematical formula that seemed to apply to determine the force of attraction between any two objects with mass. Together with Newton's laws of motion, it created a formal system of gravity and motion that would guide scientific understanding unchallenged for over two centuries.
-
- The next major step in our understanding of gravity comes from Albert Einstein, in the form of his “general theory of relativity“, which describes the relationship between matter and motion through the basic explanation that objects with mass actually bend the very fabric of space and time.
-
- This changes the path of objects in a way that is in accord with our understanding of gravity. Therefore, the current understanding of gravity is that it is a result of objects following the shortest path through spacetime, modified by the warping of nearby massive objects.
-
- In the majority of cases that we run into, this is in complete agreement with Newton's classical law of gravity. There are some cases which require the more refined understanding of general relativity to fit the data to the required level of precision. Then we have to use Einstein’s equations.
-
-
There are the few cases where not even general relativity can quite give us meaningful results. Specifically, there are cases where ‘general relativity” is incompatible with the understanding of “quantum physics“.
-
- One of the best known of these examples is along the boundary of a blackhole, where the smooth fabric of spacetime is incompatible with the granularity of energy required by quantum physics. This was theoretically resolved by the physicist Stephen Hawking, in an explanation that predicted blackholes radiate energy in the form of “Hawking radiation“.
-
- What is still needed is a comprehensive theory of gravity that can fully incorporate quantum physics. Such a theory of quantum gravity would be needed in order to resolve these questions.
-
- Physicists have many candidates for such a theory, the most popular of which is “string theory“, but none which yield sufficient experimental evidence to be verified and broadly accepted as a correct description of physical reality.
-
- In addition to the need for a quantum theory of gravity, there are two experimentally-driven mysteries related to gravity that still need to be resolved:
-
- Scientists have found that for our current understanding of gravity to apply to the universe, there must be an unseen attractive force (called dark matter) that helps hold galaxies together and an unseen repulsive force (called dark energy) that pushes distant galaxies apart at faster rates.
-
- “Quantum gravity” is an overall term for theories that attempt to unify gravity with the other fundamental forces of physics . It generally postulates a theoretical entity, a “graviton“, which is a virtual particle that mediates the gravitational
-
- The standard model of quantum mechanics, developed between 1970 and 1973, postulates that the other three fundamental forces of physics are mediated by “virtual bosons“. Photons mediate the electromagnetic force, W and Z bosons mediate the weak nuclear force, and gluons (such as quarks) mediate the strong nuclear force.
-
- The “graviton“, therefore, would mediate the gravitational force. If found, the graviton is expected to be massless (because it acts instantaneously at long distances) and have spin 2 (because gravity is a second-rank tensor field).
-
- The major problem in experimentally testing any theory of quantum gravity is that the energy levels required to observe this conjectures are unattainable in current laboratory experiments.
-
- Even theoretically, quantum gravity runs into serious problems. Gravitation is currently explained through the theory of general relativity, which makes very different assumptions about the universe at the macroscopic scale than those made by quantum mechanics at the microscopic scale.
-
- Attempts to combine the two theories generally run into the "renormalization problem," in which the sum of all of the forces do not cancel out and result in an infinite value. In “quantum electrodynamics“, this happened occasionally, but one could renormalize the mathematics to remove these issues. Such renormalization does not work in a quantum interpretation of gravity.
-
- The assumptions of quantum gravity are generally that such a theory will prove to be both simple and elegant, so many physicists attempt to work backward, predicting a theory that they feel might account for the symmetries observed in current physics and then seeing if those theories work.
-
- Some unified field theories that are classified as quantum gravity theories include:
-
----------------------------- String theory / Superstring theory / M-theory
-
----------------------------- Supergravity
-
----------------------------- Loop quantum gravity
-
----------------------------- Twistor theory
-
----------------------------- Noncommutative geometry
-
----------------------------- Euclidean quantum gravity
-
----------------------------- Wheeler-deWitt equation
-
- It is possible that an understanding of quantum gravity will not merely consolidate the theories, but will rather introduce a fundamentally new understanding of space and time. Stay in school, there is much more to learn.
-
- July 18, 2021 GRAVITY - how can we explain it? 3224
----------------------------------------------------------------------------------------
----- Comments appreciated and Pass it on to whomever is interested. ----
--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
-- email feedback, corrections, request for copies or Index of all reviews
--- to: ------ jamesdetrick@comcast.net ------ “Jim Detrick” -----------
--------------------- --- Wednesday, July 21, 2021 ---------------------------
No comments:
Post a Comment