The Secret Truth About Gravity
Psst. You’re about to see how gravity really works, probably for the first time. Albert Einstein realized it while riding an elevator in 1907. But it was hard to visualize, even harder to accept, and frankly, so puzzling it was embarrassing to repeat. Because the theory says that when you drop an object on earth, there’s no downward pull of gravity on it — and that gravity is essentially the opposite of what we all think. It has been misunderstood and even suppressed for over a century. But now it’s easy to prove with a smartphone, and easier than ever to explain with computer graphics — and a revolution is beginning.
Relativity Reigns
Today the world celebrates a scientific centennial. One hundred years ago, the news was broadcast around the world: the most exciting physics experiment in history had been conducted and its results confirmed — proving Einstein’s Theory of General Relativity.
Ordinary people couldn’t easily repeat the experiment — which required telescopes and a total solar eclipse — and the math looked complicated. So most people barely understood it, and had to take the scientists’ word for it. Ever since, the physics and math of general relativity, and how the theory explains gravity, has remained the province of physics students and career faculty committed to studying many pages of four dimensional symbolic math. And even then, they often weren’t certain they’d gotten it.
That’s about to change — forever. Nowadays we don’t have to wait for an eclipse or a supernova, and we don’t need a telescope, a particle accelerator, expensive equipment or any complicated math to prove the core of Einstein’s theory of gravity! Mobile electronics has turned every phone into a programmable physics laboratory with multiple sensors. And animated 3D computer graphics let us visualize Einstein’s ideas about curved spacetime in ways he never could.
The revolutionary approach outlined in this article — with its companion app software and 3D animation — lets anyone run physics experiments that prove Newton was wrong about gravity and Einstein was right. Easily repeatable experiments and startling animated computer graphics show us directly how gravity works, where our weight comes from, and how matter causes the space containing it to expand so it looks like nearby objects are attracted to it.
The Secret About Gravity
This lets us confront and celebrate an unexpected truth about relativity and gravity that has been all but suppressed for the past hundred years:
Gravity is, in a way, exactly the opposite of what we usually think it is.
The surface of a massive body like our planet earth accelerates outward, making it look like objects are falling toward it! It’s a key difference between Einstein’s and Newton’s theory of gravity. The theory of space and time that makes this peculiar circumstance possible is known as spacetime curvature.
In Einstein’s theory, gravity is what physicists call a fictitious force. Though it sounds implausible, it’s easy to measure that in fact we’re continuously being pushed upward by the surface of the earth, away from its center:
• Anything nearby that isn’t supported by the earth’s surface looks like it’s falling. That’s what g, earth’s (upward) surface acceleration due to gravity, is.
• And when we are in contact with earth’s surface, it pushes upward against us at g, with a force proportional to our mass. That’s what our weight is.
We’re not being pulled down by gravity. There is no downward pull of gravity. We’re really being pushed upward by the surface of the earth.
In fact, as absurd as it sounds, the upward acceleration we experience when standing on earth — which causes our weight — is the push we’d feel if earth was continuously, geometrically expanding from its center. It’s what we’d feel if earth’s volume was increasing at 5 million cubic kilometers per second per second, while each atom near its surface doubled in size every 13.5 minutes.¹ Einstein’s theory of spacetime curvature describes how this can happen without the earth’s size in meters actually getting bigger. In his stunning theory, meters can vary in length and seconds can vary in duration.
Every human being naturally has trouble believing this — even every physicist, deep down. It shatters our lifelong intuitions about life on earth, gravity and reality. To consider taking this idea seriously literally undermines the solid, stationary ground we like to think we’re standing on.
When we first hear space is expanding we tend to picture objects moving further apart, explosions, and big bangs — practically the opposite of gravitational attraction. But if we think it through, we realize that when space itself and rulers are expanding, the distances between things — as measured by those rulers — are shrinking. The expansion of space itself means objects moving inertially in straight lines appear to be attracting and orbiting one another, not exploding apart. So at first, spacetime curvature defies our intuition — but then it neatly explains gravity!
How Stretching Space Results in Gravity
We should point out that while this neat explanation shows how Newton’s gravity emerges when space stretches over time, it isn’t quite exact. Newton’s laws don’t account for how very fast-moving entities are pulled by matter — specifically, for how light is deflected twice as far as slow-moving matter would be, as shown in the 1919 eclipse experiment that famously helped prove Einstein’s theory. However, since in our practical lives, nothing except light travels anywhere near that speed, this approach offers an excellent qualitative description how gravity as we know it emerges when space continuously expands. It effectively describes how Einstein’s gravity approaches Newton’s predictions as the speed of light c approaches infinity.
So while this approach doesn’t deal with time dilation in gravitational fields, or why people who accelerate on starships are younger than people back home when they return, it vividly explains how gravity can be a fictitious force — providing a kind of intuitive qualitative account of how expanding space results in gravity.
Prof Thorne’s Cliff Diver
When we realize we’re really being pushed upward, we can see how a bungee jumper or a cliff diver has a clearer view of the real forces on falling objects than someone standing ‘still’ on earth. As Nobel laureate Prof Kip Thorne’s sketch shows, that’s exactly what Einstein’s general relativity theory says:
Of course, we’ve noted that to most of us, the idea that earth’s surface is pushing us upward and nothing is pulled downward by gravity is profoundly unintuitive! At the same time, this approach — showing just how the stretching of lengths causes gravity — offers a surprising, clearer, deeper understanding of the subject. We’ll contrast this with some traditional relativity summaries and curricula, where core ideas like lengths that change are often overlooked or hidden in inscrutable equations.
Standard Curricula
These ideas make us all so naturally uncomfortable that even physicists who are making their best effort to explain relativity often shy away from it. You can read a thousand page book on general relativity and gravitation and find it never mentions weight, or fictitious forces, lengths that change, or the easily measured outward acceleration of earth’s surface.
Cal Tech physics Professor Sean Carroll’s textbook Spacetime and Geometry — An Introduction to General Relativity cautiously tips readers off on page 2:
Once we become more familiar with the spirit of GR, it will make perfect sense to think of a ball flying through the air as being more truly “unaccelerated” than one sitting on a table… (which is why we feel a force on our feet as we stand on Earth).
But by the book’s end 500 pages later, he has never plainly offered the startling truth, which might appear as:
It’s easy to confirm with accelerometers that the ball that appears to be falling is barely accelerating at all, while the table is accelerating upward at 9.8 meters per second per second, known as 1 g. The seeming downward pull of gravity on the ball in the air is an illusion experienced by observers around the table — who don’t believe that they’re being pushed upward either!
In fact, by the end of Prof Carroll’s book he has never mentioned weight, earth’s surface gravity g, fictitious forces, or accelerometer experiments at all!
Instead he spends a full chapter and more explaining the four dimensional tensor math of geodesics. (A geodesic is a 4D straight line in physicists’ standard language of spacetime.) In principle, the math of geodesics can confirm the conclusion in Prof Thorne’s sketch — that when we fire the cannon ball, the nearby cliff diver can see that it is traveling at a constant velocity in a straight line — that it isn’t being pulled downward by a force of gravity. But Prof Carroll’s many pages of tensor math wind up hiding the most startling facts in Einstein’s tensor equations: rulers are increasing in length in the vicinity of matter, the earth’s surface is accelerating outward, and it’s easy to measure!
Experiment: Toss a Mobile Device
Hold a mobile device vertically, toss it up vertically in the air, and catch it. While predictably the device appears to slow down, reach a maximum height, reverse direction, and fall faster and faster downward toward the ground, the device’s accelerometers will read 0.0 in every direction throughout its flight — showing that in fact it’s moving at a constant velocity in a straight line — until it lands or you catch it.
But hold the device in your hand, or set in down on a table or the ground, and you’ll measure a constant upward acceleration of 1 g. That’s why the device appears to change direction and fall — we and the surface of the earth are accelerating upward, catching up with the device we tossed, and outpacing it.
This confirms the fictitious force theory, as well as Prof Thorne’s sketch and Prof Carroll’s introduction: the magnitude of a ball’s acceleration (yellow) is 0.0 when it’s in free fall and 1.0 g when you’re holding it or it’s on the table.
Prof Carroll’s textbook is better than most. So many physics books avoid these uncomfortable facts — even revered epics like Gravitation co-authored by Prof Thorne himself — they’ve wound up making general relativity more mysterious, needlessly complicated, and much harder to understand. I haven’t even seen an animated movie that even tries to capture the profound insight of Prof Thorne’s 1994 sketch before this — and Thorne tells me he hasn’t either.
Why I’ve Written This
After consulting many scientists and physics textbooks, I realized I might have something to contribute. So I’ve written several related computer programs and a little book explaining how this works: The Secret Truth About Gravity.
Why me? I’ve been a professor at MIT, NYU and elsewhere. I have Doctoral and Masters degrees from MIT and a BS from Yale. I’ve co-edited a book for MIT Press and have been been published in Physical Review Letters.
I’m also a software engineer and media scientist with extensive experience in computer graphics, virtual and augmented reality, mobile accelerometers, gyroscopes and other sensors. So I’m a practicing experimental physicist and media scientist, doing experiments every day. I’ve developed some into products and patented inventions — some of them outlined in the recent book Valley of Genius. But I’m not a physics professor and I rarely teach relativity classes — although people who do (like Nobel Laureate and Cal Tech Prof H. David Politzer) readily confirm these results are correct.
The Physics, Math, and Cognitive Psychology of a Scientific Revolution
In fact, my qualifications to explain relativity lie partly in cognitive psychology — and my doctorate in, and pursuit of, Artificial Intelligence. After all, the mystery is not just How can earth’s surface accelerate outward without the planet getting any bigger? It’s also: How and why have these essential elements of Einstein’s theory been avoided and overlooked for over a century? Why are physics students so often surprised by the results of simple experiments like these? What topics and experiments are routinely skipped, and why?
The truth is, our core intuitions and beliefs — that gravity on earth is a real, downward force and that the ground we’re standing is a good stationary reference frame — aren’t necessarily affected much by what we learn in relativity class. Whereas we tend to think of our belief systems as inherently self-consistent and logical — especially in matters of science — in Artificial Intelligence we embrace the fact that our minds and our ideas are diverse, compartmentalized, dynamic, and sometimes far from self-consistent. At MIT, theories like Prof Marvin Minsky’s Society of Mind and The Emotion Machine reflect the plurality of belief systems and strategies that govern our thinking. Professor Minsky was a mentor who encouraged this work.
So while I have extensive credentials in science and technology, my status as a cognitive scientist and a relative outsider amid physics faculty may wind up being advantageous here. Kuhn’s The Structure of Scientific Revolutions explains why big physics discoveries aren’t often made in physics departments. In any case, I can’t be denied tenure for advocating an unusual approach to physics that — while it embraces Einstein’s theory and is confirmed by repeatable experiments — challenges mainstream curricula.
So we wind up exploring not just physics, but scientific culture and epistemology — how belief systems do and don’t change. We’ll see how simple math can illuminate revolutionary ideas — and how complex math can sometimes obscure them. Then we can understand what it takes to fully supplant Newton’s idea of gravity with Einstein’s profoundly better ideas— and relate it all to our understanding of gravity in our daily lives.
Join the Revolution
I am finding ways to explain how spacetime curvature causes gravity — including the kind of animation Prof Thorne outlined and a new way to show how space stretches — to make general relativity much more understandable. Watch the video. Read the book and run the app when they’re available — or run any smartphone accelerometer program — to see for yourself how an object resting on the surface of the earth accelerates, compared to an object flying through the air. Learn how this approach makes it easy to see how our weight, Newton’s and Gauss’s gravity laws, Kepler’s laws for orbits, and much more result simply and directly when space itself is stretched by matter as Einstein predicted.
The material may seem implausible or radical at first. But we’ll see the core concepts confirmed in the writings of Newton, Einstein and Feynman, and in work by Professors Thorne and Carroll — as well as articles in Wikipedia and videos from the Public Broadcasting System. And you’ll see repeatable experiments you can perform yourself. They confirm this is a new but uniquely understandable approach to Einstein’s general relativity — a theory that is now a hundred years old and deserves to be more fully and widely understood.
Happy centennial!
David Levitt
29 May 2019
Coming soon:
The Secret Truth About Gravity — the book
Table of Contents
Hiding in Plain Sight for a Century
Is This Einstein’s Theory of General Relativity and Spacetime Curvature?
Is This The Standard Math of Spacetime Curvature?
What’s New in This Book
Introduction
Background and Overview
Remember Learning Relativity?
Newton Was Wrong
Too Ridiculous To Believe
Newton’s Falling Apple — Inside Out
Fictitious Forces
Fictitious Centrifugal Force vs Real Centripetal Force
Gravity is a Fictitious Force
Weight is the Real Force
We Unconsciously Knew This
What Is Weight?
Weight Arises from Physical Contact
Contact is Key — Even Newton Rejected His Own Theory!
Measure Gravity and Weight
Relativity Is a Simpler, More Elegant Theory
You’re Accelerating Upward Right Now
So the Earth is Expanding? Not Exactly.
Rulers and Rigid Objects Change Length
We Really Don’t Want to Believe This
We Tolerated Rulers That Change Length Slightly
How Relativity Students Reconcile This
“Space is Expanding” Has Too Many Meanings
Even Einstein Struggled
Space Is Not Conserved
Space and Time Dilation
How Gravity Works
Weight and Orbits
Space is a Fluid
Online Sources
Missing from Key Textbooks
Prof Carroll’s Revealing Explanation
Accelerating Upward Without Moving Upward
What’s an Accelerometer?
Schwarzchild’s Assumption
Proof of Einstein’s Theory Ignored?
Prof Feynman’s Hint
A Simple Idea Explains A Lot
Proving Newton’s Inverse Square Law
VIDEO: 3D Animation of Spacetime Curvature
Space Dilation Looks Like Gravity
Expansion Is Elusive When Observers Expand Too
Prof Thorne’s Illustration
FIGURE: The True Meaning of Newton’s G
Doubling Rates, Orbital Periods, and Kepler’s Law
Einstein Field Equation and Einstein’s Constant
Home Grown and “Fringe” Theories
12 Stages of Denial and Acceptance
Author’s Background
Technological Context Informs Discovery
Current Status and Next Steps
Further Integration with Metric Tensor Math
Detailed Integration with Classical Physics
More Animated Computer Graphics
21st Century Computing for Scientific Revolutions
Call for Recorded Video
Call for Animated Graphics
¹Where M is earth’s mass and G is Newton’s gravitational constant:
• earth’s volume acceleration is 4πGM = 5 million cubic kilometers/sec/sec
• at a radius or distance from earth’s center R,
lengths appear to double every sqrt(R³/GM) = 810 seconds or 13.5 minutes
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