The truth in physics is simple and simplifying, but in the case of how gravity causes stuff to accelerate, it’s also stunning in describing the immense power at the root of gravity, and so requires a bit of introduction.
Consider:
Clocks run about 1/50th of a second slower per year on the surface of the Earth compared to clocks in deep space far from gravitational bodies.
That seems like such a tiny thing. But it’s not. In fact, that tiny bit of one tick of a clock is the cause of the acceleration of bodies in free fall and is the reason it takes so much power to send a rocket into orbit.
To explain:
The foundational principle of all of modern physics is Galileo’s principle of equivalence of rest and uniform motion, which is borrowed by Newton for his First Law of Motion: An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force.
And if you ever experience free fall long enough, then you’ll notice that there is not any force at all acting upon you.
These facts and all of science agree that when we are accelerating in free fall then we are in a condition indistinguishable from being at rest or in uniform motion …
…but at rest relative to what? Our acceleration is uniform motion compared to what?
Or, in other words, how is the experience of being inert conserved in free fall: where/how is inertia conserved in free fall?
And the answer is that inertia is conserved in time: absent an external force, bodies remain at rest in time.
What does that mean? How do we know that? And how does that lead to bodies accelerating in free fall?
It means that clocks in uniform motion and clocks at rest are in an equivalent state (according to Galileo and Newton and Einstein), signified to Einstein by the clocks ticking at an unchanging/constant rate:
If you accelerate a clock through space, you’ll observe the ticking will slow until the clock stops accelerating through space, and then the clock will have uniform motion through space at the acquired velocity and the ticking remains at a constant slowed rate, and that constant rate of ticking is the conservation of the energy of acceleration that was used to accelerate the clock — the conservation of time, updating Newton’s conservation of inertia.
And so, the answer to “how does gravity pull stuff into it?”, and “how is inertia conserved in free fall?”, and “gravitational acceleration is uniform compared to what?”, is: bodies in free fall are at rest in time and the universal speed of time is accelerating — the rate at which clocks tick is accelerating uniformly everywhere in the Universe — and so bodies near the slower time of a gravitational body accelerate into that slower time at exactly the rate that allows them to remain at rest in time: to conserve/hold constant their speed of time and thus conserve their inertia as required by Newton’s Law.
This universal acceleration of the speed of time is easy to prove. But to understand this speed of time cosmological model, we must first update our understanding of time.
The acceleration of time/ticking doesn’t mean what Einstein thought it meant. Einstein thought that time is what we measure with clocks, such that clocks ticking slower because they are either in a gravitational well or traveling at high velocity were lagging further and further in the past. What else could it be? If a clock is working normally but ticking slower or faster, then our assumptions about clocks measuring time lead us to immediately suppose that the clocks are advancing on a timeline at a different rate than we are, either lagging in the past or advancing into the future. This is even easier to prove wrong than it is to prove that the universal speed of time is accelerating: When we talked using the radio with astronauts on the Moon, their clocks were ticking faster than our clocks on Earth because gravity slows time/ticking less on the Moon, and orbital motion also slows time on the Moon as viewed from Earth. But we could not have been sending radio messages back and forth between the past and the future. What is happening instead is that different rates of ticking correspond to the different rates at which we simultaneously experience events. That’s a tricky idea because it’s new, so let me put it another way: Our clock on Earth ticks a 50th of a second less during a year than a clock in outer space, thus astronauts in space will age a 50th of second more each year, but we are all always in the same now. The different ticking of clocks in gravitational wells and at high speed measures the different durations experienced of the same events. This means that if you lived 50 years on Earth (50 orbits of the Sun) then you are about one second younger than you would be if you had instead lived simultaneously in space, but you are not one second in the past. In fact, when your clock ticks slower in gravitational wells or at high velocity, it measures the duration of events as faster: fewer seconds pass. And yet, everyone and everything everywhere is in the same now…but not necessarily experiencing now at the same rate.
In the conversation below in the comments, we found a good way to illustrate this. If someone on Earth and someone in outer space each measures the duration of the orbit of the Earth around the Sun, then they can start their measurements simultaneously and end their measurements simultaneously, and they will agree that the astronaut measured the orbit as 1/50th of a second longer. The astronaut’s seconds were shorter due to gravity slowing (lengthening) seconds on the Earth.
So time is what we measure with clocks, such that the ticking signifies the speed at which events occur, not the speed at which we move from the past to the future, as Einstein assumed. It was an understandable mistake to make, imho, and understandable if this takes you awhile to get your head around. To understand why it is an easy mistake to make and exactly where Einstein makes this error, consider first that HG Wells published The Time Machine a few years before Einstein published his time theory in 1905, and Wells represented time travel by clocks ticking faster or backwards. It just seems natural because we assume that’s what clocks measure. And Einstein’s theory theorized a failure of synchronous running of clocks in motion and we have since proven clocks at rest are asynchronous compared to clocks in motion, but Einstein named it a failure of simultaneity, and we know now that is incorrect as described in the example above comparing clocks on the Moon with those on the Earth.
Now, returning to the proof that the universal speed of time is accelerating, we can begin by noting that this means that all clocks everywhere are continuously ticking faster. Intuitively, we may assume this means the clock is continuously advancing into the future faster because we are accustomed to assuming that’s what clocks measure. But, as in the circumstance of faster ticking of clocks on the surface of the Moon than on the surface of the Earth, and in the example of measuring the duration of an orbit around the Sun, the clocks are always in the same now, but the faster-ticking clock is aging faster as it experiences more seconds passing. Seconds are getting shorter universally.
Accordingly, light coming to us from distant galaxies can be predicted to show a frequency shift corresponding to how much the universal speed of time has accelerated since the light began its journey, because our clocks we use to measure frequency of light are ticking faster here when the light arrives than clocks were ticking when the light began its transit — the seconds are shorter when the light arrives. Thus, a light wave that was one second long when the light started from a distant galaxy might be 2 seconds long measured with our faster-ticking clocks now when the light arrives, or, put inversely, two light waves per second would become one light wave per second when the clock ticks twice as fast. And fewer light waves per second is a redshift of frequency — a redder light. And this means that the rate of this cosmological redshift (associated with the Hubble value (H) that supposedly describes the rate of universal expansion of space) must have the same value as the rate of acceleration of universal time, which acceleration of time must also determine the value of the Gravitational constant (G) that describes the rate at which bodies accelerate in free fall due to the different degree of slowing of time by different masses of planets…
…and it does.
The math is simple, not to say easy to understand without a little explanation.
If bodies fall in gravity to remain at rest in universally accelerating time, and we call the rate of accelerating time T, then the universal constant value for determining gravitational acceleration that we call G relates to T via a simple formula:
G=Tc
To understand this formula, consider that you could accelerate towards a light source in able to increase the frequency of waves of light you encounter, like encountering more ocean waves per second by running into the surf. So if the rate of your clock’s ticking is accelerating at rate T, and thus causes the number of waves you encounter to be fewer per second — redshifted — because your seconds are shorter, then Tc (where c is the constant velocity of light) is the rate at which you must accelerate towards the light to compensate for the universal acceleration of the speed of time, to keep the wave frequency constant.
Now, solving for T, by dividing G with c (using the standardized estimates of G and c from the world’s best measuring devices) we get a value for the universal rate of acceleration of time T of
I think it’s worth noting for physicists reading this that it’s highly significant that the units for T correspond to transformations of measurements of space and mass, as well as time, that must be present with universally accelerating time, and thus at last account for the unexplained units that have been necessary to attach to G simply to normalize it.
But to prove this equation and this value for T are correct, then the value for T must also correspond to the apparent rate of cosmological expansion of space in units of meters of expansion per second per meter, thus accounting for cosmological redshift without expansion of space. This universal rate of expansion is called the Hubble constant (H). The exact vale of the Hubble constant is somewhat uncertain. but is generally believed to be between 50 and 100 kilometers per second for every megaparsec in distance — km/sec/Mpc
.
The current estimates for H from our best science are approximate, and sometimes vary slightly from year to year, because they are derived from different ways of estimating how fast galaxies seem to be receding from us, with different ways discovered occasionally and old measuring instruments replaced with better ones. And it is highly significant that the value for H derived using T, is about 6.8 kilometers per second per megaparsec, varying from the current estimates by about one quintillionth of the value in the middle of the 50 to 100 kilometer range (to find value of H predicted by T, convert Megaparsecs of H to kilometers, and then multiply by T to find the corresponding kilometers of change per Megaparsec).
The incredible accuracy of this prediction derived from first principles can’t be dismissed, even if it leaves unanswered questions about why this quintillionth of a Megaparsec variance from the value of H is consistently observed beyond measurement error. For example, this value of 6.8 km/s/Mpc is about 1/10th the measured recession speed observed as cosmological redshift, and measured redshift may provide a more accurate approximation of T, but the slowing of time in gravitational fields may lag an order of magnitude behind T because it is an effect of T.
This new metaphysical understanding, unrefuted anywhere in the 15 years since its acceptance for publication in a peer-reviewed physics journal, provides an explanation for gravitational acceleration missing from the currently accepted theory; it also accounts for the units of G, which are utterly unaccounted for by the currently accepted theory; and it predicts the Hubble value closely enough to explain cosmological redshift without the paradoxes of the expansion theory. The metaphysics describing the derivation of the underlying principle of conservation of time, as an update of Newton’s understanding of conservation of inertia, is presented in the original paper, written in plain language (borrowed from Einstein), and available online free at The Speed of time, or On the Failure of Simultaneity
. This answer and discussion on Quora significantly updates that twenty year old theory by clarifying the changed understanding of the meaning of change in speed of time, describing change in length of seconds rather than describing change of rate of advancing into the future, and thus clarifies that we are all in the same now no matter our relative velocities or locations in gravitational wells, with different readings simultaneously on our different clocks but no failure of simultaneity on a timeline into the future.
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