Is Freefall Really Not Accelerated Motion? Debunking Common Misconceptions

[inertiaforce]

According to this video, freefall isn't accelerated motion and is actually stationary. In other words, you aren't moving in freefall:

 

[Orodruin]

According to this video, freefall isn't accelerated motion and is actually stationary. In other words, you aren't moving in freefall

No, this is the wrong interpretation. You are not accelerating in free fall. (We are here talking about what is called proper acceleration, which is what an accelerometer measures.) Movement is relative.

 

[jedishrfu]

When you free fall you are in an inertial frame of reference.

Consider the Vomit Comet where the plane is in feefall for a few seconds. Inside you feel weight-less and move around without any feeling of gravity. If you kick off from the wall you will travel at constant velocity toward the other wall.

 

[inertiaforce]

No, this is the wrong interpretation. You are not accelerating in free fall. (We are here talking about what is called proper acceleration, which is what an accelerometer measures.) Movement is relative.

You are correct Orodruin. I meant to say that you are not "accelerating" in freefall. Thank you for catching the mistake.

 

[inertiaforce]

When you free fall you are in an inertial frame of reference.

Consider the Vomit Comet where the plane is in fretful for a few seconds. Inside you feel weight-less and move around without any feeling of gravity. If you kick off from the wall you will travel at constant velocity toward the other wall.

An "inertial frame of reference" is one in which you aren't experiencing accelerated motion, correct?

 

[Orodruin]

Also observe that this does not mean that "gravity is an illusion" as stated in the video title. All it means is that you need proper acceleration to remain stationary.

 

[inertiaforce]

All it means is that you need proper acceleration to remain stationary.

Lol. "You need proper acceleration to remain stationary." I'm sure that's going to go over well with the general public...

 

[Orodruin]

Lol. "You need proper acceleration to remain stationary." I'm sure that's going to go over well with the general public...

What is the point of this post? Do you understand what stationary and proper acceleration means?

 

[inertiaforce]

What is the point of this post? Do you understand what stationary and proper acceleration means?

No not exactly lol. I'm a layperson who has studied gravity on his own lol.

But I have come to realize that free fall is not an accelerated frame of reference. Prior to today, I was always told that freefall was an accelerated frame of reference. It is only today that I started to realize that freefall is not an accelerated frame of reference. It is an inertial (nonaccelerated) frame of reference because there are no net forces acting on a person in freefall. Therefore, it is not an accelerated frame of reference. This is so nonintuitive for the layperson. It has taken me a lot of research on my own just to come to this understanding. Is this understanding correct?

I am not familiar with the term "proper acceleration." Please educate me.

 

[inertiaforce]

What is the point of this post? Do you understand what stationary and proper acceleration means?

It seems like I may have inadvertently insulted you with my comment about not going over well with the general public. That wasn't my intention. I did not intend it to be an insult. I intended it to mean that this stuff is complicated for the average person to understand. It was a joke about the average person not being able to understand the terms "acceleration to remain stationary." The terms "acceleration to remain stationary" will cause confusion for the general public.

 

[atyy]

But I have come to realize that free fall is not an accelerated frame of reference. Prior to today, I was always told that freefall was an accelerated frame of reference. It is only today that I started to realize that freefall is not an accelerated frame of reference. It is an inertial (nonaccelerated) frame of reference because there are no net forces acting on a person in freefall. Therefore, it is not an accelerated frame of reference. This is so nonintuitive for the layperson. It has taken me a lot of research on my own just to come to this understanding. Is this understanding correct?

Both are right.

In Newtonian gravity, one usually talks about 3-acceleration, which is not zero in free fall.

In general relativity, one usually talks about 4-acceleration, which is zero in free fall.

The proper acceleration is the acceleration read by an accelerometer, and it is more closely related to the 4-acceleration.

 

[DrGreg]

I am not familiar with the term "proper acceleration." Please educate me.

Like almost everything else in relativity, acceleration is relative. Proper acceleration means "acceleration relative to a freefalling object that you are momentarily at rest with". Practically, it is acceleration as measured by an accelerometer. Other "non-proper" sorts of accleration are possible, usually referred to as "coordinate acceleration", which depends which coordinates you choose to use.

When someone talks about acceleration, you need to be clear whether they mean proper acceleration or coordinate acceleration. Usually, in relativity, they mean proper accleration, but they might not.

 

[A.T.]

I am not familiar with the term "proper acceleration."

https://en.wikipedia.org/wiki/Proper_acceleration

 

[PWiz]

It is only today that I started to realize that freefall is not an accelerated frame of reference. It is an inertial (nonaccelerated) frame of reference

Strictly speaking, free-fall is an inertial frame of reference locally only. This distinction becomes important when we're talking about frames in regions of space close to a massive object.

 

[harrylin]

According to this video, freefall isn't accelerated motion and is actually stationary. In other words, you aren't moving in freefall:

I don't think that that video is an acceptable source for discussion on this forum.

Apart of that, motion is defined wrt your chosen reference system. Consequently, freefall is accelerated motion wrt to a Newtonian (or "Galilean") reference system, but it's stationary wrt another free falling reference system. In principle it's as simple as that, and it's the consideration of free falling reference systems that led the way to GR.

Note however: falling reference systems behave locally just like Galilean reference systems (and in absence of gravitation there is no difference). As Galilean reference systems are called "inertial frames", such free falling frames are also called "local inertial frames".

 

[bahamagreen]

Video starts off without even pausing to think about anything, continues without follow-up or follow-through, just steady non-stop hipster blather like a cleaning product sales pitch.

One might wonder that if it were the Earth that accelerates to the apple at rest, the apple must observe that the whole universe must instantaneously and universally accelerate at 10m/S^2 wrt the resting apple in the apple's "up" direction for the duration of the Earth's fall, and this acceleration of the whole universe will stop suddenly, instantaneously, and universally when the Earth is stopped when it contacts the apple.

How did the distant regions of the universe conspire to coordinate their differential sudden onset of acceleration and subsequent cessation of motion over billions of years in order to appear to be instantaneous and universal to the apple just when the stem that held the Earth broke?

How much energy does it take to present the appearance to the apple that the whole universe has accelerated, and then stopped suddenly, and what stops the universe since it is only the Earth that is stopped when it contacts the resting apple?

I don't disagree that gravitation was ready for rethinking, but the video has nary a single real thought throughout the whole high speed irritating un-shaven narrative.

 

[Orodruin]

One might wonder that if it were the Earth that accelerates to the apple at rest, the apple must observe that the whole universe must instantaneously and universally accelerate at 10m/S^2 wrt the resting apple in the apple's "up" direction for the duration of the Earth's fall, and this acceleration of the whole universe will stop suddenly, instantaneously, and universally when the Earth is stopped when it contacts the apple.

You are trying to take a global view, which is not as straightforward as it seems in GR. The ground has a proper acceleration of ca 10 m/s^2. This does not mean that it is not stationary. The Earth is (essentially) stationary as well. The apple is not stationary, but at rest in a local inertial frame.

 

[stevendaryl]

Video starts off without even pausing to think about anything, continues without follow-up or follow-through, just steady non-stop hipster blather like a cleaning product sales pitch.

I don't think your characterization is at all accurate. There is nothing "hipster" about it. He pretty much defines all the terms that he uses. But I agree that it's too fast to do anyone any good unless they already know this stuff.

 

[harrylin]

Video starts off without even pausing to think about anything, continues without follow-up or follow-through, just steady non-stop hipster blather like a cleaning product sales pitch.

So hip that it's simply wrong:

"according to Einstein there is no such thing as a gravitational force, instead it's more appropriate to think of the apple as stationary and the ground [..] as accelerating upward"" - video (emphasis mine)

"The general theory of relativity renders it likely that the electrical masses of an electron are held together by gravitational forces."
"I must warn the reader against a misconception [..] we might easily suppose that the existence of a gravitational field is always only an apparent one. [..] This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the Earth (in its entirety) vanishes - Einstein 1916, Relativity: The Special and General Theory (emphasis mine)

However:

One might wonder that if it were the Earth that accelerates to the apple at rest, the apple must observe that the whole universe must instantaneously and universally accelerate at 10m/S^2 wrt the resting apple in the apple's "up" direction for the duration of the Earth's fall, and this acceleration of the whole universe will stop suddenly, instantaneously, and universally when the Earth is stopped when it contacts the apple.

How did the distant regions of the universe conspire to coordinate their differential sudden onset of acceleration and subsequent cessation of motion over billions of years in order to appear to be instantaneous and universal to the apple just when the stem that held the Earth broke? [..]

Regretfully Einstein's original GR did correspond to somewhat similar views, see:
- https://en.wikisource.org/wiki/The_...ain_the_extension_of_the_relativity-postulate.
- https://en.wikisource.org/wiki/Dialog_about_Objections_against_the_Theory_of_Relativity

 

[A.T.]

Similar issues discussed here:
https://www.physicsforums.com/threads/einstein-says-objects-do-not-fall-to-the-earth.781200/

 

[A.T.]

One might wonder that if it were the Earth that accelerates to the apple at rest, the apple must observe that the whole universe must instantaneously and universally accelerate at 10m/S^2 wrt the resting apple...

Non inertial frames are like that even in classical mechanics. And in GR inertial frames exist only locally, over regions where effects of tidal gravity are negligible. So even though the apple is inertial, you cannot extend it's inertial rest frame to infinity or even to a substantial fraction of the planet's size.

 

[harrylin]

Similar issues discussed here:
https://www.physicsforums.com/threads/einstein-says-objects-do-not-fall-to-the-earth.781200/

Yes indeed, #37
Thanks for finding it back!

 

[stevendaryl]

So hip that it's simply wrong:

"according to Einstein there is no such thing as a gravitational force, instead it's more appropriate to think of the apple as stationary and the ground [..] as accelerating upward"" - video (emphasis mine)

"The general theory of relativity renders it likely that the electrical masses of an electron are held together by gravitational forces."
"I must warn the reader against a misconception [..] we might easily suppose that the existence of a gravitational field is always only an apparent one. [..] This is by no means true for all gravitational fields, but only for those of quite special form. It is, for instance, impossible to choose a body of reference such that, as judged from it, the gravitational field of the Earth (in its entirety) vanishes - Einstein 1916, Relativity: The Special and General Theory (emphasis mine)

It is not "simply wrong". Einstein was speaking loosely about "gravitational forces". Strictly speaking, there are no such things in GR.

 

[harrylin]

It is not "simply wrong". Einstein was speaking loosely about "gravitational forces". Strictly speaking, there are no such things in GR.

Yes it is simply wrong to pretend what is not true. And the video claims that according to Einstein it is more appropriate to think of the apple as stationary and the ground as accelerating upward", which is also wrong and effectively propagating the misconception that Einstein warned for. Einstein's explanation implies that one may equally well hold that the apple is stationary and the Earth accelerating upward, so that only locally the gravitational field vanishes.
Thus Einstein claimed that "The Earth produces in its surrounding a gravitational field, which acts on the stone and produces its motion of fall" while this video pretends that according to Einstein it is more appropriate to interpret it as the ground accelerating upward!

 

[A.T.]

...video pretends that according to Einstein it is more appropriate to interpret it as the ground accelerating upward!

In terms of frame invariant proper acceleration that is true. One just shouldn't conflate the frame invariant proper acceleration (or lack of it) with frame dependent properties like moving (or being stationary).

 

[harrylin]

In terms of frame invariant proper acceleration that is true. One just shouldn't conflate the frame invariant proper acceleration (or lack of it) with frame dependent properties like moving (or being stationary).

Apparently the video doesn't pretend to present proper acceleration there. And I had overlooked the title "Is gravity an illusion?". No, GR is not a theory about an illusion!

 

[stevendaryl]

Yes it is simply wrong to pretend what is not true. And the video claims that according to Einstein it is more appropriate to think of the apple as stationary and the ground as accelerating upward", which is also wrong and effectively propagating the misconception that Einstein warned for. Einstein's explanation implies that one may equally well hold that the apple is stationary and the Earth accelerating upward, so that only locally the gravitational field vanishes.
Thus Einstein claimed that "The Earth produces in its surrounding a gravitational field, which acts on the stone and produces its motion of fall" while this video pretends that according to Einstein it is more appropriate to interpret it as the ground accelerating upward!

In explaining how GR differs from Newtonian gravity, the video's explanation is exactly appropriate. Saying "the Earth produces a gravitational field, which acts of the stone" is Newtonian gravity. That doesn't tell us anything about GR.

I think you're wrong about this.

 

[stevendaryl]

Apparently the video doesn't pretend to present proper acceleration there.

It certainly does talk about proper acceleration, when it talks about acceleration relative to an inertial frame. That's what proper acceleration is.

 

[stevendaryl]

Apparently the video doesn't pretend to present proper acceleration there. And I had overlooked the title "Is gravity an illusion?". No, GR is not a theory about an illusion!

It depends on what you're calling "gravity". The "force" of gravity is an illusion in GR. Or more strictly speaking, it is an artifact of using a noninertial coordinate system. The fact that noninertial coordinate systems are forced on us---we don't have any options, since there are no global inertial coordinate systems--is certainly not an illusion.

That's what made GR such a difficult problem for Einstein. Newtonian physics characterized gravity by the acceleration of a particle under its influence. Newton's law of gravity related that quantity (or its divergence, actually) to mass density. Einstein knew, from the equivalence principle, that that was the wrong way to characterize gravity, because the apparent force of gravity is completely subjective; it depends on an arbitrary choice of coordinate system. There is an apparent force of gravity aboard an accelerating train, and that is certainly not due to the attraction between different hunks of matter. Einstein needed to find some way to characterize gravity in terms of something objective, and that thing turned out to be spacetime curvature. Curvature is certainly not an illusion.

This video is about what makes GR different than Newtonian gravity, and you keep complaining that it doesn't reassure the nervous that much is the same as with Newtonian gravity. Yes, there is a lot that is the same, and with gravity in mild conditions such as near the Earth, GR can be thought of as a tweak on Newtonian gravity. But if you're trying to learn GR, you want to know what's different about it.

You say that the presentation in this video is somehow contrary to Einstein, but it is thought experiments such as those described in this video that inspired Einstein's theory. He's the one who introduced the idea of trains and elevators to relate the force of gravity to the feeling of being inside an accelerating vehicle. That insight is what led to his theory, and it's what's different about his theory than Newtonian physics.

I think you're completely wrong about this.

 

[harrylin]

It certainly does talk about proper acceleration, when it talks about acceleration relative to an inertial frame. That's what proper acceleration is.

Acceleration relative to an inertial frame is commonly defined by means of a to the inertial frame attached coordinate system as d²s/dt². At least, that's what textbooks teach and how most ordinary people understand it. However, that is irrelevant as at that point no mention at all is made of inertial frames; just a comparison between what Newton allegedly said and what Einstein allegedly said about the same phenomenon. For sure Newton was speaking of coordinate acceleration, and logically this is also how at least 90% of the intended viewers will interpret the comparison in the introduction between "accelerating down"(Newton) and "accelerating up" (Einstein).

It depends on what you're calling "gravity".

Perhaps you mean, it depends on what the video calls "gravity", or what most people understand by that word (that emission was obviously aiming at the general public). Compare https://en.wikipedia.org/wiki/Gravity

[edit insert:]

[..] Saying "the Earth produces a gravitational field, which acts of the stone" is Newtonian gravity. [..]

[..] Newtonian physics characterized gravity by the acceleration of a particle under its influence. Newton's law of gravity related that quantity (or its divergence, actually) to mass density. Einstein knew, from the equivalence principle, that that was the wrong way to characterize gravity [..] There is an apparent force of gravity aboard an accelerating train, and that is certainly not due to the attraction between different hunks of matter.

Sorry, but to me it really sounds as if you saying here that according Einstein the Earth does not produce in its surrounding a gravitational field, which acts on the stone and produces its motion of fall.
Einstein emphasized that GR is a field theory; that is not "Newtonian gravity"! Different from Newtonian theory there is no "direct action at a distance"; Einstein stresses that an important difference is that "the action of the Earth on the stone takes place indirectly". Another important difference is of course the extension of the equivalence principle to all physical phenomena.

[...]
This video is about what makes GR different than Newtonian gravity, and you keep complaining that it doesn't reassure the nervous that much is the same as with Newtonian gravity.

I'm sorry to hear that you misunderstood my agreement with bahamagreen as a complaint; my criticism on the video has nothing to do with complaining. I do wonder why you are so keen on defending wrong information which I exposed. Even more, I'm puzzled why you would think that according to me that video should reassure the nervous that much is the same as with Newtonian gravity; why would you think such a thing??

Yes, there is a lot that is the same, and with gravity in mild conditions such as near the Earth, GR can be thought of as a tweak on Newtonian gravity. But if you're trying to learn GR, you want to know what's different about it.

That's fine of course; I clarified a misconception that the video seems to promote and against which Einstein warned.

You say that the presentation in this video is somehow contrary to Einstein, but it is thought experiments such as those described in this video that inspired Einstein's theory. [..]

Yes of course; a misleading title and introduction do not mean that all the information in the video is wrong.

Now this discussion has deviated a lot from the question, which has been fully answered; I'm out of this thread.

 

[WannabeNewton]

The video in the OP is perfectly accurate; in particular, the video title and introduction are also as such.

I don't see what the issue is with it. This is pointless pedantry and an argument over semantics, which for some reason seems to be the norm as of late in the GR forum.

 

[stevendaryl]

Acceleration relative to an inertial frame is commonly defined by means of a to the inertial frame attached coordinate system as d²s/dt². At least, that's what textbooks teach and how most ordinary people understand it. However, that is irrelevant as at that point no mention at all is made of inertial frames

If we're talking about the same video, he makes a big point about inertial frames. At 1:35 into the video.

I think you're completely wrong about this. Certainly there can be criticism of the video, its accuracy, whether it's misleading, etc. But the points that the presenter tries to get across (and fails in your case) are absolutely essential in understanding GR and how gravity is treated differently by Newton and by Einstein. As I said, the thought experiment about the accelerating train is the same sort of thought experiment that led Einstein to GR in the first place.

 

[Wes Tausend]

...

According to this video, freefall isn't accelerated motion and is actually stationary. In other words, you aren't moving in freefall:

In other words, you aren't moving in freefall: ... is not necessarily always correct, but it could be in some instances. It would be more proper to say, "You (or an object) cannot ever feel any motion whatsoever in free-fall:"

Otherwise the video is a pretty good rendition of gravity as Einstein saw it. The easiest way to understand this is to simply consider Einstein's thought experiment that he himself used to form a slightly different perspective (conjecture) of gravity from that of Newton's now failed "attraction" theory. Now, in all fairness, Newton was pretty close, and Einstein certainly couldn't have completed his gravity theory (GR) without Newton's help.

The trick to understanding (as employed by Einstein himself) is to use a pictoral imagination of geometry to see why the above video rings so close to true. Math is important to our overall quantative understanding, and imperative to our conclusive geometric proofs, but there are those times, in my opinion, when the tedious intricate symbolic language of math tends to clutter up clear understanding of the stark geometric quality of natures basic principles.

To simplify his understanding, what Einstein did is imagine a chest (elevator) being drawn up in an earth-like accelerated manner (32 ft/sec/sec) in gravity-free space by a cable (A modern version might use a rocket ship under power with the scientists standing on the back wall, directly in front of the engine). Within the chest, he imagined two scientists that conduct simple experiments to see if they are in a gravitational field or not.

For example, one of the "thought" experiments might be to let a couple of balls fall off a table which is standing on what seems to be the floor. Let us do that. To make things interesting, we will have one ball heavier than the other. Next, both balls appear to roll off the table and fall to the floor, arriving simultaneously, just what the two scientists would expect in normal gravity, since that key point is exactly what is observed on earth.

In the previous above case, the balls, being accelerated by the chest and table top, initially stick to it as though they are attracted like Newton's erroneous "attraction-take" on gravity. In reality the balls are held there by the same inertial force one feels when a hotrod (or Space Shuttle) accelerates, pasting one to the seatback. We have always regarded this inertia as a form of artificial gravity and now Einstein has regarded the two to be "equivalent"... hence his Equivalence principle.

To continue, when these balls escape (roll past) the table edge, pure inertia causes them to merely continue to coast through space at the same last speed that previous contact with the accelerating table top gave them (threw them), while the floor speeds up even more (continues accelerating) to soon strike (meet) the balls. In this "chest" case, it is not so much that the different weight balls fall at the same velocity, as it is that the one-piece floor must logically rise evenly. Note that the floor will seem to strike the heavier ball with more force, the only difference between the simultaneous impacts. In effect, the side-by-side balls could be standing still, floating like you suggested, or moving along equally in any other form of inertial motion... when the rising floor simply strikes them both at the same time.

For Einstein, to furnish a theory of general relativity (GR) to include gravity, and yet accompany the special case of light (Special Relativity, SR), more thought becomes a nagging mandate. Inevitably, the curvature of space becomes evident in the same above "chest" scenario when Einstein considers what such a chest acceleration might do to the speed of light, which is regarded as merely constant. First, I imagine smiling in an understatement, Einstein remarks that the drawn chest, "would reach unheard of speeds", and leaves it at that. But he also realizes that if Equivalence is to be true, the acceleration of the chest will slightly outrun the ability of light to travel evenly across the room in a straight line. Consider the next paragraph.

In other words, a hole drilled in one side of an inertially moving chest might allow a perfectly perpendicular light beam to shoot across the chest and strike the other wall at exactly the same height as the hole in the first wall. But it cannot hit the same spot if the chest accelerates meanwhile. The acceleration of either the chest, or "equivalent" gravity on earth, means that the perfectly straight light beam will appear to bend in a minute curve and hit the adjacent wall slightly lower than it would in a non-accelerating chest (therefore an inertial chest; a chest either moving consistantly in an inertial frame or standing still). If we are to continue to regard light as traveling in a straight line, and we do, the conclusion is that both the acceleration of the chest and gravity itself, will bend space; our dear space which is always the path of light. Voila... light will be bent and it is! And that is the essence of Einstein's General Relativity.

It gets more complicated for many of us when we apply advanced math, especially beyond our training level, but the basic principle is not so complicated and should never be forgotten. We must all dust it off on occasion. Even Einstein, the king of visualisation and thought experiment, once remarked in mock confusion:
“Since the mathematicians have invaded the theory of relativity, I do not understand it myself anymore.”
(source: In A. Sommerfelt “To Albert Einstein’s Seventieth Birthday” in Paul A. Schilpp (ed.) Albert Einstein, Philosopher-Scientist, Evanston, 1949.)

Wes
...

 

[A.T.]

Apparently the video doesn't pretend to present proper acceleration there.

From 1:35 on he explains how to determine if a frame undergoes proper acceleration.

 

[eltodesukane]

No, this is the wrong interpretation. You are not accelerating in free fall. (We are here talking about what is called proper acceleration, which is what an accelerometer measures.) Movement is relative.

Would an accelerometer measures a non-zero acceleration aboard the ISS?