The TRUE Cause of Gravity in General Relativity

Alternatively titled, "Physics Myth-Busters: why time dilation does NOT cause gravity" this video explores an explanation of General Relativity whose monumental erroneousness is commensurate only to its popularity. What is this explanation, why is it so wrong, and how did so many high-profile YouTubers get suckered into promoting it? Prepare for a heavy dose of critical thinking, as we attempt to sift facts from falsehood and approach a deeper, more meaningful understanding of the actual nature of gravity.

Contents:
00:00 - Introduction
01:17 - Interpreting Curvature
01:55 - The "Time Dilation Causes Gravity" Explanation
04:00 - First Confusions
05:30 - Distinctions between Gravity & Gravitational Attraction
07:36 - The Problem of the Uniform Gravitational Field
11:03 - "Gravity" at the Surface of the Earth
13:30 - Spacetime Diagrams vs. Spacetime
16:39 - Testing for Curvature
19:19 - A Hidden Coordinate Transformation
21:11 - The True Cause of Gravity
24:09 - Planes of Simultaneity
24:50 - We Need Your Help!

[dialectphilosophy]

Good news: the follow-ups to this video hav arrived! Still confused about how the ground can be accelerating up? We now take a deeper look into this concept in our new videos, "The Sky Is Falling Up" and "The River Model of General Relativity". Check them out!

[EugeneKhutoryansky]

You quoted and showed animations from my video, but what you didn't mention is that my video also included statements such as "This visualization does not accurately represent the full picture, since a point particle of zero volume would still follow the exact same path." With regards to where I got the idea for my visualization, this did not come from any other YouTuber, but from Einstein's Field Equations. I have another video explaining the mathematics of Einstein's Field Equations, which is what one needs in order to get the full picture. (Your video doesn't explain the full picture either.)

[ScienceAsylum]

This kind of response is an all-too-common response that science communicators receive from scientists. You _think_ your issue with my video (and other similar videos) is that the physics is wrong, but that's not the case. You're just offended by an approximation/analogy I used because it ignores some detail that you _personally_ feel is vital to the topic.

When you're a science communicator, you have to consider your audience. Where are they currently in their understanding of the topic? What ideas are they going to accept or immediately reject? A fully detailed explanation of general relativity is going to be pretty abstract and full of jargon, which is unapproachable for most viewers. Teaching is the art of knowing exactly what details you can leave out while still communicating the topic.

I'll admit that if you leave out too much or make it seem like nothing is missing, then you're just wrong. However, if you watch the cold open of my video, I clearly stated I was about to explain the gravity humans experience here on Earth. In jargon terms, that's the weak-field approximation. I'm saying the analogy I'm about to use is limited. It's not meant to give someone a deeply detailed understanding of gravity. It's just meant to take them a little outside their comfort zone and give them an idea of how spacetime can cause the motion that non-physicists attribute to gravity. By using the weak-field approximation, it allowed me to ignore space and focus on time. The fluid analogy then makes the idea of time more tangible and familiar. Things like approximations and analogies are perfectly fine as long as you're open about it.

The other issue is language. Are the words I use in my videos following the strictest scientific definitions of those words? Absolute not. In fact, if I'm going to use a strict scientific definition, I usually make a point to explain that strict definition within the video. Otherwise, I'm likely using a more common definition for that word. In the video you're referencing, I'll admit I'm pretty lax with my use of words like "gravity" and "curvature."

Again though, I wasn't teaching an upper-level physics course on GR. I made an educational video for a general audience. Saying I'm "100% unequivocally wrong" or "all wrong on all counts" is too harsh. I just took some educational liberties you don't like. Personally, the "ground is accelerating upward" explanation bothers _me._ Is that explanation mathematically true? Kind of, depending on your definition of acceleration, but I also wouldn't say it's "100% unequivocally wrong" because that's too harsh. Different explanations make sense to different people. We're all trying to educate here. You don't have to be so divisive.

[GrapplingwithPhysics]

I wish Einstein was still alive and had his own YouTube channel to clear this all up.

[budweiser]

No clue what this video was about. It had spent the first 7 mins arrogantly shitting on other You Tubers when I turned off. All I know is we're all travelling in a straight line through spacetime at the speed of light. Mass curves spacetime, so we're drawn together, that's gravity.

[ScienceClicEN]

This problem needed to be addressed, it is indeed a wrong explanation for general relativity. It is, in my opinion, a really wrong explanation, since in reality things are the other way around : it is gravity that causes a difference in time dilation. And more precisely it is the fact that the person on Earth has to constantly accelerate upwards, to compensate the curvature of spacetime, which causes the gradient in time dilation.

However I don't think any YouTuber is to blame since I have also seen this explanation told by physicists themselves, such as Carlo Rovelli who sometimes wrote in his books that "gravity is the tendency for objects to go where time slows down". It is similar to how misconceptions about black holes spread : some of them were popularized by physicists themselves. It is then pretty difficult knowing what is right and what's not.

For instance, another example is the fact that one would see the universe accelerate to infinity as one would cross the horizon of a black hole. This is completely wrong, but this explanation shows up all the time and it is sometimes told by physicists themselves.



I want to clarify my point of view. There are two distinct phenomena that one could call "gravity" :

1. First one is the fact that when we drop an apple, it seems to accelerate towards the ground. This is a matter of kinematics, apparent movement. The apple seems to experience a "force", which we could call "gravity".

2. The second one is the fact that when we drop an apple, the distance between the apple and the Earth becomes smaller : the apple moves closer to the center of the Earth. This is a different kind of observation, not linked to kinematics this time, but to geometry : we look at the distance between the two objects. This could also be referred to as "gravity" : it is the fact that massive bodies seem to mutually attract each other.

It is important to note that there is a clear distinction between both phenomena :

The first observation depends on the observer. A free-falling observer would not see the apple experiencing any acceleration, since from their point the view the apple seems to float, weightless, as they both fall. This first notion of "gravity" is "observer-dependent". In particular, in a free-falling frame of reference, this type of "gravity" disappears (this is the equivalence principle).

The second observation however is not observer-dependent : all observers will agree that the apple and the Earth tend to move closer to each other (and eventually collide). This notion of "gravity" is "absolute".

If I understood correctly, what Dialect and myself consider to be "true gravity" is the second observation : the fact that massive objects attract each other.

In this case, it is impossible to explain this phenomenon with a time dilation gradient. Why? Because the gradient of time dilation is observer-dependent : a free-falling observer doesn't measure any gradient ! However the phenomenon of gravity (the apple coming closer to the Earth) is still there, it is not observer-dependent.

The explanation comes from the fact that this second notion of "gravity" is fundamentally non local. (mathematically, it is of "order 2", it involves how the gradient of time dilation itself changes from place to place). What explains this type gravity (i.e. the fact that massive objects attract each other) is the curvature of spacetime. And indeed, curvature is not observer-dependent, all observers, even the free-falling one who doesn't observe a time dilation gradient, will still agree that spacetime is curved.

So to sum up, the gradient of time dilation is linked to the first type of "gravity". Which, more simply, is explained in general relativity by the fact that the ground constantly accelerates upwards (and inside an accelerating frame, you will indeed observe a time dilation gradient, whether you are on Earth or in a spaceship, whether you are in curved spacetime or not).

That's the key point : the gradient of time dilation is not linked to "gravity", at least not directly, it is actually more general, and is present in all accelerated frames of reference. So, also in your car when you accelerate, or in a spaceship with fake/artificial gravity. The curvature of spacetime on the other hand is ONLY present when there is "true gravity", in the sense that two massive bodies attract each other. In other words, there is "true gravity" only when the gradient of time dilation is not the same everywhere in space. Or yet in other words, there is "true gravity" only when there are tidal forces.

[jppcasey]

It would be nice to see a straightforward video just laying out your theory with less drama and personal attacks punctuated with giant text. Sometimes in life people have to eat their own words, and big bold text can be hard to swallow. Let your work stand on its own. Be humble.

[phillustrator]

PhD physicist here. I am not exaggerating when I say that I don't understand 90% of the popsci videos/articles about physics, of topics I have a good grasp of. I just don't understand what they mean and oftentimes, it sounds like gibberish to me. I can't imagine what the layperson understands from them. It's probably worse than not understanding. No wonder we have so many misconceptions about science going around.

[aarondyer.pianist]

I like exploring this topic but frankly this video is more about “I’m right and they’re all wrong” than actual education. You could have given a better presentation without making it so personal.

[arbitraryconst]

Alesandro Russel did a great job explaining gravity in his videos, which I really recommend to watch.

[Armoterra]

I’m fairly certain this video could have been done in a more polite manner.

Sometimes on your journey North, you end up going East because you don’t yet have the strength to plow through a mountain.

These concepts are not easy for the non-scientific person to understand. The other explanations, wrong as they may have been (East), still took us in the right direction (North) without overwhelming us with concepts that we’re not familiar with (the mountain). Thanks to them, I was able to understand your video more easily than I would have with my layman’s blank slate.

I think, rather than being a stab at the other channels, you could have treated this video as “the next step” in the layman’s journey to understanding gravity. Even Newton’s explanation, wrong as it is, is a good stepping stone to understanding Einstein’s explanation.

Maybe I’m wrong, but that’s my two cents.

[zyrphath]

This was an ambitious video, but unfortunately it falls a bit short off of its goal. There are several wrong statements made herein, though not all of them terribly impact the topic at hand, but being a type of fact-checking video it's not a pristine look.

For instance, the narrator asks with incredulity whether the travelling spaceship creates curvature as it passes through space, ending with the statement that this would be absurd. But it's really not. Any stress-energy tensor creates curvature wherever it exists, so a spaceship moving through spacetime will most certainly create curvature, and the faster it moves, the more kinetic energy it has, meaning it creates more curvature at higher speeds. A collection of photons would create curvature - hell, you could in theory create a black hole out of nothing but photons.

Another example is the spacetime diagrams vs. spacetime segment, where the narrator seems to disregard the geodesics explanation. It's unclear what they really meant here, but it's uncontested fact that objects *do* follow geodesics - geodesic trajectories map to Newtonian trajectories so closely that they're nigh indistinguishable - so it's unclear what the real objection here is supposed to be. The narrator also doesn't discuss the relationship between geodesics and observed time dilation (I'll return to that topic later on), which is the thing that is the real "cause" of these videos they're trying to debunk, contrary to the explanation they're giving about misunderstanding a coordinate translation.

It also rather seems that the narrator didn't fully understand the point made in the videos he's attacking. At the heart of the presented argument is that matter alters the flow of time (putting a dent in local spacetime), and the altered flow of time is then what causes the realization of why an object moving through that field seems to change directions. It can't be the space component, because if it was only the space component then gravitational pull wouldn't create inertial trajectories, objects of all velocities would follow the same path. What they are referencing here is Einstein's gradient for how proper time evolves in a gravitational field - and it does so in a manner that maps to the evolution of the geodesic.

So what they are *really* saying, is that gravity in non-relativistic frames of reference occur because *time* is curved (space is also curved since it's a part of spacetime, but the curvature of the space component is irrelevant for macro objects and indeed with a velocity not approaching c), and it's uncomplicated to show why that must be - I'm sure there's an equal explanation in GR, but it's much easier to realize that special relativity basically says so without any complexity at all. Under SR, basically all macro objects move more in the time component than they do the space component - significantly more so - so any curvature strong enough to affect them in the way we see has to arise in the component their delta is large in. Meaning the time component, because the space component delta is disappearingly tiny in comparison.

To go from all of this to "gravity is caused by time-dilation", is a little bit of a leap... I'll give the narrator that. It's not a strong explanation, but it's also not nearly as wrong as the narrator is trying to make it out to be in this video, certainly far from being "100% unequivocally wrong".

[jnbfrancisco]

This reminds me of the time when I was an instructor for the USAF. I noticed that some of my fellow instructors would change some of the well written systems explanation to try to make them easier to understand for their students. In that attempt they taught some things wrong. I don't think that they understood the subject either. Fortunately it was a rare event that a good understanding of systems operation and purpose was required to fix the airplanes we worked on.

[silvercam]

My problem with this video is the general message that is conveyed. In particular, while Dialect attempts to make a 'fuller picture' of gravity, it seems the main point is to actually put down other physics content creators for what Dialect deems as wrong. The ending completely rubbed me the wrong way, and is harmful to the physics content space.

ScienceClic has been brought up a couple times in the comments, and I think their channel is the perfect example of how Dialect should've communicated this video. Instead of focusing on other content creators---and in certain cases making a straw man of their videos--the focus should be on the actual physics and the creativity of how Dialect wants to communicate such physics. Not to say that Dialect doesn't include a lot of physics in this video, but I believe the tone of this video is not effective.

I'll end this by saying; if Dialect believes they have a better to make than what is currently on Youtube, then they should make a video solely on that. Believe me, if their video is well-made and does the physics service more than other content creators it will get praise. However, the only takeaway I can get from this video is a sad attempt to recruit viewers and gain popularity by putting other content creators down. In such a small space as physics visual education, it is sad to see a video such as this.

[maxwell]

Anyone who says that something is “100% unequivocally wrong” in this high end level topic that is still being studied is a narcissist who doesn’t understand anything.

[seandavies]

I would be cautious with certainty. General Relativity is very difficult, I've studied it myself and at university and have become quite familiar. But every so often I encounter a situation that you'd think is quite simple, but turns out to have a lot of nuance I had missed before. Given that the theory explicitly demands that there is no privileged coordinate system, we know that there is a huge amount of flexibility in how we can describe any given spacetime and I would argue that disentangling those features which are absolute and "real", from those which are artifacts of a given description, is a very difficult task. At the end of the day, we still need to use the coordinates, we can't analyse a spacetime without them, but you cannot just look at a metric and know immediately what's real and what's subjective. It takes a lot more work than that. If your view is that anything less than an unabridged explanation of GR that is then fully absorbed by the layperson audience, is misinformation, then I'm afraid you haven't met enough people to gauge how unrealistic that is

[PSG_Mobile]

I understood the arguments against other YouTube channels, but didnt understend his explanation about thr real cause of gravity. ScienceClick and Veritasium did this same explanation.

[adrianstephens]

My head hurts when I watch this. My head hurt 45 years ago when I took relativity at Uni despite being good at maths & theoretical Physics. The maths is hard, and I think all kinds of issues arise when people try and make the explanations simple - perhaps simpler than possible.

I would no more uncritically trust your explanation than the other YouTubers without being able to verify against the maths, and I'm too old to do that now.

[AIBfan]

When you said "100%, unequivocally, completely wrong", I think you could've done a better job of driving home the point that other channels are wrong. Maybe add a few more adjectives? Like "100%, unequivocally, decidedly, undoubtedly, utterly, completely wrong" would've been better. Plus you could've put flame effects around the "WRONG" in red text to hammer your point into the viewers' heads. In your next video, make sure to leave no stone unturned in putting down other science channels 😊

[GrahamAstles]

This is a very mean-spirited video - there are other ways to present your preferred interpretation than dissing those you disagree with. That is a tactic worthy of an "Egyptologist", and has no place in real science. Alternate hypotheses lead to breakthroughs eventually, ideological dogma does not. Still, from the comments below (from those more qualified than me in physics) it seems your certainty is fueled by your naivety, so I guess there is room for giving you some latitude. Very poor show indeed.