HN Theater @HNTheaterMonth

Albert Einstein said, “Intellectuals solve problems, geniuses prevent them”; he also likely was the main reason the US became aware of the need to enter the race to create nuclear weapons. [1]

At some point problems become impossible to avoid, especially if you’re biased [2] — though in the end, at a given scale, most problems (and their solutions) become as predictable as watching a man falling through the air. [3]

- Second-order thinking is not result of being smart, but not being lazy, biased, egotistical, etc.

_________

[1] https://en.m.wikipedia.org/wiki/Einstein–Szilard_letter

[2] https://youtube.com/watch?v=Va5T2KcYiOw

[3] https://youtu.be/XRr1kaXKBsU?t=49

While falling, the cat is in an inertial reference frame, so it is not accelerating. The ground is actually accelerating upward at 9.8 m/s2, counteracting the flow of spacetime.

This Veritasium video gives an intuitive explanation: https://youtu.be/XRr1kaXKBsU

⬐ IncRnd

A cat is not a rigid body and is absolutely accelerating towards Earth, experiencing the physical effects of gravity.

⬐ metrognome

There's a difference between accelerating compared to a coordinate system, and accelerating compared to an inertial rest frame. If you set a fixed coordinate system relative to the ground, then yes, the cat is accelerating towards the ground.

Consider this: if you hold an accelerometer while stationary on the ground, it will read 1g (accelerating). If you read an accelerometer while in free-fall, it will read 0g (ignoring wind resistance). In the first scenario, you are accelerating compared to your rest frame, even if you are standing still.

This is not an arbitrary distinction either. The 1g of acceleration while stationary on the ground produces measurable relativistic effects.

⬐ IncRnd

An accelerometer doesn't measure what you think. It measures the difference between the acceleration of the casing and the internals, for an example. It is designed to show 0 in free-fall. Yet, it still is under the force of 1G towards the center of the Earth.

⬐ metrognome

There's more nuance to it than that, so let me try to clarify again. Like you said, the accelerometer shows the difference between the casing's reference frame and the internal's reference frame. The internals ideally maintain an inertial reference frame. It's only "designed" to show 0g in free-fall because the difference between the reference frames of the casing and internals _is_ 0 when you are falling. Because, again, when you are falling, you are in an inertial reference frame - you are not accelerating against the flow of spacetime.

The only time the accelerometer reads something other than 0 is when something pushes it away from an inertial reference frame. This is true when you're on the ground: spacetime is curved, and "flows" towards the center of the Earth. The ground pushes you against the flow of spacetime, and the difference between these two frames of reference is 1g. Note that this 1g is not "caused" by gravity, it's caused by the electromagnetic force. Forces push matter away from an inertial reference frame. Gravity is different. It decides where an inertial reference frame goes by curving spacetime - it is not a force.

Hypothetically, if you were are the center of the Earth, the accelerometer would read 0g. There would be nothing pushing you in any particular direction - you would be in an inertial reference frame (ignoring the minor detail of being crushed by all of the Earth's mass).

Again, I'm trying to show the subtle difference between accelerating compared to a fixed coordinate system, and accelerating compared to an inertial reference frame. The fixed coordinate system does not take into account the curvature of spacetime. If it did, the coordinates would be "accelerating" towards the center of the Earth at 1g - congratulations, you've defined an inertial reference frame.

I hope that makes sense. It blew my mind when this idea clicked: that space and time are not two distinct things, they are two parts of the same thing.

⬐ IncRnd

Well, yes, but I never disputed any of that. I wrote, "A cat is not a rigid body and is absolutely accelerating towards Earth, experiencing the physical effects of gravity." Are you saying that is wrong?

I think he is easily one of the smartest people on YouTube and is producing some of the very best videos. He is also a brilliant educator. For examples, his videos on special [1] and general relativity [2] were real eye-openers for me, even though I have studied physics and have seen this stuff explained a million times before.

What I really don't like are the new video titles. At some point he started A/B testing them to maximize his revenue. He even made a video about it [3]. Of course, this selects for the most outrageous clickbait in order to get that sweet engagement. I am starting to wonder if he took into account, how much this is starting to alienate long-term followers.

[1] https://www.youtube.com/watch?v=pTn6Ewhb27k

[2] https://www.youtube.com/watch?v=XRr1kaXKBsU

[3] https://www.youtube.com/watch?v=S2xHZPH5Sng

Great video. Another one I like is "Why Gravity is NOT a Force" from Veritasium https://www.youtube.com/watch?v=XRr1kaXKBsU

⬐ drran

It uses gravity force to demonstrate that gravity force is not a force. ;-)

> Considering any disagreement with the idea that gravity is a force would end your academic career

Gravity is not a force. So thinking it is a force would end one’s academic career. See, even scientists can live in their own “bubbles” once things get complicated enough :-)

Gravity is the effect of masses moving along geodesic lines in curved space-time.

Edit: Since you probably don’t believe me https://m.youtube.com/watch?v=XRr1kaXKBsU

⬐ chaxor

I don't think there is a conflict between gravity being a force (and therefore having an associated gauge boson to mediate the force) and mass causing deformations in spacetime.

IIRC, Einstein's Field Equation equates the stress energy tensor to the spacetime curvature - so I think logic is that if the stress energy tensor has quantized levels, then the spacetime curvature should as well.

⬐ rdtsc

But that's not the current most accurate working theory? We have not detected any gravitons yet. So the general theory of relativity is still the top contender, with some cracks showing in it as well. But the comment is tongue and cheek, of course, mostly illustrating how we were ready to fire any academic who didn't believe gravity is a force, when the leading explanation at the academic is the GR explanation.

The idea being, is that perhaps some phenomena are multifaceted and complicated and climate study is the same. Moreover, it is compounded by a multitude of political and social interests trying to insert themselves into the discussion.

⬐ ars

That video is not persuasive - every single thing described there would apply to a charged object in an electrical field.

So why is charge a force and gravity not?

⬐ rdtsc

Charge is not a force either! It’s a property of objects.

In that respect you may be interested in the https://en.m.wikipedia.org/wiki/Paradox_of_radiation_of_char...

⬐ nsonha

You could've said "no such thing as a force" instead. I believe the person meant "electromagnetic force" instead of "charge". Question remains.

⬐ rdtsc

> I believe the person meant "electromagnetic force" instead of "charge". Question remains.

But those are different things...

The question is answered by stating that all the other forces are explained by the standard model, with gauge bosons mediating their interactions, while gravity is currently best explained using general relativity.

The funny part is, of course, that we were ready to let go, without a severance package even I presume :-) those academics who did not believe that gravity is a force. Given that the leading theory states that gravity is indeed not a force.

The Veritasium youtube channel has a great video about this: https://www.youtube.com/watch?v=XRr1kaXKBsU

Light is not keeping the Earth and Moon "in sync". Gravitational force is.[1] That force propogates at the speed of light. It isn't light itself.

Light does play a role in illuminating the system, rendering it observable. It's a signalling component. But again, that's exogenous to the system itself. (See discussion of Maxwell's Daemon and the resolution of that paradox for the role of light in that specific thermodynamic process.)

But, and this is key: ANY clock needs some way of passing information on to the observer. From TFA, that might be light, or sound, or chemical potential (received as the smell of rotting garbage). But that is a common element, part of the signalling mechanism, but not of the timekeeping mechanism itself.

For an astronomical clock, in most cases, the regular rotational or orbital motion is the timekeeping mechanism. The signalling mechanism is entirely separate.

E.g., in the Earth-Moon system, timekeeping is noted by both the daily rotation of the Earth, and the monthly orbit of the Moon. The kinetic energy of both those regular motions is not driven in any sense, nor is it measuring a decay.[2]

The signalling comes from an exogenous light source (the Sun) which illuminates both bodies. But the Sun isn't powering the timekeeping mechanism in question. And is equally significant in providing the light necessary for you to read a wristwatch or wall clock (of a non-illuminated variety).

________________________________

Notes:

1. I'm going to leave discussion of just what gravity is and/or how it's manifested out of this discussion. Veritassium tackled this recently though: https://youtube.com/watch?v=XRr1kaXKBsU

2. Yes, there's tidal friction. That's removing energy from the system, and is measurable with sufficiently precise clocks and over time. But that's also incidental and secondary to the motions in quesiton. We could use far more remote bodies (e.g., the Earth and another planet) in which mutual tidal forces are minimal, to eliminate this consideration. Or presume tidally-locked bodies in which orbital / rotational kinetic energy and angular momentum are constant.

I’d highly recommend this explanation: https://m.youtube.com/watch?v=XRr1kaXKBsU

This video really helped me understand why it’s not a force. Also awesome physics channel as well.

https://youtu.be/XRr1kaXKBsU

Veritasium made a cool video about this some months ago: https://www.youtube.com/watch?v=XRr1kaXKBsU

Here's [0] a bit more thorough explanation of this concept for those who are interested.

[0] https://www.youtube.com/watch?v=XRr1kaXKBsU&t=504 (Veritasium on Youtube: Why Gravity is NOT a Force @ 8:24)

⬐ pcmaffey

One thing I don’t understand is how does matter bend spacetime? Isn’t this video describing the force of gravity on spacetime instead of on things moving through spacetime?

Original page kinda doesn’t explain the idea in simple terms. This Veritasium video gives detailed “popsci” explanation:

https://youtu.be/XRr1kaXKBsU

Energy is Force times Distance. You're not moving (even though you're accelerating) so there's no energy expended.

See Veritasium's video at 10m17s:

https://youtu.be/XRr1kaXKBsU?t=617

He addresses that basic science makes this confusing because there's a curvature term that is usually left out of acceleration equations which balances out when you're accelerating along your curvature (e.g. against gravity), instead of along your spatial coordinates.

⬐ colordrops

Nice, that's it, thank you

Veritasium recently posted an excellent video on the subject: https://youtu.be/XRr1kaXKBsU

⬐ manigandham

That's linked at the bottom of the article.

Another good video to visualize GR is this by ScienceClic: https://www.youtube.com/watch?v=wrwgIjBUYVc

Veritasium just put out a great video on this: https://www.youtube.com/watch?v=XRr1kaXKBsU

⬐ avmich

A great video :) . Makes you wish he'd include more explanations of mathematical details...

⬐ lucb1e

I'm happy they didn't, making it a great video for me as well :)

Not that I'm afraid of math, I just don't feel like I need to see equations when it's a concept that is being explained rather than how to numerically calculate something. Math helps me as much as a code implementation does to explain the concept of a variable: I can observe its behavior but it doesn't necessarily teach me the concept.

⬐ seaish

It's linked at the bottom of this article.

⬐ trungdq88

This, and the one by Vsauce [1] are the 2 best videos explain spacetime on Youtube I have seen.

[1] https://youtu.be/Xc4xYacTu-E

When you are at rest on the surface of the earth, you’re actually accelerating. I barely understand it but veritasium explains it in the newest vid

https://youtu.be/XRr1kaXKBsU

The video for this page provides great context https://youtu.be/XRr1kaXKBsU

⬐ umvi

Great Veritasium video.

So some flat earth arguments are actually correct if general relativity is correct, namely that gravity is an illusion and that the real reason we are stuck to the earth is that the earth is accelerating toward us at 9.8 m/s^2

⬐ vbezhenar

I don't understand it. How can it accelerate towards anyone on its surface? Where does it get energy to accelerate? We generally need to burn fuel to accelerate something in the space.

⬐ umvi

"in curved spacetime, an object needs to accelerate just to stay still"

I don't really understand why, that was just the explanation Derek gave.

⬐ Smaug123

This amounts to a confusion over notation: "proper acceleration" (e.g. as measured by an accelerometer) vs "coordinate acceleration" (the acceleration an observer observes an object to be undergoing).

The acceleration an observer sees you undergoing is the same as the inherent "proper" acceleration you're undergoing, minus the acceleration of their coordinate frame with respect to yours. For me to stay still with respect to you, if you're in a frame that is accelerating away from me, I need some proper acceleration to catch up and counteract the fact that our frames are diverging. But if spacetime is curved, your frame probably is accelerating relative to mine - c.f. the example on the Earth's surface, where our frames inexorably accelerate towards each other as we move parallel to each other. So for me to stay still with respect to you, I need to have some proper acceleration to balance out the coordinate acceleration derived from the fact that our frames are moving in a curved space.

⬐ nobody9999

My understanding of General Relativity[1] is that mass distorts space-time, so an object traveling in a "straight line" through distorted space-time will curve with that distortion.

If the object's velocity isn't enough to traverse the curved space-time, it will move toward the center of the mass generating the distortion and fall out of the sky.

If the object is traveling quickly enough, it can continue traversing the distorted space-time and orbit that mass.

If the object is traveling even more quickly, it will traverse the distorted space-time and continue on without orbiting the mass.

In all three cases, from the perspective of the object traversing the distorted space-time, it continues to travel in a straight line, as it's the space-time that's distorted.

A (flawed) analogy would be riding a bicycle between the peaks of two identically sized hills. Starting at the top of the first hill, you coast down increasing your velocity.

Once you reach the bottom of the first hill and head up the second, your velocity decreases.

If your velocity at the bottom of the first hill is too small, you'll go up the second hill and as your velocity reaches zero, you roll back toward the bottom of the hill.

You will pick up velocity and then roll back up the first hill, then down again, then back up the second, etc. until you end up stopped at the bottom of the hill. This is akin to falling to the center of the distorting mass.

If your velocity is high enough to carry you back up to the top of the second hill and then stop, you'll roll back down and get to the bottom with the same velocity you had coming down the first hill. You'll then oscillate between the tops of both hills. This is akin to orbiting the mass.

If your velocity at the bottom of the first hill is enough to carry you past the top of the second hill, you'll just keep going after reaching the top of the second hill. That's akin to flying by the mass.

It's a flawed analogy, because in a curved space-time the directional portion of the motion vector doesn't change.

As John Wheeler[0] simplified it: "Mass tells space-time how to curve, and space-time tells mass how to move."

[0] https://en.wikipedia.org/wiki/John_Archibald_Wheeler

[1] https://en.wikipedia.org/wiki/General_relativity

⬐ manigandham

This is about relativity. We're going straight in space-time, but space-time itself is curved because of the heavy mass nearby (the Earth). This is visualized by the rocket curving towards the planet in the video, and the bent sheet experiment where the balls spiral towards the center.

So we're curving in towards the center of the mass of the Earth, but the reason we don't end up in the core of the Earth is because the surface stops us. The Earth is "pushing" us away from the center, and that's the acceleration. It's accelerating you off your straight line path, and this is the deviation from the geodesic.

⬐ strken

I thought that the Earth curved spacetime, and since an inertial observer approaching the earth would follow a straight line through curved spacetime, they'd appear to be following a curved line through space towards the earth.

What's confusing me here is the notion that when two objects collide, they accelerate into each other. Why and how is force constantly applied after the collision? My intuition is falling down here, and none of the resources I've looked at so far have explained why the acceleration happens.

⬐ manigandham

1st sentence is correct and is the same as what I said, sorry if I wasn't clear.

Remember that spacetime = space + time dimensions. The object is always travelling through time, and the curvature of spacetime is converting some of that speed through time into speed through space. That's what you perceive as motion (caused by gravity).

Time and space are linked together. The faster you go through space, the "slower" you go through time (as in you experience it slower). This is very measurable and even used to alter timings for satellites GPS readings. You can take an atomic clock on a plane and age slower than someone who just stayed on the ground.

So the spacetime curvature is continuously converting some of your temporal motion into spatial motion, until that's stopped by the surface of the Earth which is constantly "accelerating" to stop you from going further.

As to why we always move through time, that's beyond my understanding at this point but it's a fundamental axiom of physics.

⬐ strken

Yeah, sorry, I was restating what I understood you had said, in case I'd misunderstood any part of it, and then explaining where I was lost.

Your explanation makes sense, and it sounds like I'd need to understand the maths behind relativity to be able to really understand how objects behave in spacetime.

⬐ c06n

I do not understand how you can have acceleration without changing position (at 10:06). Acceleration is the derivative of speed, which is the derivative of position change. If the position change is zero, how can the acceleration be non-zero?

⬐ manigandham

Your position in spacetime is changing. You're going straight in spacetime, but spacetime is curved by the mass of the Earth so you're following that curve into the center.

The surface of the Earth keeps you from actually falling in, and is therefore pushing you away or upwards from the center. This is the acceleration acting on your straight line path through curved spacetime. This is the deviation from your geodesic.

⬐ saagarjha

If you watch a bit more, there's another term that is added.

⬐ Smaug123

Position is not an absolute notion: you need to answer "position with respect to what?".

If the thing you're measuring position against is also accelerating, then you need to apply some acceleration of your own to stay still with respect to it.

The terms you want to look up are "proper acceleration" and "coordinate acceleration". The curvature of spacetime means the thing I'm measuring position against is moving relative to me (c.f. the example of two people walking in parallel across the Earth, nevertheless eventually meeting: the curvature means that even though neither of them is measuring an acceleration, nevertheless they are accelerating towards each other), so I need to have some internal ("proper") acceleration of my own to counteract the fact that our geodesics are moving away from each other.

⬐ charles_f

This is the first explanation of relatively I actually feel I understand

⬐ deostroll

Makes one wonder how the star trek enterprise "works". They travel warp 8 or 9 and the crew "just" stand there like we do on earth...Why don't they experience any G-Forces? :)

⬐ Doxin

Well that's easy, they have inertial dampeners of course ;-)

⬐ kristianpaul

The General Theory of Relativity tells us gravity is not a force, gravitational fields don't exist. Objects tend to move on straight paths through curved spacetime.