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alukyane t1_j0f61ht wrote

Reply to comment by Weed_O_Whirler in Does rotation break relativity? by starfyredragon

Something's weird in this explanation. Isn't freefall/orbiting indistinguishable from 0-gravity because everything in your environment is experiencing the same forces?

9

OathToAwesome t1_j0fgncy wrote

really pedantic note: it's "zero-g" (as in g-forces, the force you feel from acceleration) and not "zero-gravity"

23

rexsilex t1_j0g0sxw wrote

Weightlessness is called all those things. They're interchangeable terms. Though none imply that gravity isn't still working on you

0

Alis451 t1_j0ghk8i wrote

The Space station is at 0g, but they aren't far enough outside of the Earth's Gravity well to be at Zero Gravity, because if it were it would start sticking itself, Earth's Gravity supersedes your own in relation to other nearby objects. So YES, there is a VAST difference between 0g and Zero Gravity. The space station is in a constant freefall and needs to continuously adjust.

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Max-Phallus t1_j0gk2us wrote

If anyone is interested, gravity on the IIS is still 88.33% of earth's gravity. The IIS orbits around 408KM from sea level.

Here is a PowerShell script I wrote that will calculate earth's gravity at any given altitude (if you have windows, you can open it and copy&paste it in):

function Get-GravityAtAltitude
{
	[CmdletBinding()]
	Param
	(
		[double]$AltitudeKm,
		[switch]$ReturnStats
	)
	BEGIN
	{
		$EarthParams = @{
			MeanRadius = 6371.009 # KM
			SeaLevelGravity = 9.80665 # m/s^2
		}
	}
	PROCESS
	{
		$AltLevelGravity = $EarthParams['SeaLevelGravity'] * [Math]::Pow($EarthParams['MeanRadius'] / ($EarthParams['MeanRadius'] + $AltitudeKm), 2)
		if($ReturnStats.IsPresent)
		{
			$EarthParams.Add('Altitude', $AltitudeKm)
			$EarthParams.Add('AltLevelGravity', $AltLevelGravity)
			$EarthParams.Add('GsAtGravity', ($AltLevelGravity / $EarthParams['SeaLevelGravity']).ToString('.00%'))
			return [PsCustomObject]$EarthParams
		}
		else
		{
			return $AltLevelGravity
		}
	}
	END {}
}
Get-GravityAtAltitude -ReturnStats -AltitudeKm 408
7

SomeoneRandom5325 t1_j0fa7th wrote

Yes, and a freefalling/orbiting reference frame is inertial and a reference frame on the planet is not, but general relativity makes things weird

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obog t1_j0fdv6c wrote

Wouldn't an orbital frame not be inertial? I mean in a small scale it would appear so, but an orbital reference frame would be the same as a rotational frame which is non-inertial. That can be proven by the fact that if you stick two object close to each other in orbit, they will drift around from where they were relative to each other. That wouldn't happen in a fully inertial frame of reference.

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mnvoronin t1_j0gbi6c wrote

Technically not and you can perform some tests confirming that (objects on the far wall will be accelerating ever so slightly compared to the objects on the inside wall), but the effect on the typical spacestation scale is very small (in order of nanometers per second squared).

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obog t1_j0gjarp wrote

Well yeah, as I said on small scales it does seem to be inertial but it isn't quite and those effects are noticeable between multiple objects in similar orbits.

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alukyane t1_j0get2c wrote

So if they're indistinguishable, I shouldn't be able to measure different "absolute acceleration" in the two, right?

1

ableman t1_j0hd6f8 wrote

Yes, though I feel like there's some confusion here. An orbiting/freefalling reference frame is indistinguishable from a non-accelerating one. The reason they're indistinguishable is because of general relativity. Or phrased another way, them being indistinguishable is really weird and why we need general relativity.

1

Game_Minds t1_j0gr3sv wrote

Any hypothetical measurements of acceleration would be skewed by miniscule differences in things like local gravity and additional undetected rotations, so it would be hard to pinpoint why they don't match up

0

Weed_O_Whirler t1_j0h98fa wrote

A truly uniform gravitational field is inertial, yes. But those don't really exist (we say, for instance, that on Earth the acceleration due to gravity is -9.8 m/s^(2) but there is a slight height dependence on it). So, things in free fall (without air resistance- including orbits) we will say they are in "locally inertial" frames. But even in the ISS, there will be slight tidal forces acting on you- aka, the side closer to Earth will have ever so slightly more gravity than the side further away.

1

alukyane t1_j0hbl7w wrote

Ok so then what is measurable is local variations in acceleration, not some global acceleration relative to all inertial frames.

And sure in reality uniformly-accelerating frames don't actually exist, but that also includes the zero- acceleration case, since there's always some galaxy far far away applying a force...

1

Derekthemindsculptor t1_j0gkszd wrote

Freefall just means falling without anything to help you maneuver. The second you jump from a plane, you're in freefall. Technically, when you do jumping jacks, you're in freefall on the way down.

What you're asking for is, falling at your terminal velocity. Where you are falling but aren't accelerating because that's the fastest you can go due to friction. At this point, all the forces are balanced out, similar to being stationary. You're moving, but not accelerating.

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Aescorvo t1_j0gg0dn wrote

It’s because an orbit isn’t a rotating frame - the pull of gravity is always down, you’re just moving sideways so fast you miss the planet, by which time the angle of gravity has shifted so that it’s still down.

−2

dogninja8 t1_j0gi45t wrote

Isn't orbit always a non-inertial (accelerating) reference frame, since your velocity vector is always changing direction? The only way to keep your velocity vector pointing in the same direction (relative to you) is to be rotating yourself, which is also a non-inertial reference frame.

5

alukyane t1_j0ggjgs wrote

It's definitely an accelerating frame, since gravity is acting on it. Probably rotating, too, at least around the planet. In any case I'm mostly interested in how free fall could be distinguished from 0 gravity.

4

Aescorvo t1_j0gh3zo wrote

I didn’t say it wasn’t accelerating.

Maybe I guessed wrong at what part you thought was weird. There’s no different between freefall and zero gravity. Although, for the special case of an orbit there are slight differences you can detect at different heights.

3

Game_Minds t1_j0grywh wrote

Can't all sublight paths through a relativistic spacetime be characterized as orbits? Even in intergalactic space objects' paths are curved by gravity. There would still be slight angular accelerations on basically any "straight" path even if they even out over time

1

Moikle t1_j0gu1xq wrote

Free fall IS zero gravity (although tehnically there is no such thing as zero gravity and the concept doesn't really even make sense physically)

0

Alis451 t1_j0ghvj9 wrote

>how free fall could be distinguished from 0 gravity.

The Space station is at 0g, but they aren't far enough outside of the Earth's Gravity well to be at Zero Gravity, because if it were it would start sticking itself, Earth's Gravity supersedes your own in relation to other nearby objects. So YES, there is a VAST difference between 0g and Zero Gravity. The space station is in a constant freefall and needs to continuously adjust.

−2

Aescorvo t1_j0gs39z wrote

Actually, let me amend what I said. Without looking out the window (metaphorically) you couldn’t tell. However, if you had a clock on board, and and an identical clock far enough away that it was effectively in zero gravity, AND you could view it through a telescope each revolution, you would (eventually) see that the clock runs faster than yours. That would at least tell you that it was experiencing lower gravity than you were.

This is akin to one of the effects that we have to account for with GPS satellites. Putting a clock in orbit (with a big enough display) would let us see the discrepancy compared to an identical clock on the surface, deeper in the gravity well.

1

alukyane t1_j0h92dr wrote

We then seem to agree that the top-level claim above about acceleration is wrong: you can't actually tell whether you're in an inertial or accelerating frame, if the acceleration is the same for all observable objects. Right?

1