Does rotation break relativity?

Submitted by starfyredragon t3_zmt3lg in askscience

Something has always bugged me with relatively... and that's... rotation. I can see how something moving up is mathematically the same as other stuff moving down... but rotation seems like it breaks that concept.

If I stand in place, and do a 360 rotation, I'm spinning in place. But once we interpret it as, instead, the world revolving around me, it becomes a problem. Distant galaxies, following that wide arc, have traversed billions of lightyears a second, relative to my rotation. Just centripetal force alone should reduce them to dust.

So how does relativity of motion reconcile with things rotating?

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Weed_O_Whirler t1_j0dlrc3 wrote

Relativity doesn't say "all frames of motion are equally valid." It says "all inertial frames of motion are equally valid." That word "inertial" is doing a lot of work- it means the reference frame cannot be an accelerating frame. When you're spinning, you are in an accelerated frame, so we do not consider that "just as valid" as any other frame.

Now, that might seem like an arbitrary distinction- but it's really not. The reason is- acceleration can be measured, speed cannot. That is, if you are accelerating, you can perform an experiment/take a measurement that would determine if you were undergoing that acceleration. That is, if you were in a car, accelerating towards a wall, you don't have to say "I wonder if I'm accelerating towards the wall or the wall is accelerating towards me." If you're accelerating, you can feel it- you get pressed back into your seat, etc. But if you are moving at a constant speed towards a wall, there is not experiment/measurement you can do which answers "am I moving towards the wall, or is the wall moving towards me?" Sure, you might have to say "man, the wall, the ground, and everything around me is moving towards me" so in some logical way you could say "so obviously I'm the one moving" but there isn't anything that you can do to "prove" it. Another way of putting it, accelerations are absolute, speed is always measured in relation to something.

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starfyredragon OP t1_j0dr6ed wrote

Thanks, that makes it clear!

So movement is relative, but changes in movement aren't. Weird, but makes sense.

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littlebitsofspider t1_j0dvema wrote

This is actually a super deep question you've asked, OP. Check out Mach's Principle and absolute rotation, these things are being debated even today.

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Experienced_AP t1_j0gl0ys wrote

Explanations of Mach's principle and the analogy of the bucket rotating with the water inside (was that Newton?) make my brain go blue screen.

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obog t1_j0fdjxw wrote

Linear movement continues without any force, however rotation needs a constant force to continue, that being the centripetal force. Seems odd because a force doesn't seem to be applied to a spinning object in space, but technically there has to be some force on anywhere outside of the center of mass, likely some sort of tension force (or in the case of larger objects like planets, gravity).

Edit: I want to clarify that that necessary force isn't necessarily external. A rotating object will continue rotating without an external force, however within the single object, particles near the outside of rotation will have a force of tension pulling them back towards the center, which is also the centripetal force. If that force didn't exist, the object would break apart.

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JCSterlace t1_j0ggxgj wrote

The rotation of an isolated solid object does not need a constant force to continue - angular momentum is conserved. Some internal forces holding the object together (to make it solid) would be acting centripetally, but that's internal to the object, not acting on the object.

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obog t1_j0gjdr9 wrote

Those internal forces are what I meant, as such the object is still technically accelerating

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VoilaVoilaWashington t1_j0gmqc8 wrote

The internal forces are causing the outer parts to accelerate around the center. If you swing a hammer while you spin in a circle, it's your hand that's accelerating the hammer and keeping it moving around you. Let go, and it goes flying.

That was their point - you need something keeping it all together outside the center of mass, or it will just fall apart.

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[deleted] t1_j0fkpbk wrote

[removed]

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Quartersharp t1_j0fnrfp wrote

It does… a little bit. A rotating object will have its outer parts under tension because of the centripetal acceleration, which wants to pull them outward. Unless the object flies apart, it is doing a tiny amount of work by staying intact while rotating, because its outer parts have to keep changing direction. Because of this, the object will also radiate gravitational waves, and will very gradually slow down, probably over thousands and millions of years.

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rpetre t1_j0forax wrote

Nitpick: "centripetal" means "towards the centre", so in your example the tension IS the centripetal acceleration, it's what keeps the parts of the body on their respective circular trajectories.

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wasmic t1_j0g22v9 wrote

No. There's no work being done from the rotation itself, if the object is perfectly rigid. Of course, in the ideal case there is work being done due to tiny stretches all over the place, but that should all cancel out because it both stretches and contracts to keep the same shape. With no overall radial motion coaxial with the force, there is no work being done.

Also, a spinning sphere, or a cylinder spinning around its own axis, will not emit gravitational waves. But something like a rotating cog would emit gravitational waves.

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Putnam3145 t1_j0g7zxw wrote

A perfectly rigid object has a faster-than-light speed of sound, among other problems, and is thus unphysical

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PefferPack t1_j0fu27b wrote

>it is doing a tiny amount of work by staying intact while rotating, because its outer parts have to keep changing direction

Really? This is kindof mind-blowing if true. Work is force dot displacement, so if there's no displacement in the radial direction, then there's no work done. If the body can be considered rigid then there is no radial displacement. Even in a flexible body, there would only be a short transient period of radial displacement as it stretches out from the rotational forces.

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wasmic t1_j0g1tt6 wrote

Yeah, I don't think there's a connection between gravitational waves here, and there's no work being done to keep a rotating object together.

Gravitational waves are not emitted from spherically or cylindrically symmetric objects, either (provided the cylindrical axis is the rotational axis).

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TheSmartestBanana t1_j0gjwf6 wrote

I believe the net centripetal force (toward the center) is balanced out by the net centrifugal force (toward the outside/perimeter). Therefore there is zero net force and zero net acceleration on an object rotating at constant angular velocity.

A force does not need to be applied to keep an object spinning in space (think planets and moons). It is valid to say that an object that is rotating will remain in rotation unless acted upon by an outside force (conservation of angular momentum).

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obog t1_j0gnqyo wrote

The centrifugal force isn't real, it's only an observed force when you're inside the rotating frame of reference. A constant force does still have to be applied, it's just often an internal force, not an external one. In the case of an object rotating in space, there is a force of tension acting in the outer parts of the object pulling them back towards the center. Due to that the object will stay in rotation without any outside force, but the key term there is outside. there still has to be some force to keep an object rotating, it can just be an internal force. That internal force (as I said, the force of tension for an object rotating) is the centripetal force.

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TheSmartestBanana t1_j0gqq4u wrote

Centripetal force can only be felt by a point inside the rotating body as well. That point obviously has an acceleration because the directional component of its velocity is changing constantly. The rotating object as a whole is not accelerating and therefore requires no net force. There are a lot of forces that hold an object together, but those forces do no cause an acceleration on that object and therefore cause no force to act on the object itself as a whole.

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atmsk90 t1_j0hgbik wrote

I think there is some confusion between static and dynamic equilibrium here. Centrifugal "force" is simply inertia. It's much easier to visualize by considering a point mass on a massless arm. The arm has to exert a force on the mass not as a result of any applied force from somewhere else, but just as a result of needing to accelerate the mass toward the center of rotation.

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hydroxypcp t1_j0kuo20 wrote

for this question you do have to look at individual particles of the rotating body though. If we take a human body as the rotating body, then the eyes are accelerating and thus not an inertial frame of reference

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_Jaquen_Hgar_ t1_j0giqrw wrote

Yes it is very weird. It highlights the fact that there is no such thing as absolute position, only relative position.

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Aedene t1_j0gmuxi wrote

I also would think that rotational movement is not movement through spacetime, even if it changes the "1st person" frame of directionality (forward is now right, then behind you, then left, etc.), it doesn't change the reference of ones position in spacetime. So, even if you were rotating at a constant rate in the vacume of space, not accelerating, your frame of reference is the same as that of a rotating planet or star.

The night sky "spins" at well above the speed of light for the furthest stars, but that's never contradicted relativity because that's based on position, not rotation, over time.

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MagicSquare8-9 t1_j0jjg0g wrote

The above poster are talking about special relativity only. It's "special" because it's only specific to inertia frame. So for the sake of completeness I will talk about general relativity, which follows the principle of general covariance, in which all frame of references are valid; in other word, all time and distance measurement are relative. This should show you the real crux of the problem: it's not that "acceleration is absolute and speed is relative", but rather "physical constants are differed between accelerating frame".

To achieve general covariance, general relativity comes with metric tensor. The metric tensor measure proper time and proper length, and this way we unshackle the concept of length and time from the coordinate. The metric tensor is obviously needed since an arbitrary coordinate system means you can pick system in which directions are distorted (e.g. 2 units to the right has equal length as 10 units to the front).

So you're now free to rotate your coordinate system at will. These distance galaxy will move at insane speed in your system of coordinate, but if you look at the metric their speed is completely normal.

So what's the lesson from this? Coordinate system is a completely arbitrary construct that has nothing to do with actual physic. There is nothing wrong with an object moving fast according to the coordinate system. If you have a rotating coordinate system, it's as strange as having a scaling coordinate system where the length of the axes changes over time; in either case, far away objects will appear to move too fast according to the coordinate system, but nothing actually physically change.

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nsjr t1_j0f3jhe wrote

I'm just trying to wrap my mind around this... It's 2am, so please give some slack:

Can we consider that Earth is "accelerating"/spinning referent to the sun, because it's rotating around the sun? And the sun around the galaxy? And the galaxy around some center of mass of local group?

Because as far as I understand, the same "rotation" movement is done from the surface of the Earth to its center of mass, as a simple orbit from Earth to the sun.

So calculate the real time/speed of something really far, like another galaxy, can become very tricky because we are accelerating in many different referentials

Or I'm making mistakes with different concepts here?

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girhen t1_j0f8rc8 wrote

No, you're onto the idea.

Reference frames are in relation to what makes sense. It's easy to choose either an inertial frame with reference to your car or the ground when you want to talk about movement in relation to the two. It's hard to make a fixed point for something as big as galaxies.

The thing is, when it comes to calculating position of objects that are as far away as galaxies, you can basically consider their position fixed. If I put you on a merry go round and said there was a car moving at 5mph 1000 miles away, its movement wouldn't be enough to matter to you much over the course of a couple hours.

The solar system moves at 140 miles per second. That's fast. But the closest star to us is Proxima Centauri - 4.25 light years away. 140 miles per second is .000751547c (c is speed of light). It would take 5,655 years for us to reach Proxima Centauri if it were stationary and we moved directly at it.

So yes, there are many differentials to consider. One of the ways we consider speed is by using the doppler effect of light - red shift and blue shift - to determine speed we're gaining/closing on it based on known colors we expect from stars.

So yeah... it's complicated. Astrophysics is not known to be an easy field to comprehend, much less do.

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Game_Minds t1_j0gt1ju wrote

We can actually use the doppler effect on nearby stars to determine how fast they are rotating! The light from one half of the star is bluer than the other because part of the surface of the star is rotating towards us, and part away! Add up the difference in frequency (and do math), that tells you the difference in their speeds, half of that is how fast it's rotating! This is also handy when determining things like a star's absolute color and luminosity, as its spectrum can appear shifted or blurry if the star is rotating very fast or at a funny angle

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NeverPlayF6 t1_j0gizvi wrote

> If I put you on a merry go round and said there was a car moving at 5mph 1000 miles away, its movement wouldn't be enough to matter to you much over the course of a couple hours.

This is a small angle approximation, right?

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girhen t1_j0gxhv7 wrote

Absolutely. There are two orders of power between the observer and the two objects. Typically, 15 degrees is acceptable to use it, or 10+ times the distance of the observer from the two objects. 100+ times the distance is preferable, which is still the case of the car after moving for 2 hours.

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aaeme t1_j0fcos6 wrote

>Can we consider that Earth is "accelerating"/spinning referent to the sun, because it's rotating around the sun? And the sun around the galaxy? And the galaxy around some center of mass of local group?

Absolutely we can and should but with a big caveat: general relativity tells us that the earth, the sun, the galaxy and everything that is only 'moving' due to gravity is following geodesics: 'straight' lines in a curved spacetime; they're not accelerating relative to spacetime; they're weightless in free fall. Just like a 'stationary' object infinitely far from any gravitational effects.

Nevertheless, things are definitely in motion. There is no frame of reference where everything is still. The earth is definitely rotating around the sun (and the sun around the galaxy) relative to any outside observer. But from the earth's frame of reference we can consider ourselves 'stationary' because we don't experience any acceleration from our orbit.

If we never saw the night sky it would be difficult if not impossible to prove by experiment that the earth is in motion around the sun. We could prove it's spinning at 1 rev per day because that's not due to gravity, the real forces of atoms in the Earth's crust are constantly accelerating us upwards against the freefall flow of spacetime and that allows us to spin at less than orbital speeds and feel the weight of things.

>So calculate the real time/speed of something really far, like another galaxy, can become very tricky

It is tricky to measure distant velocities but not really because of our motion or any acceleration: accelerations are rarely big enough to make massive changes in velocity quickly. There are exceptions: measuring the speed of expansion of a supernova would be difficult because it's accelerating (or decelarating) hard but not so for the motion of a star or galaxy across the sky. The difficulties in measuring that are nothing to do with its acceleration or ours. They are because all velocities are relatively small compared to the enormity of space. They might take a century to move a thousandth of a degree across the sky.

Nevertheless, for really accurate measurements of, say, the velocity of Andromeda relative to the Milky Way, we do have to take into account the movement of the Earth round the Sun and the movement of the Sun around and within the Milky Way but that's pretty easy to do, we know those velocities well and we just subtract them from the raw measurements.

I hope all that makes sense and I haven't laboured the point.

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pelican_chorus t1_j0gmcvb wrote

>If we never saw the night sky it would be difficult if not impossible to prove by experiment that the earth is in motion around the sun.

Am I right in thinking that there is, in fact, no experiment that could tell whether we were moving around the sun, or the sun moving around us (in the same way that a car moving at constant speed towards a wall can say whether it is moving or the wall is moving)?

Spinning, however, seems different, right? We can tell that the Earth is spinning on its own axis using Foucault's Pendulum, right?

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Game_Minds t1_j0gtonc wrote

Well no. If we had no other ways of determining the mass of the sun, then maybe. But we can do gravitational lensing measurements and things, and the whole eclipse thing, and fusion doesn't happen at earth sized masses, etc etc etc. There are many ways to determine that the sun is millions of times the size of earth, and that the system is spinning, and put both together and we are orbiting the sun

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aaeme t1_j0h75op wrote

The hypothetical scenario is something like if we always lived in caverns miles beneath the surface so didn't even know the sun existed. Could we tell by local experiments that the earth was in motion and in orbit and the metrics of that orbit?

I think in theory we could from tidal forces and we'd notice a periodicity of a year for those and could in theory work the rest out.

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pelican_chorus t1_j0ha7gb wrote

But isn't an orbiting body in free-fall? And a body in free-fall can't feel the force of the body it's falling towards, right? That was my point. Isn't it an inertial frame?

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aaeme t1_j0hb31e wrote

At an infinitesimal point yes but across a volume tidal forces exist. There would be a slight stretching in the direction of the Sun and squeezing tangential to the Sun.

Across a year those forces would rotate 360° and not uniformly (we could calculate the eccentricity of the elliptical orbit from that).

Edit: measuring all that would be a lot more difficult because the earth is spinning so would have to figure that out so we can subtract the effects of that on our measurements.

It's hard to imagine how any species could properly understand physics well enough to do this without seeing the night sky so perhaps a better mind experiment is if a scientist got teleported to a sealed windowless box on a random planet somewhere in the universe could they tell by measurement whether the planet was in orbit around a star or not and the details of that orbit. I'm 99% sure they could from tidal forces and possibly by other means too.

Edit 2: another way to tell would be from the time-dilation differential from one side of the room to the other. A clock slightly deeper in the star's gravity well would run slightly slower.

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obog t1_j0fdou9 wrote

Yeah, the earth is accelerating, exactly in the direction of the sun. It's falling towards it, but it's momentum keeps it from actually getting any closer. Even though the earth has reached a kind of equilibrium where it will stay in orbit, its still constantly accelerating towards the sun.

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Weed_O_Whirler t1_j0h7az2 wrote

So, it is true. Trying to calculate the time dilation between Earth, and say, some planet on the opposite side of our Milky Way would be very, very difficult. But at the same time, compared the the speed of light, we know those other planets are moving really, really slowly, and thus the differences are minute.

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Sparlingo2 t1_j0emma4 wrote

Einstein got on a train from New York to Boston and after fifteen minutes he asked "when does Boston Arrive?" From his perspective all was moving outside and he was standing still.

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alukyane t1_j0f61ht wrote

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

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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"

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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

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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
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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?

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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.

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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

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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.

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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...

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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.

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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.

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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.

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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.

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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

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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)

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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.

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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?

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Alt-One-More t1_j0f95lk wrote

So what about the alternative where OP is in space and spinning but not under acceleration?

Or are you always experiencing acceleration when rotating?

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Weed_O_Whirler t1_j0ghrs4 wrote

Rotation is an acceleration. Your velocity is always changing.

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GearRatioOfSadness t1_j0g1d4j wrote

The classic example of time dilation is "an astronaut flies around the earth at near the speed of light a few times and returns. Everyone has aged more than him." I assume that holds true but I always wondered, if speed is relative why is the astronaut the one moving fast and not the earth or both.

Are you saying that because the act of orbiting or going in a circle around the earth involves acceleration perpendicular to the direction of travel, that the speeds are not relative and the astronaut can be determined as the one that is in fact moving fast?

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man-vs-spider t1_j0g1yab wrote

Can I ask a follow up / clarification?

Is a free fall frame of reference inertial? You are accelerating, but it seems like it is equivalent to just floating around in free space

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WittyUnwittingly t1_j0g6vpt wrote

So, how then, would one account for mathematically, something spinning at relativistic speeds. Black holes do this, don't they?

Is it just a transformatiom of the time dilation equation into circular [spherical/cylindrical] coordinates?

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saschaleib t1_j0hp3pw wrote

Oh wow, an explanation of a really complex issue that even I could understand. I tip my hat with respect!

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Ancquar t1_j0f9xbf wrote

Let's say you have a fast-moving object in space and two points on opposite ends of the earth, one which is currently carried by earth's rotation towards it, and one away. If the scale of time dilation depends on relative speed, then the two points on Earth should have different relative speeds and thus time, however you'd expect their time to be the same. In a short time frame that system would be more or less inertial, no? This is something I never understood

0

MagicalSkyMan t1_j0fatgf wrote

So I'm a total noob at physics but I disagree with your claim about acceleration being more measurable than speed.

Being pressed back to your seat is a consequence of the car seat being the source of the force that is making you accelerate while the force is not being distributed to each of the particles (that you are composed of) at the same instant. If the car was transferring that same force (like magnetically or something) to each of your particles uniformly then you wouldn't be noticing the acceleration as there would be no compression happening within your body.

Maybe there is some other effect that happens during acceleration that could be used to determine acceleration is taking place? I'm trying to think in terms of a single electron or neutron or something for simplicity. Not coming up with anything so far.

−9

Amadex t1_j0fvu4q wrote

>If the car was transferring that same force (like magnetically or something) to each of your particles uniformly then you wouldn't be noticing the acceleration as there would be no compression happening within your body.

It's paradoxical because would need an accelerometer to know the intensity of the field that you would need to cancel acceleration (putting aside the fact that it is relatively trivial to measure magnetic fields and therefore distinguish it from acceleration).

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midnight_mechanic t1_j0e1sr1 wrote

In case you are unaware, circular motion is acceleration. Technically circular motion is described as acceleration perpendicular to your direction of constant velocity.

You can feel this lateral acceleration when you are in a car and make a hard turn at speed. Your car is still traveling at the same speed the whole time but you feel the car try to move out from under you and you feel forced to the outside of the turn.

Also with gravity we feel the force of gravity pushing us downward, but we don't feel the lateral velocity component of the Earth's rotation.

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starfyredragon OP t1_j0ekbul wrote

So motion is relative, but acceleration is not.

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BadBoyJH t1_j0f3tln wrote

Not quite, this isn't about acceleration vs motion, but that motion is relative in an inertial frame of reference.

If you are accelerating, then you're not in an inertial frame of reference, so it's not the same as the distant stars exceeding c.

If something else is accelerating, then that is relative, and can be viewed differently depending on the frame of reference.

As a semi-practical example, a plane taking off on an aircraft carrier. Let's have him take off backwards for the purposes of explanation, even though they wouldn't practically.
Let's say an outside observer is watching the plane take off, going back off a ship travelling forwards at 30kph. To the outside observer the plane accelerating from 30kph, through 0, and then out to -170kph.
But from someone on the ship, the plane has gone from 0 to 200kph.
That acceleration is still relative in both of those frames of reference.
But the pilot in the plane is not in an inertial frame of reference, and so it's not the same as the world accelerating under him (eg he experiences inertial G forces).

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Octolavo t1_j0ez4cj wrote

Mach’s principle states that rotation is absolute with respect to the distant stars and the overall distribution of matter in the universe.

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Game_Minds t1_j0guh1t wrote

And it's a principle because while it's handy and you can use it to make correct assumptions about the universe, it's also sort of fundamentally unprovable

Gödel is one of my all time faves lol

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larikang t1_j0h6gbc wrote

> Also with gravity we feel the force of gravity pushing us downward

Isn’t it the other way around in relativity? When you are in free fall, you feel no force from gravity. When you are standing still you feel the force of the ground accelerating upwards into you.

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midnight_mechanic t1_j0h9sxi wrote

No. That's all backwards.

In free fall you are accelerating towards the earth (or whatever massive object). As you get closer to the massive object your rate of acceleration will actually increase. The acceleration due to gravity is 10 or 12% less at the international space station than it is on earth, for example.

You can always measure when you are in a gravitational field (or being accelerated). For that reason, it wouldn't be correct to say you don't feel any force from gravity.

The force due to gravity you feel on earth is you are being pressed/accelerated into the surface of the earth. Whatever you are standing on is strong enough to resist the pressure from you being forced into it, unless the floor collapses.

The ground is *stationary. It doesn't accelerate into you.

Stationary is a relative term that means whatever I want it to mean as long as my frame of reference is consistent. For this reason, for these examples, the earth is unmoving, stationary and eternal.

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EuphonicSounds t1_j0kyyzz wrote

Actually, you have it backwards: an accelerometer in free-fall measures 0. If you're interested, read up on the equivalence principle, which is a cornerstone of general relativity.

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larikang t1_j0hdt9a wrote

> You can always measure when you are in a gravitational field (or being accelerated). For that reason, it wouldn't be correct to say you don't feel any force from gravity.

When astronauts do low-G training in the vomit comet they are definitely in a gravitational field and (in your words) are "being accelerated". But they feel no force of gravity.

Or as a similar thought experiment: imagine that I am in a spaceship with no windows traveling at a constant speed far from any large mass. If the spaceship started to approach a large planet, would I feel a force of gravity pulling me in toward the planet stronger and stronger as I approached it? My understanding of relativity is no: since I am in an inertial reference frame it makes no difference to me.

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Dave37 t1_j0fryc6 wrote

You are absolute correct. Relativity only holds for inertial reference frames, and rotation is not. Rotation happens under constant acceleration, so it's not hard to notice which part is rotating and which is still.

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HonestWestern8594 t1_j0gs0p3 wrote

All the other answers here are good but I want to add that for your example General Relativity which handles accelerating frames of reference is the theory that deals with accelerating frames. The relativity you're referencing is special relativity. So, although rotation isn't handled by special relativity, general relativity can handle rotating frames of reference or any other accelerating frames of reference.

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starfyredragon OP t1_j0ibyin wrote

Interesting... so does that means even though something can't move faster than the speed of light, it could hypothetically spin faster than light if it could maintain structural integrity?

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FutureMartian9 t1_j0hpa6e wrote

Tangent of the actual question, but with a spinning object, the parts furthest away from the center of rotation should experience a time shift relative to the parts at the center of rotation, right?

For example a large disk of radioactive material spinning - the radioactivity at the outside edge would decay faster than the radioactivity at the center?

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starfyredragon OP t1_j0iauow wrote

I thought the edge of a disc would decay slower, since they're going closer to accelerating to lightspeed, therefore they're the ones experiencing time dialation?

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Pipe_Dry t1_j0gs6t0 wrote

I remember when I was in the eighth grade, thinking about how I could defy the speed of light. Why not take scissors as long as the distance between the Earth and the sun and attempt to close them. The arc that would be produced would exceed the speed of light. I’m sure most preadolescents interested in science contemplated this. Even if you could produce a material, that could take such a force the amount of energy capable of swinging the arms would probably exceed the output of the sun. After that exercise, I went back to thinking about why I just got a D on my algebra test.

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Chemomechanics t1_j0icfp0 wrote

The scissors example is particularly interesting (relative to the thought experiment of pushing a long rod) because no part of the scissors can move faster than light, and the mechanical wave that causes the scissors to close can't move faster than light, but the point of first intersection or overlap between the two blades (i.e., the point where cutting starts with ordinary scissors) can move faster than light. There's no object corresponding to that point, just a geometrical abstraction.

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linkjo100 t1_j0hfquj wrote

The premise is flawed. You are assuming that the information between each atoms is instantaneous which it’s not.

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avian_aficianado t1_j0ly7wp wrote

The propogated acceleration outputs between electromagnetic forces of the superliminal scissors atoms would degrade before the point of intersection( The energy needed to accelerate anything to light speed is infinite, but the amount of energy needed to break a particular thing is finite.)This entire thought expirement is contingent on infinite energy being available, however, and the amount of paradoxes that woud arise must be considered. Time dilation and the asynchronization between the inertial frames would affect the time flow frame as well.

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aether_drift t1_j0fmfal wrote

You are not a point particle. At least I assume.

Your arms etc. would have to accelerate at a different rate than say your liver to achieve the same RPM as they would end up moving faster.

Therefore your arms and liver experience different relativistic time frames - your liver be getting older faster dude.

Boy isn't that the truth, I regret every cocktail. No I don't.

If only the world were to rotate, then no part of you would undergo any acceleration.

I'm probably wrong. I am drunk.

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