[Mertax]

There is no absolute direction for a galaxy’s spin—it’s always relative to the observer’s perspective.

So I’d suspect they’re saying time and distance would need to be factored in rather than just looking at static images relative to our position today since our own spin may have caused a particular galaxy to appear to have been spinning in a different direction at another point in space-time

[tremon]

I don't understand this logic. To me, that's equivalent to saying "there's no absolute direction for which way a wheel spins, it's relative to the speed of the observer". Which makes no sense to me, because my definition of spin is measured against the axis of rotation of the object itself.

I don't see how time-intermittent frame captures from our own position affect that interpretation. Or are we using an astonomy-specific definition of spin here?

[doug-moen]

It's true, there is no absolute direction for which way a wheel spins. It's relative to the observer.

If you are standing on the side of a road, and a bicycle goes by, then you may observe the wheels to rotate clockwise, while an observer on the other side of the road will observe the same wheels to rotate counter-clockwise.

The sun is said to rotate around the centre of the milky way galaxy once every 225 million years. Over that time frame, some of the galaxies we observe will flip between clockwise and counterclockwise rotation as our viewpoint changes.

But that isn't relevant here. The Space article is too vague and handwavy to make any conclusions about the research, and should be ignored. Only the original scientific paper is worth reading: https://academic.oup.com/mnras/article/538/1/76/8019798?logi...

Section 5.2 "Physics of Galaxy Rotation" seems particularly relevant.

> due to the Doppler shift effect galaxies that rotate in the opposite direction relative to the Milky Way are expected to be slightly brighter than galaxies that rotate in the same direction relative to the Milky Way. Therefore, more galaxies that rotate in the opposite direction relative to the Milky Way are expected to be observed from Earth, and the difference should peak at around the Galactic pole. That observation is conceptually aligned with the empirical data of Fig. 10, and the observation using JADES described in Section 3.

You should read the paper for the full argument.

[jacksnipe]

But what if you, the observer, are also rotating? What if you’re rotating faster than the thing you’re observing?

[cwillu]

I don't see how that could be so prominent as to reverse the visible arms of a spiral galaxy, what am I missing?

[d1sxeyes]

It’s the same as saying you can only work out how fast something is moving in relation to something else. Your car is doing 50mph on the highway, but the earth is spinning round the sun and the sun is moving around the centre of the galaxy and the galaxy is moving compared to other galaxies and so on.

[cryptonector]

> There is no absolute direction for a galaxy’s spin—it’s always relative to the observer’s perspective.

Not entirely. The galaxy is bound by gravity and the stars rotate in the galaxy around its baricenter. We can compute how fast it must be rotating from the amount of visible matter. Enter dark matter and various complications, but still, you can tell that it's rotating and which way.

And for galaxies we see edge on we can use the difference if redshift on one end versus the other to tell which way it is turning.

[Y_Y]

I don't necessarily agree, in the presence of a universe (and under some reasonable cosmological assumptions) you can't just get rid of an observed rotation by a change of inertial frame. You can rotate along with it, but you'll produce a tell-tale fictitious force.

See e.g. https://en.wikipedia.org/wiki/Mach's_principle