Relativity can make sense, once you get used to it.
But nobody ever gets used to quantum mechanics.
Right. It’s very much like using the local direction of down as a preferred direction- everyone on Earth can do it, but everyone has a different preferred direction - since “down” is a different direction at every different point on Earth.
"If you think you understand quantum mechanics, you don’t understand quantum mechanics.” ~ Richard Feynman
“Those who are not shocked when they first come across quantum theory cannot possibly have understood it.” ~ Niels Bohr
Yeah I’d actually disagree that relatively is particularly counter intuitive. It has its weird parts but it basically flows from these bsic facts:
- Everything is relative. There is no global frame of reference. Without an external reference point you fundamentally cannot tell the difference between two objects enclosed in a space ship going 90% the speed of light, and the same objects stationary in space.
- You similarly cannot tell the difference between an object in a spaceship accelerating at 1g and an object sitting on an earth sized planet and experiencing gravity (this was what special? relativity added IIRC)
- The speed of light is the same in all frames of reference. it doesn’t behave like sound waves where they appear longer or shorter to observers moving through them compared to a stationary observer.
And I’d definitely strongly disagree you should just give up if you don’t get it. There are some complicated concepts but nothing completely incomprehensible to a layman. I’d recommend this book:
Quantum physics on the other hand is just a freak show 
The thing is, our intuitive understanding of physics - motion, velocity, acceleration, etc - is messed up by friction, gravity and air resistance. Even Aristotle, trying to explain why an arrow could travel fast and far compared with things along the ground, hypothesized something along the lines of “bird feathers want to fly, so the feathers on an arrow help it fly fast by pulling the air past.” It was a major leap for Newton to say things in motion tend to stay in motion, and that they don’t in the real world is due to friction and air resistance.
And that’s not even accounting for relativity.
Relativity, as Einstein imagined it - I measur light travelling toward me at c velocity. You zip by me riding a spaceship that I see as going 0.5c - what do you see? You also measure the same light also travelling toward you at c velocity. that means time is travelling slower for you than me. Follow this to its extreme, and time goes slower for the person moving wrt you, distances in the direction of travel measure less and mass increases. Yet from their point of view, the problem is you - your time goes slower, your mass increases and distance decreases. How do we reconciles these? we don’t. “The same time” (simultanaeity) does not exist in relativity, unless it’s at the same place too.
hence…
BTW, that’s simple relativity. Add in acceleration or deceleration - as in “then the astronaut returns to where he started on earth” and things get even more weird - special relativity.
Actually it was Galileo who said that, a generation before Newton. Galileo also proposed Galilean relativity, that all motion is relative to a reference frame. Galilean relativity is pretty intuitive to understand. The nonintuitive thing about Special Relativity is that it seems to contradict Galilean relativity – SR says that the speed of light is the same for all observers; that is, it is NOT relative to a reference frame.
I thought Galileo’s contribution was to show that gravity accelerated all objects equally (minus air resistance, friction, etc…) Newton expanded that to say all objects in motion tend to stay in motion, etc. etc.
Galileo made many contributions to physics of course. One of them was the idea what we now call inertia, that objects in motion retain their motion unless acted on by a force. See the quote here in the Wikipedia page;
Imagine any particle projected along a horizontal plane without friction; then we know, from what has been more fully explained in the preceding pages, that this particle will move along this same plane with a motion which is uniform and perpetual, provided the plane has no limits.
Another was the relativity of motion.
A lot of Newton’s ideas were anticipated by Galileo. Newton put them all into a much more coherent mathematical framework than Galileo developed. Also Galileo never developed the idea that gravity extends beyond the Earth; he thought that there was a kind of “circular inertia” that kept the planets moving in their orbits.
So how is CMB moving relative to Earth at Earth’s position?
It depends on time of year, as the Earth moves in an (almost) circle around the Sun. But on average, or equivalently from the reference point of the Sun, it’s about 600 km/s.
So are there objects that stand still compared to the CMB?
I don’t know of any specifically off the top of my head, but there’s no reason there couldn’t be, and there’s almost certainly something.
Is it correct to think that early in the history of the universe before gravity started clumping things together into orbiting masses, that everything (or almost everything) was very close to motionless relative to the (local) CMB?
As in the OP the CMB is just “the other spaceship” so sure…you could be “not moving” relative to it.
That does not mean you are not moving. Indeed, you are definitely moving from some other perspective.
For example, you are driving on the expressway at 60 mph. The car next to you is driving at 60 mph. From your perspective the other car is not moving. From the perspective of someone on the side of the road you are both doing 60 mph.
The ship will also observe all the weird relativistic effects, only in this case the whole universe will be squished shorter (in the direction the ship is moving), and blue-shifted.
At all stages of the Universe’s history, it’s always been true that most things are very close to motionless relative to the local CMB (or rather, to the local comoving cosmological frame, to which the CMB itself is just a (very good) approximation). And this trend actually gets more true with time, as things that are moving quickly relative to the local reference frames will move into other localities that agree more closely with them.
A ‘free-floating’ planet in the middle of one of the big cosmic voids might come close?
No star to orbit, no galaxy for the (nonexistent) star to orbit…?
Not something we could probably ever detect, though.
Thanks
Actually…we can and have. Rogue planets are definitely a thing:
Not in the middle of a Great Void, we haven’t.
But even within a galaxy, there are probably going to be some things for which the various orbital motions just happen to cancel out. It’s not a matter of there being some things at rest relative to the CMB because they’re at rest relative to the CMB; it’s just that, of all the wide variety of velocities objects can have, that’s one of them. While “at rest relative to the CMB” is a convenient reference frame to use, the CMB exerts very little actual influence on anything.