That wouldn't work.

If you dropped something it would simply stay right next to you, even if you were accelerating - both of you are accelerating at exactly the same rate.

Why? What's making it accelerate? I wasn't trying to talk about some kind of universal/aether acceleration. I meant you and only you should accelerate.

And how do you accelerate - do you have a rocket attached to your feet?

It doesn't really matter. But there should be some mechanism. A rocket attached to your feet works quite well if you ignore the exhaust for purposes of reference.

If you want a useful thought experiment about physics then you need to give reasonable physics tools. Otherwise you get something like "Pretend you are an inanimate green blob. You can't climb a hill, can you? Therefore climbing a hill is an illusion."

If you have a rocket on your feet why do you need to drop something to figure out if you are accelerating? You can feel the pressure on your feet.

The only type of acceleration that is undetectable is gravity - and gravity will effect the dropped object exactly as much as you.

Great! You just simplified the experiment!

Dropping something was only one idea, but feeling pressure is much simpler.

Yes, but when you drop something, you're creating a reference frame, aren't you?

Ah, you're misunderstanding the term reference frame as it's being used here. A reference frame is like a coordinate system for velocity; it's not something physical.

No. When you drop something nothing whatsoever happens.

And as shardling said, a reference frame isn't a physical thing that is created and destroyed - it's simply the zero point for your system of measurement - but the measurement would work just fine if you started with any number, zero is just convenient.

I understand a frame of reference doesn't have to be physical. However, if you're in an apparently empty and dark universe, and you're interested in knowing whether you're accelerating or not, postulating a frame of reference is not going to be of much help.

I may have missed the point of dropping an object, though. I thought the idea was to create a physical point we could anchor the coordinate system to, and in that way be able to measure acceleration.

Dropping an object that won't get accelerated is one way to measure acceleration, but not the only way. If you are the actor causing the acceleration on yourself, as I originally intended, then presumably you already know / measure-by-doing how much you have accelerated, and you can use dead reckoning to watch the earlier reference frame fly by.

If the force is external and doesn't apply pressure then you have to drop an object.

If the force affects the entire universe the same way then that's aether and can't be measured / doesn't exist. But that was not the kind of force I was using to disprove "you can't move".

> If the force is external and doesn't apply pressure then you have to drop an object.

What kind of force is external and doesn't apply pressure, and also doesn't affect the dropped object? There is no such force, and therefor no such test.

> However, if you're in an apparently empty and dark universe, and you're interested in knowing whether you're accelerating or not

You can't. There is absolutely no test that can tell you this.

You can't "drop" anything - whatever force is accelerating you also accelerates the object, so letting go of it tells you nothing.