I guess the GP understood special relativity, but not general ;)

Well, to be really pedant, if you were spinning around yourself, you'd feel an acceleration.

There will be 'gravity' -a force- in the opposite direction you are accelerating in. You should know the feeling, it happens when you accelerate in a car, or when a plane lifts off. If it's really you floating around, you will be able to feel it. (I reckon you could even guess at the amount by comparing it to the acceleration of 9.81 m/s^2 that you're used to.)

>There will be 'gravity' -a force- in the opposite direction you are accelerating in.

Wouldn't there be a force in the direction you are accelerating in?

Anyway, when you accelerate a car, you are not feeling acceleration, you are feeling the force of the car on you. Imagine being inside of a box that is in free fall. What experiment could you run to determine if you were accelerating?

>Wouldn't there be a force in the direction you are accelerating in?

Well, yes, that would be the standard way to look at it. Sorry for being confusing.

>Anyway, when you accelerate a car, you are not feeling acceleration, you are feeling the force of the car on you.

According to [Force = Mass * Acceleration] you can take them as equal (apart from the constant factor of your mass). In the freefalling box that is accelerating (and thus no longer in free fall) you'd also feel a force. No need for experiments if you have your normal senses. You'll be pressed against one side of the box. (And if the acceleration is 9.81 m/s^2 you couldn't distinguish it from a box on earth.)

That is actually the basic premise of general relativity - the equality of 'inertial mass' that resists against acceleration and 'gravitational mass' that happens because of, well, gravity. It's like the idea of constant speed of light for special relativity.

edit: Unless you're talking about gravity, which accelerates all parts of the setup equally, thus no force results.. but that would need something else in the experiment, either a great amount of matter or energy. But then you'd have a reference frame right there. (Mathematically. As for an experiment.. hm. Check for perturbances from space-time distortions that result from rotating gravity sources, maybe.. most gravity sources in this universe are rotating. There's an experiment done that checked that for Earth.[1])

[1] https://en.wikipedia.org/wiki/Gravity_Probe_B

drop something? If it accelerates away from you, you are accelerating (or held stationary in a gravity field, but AFIAK, those are the same things).

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.