If what you are suggesting is correct, then even on a qualitative front, when I pick something up inside the evacuated chamber it should instantly feel lighter than it does at atmospheric pressure. However, no object ever does - because the weight is the same either way.
How do you propose to pick something up to measure if it feels lighter to you. You say that no object ever does. How have you managed to do this?
I tell you what, I didn't want to muddy the water here but for the sake of trying to convince you one last time - I'm going to make a concession with regards to pressure affecting weight and tell you that things are actually a bit lighter than their true weight when in our atmosphere. Before I go any further, the weight of an object is the force measured on the object due to gravity. This is constant for any given gravitational environment.
Your admittance that air pressure has the effect on the object, should make you question your gravity.
Allow me to digress - and I apologise to all for the can of worms I may be about to open.
You surely acknowledge that a brick in a pond weighs less than a brick sitting on the ground in the atmosphere? Do you know why this happens? I'll explain. As you should know, pressure increases in a body of water as you go deeper. This fact is indisputable and if you even try to deny it I will abandon this thread and ignore you immediately (it's something you will surely have experienced yourself). In other words, at any given moment when an object is submerged in water, the water is actually pressing up on it due to the pressure difference above and below it, and this force is equal to the weight of the water displaced by the object so things are lighter underwater - surely you know this?). This is actually known as Archimedes' principle and actually it extends to all fluids - fluids including gases, i.e. our atmosphere.
A brick in a pond would weigh heavier than it would on land. Not because the brick itself is heavier but because the pressure of the water on it would be more than the pressure of air on it.
Try it.
Go to a swimming pool with a breeze block. If anyone stops you, just explain that it's an experiment you are doing for me. Just mention scepti and try and ignore the dropped eye brows and mean stare.
Anyway: Pick up the block outside of the pool, first, now throw it in and dive down to retrieve it and pick it up and you will see.
Can you open a car door easier against air or submerged in water?
Think about it. It's all about pressure.
So... you can imagine our atmosphere as being rather analogous to the oceans and we are at the bottom of this ocean, where the pressure is greatest. Like the oceans, the pressure is greater the further down you go, and actually just in the same way that water pushes up on an object with a weight equal to the volume of displaced water - air (or indeed any fluid) does the same thing. For things on the ground in our atmosphere, the air is actually making them marginally lighter than they would be in a vacuum, however this is virtually negligible at atmospheric pressure and is treated as such.
Water does not push up, it pushes around you, like holding a bar of wet soap in your hand in the bath and squeezing.
Now here's the clincher. Notice that this buoyancy actually makes something (anything) lighter from their 'true' weight. If I understand you correctly, you are implying that pressure is keeping things pinned to the earth, when really, in reality - the pressure is acting against gravity (it's just negligible for air - but not in water, as you surely have experienced). So, due to the fact the density of air lessens as you climb in altitude (interestingly, water's density remains pretty much constant regardless of depth - a fact we should be grateful for as it happens) then things should actually be (negligibly) heavier at great altitudes than they are at sea level - ultimately being their heaviest, or 'true' weight within a vacuum.
Your body is made up of many densities of matter/molecules. Some are intended to be lower and some are intended to be higher than sea level but you (as a mass) grew against the pressure and that pressure is trying to separate you back into the sandwich that makes up the earth, but your body is strong and builds up a strength to counteract it. It's a fight to the death, literally and the pressure wins, every time, on anything that fights against it.
If we were blow up dolls of helium, we would be squeezed into the sky, just like we would if were planted at the bottom of a swimming pool.
That being said, do not misconstrue this as pressure changing the true weight of something. Ultimately, I will re-iterate that the true weight of an object is a measurement of the force on the object due to gravity and gravity alone - and this is constant for the gravitational environment.
The true weight of something is it's own density which we can only measure in whatever environment it is in and accept it's weight for that environment as being weight, plus pressure on that weight/mass.
So there Scepti - I just made a concession for you that I didn't really want to make because air buoyancy is negligible. However, the truth is, with regards to your own theory - pressure actually works in the opposite way to which you suggest. The pressure of a surrounding fluid actually makes things lighter than they really are - not heavier. I've clearly spoke about water here so that you understand this, and surely you do. For air though, this buoyancy is negligible.
So - how does that go down with your way of thinking?
Good story, but it is incorrect. Your gravity is nothing other than pressure on earth. It's all pressure.
We...and every other thing, exists because of it, in whatever forms it takes, from vibration and so on.
Accept it or don't, it's your choice.