Displacement

Traveller ships are expressed in dTons.

That's a displacement ton.

The volume of one metric ton of gaseous hydrogen.

For a ship that spends most of its time in vacuum.

It's not displacing anything... it's just a unit of volume.

Can you "displace" vacuum?

Buoyancy is an issue that's arisen in Traveller a couple of times.

Buoyancy is a function of weight and volume and thus density.

Traveller has a physics shredding device called contragravity.  This machine allows you to reduce the weight of anything wrapped inside it's field.

Dial the weight to zero in an atmosphere and you're going to rise because you're now displacing the gas of the atmosphere with the zero-weight volume of your ship.

Yes, it's handwavium magic, but it's canon.

What contragrav doesn't do is affect the mass of the ship, it's not going to shoot off the ground because it still has inertia.  But it's going to rise faster and faster... then slower and slower as the atmosphere gets thinner.  But eventually it will pop to the top of the atmo and float there.

I never bothered to calculate how fast because, in game, you dialed it to zero floated up a bit, pointed the ship the desired direction with the reaction control system then kicked in the (also physics shredding) reactionless motors to power to orbit.

Contragrav hurts a lot of practical brains because the only "material" we have with no weight, also has no mass.  Vacuum.

Where I learned my first lessons in buoyancy was from using vacuum to displace air.

Make a balloon filled with vacuum.  In reality any container which can hold back the atmosphere against your vacuum will be too heavy to float in air.  But what if you could "fill" a normal sized balloon with vacuum?  It'd have a lot of lift, wouldn't it?  A lot more than a balloon filled with hydrogen because vacuum has no mass.  As an aside, that mass is why you can make a hydrogen balloon and not a vacuum one; the gas provides pressure to hold the light enough to float container from collapsing against the atmosphere.

Another aside, doing buoyancy calculations of heavier than air things is MUCH easier to wrap your brain around than lighter than air things.  One tends to think that lighter than air things don't have weight.  You can't put a helium balloon on a scale, can you?

That whole scientific STP deal sometimes works against us because it gets us to assume a set of conditions that makes other scenarios counter-intuitive.