| Nanopascals (nPa) | Gigapascals (GPa) |
|---|---|
| 1 Nanopascal | 1 × 10-18 GPa |
| 2 Nanopascals | 2 × 10-18 GPa |
| 3 Nanopascals | 3 × 10-18 GPa |
| 4 Nanopascals | 4 × 10-18 GPa |
| 5 Nanopascals | 5 × 10-18 GPa |
| 10 Nanopascals | 1 × 10-17 GPa |
| 20 Nanopascals | 2 × 10-17 GPa |
| 25 Nanopascals | 2.5 × 10-17 GPa |
| 50 Nanopascals | 5 × 10-17 GPa |
| 100 Nanopascals | 1 × 10-16 GPa |
| Reference | Nanopascals (nPa) | Gigapascals (GPa) |
|---|---|---|
| Atmospheric pressure at sea level | 1.01325 × 1014 nPa | 0.000101325 GPa |
| Healthy blood pressure (120 mmHg) | 1.6 × 1013 nPa | 0.000016 GPa |
| A car tyre | 2.2 × 1014 nPa | 0.00022 GPa |
| A racing bicycle tyre | 6 × 1014 nPa | 0.0006 GPa |
The nanopascal is a unit of pressure equal to one billionth of a pascal. Its symbol is nPa. It marks the far end of the pressure scale, the region where the idea of pressure as a push on a surface stops being useful and becomes a statement about how few particles are present.
The clearest home for the unit is extreme high vacuum. Ordinary laboratory vacuum reaches millipascals; ultra-high vacuum, used for surface physics and for the beam pipes of particle accelerators, reaches micropascals. Below that lies extreme high vacuum at nanopascals and lower, and reaching it takes a sealed chamber, hours of baking at two hundred degrees to drive gas out of the metal itself, and pumps that trap molecules rather than push them.
At those pressures a chamber is not empty. A nanopascal still contains something like a quarter of a million molecules per cubic centimetre, which sounds like a great many until you compare it with the twenty-five billion billion in the same volume of room air. What matters is not the count but the mean free path: a molecule now travels thousands of kilometres before striking another, so it hits the walls long before it meets a neighbour.
That is exactly the point. Surface science needs a sample to stay clean for the length of an experiment, and at ordinary pressures a fresh surface is covered by a layer of adsorbed gas in about a nanosecond. At a nanopascal the same surface stays clean for days. The vacuum is not there to remove air but to buy time.
Space provides the natural comparison. Low Earth orbit is around a micropascal, still dense enough that the atmosphere drags on satellites and eventually pulls them down. Interplanetary space is nanopascals. Interstellar space is far lower still, roughly a femtopascal, which no terrestrial pump has ever matched — the best laboratory vacuums are still denser than the space between the stars.
Radiation pressure lands in similar territory. Sunlight falling on a perfectly absorbing surface at Earth's distance exerts about 4.5 micropascals, and at the distance of the outer planets it falls to nanopascals. Solar sails work with these numbers, which is why they must be enormous and light to gather a usable force.
One nanopascal equals 0.000000001 pascals, 0.001 micropascals, 1,000 picopascals, or about 0.000000000000145 pounds per square inch.
The gigapascal is a unit of pressure equal to a billion pascals, written GPa. It is a thousand megapascals and ten thousand bar. It is the unit in which materials science states stiffness rather than strength, and in which the pressures inside planets and inside high-pressure laboratories are described.
Elastic modulus is its principal use. Steel has a Young's modulus of about 200 gigapascals, copper 117, aluminium 70, ordinary glass 70, concrete around 30, timber along the grain 10 to 15, and rubber less than a tenth of one. Those numbers describe how much a material stretches under load, not when it breaks, and they explain why a steel beam of the same strength as an aluminium one still deflects less.
Diamond marks the top of the ordinary scale. Its modulus of roughly 1200 gigapascals is the highest of any natural material, which is why it is used to make anvils for high-pressure work: nothing else can push that hard without deforming first. Synthetic diamond is made industrially at about five gigapascals and high temperature, in presses built specifically to hold that pressure.
The interior of the earth is described in the same unit. Pressure at the base of the crust is around one gigapascal, at the boundary between mantle and core about 136, and at the centre of the planet roughly 360. Laboratory diamond anvil cells now reach and exceed the central value, which allows the behaviour of iron and silicates under planetary conditions to be studied directly rather than inferred.
For contrast, the deepest point in the ocean exerts only about 0.11 gigapascals. That comparison is worth keeping, because it shows how much larger geological pressures are than anything associated with water: the bottom of the Mariana Trench is a thousandth of the pressure at the centre of the earth.
Below the gigapascal, the megapascal describes strength, and above it there is little except stellar and theoretical physics. The terapascal appears mainly in the elastic modulus of carbon nanotubes and in the interiors of giant planets, so the gigapascal is effectively the top of the range that laboratories and engineers work in.
One gigapascal equals 1,000,000,000 pascals, 1000 megapascals, 10,000 bar, or about 145,000 pounds per square inch.