Can we proceed with theory alone as guide ? It resemble more a top - of - the - line research microscope in the precision machining of it minute , elegant part and is not unlike it as well in bulk , weight and cost .

In Victorian time Michael Faraday gingerly liquefied gase inside thick - walled glass tube . By World War I , ingenuity and boldness of design allowed Harvard ' Percy W. Bridgman to reach much higher pressure along semi - industry lines . His large press disclosed innumerable new phase in familiar condensed matter , most famously several new crystalline phase of H2O . Eventually he came to water ice VI , which at the highest pressure of the time remained an ice solid even at the temperature of a very hot kitchen oven .

The price of this mastery was not low: in 1922 a pressure container failed , steel flew like shrapnel and two men were killed , although the Harvard laboratory went on safely for decade more . Step by step , industry developed high - pressure technology for large - scale synthes . The most important early product was ammonia , made in the million of ton by reacting hydrogen and nitrogen gase at modest pressure . The most celebrated is synthes diamond , by now a high - pressure commodity .

Set some simple marker along the pressure scale . Infant breath and puff their cheeks , displaying small change in local air pressure . Mountaintop lure other among us into pressure lower by a factor of two or so . The weather bring stay - at - homes incessant , less noticeable variation in pressure , but sea - level air still afford us a meaningful natural unit . Call the typical pressure at sea level one atmosphere .

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Like many another kid , I was disappointed year ago by the poor result of taking a long rubber tube into the pool , in an attempt to tap surface air for breath on the bottom . No way! The added weight of even a foot of water is too much for chest muscles . To breath at any depth underwater , you need either to be housed within some sealed submersible or to sip breath of air fed from a portable tank at regulated matching pressure . That is SCUBA - self - contained underwater breath apparatus .

The ocean define a second natural pressure marker . Call 1,000 atmosphere one " ocean , " by analogy . That value is a ballpark fit to seawater pressure six mile down in the deepest undersea trenches . The earliest physical successe called for pressures reaching one ocean or so , and the ice for pressure half a dozen time greater . The diamond appeared after World War II up at 50 or 100 oceans , at which graphite dissolved in molten metal at white heat crystallize to diamond . Such condition have become routine in the special multiton presse that yield synthetic diamond around the world .

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Solid rock is denser than water , and our planet ' center lie a decisive 1,000 times deeper than the average - depth sea bottom . We can plausibly define a third natural benchmark for core pressures , one equal to 1,000 ocean of 1,000 atmosphere each , or a million atmospheres . Calling one atmosphere by the crisper name " bar , " it informal equivalent , we use it multiple , one megabar , as a convenient unit for describing the tremendou pressure at the core .

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It turn out to be surprisingly easy to estimate the pressure so deep down . The symmetrical Newtonian pull of all the interior part on all the layer above compact Earth into a near sphere . Even the expected increase in density as depth approache the center has at most a modest effect on a calculation that treat the globe as a uniform fluid . The central pressure approximate the simple product of three well - known quantities: the average density of Earth ; the planet ' radius ; and the familiar constant g , the value of surface gravity signaled by every apple that falls . That central pressure is reliably estimated at about four megabar whatever the internal composition , if only all the strata are both stable ( not undermined by vast buried caverns ) and uniform from place to place , differing only as their depth differs . What is down there anyhow ? This rich question bear strongly on the literal root of all geology .

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Can we proceed with theory alone as guide ? Can we ignore unknown minerals and fluids , some sure to be as strange as hot ice was ? The geophysicist wisely chose to experiment . One megabar of pressure was first reached 20 year ago at the Geophysical Laboratory of the Carnegie Institution of Washington . That lab and other can now summon the conditions of Hade itself right to the bench top . The megabar apparatu in no way resemble the heavy , house - size hydraulic presse of old , stowed behind safety wall and served by a team of cautiou engineers . It resemble more a top - of - the - line research microscope in the precision machining of it minute , elegant part and is not unlike it as well in bulk , weight and cost . No monster machine at all , this device is mostly made in house by a few master machinist as the prized tool of a few physicists .

How do they evade danger , weight and expense ? Pressure is a local quantity . Even a handheld steel sewing needpushed firmly into a surface can exert oceanic pressure if only over a tiny area . Megabar of pressure in a minute container , a tenth of a millimeter on edge , store up something like the energy released by a pencil falling off the workbench . If that holder fractures , expect only a small pop , a source more of sharp fiscal pain than of physical injury . Work small ; a miniature - scale apparatu produce pressure to match a planet ' core simply by hand - tightening a single bolt!

But can you learn about Earth ' center from so small a sample ? Indeed you can , for matter is atomic . Its propertie are well developed whenever sufficient atom are present long enough to arrange themselve as the laws require . That small pressure vessel holds some million billion atoms , more than enough to allow matter to fulfill its subtlest intentions . A few microgram are ineffective for making diamond commercially but brilliantly suited to yield knowledge .

How will you learn what happen to matter compressed under megabar ? First , use a transparent container , just as Faraday did . Probe it adroitly with infrared and optical lasers , with x - rays , neutrons , microwaves , even sound . Tight , high - intensity beam may be needed for a scant sample ; for x - ray diffraction studies , the geophysicist often subject their pressure cell to a fierce synchrotron source . The limit are set mainly by the changing state of condensed - matter physics .

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How to contain the sample ? Squeeze it inside a tiny box of the hardest stuff known , diamond . Find two flawless gems - single crystal selected for purity and extratransparency - then cut in facet to order . Less than one carat in weight , they will be dear but affordable . Align the tip end of the two truncated diamond cone with a thin ring gasket of steel alloy held between the two tiny flattened tips . The sample is confined within the hole in the gasket . Press diamond hard against diamond , and the whole assembly seal tight .

Megabar come easily . A diamond - diamond total force of only a couple of hundred kilograms is adequate , for that force is concentrated over an extremely small area . Up - to - date diamond - anvil cell may have contact faces only a thirtieth of a millimeter across . Sophisticated mounting design and virtuoso fabrication keep the gem nearly parallel for final optical alignment . Cryogenic cooling and resistance heating are both quite manageable .

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The theorist opine that at about 12 megabar , diamond undergoe a change in phase , and some anvil will break . That limit is still ahead . About half that pressure is expected to be enough to press molecular hydrogen into hydrogen metal , a goal that has long eluded the labs . In the past few months there has been a credible report of making the metal hydrogen using a big , noisy gas gun , but the stuff lasted the mere microsecond it took a shock wave to cross the cryogenic hydrogen target .

Fortune may favor us with some new anvil substance harder than pure diamond , so we will be able to see the luster of that simplest of metals . In this subtle corner of condensed - matter physics , the pressure of a planet 's core is literally at hand , through an art once awkward but now as harmonious as a quiet song .

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