Swot up on your physics wi th our handy glossary, by pop­u­lar science writer

Focus-Science and Technology - - Gravity - Brian Clegg


Cen­tral to the se­cond law of ther­mo­dy­nam­ics, en­tropy is a mea­sure of the dis­or­der in a sys­tem. It re­flects the num­ber of dif­fer­ent ways the com­po­nents of a sys­tem can be re­ar­ranged. The let­ters mak­ing up the words on this page have low en­tropy – there’s only one way to ar­range them (as­sum­ing each in­di­vid­ual a,b, c, etc. is unique) to pro­duce the text you’re read­ing. But if you scram­ble the let­ters, it will have higher en­tropy, as there are lots of ways to ar­range them jum­bled up. The se­cond law of ther­mo­dy­nam­ics re­flects that it’s eas­ier to go from an or­dered page to scram­bled let­ters than it is to go from a pile of let­ters to the con­tents of this mag­a­zine. Sim­i­larly, it’s eas­ier to break an egg than to un­break it.


Physics recog­nises four fun­da­men­tal forces: elec­tro­mag­netism, which deals with in­ter­ac­tions in mat­ter and light; the strong nu­clear force, which holds the par­ti­cles of atomic nu­clei to­gether; the weak nu­clear force, which is in­volved in nu­clear de­cay; and grav­ity. All ex­cept grav­ity fit with quan­tum the­ory.


The Gen­eral The­ory of Rel­a­tiv­ity, pub­lished by Ein­stein in 1915, ex­plains how mass warps space and time, and how these warps in­flu­ence the way that mat­ter moves. It pro­vides equa­tions that give us a pre­cise de­scrip­tion of grav­ity, in­di­rectly pre­dict­ing phe­nom­ena like black holes, grav­i­ta­tional waves and the Big Bang.


Ein­stein’s Gen­eral Rel­a­tiv­ity pre­dicts that mas­sive ob­jects warp space enough to make pass­ing light curve around them. This means that large cos­mic struc­tures like gal­ax­ies can act like lenses. Light com­ing from be­hind the galaxy is bent around it to­wards the viewer, bring­ing dis­tant bod­ies into fo­cus.


This stands for Mod­i­fied New­to­nian Dy­nam­ics – a the­ory that ex­pands on New­ton’s laws of mo­tion. It of­fers a po­ten­tial ex­pla­na­tion for the un­ex­pected be­hav­iour of spi­ral gal­ax­ies and galac­tic clus­ters usu­ally at­trib­uted to dark mat­ter. It is based on the idea that the ef­fect of grav­ity be­haves in a sub­tly dif­fer­ent man­ner on a vast scale. Even so, it still doesn’t ex­plain all the ob­served odd­i­ties – but then nei­ther does dark mat­ter.


Ger­man physi­cist Max Planck math­e­mat­i­cally de­rived the Planck length, a unit of dis­tance around 100 bil­lion bil­lion times smaller than the nu­cleus of an atom, us­ing con­stants of na­ture such as the speed of light. If space is not con­tin­u­ous but made up of quanta – the min­i­mum amount of a phys­i­cal prop­erty that can be in­ter­acted with – it has been sug­gested that its quanta might be a Planck length across (see be­low for more on quan­tum the­ory). Be­low this dis­tance, mea­sure­ment would not be pos­si­ble. A Planck area is a Planck length squared. In black hole the­ory, when a black hole ab­sorbs a sin­gle bit of in­for­ma­tion, its event hori­zon – the bound­ary around it from which not even light can es­cape – ex­pands by one Planck area.


This the­ory de­scribes the be­hav­iour of light and mat­ter on a very small scale – that of in­di­vid­ual par­ti­cles such as atoms, elec­trons and pho­tons. The the­ory takes its name from its cen­tral idea that phe­nom­ena are not con­tin­u­ous in na­ture but are in­stead bro­ken down into tiny in­di­vis­i­ble chunks or pack­ets called quanta. In clas­si­cal me­chan­ics, ob­jects al­ways ex­ist in a spe­cific place at a spe­cific time. But in quan­tum the­ory we can only de­ter­mine the prob­a­bil­ity of an ob­ject be­ing in a cer­tain place at a cer­tain time. This seems coun­ter­in­tu­itive, but the the­ory is in­cred­i­bly suc­cess­ful in ex­plain­ing the in­ter­ac­tions of light and mat­ter.


String the­ory was de­vised to ex­plain in­con­sis­ten­cies in par­ti­cle physics. It is a lead­ing ap­proach in the at­tempt to pro­duce the so-called The­ory of Ev­ery­thing. In string the­ory, par­ti­cles are re­placed with vi­brat­ing strings, but for the maths to work there need to be nine spa­tial di­men­sions rather than the three we ob­serve.


Orig­i­nally de­vel­oped to pro­vide a the­o­ret­i­cal ba­sis for the de­sign and op­er­a­tion of steam en­gines, ther­mo­dy­nam­ics – lit­er­ally the move­ment of heat – is now a fun­da­men­tal area of study in physics. It has four laws, of which the most im­por­tant are the first ‘en­ergy is al­ways con­served’, and the se­cond ‘heat al­ways moves from a hot­ter to a colder body’. The se­cond law also shows that, on av­er­age, in a sys­tem that’s iso­lated from its sur­round­ings, en­tropy stays the same or in­creases – to de­crease it re­quires en­ergy.


The amount of light en­ergy emit­ted by a spi­ral galaxy such as the Milky Way is roughly pro­por­tional to its speed of ro­ta­tion. The faster the gal­ax­ies spin, the brighter they are. This is known as the Tully- Fisher re­la­tion, named af­ter the as­tronomers Brent Tully and Richard Fisher who dis­cov­ered it.

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