32:
282:
are used to describe their magnitude or quantity. Many of these quantities are related to each other by various physical laws, and as a result the units of a quantities can be generally be expressed as a product of powers of other units; for example, momentum is mass multiplied by velocity, while
406:
issue is whether these fundamental constants really exist as dimensional or dimensionless quantities. This is equivalent to treating length as the same as time or understanding electric charge as a combination of quantities of mass, length, and time which may seem less natural than thinking of
393:
could be used instead of length. Some physicists have not recognized temperature as a base dimension since it simply expresses the energy per particle per degree of freedom which can be expressed in terms of energy (or mass, length, and time). Duff argues that only dimensionless values have
397:
There are other relationships between physical quantities that can be expressed by means of fundamental constants, and to some extent it is an arbitrary decision whether to retain the fundamental constant as a quantity with dimensions or simply to define it as unity or a fixed
426:, which relates energy (with dimension expressible in terms of mass, length and time) to frequency (with dimension expressible in terms of time). In theoretical physics it is customary to use such units (natural units) in which
553:
The preferred choices vary by the field in physics. Using natural units leaves every physical quantity expressed as a dimensionless number, which is noted by physicists disputing the existence of incompatible base quantities.
344:
In 2019 the seven SI base units were redefined in terms of seven defining constants. Therefore the SI base units are no longer necessary but were retained because for historical and practical reasons. See
418:, which is a fundamental constant. It is possible to use this relationship to eliminate either the base unit of time or that of distance. Similar considerations apply to the
838:
in the number of fundamental units and in the dimensions of any physical quantity in terms of those units has been emphasized by
Abraham, Plank, Bridgman, Birge, and others.
365:
A set of base dimensions of quantity is a minimal set of units such that every physical quantity can be expressed in terms of this set. The traditional base dimensions are
605:
526:
That leaves one base dimension and an associated base unit, but there are several fundamental constants left to eliminate that too – for instance, one could use
898:
Because, however, of the arbitrary character of dimensions, as presented so ably by
Bridgman, the choice and number of fundamental units are arbitrary.
49:
785:
851:
516:
Similarly, one could eliminate the candela, as that is defined in terms of other physical quantities via a technical scaling constant,
96:
68:
115:
807:
20:
75:
651:
53:
921:
310:
82:
609:
64:
288:
499:
One could eliminate the kelvin as it can be argued that temperature simply expresses the energy per particle per
503:, which can be expressed in terms of energy (or mass, length, and time). Another way of saying this is that the
407:
temperature as measuring the same material as energy (which is expressible in terms of mass, length, and time).
725:
347:
42:
852:"On the establishment of fundamental and derived units, with special reference to electric units. Part I."
531:
399:
174:
546:
220:
89:
916:
866:
753:
568:
563:
284:
730:
441:
242:
141:
777:
743:
686:
542:
504:
882:
704:
674:
500:
487:
477:
225:
151:
874:
769:
761:
696:
386:
304:
214:
208:
419:
378:
19:
This article is about the type of unit of measure. For the concept in number theory, see
870:
818:
757:
636:. Washington, D.C.: U.S. Department of Commerce. p. 56 (10th CGPM, 1954, Resolution 6).
631:
411:
910:
781:
573:
360:
300:
190:
155:
146:
647:
287:. Those that can be expressed in this fashion in terms of the base units are called
247:
765:
513:
is a technical scaling constant and could be set to a fixed dimensionless number.
403:
382:
341:. Several derived units have been defined, many with special names and symbols.
338:
267:
237:
195:
186:
31:
700:
154:, where no quantity in the subset can be expressed in terms of the others. The
578:
708:
162:, consists of the metre, kilogram, second, ampere, kelvin, mole and candela.
773:
283:
velocity is distance divided by time. These relationships are discussed in
601:
469:
One could eliminate the ampere by setting either the vacuum permittivity
314:
259:
255:
199:
are common base-10 or base-2 powers multiples and submultiples of units.
212:, involving the combination of quantities with different units; several
322:
878:
370:
334:
330:
326:
275:
271:
202:
While a base unit is one that has been explicitly so designated, a
691:
748:
390:
318:
410:
For instance, time and distance are related to each other by the
402:, and reduce the number of explicit base quantities by one. The
394:
physical meaning and all dimensional units are human constructs.
374:
366:
263:
251:
455:
One could eliminate either the metre or the second by setting
150:. A base quantity is one of a conventionally chosen subset of
25:
496:
to 1. This is natural as it is a technical scaling constant.
16:
Unit of measurement adopted by convention for a base quantity
486:
One could eliminate the mole as a base unit by setting the
385:, but in principle, other base quantities could be used.
459:
to unity (or to any other fixed dimensionless number).
56:. Unsourced material may be challenged and removed.
817:. John Wiley and Sons. p. 775. Archived from
729:
720:
718:
462:One could then eliminate the kilogram by setting
673:Quincey, Paul; Brown, Richard J C (2017-08-01).
675:"A clearer approach for defining unit systems"
606:International Organization for Standardization
596:
594:
8:
731:"How fundamental are fundamental constants?"
630:Taylor, Barry N.; Thompson, Ambler (2008).
440:. A similar choice can be applied to the
747:
690:
116:Learn how and when to remove this message
590:
633:The International System of Units (SI)
7:
54:adding citations to reliable sources
389:could be used instead of charge or
808:"Appendix on Units and Dimensions"
654:from the original on 19 April 2021
313:(SI), there are seven base units:
14:
648:"9th edition of the SI Brochure"
30:
21:Fundamental unit (number theory)
788:from the original on 2021-11-22
612:from the original on 2019-07-02
41:needs additional citations for
250:aspects of the world, such as
160:Systéme International d'unités
1:
311:International System of Units
295:International System of Units
806:Jackson, John David (1998).
766:10.1080/00107514.2014.980093
177:of a given unit; likewise a
859:American Journal of Physics
938:
850:Birge, Raymond T. (1935).
483:to a dimensionless number.
466:to a dimensionless number.
358:
298:
65:"Base unit of measurement"
18:
815:Classical Electrodynamics
701:10.1088/1681-7575/aa7160
130:base unit of measurement
348:2019 revision of the SI
218:are specially named. A
132:(also referred to as a
532:gravitational constant
547:cosmological constant
221:coherent derived unit
183:submultiple of a unit
922:Dimensional analysis
888:on 23 September 2015
736:Contemporary Physics
569:Dimensional analysis
564:Characteristic units
400:dimensionless number
285:dimensional analysis
50:improve this article
871:1935AmJPh...3..102B
758:2015ConPh..56...35D
442:vacuum permittivity
243:physical quantities
236:In the language of
152:physical quantities
142:unit of measurement
824:on 13 January 2014
602:"ISO 80000-1:2009"
543:electron rest mass
505:Boltzmann constant
226:conversion factors
193:of a given unit.
171:multiple of a unit
879:10.1119/1.1992945
501:degree of freedom
488:Avogadro constant
478:elementary charge
126:
125:
118:
100:
929:
901:
900:
895:
893:
887:
881:. Archived from
856:
847:
841:
840:
831:
829:
823:
812:
803:
797:
796:
794:
793:
751:
733:
722:
713:
712:
694:
670:
664:
663:
661:
659:
644:
638:
637:
627:
621:
620:
618:
617:
598:
439:
432:
387:Electric current
305:SI derived units
215:SI derived units
209:derived quantity
179:unit submultiple
173:) is an integer
138:fundamental unit
121:
114:
110:
107:
101:
99:
58:
34:
26:
937:
936:
932:
931:
930:
928:
927:
926:
907:
906:
905:
904:
891:
889:
885:
854:
849:
848:
844:
827:
825:
821:
810:
805:
804:
800:
791:
789:
724:
723:
716:
672:
671:
667:
657:
655:
646:
645:
641:
629:
628:
624:
615:
613:
600:
599:
592:
587:
560:
540:
522:
512:
495:
475:
450:
434:
427:
420:Planck constant
363:
357:
307:
299:Main articles:
297:
234:
122:
111:
105:
102:
59:
57:
47:
35:
24:
17:
12:
11:
5:
935:
933:
925:
924:
919:
909:
908:
903:
902:
865:(3): 102–109.
842:
798:
714:
685:(4): 454–460.
665:
650:. BIPM. 2019.
639:
622:
589:
588:
586:
583:
582:
581:
576:
571:
566:
559:
556:
551:
550:
538:
524:
520:
514:
510:
497:
493:
484:
473:
467:
460:
448:
412:speed of light
359:Main article:
356:
353:
296:
293:
233:
230:
206:is unit for a
144:adopted for a
124:
123:
38:
36:
29:
15:
13:
10:
9:
6:
4:
3:
2:
934:
923:
920:
918:
915:
914:
912:
899:
884:
880:
876:
872:
868:
864:
860:
853:
846:
843:
839:
837:
836:arbitrariness
820:
816:
809:
802:
799:
787:
783:
779:
775:
774:10044/1/68485
771:
767:
763:
759:
755:
750:
745:
741:
737:
732:
727:
721:
719:
715:
710:
706:
702:
698:
693:
688:
684:
680:
676:
669:
666:
653:
649:
643:
640:
635:
634:
626:
623:
611:
607:
603:
597:
595:
591:
584:
580:
577:
575:
574:Natural units
572:
570:
567:
565:
562:
561:
557:
555:
548:
544:
537:
533:
529:
525:
519:
515:
509:
506:
502:
498:
492:
489:
485:
482:
479:
472:
468:
465:
461:
458:
454:
453:
452:
447:
443:
437:
430:
425:
421:
417:
413:
408:
405:
401:
395:
392:
388:
384:
380:
376:
372:
368:
362:
361:Natural units
355:Natural units
354:
352:
350:
349:
342:
340:
336:
332:
328:
324:
320:
316:
312:
306:
302:
301:SI base units
294:
292:
290:
289:derived units
286:
281:
277:
273:
269:
265:
261:
257:
253:
249:
245:
244:
239:
231:
229:
227:
223:
222:
217:
216:
211:
210:
205:
200:
198:
197:
196:Unit prefixes
192:
191:unit fraction
188:
184:
180:
176:
172:
168:
167:unit multiple
163:
161:
157:
156:SI base units
153:
149:
148:
147:base quantity
143:
139:
135:
131:
120:
117:
109:
98:
95:
91:
88:
84:
81:
77:
74:
70:
67: –
66:
62:
61:Find sources:
55:
51:
45:
44:
39:This article
37:
33:
28:
27:
22:
897:
890:. Retrieved
883:the original
862:
858:
845:
835:
833:
826:. Retrieved
819:the original
814:
801:
790:. Retrieved
742:(1): 35–47.
739:
735:
726:Michael Duff
682:
678:
668:
656:. Retrieved
642:
632:
625:
614:. Retrieved
552:
545:, or Λ, the
535:
527:
517:
507:
490:
480:
470:
463:
456:
445:
435:
428:
423:
415:
409:
396:
364:
346:
343:
308:
279:
248:quantifiable
241:
235:
224:involves no
219:
213:
207:
204:derived unit
203:
201:
194:
182:
178:
170:
166:
164:
159:
145:
137:
133:
129:
127:
112:
103:
93:
86:
79:
72:
60:
48:Please help
43:verification
40:
917:Measurement
404:ontological
383:temperature
268:temperature
238:measurement
187:submultiple
911:Categories
892:13 January
828:13 January
792:2020-04-03
692:1705.03765
679:Metrologia
616:2019-09-15
585:References
579:One (unit)
232:Background
106:March 2013
76:newspapers
782:118347723
749:1412.2040
709:0026-1394
134:base unit
786:Archived
728:(2015).
652:Archived
610:Archived
558:See also
315:kilogram
260:velocity
256:distance
175:multiple
867:Bibcode
754:Bibcode
476:or the
323:candela
309:In the
185:) is a
140:) is a
90:scholar
780:
707:
658:20 May
541:, the
530:, the
381:, and
379:charge
371:length
337:, and
335:kelvin
331:ampere
327:second
278:, and
276:weight
274:, and
272:energy
92:
85:
78:
71:
63:
886:(PDF)
855:(PDF)
822:(PDF)
811:(PDF)
778:S2CID
744:arXiv
687:arXiv
391:speed
319:metre
280:units
189:or a
158:, or
97:JSTOR
83:books
894:2014
834:The
830:2014
705:ISSN
660:2019
433:and
375:time
367:mass
339:mole
303:and
264:mass
252:time
246:are
181:(or
169:(or
69:news
875:doi
770:hdl
762:doi
697:doi
438:= 1
431:= 1
136:or
52:by
913::
896:.
873:.
861:.
857:.
832:.
813:.
784:.
776:.
768:.
760:.
752:.
740:56
738:.
734:.
717:^
703:.
695:.
683:54
681:.
677:.
608:.
604:.
593:^
534:,
521:cd
451:.
444:,
422:,
414:,
377:,
373:,
369:,
351:.
333:,
329:,
325:,
321:,
317:,
291:.
270:,
266:,
262:,
258:,
254:,
240:,
228:.
165:A
128:A
877::
869::
863:3
795:.
772::
764::
756::
746::
711:.
699::
689::
662:.
619:.
549:.
539:e
536:m
528:G
523:.
518:K
511:B
508:k
494:A
491:N
481:e
474:0
471:ε
464:ħ
457:c
449:0
446:ε
436:ħ
429:c
424:h
416:c
119:)
113:(
108:)
104:(
94:·
87:·
80:·
73:·
46:.
23:.
Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
