- Inertial frame of reference
physics, an inertial frame of reference is a frame of referencewhich belongs to a set of frames in which physical laws hold in the same and simplest form. According to the first postulate of special relativity, all physical laws take their simplest form in an inertial frame, and there exist multiple inertial frames interrelated by uniform translation: anchor|principle cite book |title=The Principle of Relativity: a collection of original memoirs on the special and general theory of relativity |author=Einstein, A., Lorentz, H. A., Minkowski, H., & Weyl, H. |page=p. 111 |url=http://books.google.com/books?id=yECokhzsJYIC&pg=PA111&dq=postulate+%22Principle+of+Relativity%22&lr=&as_brr=0&sig=ACfU3U2GFu6cWo5WHzgumuNEqum_fBiTiw
isbn=0486600815 |publisher=Courier Dover Publications |year=1952 ] Quotation|"Special principle of relativity": If a system of coordinates K is chosen so that, in relation to it, physical laws hold good in their simplest form, the "same" laws hold good in relation to any other system of coordinates K' moving in uniform translation relatively to K.|Albert Einstein: "The foundation of the general theory of relativity", Section A, §1The principle of simplicity can be used within Newtonian physics as well as in special relativity; see Nagelcite book |title=The Structure of Science |author=Ernest Nagel |page=p. 212 |url=http://books.google.com/books?id=u6EycHgRfkQC&pg=PA212&dq=inertial+%22Foucault%27s+pendulum%22&lr=&as_brr=0&sig=ACfU3U1jQDxKwQROE_XK3mxm7xXfTrwhpQ |isbn=0915144719 |publisher=Hackett Publishing |year=1979 ] and also Blagojević.cite book |title=Gravitation and Gauge Symmetries |author=Milutin Blagojević |page=p. 4 |url=http://books.google.com/books?id=N8JDSi_eNbwC&pg=PA5&dq=inertial+frame+%22absolute+space%22&lr=&as_brr=0&sig=ACfU3U2Vg9oaYzXo79yholm3opU9DRpwZA#PPA4,M1 |isbn=0750307676 |publisher=CRC Press |year=2002] In practical terms, the equivalence of inertial reference frames means that scientists within a box moving uniformly cannot determine their absolute velocity by "any" experiment (otherwise the differences would set up an absolute standard reference frame).cite book |title=Relativity: The Special and General Theory |author=Albert Einstein |page=p. 17 |year=1920 |publisher=H. Holt and Company |url=http://books.google.com/books?id=3H46AAAAMAAJ&printsec=titlepage&dq=%22The+Principle+of+Relativity%22&lr=&as_brr=0#PPA17,M1 ] cite book |title=Six not-so-easy pieces: Einstein's relativity, symmetry, and space-time |author=Richard Phillips Feynman |page=p. 73 |isbn=0201328429 |year=1998 |publisher=Basic Books |url=http://books.google.com/books?id=ipY8onVQWhcC&pg=PA49&dq=%22The+Principle+of+Relativity%22&lr=&as_brr=0&sig=ACfU3U3KfqkThK26GE7jG-QEFypFxJ17eQ#PPA73,M1 ] According to this definition, supplemented with the constancy of the speed of light, inertial frames of reference transform among themselves according to the
Poincaré groupof symmetry transformations, of which the Lorentz transformations are a subgroup.cite book |title=Compendium of Theoretical Physics |author=Armin Wachter & Henning Hoeber |page=p. 98 |url=http://books.google.com/books?id=j3IQpdkinxMC&pg=PA98&dq=%2210-parameter+proper+orthochronous+Poincare+group%22&lr=&as_brr=0&sig=ACfU3U1Q-OkzdJbWVxioMFRbl1MuYt6bPw |isbn=0387257993 |publisher=Birkhäuser |year=2006 ]
The expression "inertial frame of reference" ( _de. Inertialsystem) was coined by
Ludwig Langein 1885, to replace Newton's definitions of "absolute space and time" by a more operational definition. [Cite journal
title=Über die wissenschaftliche Fassung des Galileischen Beharrungsgesetzes
volume=2] cite book |author=Julian B. Barbour |title=The Discovery of Dynamics |edition=Reprint of 1989 "Absolute or Relative Motion?" |page=pp. 645-646 |url=http://books.google.com/books?id=WQidkYkleXcC&pg=PA645&dq=Ludwig+Lange+%22operational+definition%22&lr=&as_brr=0&sig=ACfU3U0mti8md3OKWLkQ7xm4Db1dqr6kjA#PPA646,M1
isbn=0195132025 |publisher=Oxford University Press |year=2001 ] As referenced by Iro, [http://books.google.com/books?id=9a9KAAAAMAAJ&q=Inertialsystem+inauthor:%22von+Laue%22&dq=Inertialsystem+inauthor:%22von+Laue%22&lr=&as_brr=0&pgis=1 Lange proposed] :cite book |page=p.169 |title=A Modern Approach to Classical Mechanics |author=Harald Iro |url=http://books.google.com/books?id=-L5ckgdxA5YC&pg=PA179&dq=inertial+noninertial&lr=&as_brr=0&sig=ACfU3U3fCW68SLm1zalPZWi0LvLK4DrXYg#PPA169,M1 |isbn=9812382135 |year=2002 |publisher=World Scientific] Quotation
A reference frame in which a mass point thrown from the same point in three different (non co-planar) directions follows rectilinear paths each time it is thrown, is called an inertial frame. |L. Lange (1885) as quoted by Max von Laue in his book (1921) "Die Relativitätstheorie", p. 34, and translated by Iro| A discussion of Lange's proposal can be found in Mach.cite book |title=The Science of Mechanics |page=p. 38 |author=Ernst Mach |url=http://books.google.com/books?id=cyE1AAAAIAAJ&pg=PA33&dq=rotating+sphere+Mach+cord+OR+string+OR+rod&lr=&as_brr=0#PPA38,M1 |publisher=The Open Court Publishing Co. |year=1915]
The inadequacy of the notion of "absolute space" in Newtonian mechanics is spelled out by Blagojević:cite book |title=Gravitation and Gauge Symmetries |author=Milutin Blagojević |page=p. 5 |url=http://books.google.com/books?id=N8JDSi_eNbwC&pg=PA5&dq=inertial+frame+%22absolute+space%22&lr=&as_brr=0&sig=ACfU3U2Vg9oaYzXo79yholm3opU9DRpwZA#PPA5,M1 |isbn=0750307676 |publisher=CRC Press |year=2002] Quotation|*The existence of absolute space contradicts the internal logic of classical mechanics since, according to Galilean principle of relativity, none of the inertial frames can be singled out.
*Absolute space does not explain inertial forces since they are related to acceleration with respect to any one of the inertial frames.
*Absolute space acts on physical objects by inducing their resistance to acceleration but it cannot be acted upon. | Milutin Blagojević: "Gravitation and Gauage Symmetries", p. 5The utility of operational definitions was carried much further in the special theory of relativity. cite book |title=Special relativity |author=NMJ Woodhouse |page=p. 58 |url=http://books.google.com/books?id=tM9hic_wo3sC&pg=PA126&dq=Woodhouse+%22operational+definition%22&lr=&as_brr=0&sig=ACfU3U3at8NqLoJYAia9MZwvpqM4C7s9JQ |isbn=1-85233-426-6 |publisher=Springer |location=London |year=2003] Some historical background including Lange's definition is provided by DiSalle, who says in summary: Cite book
author =Robert DiSalle
chapter =Space and Time: Inertial Frames
title =The Stanford Encyclopedia of Philosophy
editor=Edward N. Zalta
Newton's inertial frame of reference
Within the realm of Newtonian mechanics, an "
inertial frame of reference", or "inertial reference frame", is one in which is valid.cite book |author=C Møller |title=The Theory of Relativity |publisher=Oxford University Press |location=Oxford UK |isbn=019560539X |year=1976 |page=p. 1 |url=http://worldcat.org/oclc/220221617&referer=brief_results |edition=Second Edition] However, the principle of special relativity generalizes the notion of inertial frame to include all physical laws, not simply Newton's first law.
Newton viewed the first law as valid in any reference frame moving with uniform velocity relative to the fixed stars; [The question of "moving uniformly relative to what?" was answered by Newton as "relative to "
absolute space". As a practical matter, "absolute space" was considered to be the fixed stars. For a discussion of the role of fixed stars, see cite book |title=Nothingness: The Science of Empty Space |author=Henning Genz |page= p. 150 |isbn=0738206105 |publisher=Da Capo Press |year=2001 |url=http://books.google.com/books?id=Cn_Q9wbDOM0C&pg=PA150&dq=frame+Newton+%22fixed+stars%22&lr=&as_brr=0&sig=ACfU3U3BgdpcUhKWUTkXYsEUOi5gaFuanQ ] that is, neither rotating nor accelerating relative to the stars.cite book |title=Physics |page=Volume 1, Chapter 3 |isbn=0471320579 |url=http://books.google.com/books?id=CucFAAAACAAJ&dq=intitle:physics+inauthor:resnick&lr=&as_brr=0
publisher=Wiley |year=2001 |edition=5th Edition |author= Robert Resnick, David Halliday, Kenneth S. Krane ] Today the notion of "
absolute space" is abandoned, and an inertial frame in the field of classical mechanicsis defined as:cite book |url=http://books.google.com/books?id=r5P29cN6s6QC&pg=PA70&dq=fixed+stars+%22inertial+frame%22&lr=&as_brr=0&sig=60wXg5_82gzOo0JVzMtvDEn5XEY |title=Introduction to classical mechanics |page=p. 70 |author=RG Takwale |publisher=Tata McGraw-Hill|year=1980 |isbn=0070966176 |location=New Delhi] cite book |url=http://books.google.com/books?id=ggPXQAeeRLgC&printsec=frontcover&dq=isbn:1852334266#PPA6,M1 |title=Special relativity |page=p. 6 |author=NMJ Woodhouse |publisher=Springer |year=2003 |isbn=1-85233-426-6 |location=London/Berlin] Hence, with respect to an inertial frame, an object or body accelerates only when a physical forceis applied, and (following Newton's first law of motion), in the absence of a net force, a body at rest will remain at rest and a body in motion will continue to move uniformly—that is, in a straight line and at constant speed. Newtonian inertial frames transform among each other according to the Galilean groupof symmetries.
If this rule is interpreted as saying that straight-line motion is an indication of zero net force, the rule does not identify inertial reference frames, because straight-line motion can be observed in a variety of frames. If the rule is interpreted as defining an inertial frame, then we have to be able to determine when zero net force is applied. The problem was summarized by Einstein:cite book |title=The Meaning of Relativity |author=A Einstein |page=p. 58 |year=1950 |url=http://books.google.com/books?as_q=&num=10&btnG=Google+Search&as_epq=%22The+weakness+of+the+principle+of+inertia+lies+in+this%2C+that+it+involves+an+argument+in+a+circle%3A+a+mass+moves+without+acceleration+if+it+is+sufficiently+far+from+other+bodies%3B+we+know+that+it+is+sufficiently+far+from+other+bodies+only+by+the+fact+that+it+moves+without+acceleration.%22+&as_oq=&as_eq=&as_brr=0&lr=&as_vt=&as_auth=&as_pub=&as_sub=&as_drrb=c&as_miny=&as_maxy=&as_isbn= |publisher=Princeton University Press]
There are several approaches to this issue. One approach is to argue that all real forces drop off with distance from their sources in a known manner, so we have only to be sure that we are far enough away from all sources to insure that no force is present.cite book |title=Introductory Special Relativity |author=William Geraint Vaughan Rosser |page=p. 3 |url=http://books.google.com/books?id=zpjBEBbIjAIC&pg=PA94&dq=reference+%22laws+of+physics%22&lr=&as_brr=0&sig=ACfU3U3Ee_ApZCzqvj_Y7hOU3M01GnDZxg#PPA3,M1
isbn=0850668387 |year=1991 |publisher=CRC Press ] A possible issue with this approach is the historically long-lived view that the distant universe might affect matters (
Mach's principle). Another approach is to identify all real sources for real forces and account for them. A possible issue with this approach is that we might miss something, or account inappropriately for their influence (Mach's principle again?). A third approach is to look at the way the forces transform when we shift reference frames. Fictitious forces, those that arise due to the acceleration of a frame, disappear in inertial frames, and have complicated rules of transformation in general cases. On the basis of universality of physical law and the request for frames where the laws are most simply expressed, inertial frames are distinguished by the absence of such fictitious forces.
Newton enunciated a principle of relativity himself in one of his corollaries to the laws of motion:cite book |title=Six not-so-easy pieces: Einstein's relativity, symmetry, and space-time |author=Richard Phillips Feynman |page=p. 50 |isbn=0201328429 |year=1998 |publisher=Basic Books |url=http://books.google.com/books?id=ipY8onVQWhcC&pg=PA49&dq=%22The+Principle+of+Relativity%22&lr=&as_brr=0&sig=ACfU3U3KfqkThK26GE7jG-QEFypFxJ17eQ#PPA50,M1 ] See the "Principia" on line at [http://ia310114.us.archive.org/2/items/newtonspmathema00newtrich/newtonspmathema00newtrich.pdf Andrew Motte Translation] ] Quotation|The motions of bodies included in a given space are the same among themselves, whether that space is at rest or moves uniformly forward in a straight line.|Isaac Newton: "Principia", Corollary V, p. 88 in Andrew Motte translation
This principle differs from the special principle in two ways: first, it is restricted to mechanics, and second, it makes no mention of simplicity. It shares with the special principle the invariance of the form of the description among mutually translating reference frames.However, in the Newtonian system the
Galilean transformationconnects these frames and in the special theory of relativity the Lorentz transformationconnects them. The two transformations agree for speeds of translation much less than the speed of light.] The role of fictitious forces in classifying reference frames is pursued further below.
Non-inertial reference frames
Inertial and non-inertial reference frames can be distinguished by the absence or presence of
fictitious forces, as explained shortly.cite book |title=Discovering the Natural Laws: The Experimental Basis of Physics |author= Milton A. Rothman |page=p. 23 |url=http://books.google.com/books?id=Wdp-DFK3b5YC&pg=PA23&vq=inertial&dq=reference+%22laws+of+physics%22&lr=&as_brr=0&source=gbs_search_s&cad=5&sig=ACfU3U33YE3keeD7lDVtQvt-ltW87Lsq2Q
isbn=0486261786 |publisher=Courier Dover Publications |year=1989 ] cite book |title=A Contemporary View of Elementary Physics |page=138 |publisher=McGraw-Hill |year=1968 |url=http://books.google.com/books?as_q=&num=10&btnG=Google+Search&as_epq=The+effect+of+his+being+in+the+noninertial+frame+is+to+require+the+observer+to&as_oq=&as_eq=&as_brr=0&lr=&as_vt=&as_auth=&as_pub=&as_sub=&as_drrb=c&as_miny=&as_maxy=&as_isbn= |asin= B000GQB02A |author=Sidney Borowitz & Lawrence A. Bornstein ] Quotation|The effect of his being in the noninertial frame is to require the observer to introduce a fictitious force into his calculations….|Sidney Borowitz and Lawrence A Bornstein in "A Contemporary View of Elementary Physics", p. 138 The presence of fictitious forces indicates the physical laws are not the simplest laws available so, in terms of the special principle of relativity, a frame where fictitious forces are present is not an inertial frame:cite book |title=Mathematical Methods of Classical Mechanics |page=p. 129 |author=V. I. Arnol'd |isbn=978-0-387-96890-2 |year=1989 |url=http://books.google.com/books?as_q=&num=10&btnG=Google+Search&as_epq=additional+terms+called+inertial+forces.+This+allows+us+to+detect+experimentally&as_oq=&as_eq=&as_brr=0&lr=&as_vt=&as_auth=&as_pub=&as_sub=&as_drrb=c&as_miny=&as_maxy=&as_isbn=|publisher=Springer] Bodies in
non-inertial reference frames are subject to so-called "fictitious" forces (pseudo-forces); that is, forces that result from the acceleration of the reference frameitself and not from any physical force acting on the body. Examples of fictitious forces are the centrifugal force and the Coriolis forcein rotating reference frames.
How then, are "fictitious' forces to be separated from "real" forces? It is hard to apply the Newtonian definition of an inertial frame without this separation. For example, consider a stationary object in an inertial frame. Being at rest, no net force is applied. But in a frame rotating about a fixed axis, the object appears to move in a circle, and is subject to centripetal force (which is made up of the Coriolis force and the centrifugal force). How can we decide that the rotating frame is a non-inertial frame? There are two approaches to this resolution: one approach is to look for the origin of the fictitious forces (the Coriolis force and the centrifugal force). We will find there are no sources for these forces, no originating bodies.For example, there is no body providing a gravitational or electrical attraction.] A second approach is to look at a variety of frames of reference. For any inertial frame, the Coriolis force and the centrifugal force disappear, so application of the principle of special relativity would identify these frames where the forces disappear as sharing the same and the simplest physical laws, and hence rule that the rotating frame is not an inertial frame.
Newton examined this problem himself using rotating spheres, as shown in Figure 2 and Figure 3. He pointed out that if the spheres are not rotating, the tension in the tying string is measured as zero in "every" frame of reference.That is, the universality of the laws of physics requires the same tension to be seen by everybody. For example, it cannot happen that the string breaks under extreme tension in one frame of reference and remains intact in another frame of reference, just because we choose to look at the string from a different frame.] If the spheres only "appear" to rotate (that is, we are watching stationary spheres from a rotating frame), the zero tension in the string is accounted for by observing that the centripetal force is supplied by the centrifugal and Coriolis forces in combination, so no tension is needed. If the spheres really are rotating, the tension observed is exactly the centripetal force required by the circular motion. Thus, measurement of the tension in the string identifies the inertial frame: it is the one where the tension in the string provides exactly the centripetal force demanded by the motion "as it is observed in that frame", and not a different value. That is, the inertial frame is the one where the fictitious forces vanish. (See rotating bucket argument for original text. See rotating spheres for mathematical formulation.)
So much for fictitious forces due to rotation. However, for linear acceleration, Newton expressed the idea of undetectability of straight-line accelerations held in common:
This principle generalizes the notion of an inertial frame. For example, an observer confined in a free-falling lift will assert that he himself is a valid inertial frame, even if he is accelerating under gravity, so long as he has no knowledge about anything outside the lift. So, strictly speaking, inertial frame is a relative concept. With this in mind, we can define inertial frames collectively as a set of frames which are stationary or moving at constant velocity "with respect to each other", so that a single inertial frame is defined as an element of this set.
For these ideas to apply, "everything" observed in the frame has to be subject to a base-line, common acceleration shared by the frame itself. That situation would apply, for example, to the elevator example, where all objects are subject to the same gravitational acceleration, and the elevator itself accelerates at the same rate.
Classical mechanics, which includes relativity, assumes the equivalence of all inertial reference frames. Newtonian mechanics makes the additional assumptions of absolute spaceand absolute time. Given these two assumptions, the coordinates of the same event (a point in space and time) described in two inertial reference frames are related by a Galilean transformation
where and represent shifts in the origin of space and time, and is the relative velocity of the two inertial reference frames. Under
Galilean transformations, the time between two events () is the same for all inertial reference frames and the distancebetween two simultaneous events (or, equivalently, the length of any object, ) is also the same.
Einstein's theory of special relativity, like Newtonian mechanics, assumes the equivalence of all inertial reference frames, but makes an additional assumption, foreign to Newtonian mechanics, namely, that in
free spacelight always is propagated with the speed of light"c"0, a defined [http://physics.nist.gov/cgi-bin/cuu/Value?c value] independent of its direction of propagation and its frequency, and also independent of the state of motion of the emitting body. This second assumption has been verified experimentally and leads to counter-intuitive deductions including:
time dilation(moving clocks tick more slowly)
length contraction(moving objects are shortened in the direction of motion)
relativity of simultaneity(simultaneous events in one reference frame are "not" simultaneous in almost all frames moving relative to the first).
These deductions are logical consequences of the stated assumptions, and are general properties of space-time, not properties pertaining to the structure of individual objects like atoms or stars, nor to the mechanisms of clocks.
These effects are expressed mathematically by the
where shifts in origin have been ignored, the relative velocity is assumed to be in the -direction and the
Lorentz factorγ is defined by:
The Lorentz transformation is equivalent to the
Galilean transformationin the limit "c"0 → ∞ (a hypothetical case) or "v" → 0 (low speeds).
Lorentz transformations, the time and distance between events may differ among inertial reference frames; however, the Lorentz scalardistance "s"2 between two events is the same in all inertial reference frames
From this perspective, the
speed of lightis only accidentally a property of light, and is rather a property of spacetime, a conversion factor between conventional time units (such as seconds) and length units (such as meters).
Incidentally, because of the limitations on speeds faster than the speed of light, notice that a rotating frame of reference (which is a non-inertial frame, of course) cannot be used out to arbitrary distances because at large radius its components would move faster than the speed of light. See Landau and Lifshitz. cite book |title=The Classical Theory of Fields |author=LD Landau & LM Lifshitz |edition=4rth Revised English Edition |page=pp. 273-274 |year=1975 |isbn=978-0-7506-2768-9 |publisher=Pergamon Press ]
General relativity is based upon the principle of equivalence:cite book |title=Physics for Scientists and Engineers with Modern Physics |author=Douglas C. Giancoli |url=http://books.google.com/books?id=xz-UEdtRmzkC&pg=PA155&dq=%22principle+of+equivalence%22&lr=&as_brr=0&sig=ACfU3U1ECpntiAMb3nvSw5WmTmKSK2Vqyw
page=p. 155 |year=2007 |publisher=Pearson Prentice Hall |isbn=0131495089 ] Quotation|"The principle of equivalence: "There is no experiment observers can perform to distinguish whether an acceleration arises because of a gravitational force or because their reference frame is accelerating|Douglas C. Giancoli "Physics for Scientists and Engineers with Modern Physics", p. 155This idea was introduced in Einstein's 1907 article "Principle of Relativity and Gravitation" and later developed in 1911.A. Einstein, "On the influence of gravitation on the propagation of light", "Annalen der Physik", vol. 35, (1911) : 898-908] Support for this principle is found in the
Eötvös experiment, which determines whether the ratio of inertial to gravitational mass is the same for all bodies, regardless of size or composition. To date no difference has been found to a few parts in 1011.cite book |title=Physics Through the Nineteen Nineties: Overview |page=p. 15 |url=http://books.google.com/books?id=Hk1wj61PlocC&pg=PA15&dq=equivalence+gravitation&lr=&as_brr=0&sig=ACfU3U3Yv2jH-RWbdEgxp_Z9BDZPnTFgSg
isbn=0309035791 |year=1986 |author=National Research Council (US) |publisher=National Academies Press ] For some discussion of the subtleties of the Eötvös experiment, such as the local mass distribution around the experimental site (including a quip about the mass of Eötvös himself), see Franklin.cite book |title=No Easy Answers: Science and the Pursuit of Knowledge |author=Allan Franklin |page=p. 66 |url=http://books.google.com/books?id=_RN-v31rXuIC&pg=PA66&dq=%22Eotvos+experiment%22&lr=&as_brr=0&sig=ACfU3U21h60xyvDwPRzyDEw83saJr9PzzQ
isbn=0822959682 |year=2007 |publisher=University of Pittsburgh Press ]
Einstein’s general theory modifies the distinction between nominally "inertial" and "noninertial" effects by replacing special relativity's "flat"
Euclidean geometrywith a curved non-Euclidean metric. In general relativity, the principle of inertia is replaced with the principle of geodesic motion, whereby objects move in a way dictated by the curvature of spacetime. As a consequence of this curvature, it is not a given in general relativity that inertial objects moving at a particular rate with respect to each other will continue to do so. This phenomenon of geodesic deviationmeans that inertial frames of reference do not exist globally as they do in Newtonian mechanics and special relativity.
However, the general theory reduces to the special theory over sufficiently small regions of spacetime, where curvature effects become less important and the earlier inertial frame arguments can come back into play. Consequently, modern special relativity is now sometimes described as only a “local theory”. (However, this refers to the theory’s application rather than to its derivation.)
* [http://plato.stanford.edu/entries/spacetime-iframes/ Stanford Encyclopedia of Philosophy entry]
* [http://www.youtube.com/watch?v=49JwbrXcPjc Animation clip] showing scenes as viewed from both an inertial frame and a rotating frame of reference, visualizing the Coriolis and centrifugal forces.
Edwin F. Taylorand John Archibald Wheeler, "Spacetime Physics", 2nd ed. (Freeman, NY, 1992)
Albert Einstein, "Relativity, the special and the general theories", 15th ed. (1954)
Henri Poincaré, (1900) "La theorie de Lorentz et la Principe de Reaction", "Archives Neerlandaises", V, 253–78.
Albert Einstein, "On the Electrodynamics of Moving Bodies", included in "The Principle of Relativity", page 38. Dover 1923
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