news and views Particles driven to diffraction Philip H.Bucksbaum Almost 70 years after it was first proposed,an experiment shows that electrons can be diffracted by light waves.This result highlights the interchangeable roles of matter and light. ave-particle duality is the concept that all particles can behave as waves. 0 and vice versa.This intellectually challenging notion,which is a fundamental prediction of quantum theory,has been tested in a new way by Herman Batelaan and co-workers,in an experiment reported on page 142 of this issue The debate over the particle versus wave B character of light is far older than quantum 22 theory.Newton was an early and active advocate for the corpuscular nature of light. 11 But it was in the first decades of the twentieth century that quantum mechanics brought E this discussion to a new plane by including matter as just another form of energy subject to the wave-particle dichotomy. What does it mean to say that matter Figure 1 Making light of the matter.Below left,diffraction of light forms a rainbow pattern on the behaves like a wave?We know waves have surface of a compact disk.Above,a drawing from Kapitza and Dirac's 1933 paper'that describes a ripples but,because atoms and electrons are proposed method for the diffraction of electrons(from the pathAE toAE)from an optical standing so small,if they are waves then their ripples wave formed by a light source,O,a collimating lens,D,and a mirror,C.Batelaan and co-workers'use must be tiny.The quantum ripples of an elec- a similar geometry in their experiment. tron in an atom are typically less than an angstrom,or one ten-billionth of a metre,in rainbow pattern of colours that you see when as described in the previous paragraph,even size.But we don't need to see ripples to detect you look at the surface of a compact disk is when the light waves are replaced by particles waves.The accepted evidence for wave-like caused by light waves diffracting from the and the material grating by light.Think behaviour is the phenomenon of diffraction. regularly spaced bands of shiny material that about how to make that compact disk out of Diffraction is easy to demonstrate for make up the tracks.This effect can be seen alight beam fora moment.Don't panicif you light.The because the wavelength of light,although haven't come upwithasolution:the Batelaan small,is large enough to be comparable to group has done it for you. the spaces between adjacent tracks. Batelaan and colleagues used a method When light from a lamp or the sun originally proposed by two brilliant physi- strikes a compact disk,each compo- cists,Paul Dirac and P.L.Kapitza,in a classic nent of colour in the 'white'light is paper2 written in 1933.Dirac and Kapitza deflected in a direction dictated by each won Nobel prizes later,but not for this the ratio of its wavelength to the work.This is the only paper that they wrote track spacing.Specifically,for together,and it seems to be an isolated light with wavelength A inci- curiosity.It was not written to resolve the dent at 90on a grating with wave-particle debate,because by the early track spacing d.diffraction 1930s this had been decided by numerous occurs at an angle given by experiments in favour of...well,both parti- sin0=λ/d2Wd3 Vd and cles and waves,as quantum theory predicts. so on.Light waves scatter- Nonetheless they wrote that the diffraction ing from all of the tracks of electrons by light would be a very interest- add coherently only at ing experiment. these special angles.The The figure in the Kapitza-Dirac paper wavelengths of visible light reveals the trick for creating a regular lattice are tiny (just 400-700 of optical radiation (Fig.1).Kapitza and nanometres)but,if the Dirac reasoned that an optical standing grating spacing d is small wave would have the correct properties.A enough,the separation of the standing wave is just wave-like motion that colours (due to the angle 6)is oscillates but doesn't travel,such as the oscil- easy to detect. lations ofa vibrating violin string.Anoptical Wave-particle duality in quan- standing wave has an oscillating electric field tum mechanics means that we should made by two counterpropagating and over- be able to perform the same observation lapping light beams.Kapitza and Dirac NATURE|VOL41313SEPTEMBER 2001www.nature.com 2001 Macmillan Magazines Ltd 117Wave–particle duality is the concept that all particles can behave as waves, and vice versa. This intellectually challenging notion, which is a fundamental prediction of quantum theory, has been tested in a new way by Herman Batelaan and co-workers, in an experiment reported on page 142 of this issue1 . The debate over the particle versus wave character of light is far older than quantum theory. Newton was an early and active advocate for the corpuscular nature of light. But it was in the first decades of the twentieth century that quantum mechanics brought this discussion to a new plane by including matter as just another form of energy subject to the wave–particle dichotomy. What does it mean to say that matter behaves like a wave? We know waves have ripples but, because atoms and electrons are so small, if they are waves then their ripples must be tiny.The quantum ripples of an elec￾tron in an atom are typically less than an ångström, or one ten-billionth of a metre, in size.But we don’t need to see ripples to detect waves. The accepted evidence for wave-like behaviour is the phenomenon of diffraction. Diffraction is easy to demonstrate for light. The rainbow pattern of colours that you see when you look at the surface of a compact disk is caused by light waves diffracting from the regularly spaced bands of shiny material that make up the tracks. This effect can be seen because the wavelength of light, although small, is large enough to be comparable to the spaces between adjacent tracks. When light from a lamp or the sun strikes a compact disk, each compo￾nent of colour in the ‘white’ light is deflected in a direction dictated by the ratio of its wavelength to the track spacing. Specifically, for light with wavelength inci￾dent at 90° on a grating with track spacing d, diffraction occurs at an angle given by sin/d, 2/d, 3/d and so on. Light waves scatter￾ing from all of the tracks add coherently only at these special angles. The wavelengths of visible light are tiny (just 400–700 nanometres) but, if the grating spacing d is small enough, the separation of the colours (due to the angle ) is easy to detect. Wave–particle duality in quan￾tum mechanics means that we should be able to perform the same observation as described in the previous paragraph, even when the light waves are replaced by particles and the material grating by light. Think about how to make that compact disk out of a light beam for a moment.Don’t panic if you haven’t come up with a solution;the Batelaan group1 has done it for you. Batelaan and colleagues used a method originally proposed by two brilliant physi￾cists, Paul Dirac and P. L. Kapitza, in a classic paper2 written in 1933. Dirac and Kapitza each won Nobel prizes later, but not for this work. This is the only paper that they wrote together, and it seems to be an isolated curiosity. It was not written to resolve the wave–particle debate, because by the early 1930s this had been decided by numerous experiments in favour of… well, both parti￾cles and waves, as quantum theory predicts. Nonetheless they wrote that the diffraction of electrons by light would be a very interest￾ing experiment. The figure in the Kapitza–Dirac paper reveals the trick for creating a regular lattice of optical radiation (Fig. 1). Kapitza and Dirac reasoned that an optical standing wave would have the correct properties. A standing wave is just wave-like motion that oscillates but doesn’t travel, such as the oscil￾lations of a vibrating violin string.An optical standing wave has an oscillating electric field made by two counterpropagating and over￾lapping light beams. Kapitza and Dirac NATURE|VOL 413 | 13 SEPTEMBER 2001 |www.nature.com 117 news and views Particles driven to diffraction Philip H. Bucksbaum Figure 1 Making light of the matter. Below left, diffraction of light forms a rainbow pattern on the surface of a compact disk. Above, a drawing from Kapitza and Dirac’s 1933 paper2 that describes a proposed method for the diffraction of electrons (from the path AE to AE) from an optical standing wave formed by a light source, O, a collimating lens, D, and a mirror, C. Batelaan and co-workers1 use a similar geometry in their experiment. STEVE PERCIVAL/SPL Almost 70 years after it was first proposed, an experiment shows that electrons can be diffracted by light waves. This result highlights the interchangeable roles of matter and light. © 2001 Macmillan Magazines Ltd