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243 244 // Homepage & listing ads 245 const t_1 = defineIfSlot('/21821800277/pw/pw_t_hp', [[970,90],[728,90],[729,90],[300,250],[320,50]], 'div-gpt-ad-3759129-18', pubads); 246 const t_2 = defineIfSlot('/21821800277/pw/pw_t1_hp', [[970,90],[728,90],[729,90],[300,250],[320,50]], 'div-gpt-ad-3759129-19', pubads); 247 const t_3 = defineIfSlot('/21821800277/pw/pw_t2_hp', [[970,90],[728,90],[729,90],[300,250],[320,50]], 'div-gpt-ad-3759129-20', pubads); 248 249 // Only map if they actually exist 250 t_1 && t_1.defineSizeMapping(leaderboard); 251 t_2 && t_2.defineSizeMapping(leaderboard); 252 t_3 && t_3.defineSizeMapping(leaderboard); 253 254 // Search results 255 256 const headerSize = googletag.sizeMapping() 257 .addSize([970, 0], [[970, 90]]) 258 .addSize([0, 0], [[320, 50]]) 259 .build() 260 261 // Header ad (always appears) 262 googletag.defineSlot('/21821800277/pw/pw_h_sw', [[970,90], [320, 50]], 'div-gpt-ad-5400963-20').defineSizeMapping(headerSize).addService(pubads); 263 264 // Only apply targeting if values exist 265 const article_og = getOgType(); 266 267 if ( article_og ) { 268 pubads.setTargeting( "articleTag", article_og ); 269 } 270 271 const article_type = getMetaContent( "oas:article-type" ); 272 273 if ( article_type ) { 274 pubads.setTargeting( "articleType", article_type ); 275 } 276 277 const article_category = getMetaContent( "oas:category" ); 278 279 if ( article_category ) { 280 pubads.setTargeting( "category", article_category ); 281 } 282 283 const article_sub = getMetaContent( "oas:sub-category" ); 284 285 if ( article_sub ) { 286 pubads.setTargeting( "subCategory", article_sub ); 287 } 288 289 pubads.setTargeting( "articleId", "130590" ); 290 291 292 293 // Add class to body if ads are visible 294 pubads.addEventListener( "impressionViewable", function () { 295 document.body.classList.add("has-ads"); 296 }); 297 298 pubads.enableSingleRequest(); 299 pubads.collapseEmptyDivs(); 300 301 // Start! 302 googletag.enableServices(); 303 }); 304</script>
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855p-content/uploads/2026/09/23-9-26-chopped-photon.jpg"><img decoding="async" class="size-full wp-image-130596" src="https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon.jpg" alt="Illustration of a truncated photon" width="829" height="537" srcset="https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon.jpg 829w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-211x137.jpg 211w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-317x205.jpg 317w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-768x497.jpg 768w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-618x400.jpg 618w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-636x412.jpg 636w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-633x410.jpg 633w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-635x411.jpg 635w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-304x197.jpg 304w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-257x166.jpg 257w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-300x194.jpg 300w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-139x90.jpg 139w, https://physicsworld.com/wp-content/uploads/2026/09/23-9-26-chopped-photon-128x83.jpg 128w" sizes="(max-width: 829px) 100vw, 829px" /></a>
855<script type="application/json">"<strong>Chop and change<\/strong> The top panel shows a photon (represented as a wave) being reflected from a mirror. The wave is incident from left to right with the solid blue line showing the portion of the wave travelling left to right and the dashed line showing the portion of the wave that has been reflected back to the left. The researchers considered what happens when the mirror is removed and the truncated wave continues to travel to the right. The lower panel shows the situation shortly after the mirror has been removed. The narrow central portion is the complicated transition region containing multiple photons. The region to the left shows a truncated photon state that is locally equivalent to a single photon. To the right is the truncated photon shown as locally equivalent to the vacuum state. (Courtesy: Isak Cecil Onsager Rukan, Jan Gulla, Johannes Skaar\/<em>Phys. Rev. Lett.<\/em>)"</script>
855<figcaption class="gallery-item__caption"><strong>Chop and change</strong> The top panel shows a photon (represented as a wave) being reflected from a mirror. The wave is incident from left to right with the solid blue line showing the portion of the wave travelling left to right and the dashed line showing the portion of the wave that has been reflected back to the left. The lower panel shows the situation shortly after the mirror has been removed. The narrow central portion is the complicated transition region containing multiple photons. The region to the left shows a truncated photon state that is locally equivalent to a single photon. To the right is the truncated photon shown as locally equivalent to the vacuum state. (Courtesy: Isak Cecil Onsager Rukan, Jan Gulla, Johannes Skaar/<em>Phys. Rev. Lett.</em>)</figcaption></figure> 856<p>Physicists in Norway have combined quantum mechanics with relativity to show that the act of âcutting a photon in halfâ has no immediate effect on how that photon appears to an observer outside of a small transition region where the truncation took place. Within that region, however, their calculations suggest that large numbers of photons are required to describe the truncated photon.</p> 857<span class="mpu align-right" aria-hidden="true"><div id='div-gpt-ad-3759129-1'class="advert" > 858
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860 861</div></span> 862<p>Light is complex. For some phenomenon, it exhibits particle-like behaviour, and for others, it exhibits wave-like behaviour. This duality is reconciled by quantum field theory, which describes light as both a particle and a wave at the same time. More precisely, in this framework, light can be considered as consisting of particles called photons, and these photons are defined as excitations in quantum electromagnetic fields.</p> 863<p>A common technique in optical experiments is to chop a beam of light into divisions called pulses, and an interesting â and until recently, unexplored – question is what happens to a single photon when it is chopped? The physicists <a href="https://www.mn.uio.no/fysikk/english/?vrtx=person-view&uid=icrukan">Isak Cecil Onsager Rukan</a>, <a href="https://www.mn.uio.no/fysikk/english/people/aca/jangull/">Jan Gulla</a> and <a href="https://www.mn.uio.no/fysikk/english/people/aca/johask/index.html">Johannes Skaar</a> at Norwayâs University of Oslo set out to answer exactly this question.</p> 864<h3>Can one cut a photon in half?</h3> 865<p>What do the researchers mean by cutting a photon in half? In the wave picture of light one can imagine an electromagnetic wave that is separated in two parts by an ideal shutter. Writing in <a href="https://journals.aps.org/prl/abstract/10.1103/94pm-hp34"><em>Physical Review Letters</em></a>, the trio considers that thought experiment with a set-up displayed in the upper portion of the figure.</p> 866<p>Here, a photon is represented by an electromagnetic wave that is moving from left to right towards a mirror. If the mirror is ideal, then the photon is reflected completely.</p> 867<p>A photon can be truncated by removing (either gradually or abruptly) the mirror while the wave is being reflected. Because some of the wave has already been reflected when the mirror is removed, the result is forward-moving and backward-moving modes â the solid and dashed lines respectively.</p> 868<p>In this way, the action of removing the mirror has effectively âcutâ the photon in half if we ignore the backward-moving modes. The outcome is surprising. In the particle picture of light, one would expect to find or not find a forward propagating photon. However, calculations done by the trio show that the outcome is a complicated state consisting of a classical mix and a quantum superposition of multiple photons.</p> 869<h3>Shifting perspectives</h3> 870<p>Returning to our earlier definition of a photon, the absence of excitations of the quantum electromagnetic field corresponds to the absence of p
870hotons. This is called a vacuum (or empty) state. However, this notion of photons (and particles in general), is not always fixed, and can be observer or scenario dependent. For instance, what looks like a vacuum to a stationary observer, might look like radiation (a bunch of photons) to an accelerating observer. This is known as the Unruh effect.</p> 871<p>In a similar manner, what is considered a vacuum state and a photon is different before and after the mirror is removed. This is because the presence of the mirror divides the physical space in half. The nature of excitations before and after the reflector is removed is different, thereby changing the notion of what is considered as a particle. The definition of vacuum in different frames (observers) is related mathematically by the Bogoliubov transformation. It is the same mathematics that also describes why black holes must eventually decay by emitting radiation. In this case the incoming vacuum state is related to outgoing thermal radiation via these transformations.</p> 872<p>Using these mathematical tools, the researchers were able to calculate the complicated final state. First, they assumed that the reflector was removed instantaneously. In such a scenario, they had the unphysical result that the expected number of photons in the final state is infinity. If they assume that the mirror is slowly removed, they get a finite number of expected photons in the final state. However, you can still observe any number of photons. âThere is a non-zero probability of observing any number of photons,â explains Skaar.</p> 873<p><strong>Nothing is faster than the speed of light</strong></p> 874<p>Understanding what happens when the mirror is removed requires the invocation of both classical and quantum physics. On one hand, the quantum state of the electromagnetic field changes on removal of the mirror, with the very definition of a particle or a vacuum state being a very non-local concept. This is because quantum mechanics requires that the excitation must be defined over the entire physical space. On the other hand, the classical principle of causality dictates that information cannot travel faster than the speed of light. As a result, the change in the physical state of the electromagnetic fields caused by the removal of the reflector cannot propagate faster than the speed of light. This means that, for an observer at a location far enough from the mirror such that light has not had enough time to travel there yet, should experience no change in physical state.</p> 875<p>So, quantum mechanics requires the instantaneous change of the entire global physical state on removing the mirror, whereas causality requires a finite speed limit on the propagation of any change. The challenge for the Oslo trio was how to reconcile these two requirements.</p> 876<article class='editors-pick'> 877 <a href='https://physicsworld.com/a/einsteins-recoiling-slit-experiment-realized-at-the-quantum-limit/' class='editors-pick__title_link'> 878 <h4 class='editors-pick__title'><p>Einsteinâs recoiling slit experiment realized at the quantum limit</p> 879</h4> 880 </a> 881 <div class='editors-pick__image'> 882 <div class='editors-pick__more'>Read more</div> 883 <a href='https://physicsworld.com/a/einsteins-recoiling-slit-experiment-realized-at-the-quantum-limit/'> 884 <img loading="lazy" decoding="async" width="154" height="90" src="https://physicsworld.com/wp-content/uploads/2026/01/28-01-26-einstein-slit-featured-154x90.jpg" class="attachment-list-image size-list-image wp-post-image" alt="Laboratory set-up of Einsteinâs thought experiment" aria-label="Read more: Einsteinâs recoiling slit experiment realized at the quantum limit" srcset="https://physicsworld.com/wp-content/uploads/2026/01/28-01-26-einstein-slit-featured-154x90.jpg 154w, https://physicsworld.com/wp-content/uploads/2026/01/28-01-26-einstein-slit-featured-211x123.jpg 211w, https://physicsworld.com/wp-content/uploads/2026/01/28-01-26-einstein-slit-featured-1024x598.jpg 1024w, https://physicsworld.com/wp-content/uploads/2026/01/28-01-26-einstein-slit-featured-317x185.jpg 317w, https://physicsworld.com/w
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