<story><title>A mirror that reverses how light travels in time</title><url>https://spectrum.ieee.org/time-reversal-interface</url></story><parent_chain></parent_chain><comment><author>tomxor</author><text>The title is very click batey.&lt;p&gt;It doesn&amp;#x27;t reverse how light travels in time, it just reverses light in terms of the order of photons, as in &amp;quot;Last in First out&amp;quot; instead of &amp;quot;First in First out&amp;quot;. The concept is quite natural to grasp. The article makes it harder to understand than necessary by making it sound like science fiction.&lt;p&gt;The &amp;quot;reflector&amp;quot; is actually a medium that encompass the entire signal being reversed... in the same way that you would need to buffer the entire segment of a signal in memory before reversing it (you can&amp;#x27;t send what you haven&amp;#x27;t yet received), you would also need the entire segment of light to be travelling through the medium at the same time in order to reverse it. This is why their experiment involves such long sections of material.&lt;p&gt;It&amp;#x27;s an interesting and useful phenomenon, effectively being able to instantly change a material&amp;#x27;s properties to reverse the direction of photons currently passing through it, which probably has good application in signal processing performance (it would be comparable to hardware signal processing using electrical components). But &amp;quot;time&amp;quot; is not reversed; the &lt;i&gt;direction&lt;/i&gt; of photon&amp;#x27;s currently traversing a medium is dynamically reversed, but unlike a reflective &lt;i&gt;surface&lt;/i&gt;, a reflective &lt;i&gt;medium&lt;/i&gt; could change the direction of multiple photons simultaneously instead of one at a time, which affects the order of that segment..</text></comment>
<story><title>A mirror that reverses how light travels in time</title><url>https://spectrum.ieee.org/time-reversal-interface</url></story><parent_chain></parent_chain><comment><author>mcdonje</author><text>&amp;gt;When a light wave enters the new time interface and the device changes its optical properties, the signal keeps moving forward in space. However, the signal gets reversed—if it was a spoken word, it would sound as if it was getting played backwards. In contrast, with a conventional reflection, a light or sound wave would travel back at its source but mostly look or sound the same as it did before&lt;p&gt;So it&amp;#x27;s reflecting in a novel way. It&amp;#x27;s not sending information into the past. The headline gave me the wrong impression. Relativity lives. Time travel doesn&amp;#x27;t exist.</text></comment>