Editing Time travel (section)

=== Experimental results ===
Certain experiments carried out give the impression of reversed [[causality]], but fail to show it under closer examination.

The [[delayed-choice quantum eraser]] experiment performed by [[Marlan Scully]] involves pairs of [[Quantum entanglement|entangled]] [[photon]]s that are divided into "signal photons" and "idler photons", with the signal photons emerging from one of two locations and their position later measured as in the [[double-slit experiment]]. Depending on how the idler photon is measured, the experimenter can either learn which of the two locations the signal photon emerged from or "erase" that information. Even though the signal photons can be measured before the choice has been made about the idler photons, the choice seems to retroactively determine whether or not an [[Interference (wave propagation)|interference pattern]] is observed when one correlates measurements of idler photons to the corresponding signal photons. However, since interference can be observed only after the idler photons are measured and they are correlated with the signal photons, there is no way for experimenters to tell what choice will be made in advance just by looking at the signal photons, only by gathering classical information from the entire system; thus causality is preserved.<ref name=Greene2004>{{cite book|last=Greene|first=Brian|title=The Fabric of the Cosmos|url=https://archive.org/details/fabricofcosmossp00gree|url-access=registration|year=2004|publisher=Alfred A. Knopf|isbn=978-0-375-41288-2|pages=[https://archive.org/details/fabricofcosmossp00gree/page/197 197–199]}}</ref>

The experiment of Lijun Wang might also show causality violation since it made it possible to send packages of waves through a bulb of caesium gas in such a way that the package appeared to exit the bulb 62 nanoseconds before its entry, but a wave package is not a single well-defined object but rather a sum of multiple waves of different frequencies (see [[Fourier analysis]]), and the package can appear to move faster than light or even backward in time even if none of the pure waves in the sum do so. This effect cannot be used to send any matter, energy, or information faster than light,<ref name="gauthier">{{cite news|last=Wright|first=Laura|title=Score Another Win for Albert Einstein|magazine=[[Discover (magazine)|Discover]]|date=November 6, 2003|url=http://discovermagazine.com/2003/nov/score-another-win-for-einstein1106|access-date=October 21, 2009|archive-date=June 12, 2018|archive-url=https://web.archive.org/web/20180612193137/http://discovermagazine.com/2003/nov/score-another-win-for-einstein1106|url-status=live}}</ref> so this experiment is understood not to violate causality either.

The physicists [[Günter Nimtz]] and Alfons Stahlhofen, of the [[University of Koblenz]], claim to have violated Einstein's theory of relativity by transmitting photons faster than the speed of light. They say they have conducted an experiment in which [[microwave]] photons traveled "instantaneously" between a pair of prisms that had been moved up to {{convert|3|ft|m|abbr=on}} apart, using a phenomenon known as [[quantum tunneling]]. Nimtz told ''[[New Scientist]]'' magazine: "For the time being, this is the only violation of special relativity that I know of." However, other physicists say that this phenomenon does not allow information to be transmitted faster than light. [[Aephraim M. Steinberg]], a quantum optics expert at the [[University of Toronto]], Canada, uses the analogy of a train traveling from Chicago to New York, but dropping off train cars at each station along the way, so that the center of the train moves forward at each stop; in this way, the speed of the center of the train exceeds the speed of any of the individual cars.<ref name="nimtz">{{cite news|last=Anderson|first=Mark|title=Light seems to defy its own speed limit|magazine=[[New Scientist]]|volume=195|issue=2617|page=10|date=August 18–24, 2007|url=https://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php|access-date=2018-09-18|archive-date=2018-06-12|archive-url=https://web.archive.org/web/20180612142609/https://www.eurekalert.org/pub_releases/2007-08/ns-lst081607.php|url-status=live}}</ref>

[[Shengwang Du]] claims in a peer-reviewed journal to have observed single photons' [[Precursor (physics)|precursor]]s, saying that they travel no faster than ''[[Speed of light|c]]'' in a vacuum. His experiment involved [[slow light]] as well as passing light through a vacuum. He generated two single [[photon]]s, passing one through [[rubidium]] atoms that had been cooled with a laser (thus slowing the light) and passing one through a vacuum. Both times, apparently, the precursors preceded the photons' main bodies, and the precursor traveled at ''c'' in a vacuum. According to Du, this implies that there is no possibility of light traveling faster than ''c'' and, thus, no possibility of violating causality.<ref>{{citation|url=http://www.ust.hk/eng/news/press_20110719-893.html|publisher=The Hong Kong University of Science & Technology|access-date=September 5, 2011|title=HKUST Professors Prove Single Photons Do Not Exceed the Speed of Light|date=July 17, 2011|archive-date=February 25, 2012|archive-url=https://web.archive.org/web/20120225022608/http://www.ust.hk/eng/news/press_20110719-893.html|url-status=live}}</ref>