Talk:Time travel

Time travel to the past via faster than light travel not possible

Scientists have proved than photons cannot go faster than light and so this method is impossible. http://news.discovery.com/space/time-travel-impossible-photon-110724.html 175.142.167.192 (talk) 13:54, 15 September 2011 (UTC)

Seems rather tautological. So they claim "Photons can't travel faster than light", but photons ARE light, meaning that 'Light can't travel faster than light'.

Also, what has the speed of a photon got to do with time travel? It seems that light can't travel faster than itself but this says nothing of the velocity of matter. HumphreyW (talk) 14:15, 15 September 2011 (UTC)

The reason it's not tautological is because they are informally using "speed of light" as a synonym for the constant c, which can be measured in ways that don't involve light (for example the constant appears in the time dilation equation, so measuring time dilation allows for a type of measure of the value of c). Relativity also says that it would take infinite energy to accelerate slower-than-light particles to faster-than-light, but neither relativity nor this new finding rules out the possibility of tachyon particles which always travel faster than light, although there are other reasons why physicists are pretty confident they don't exist. Hypnosifl (talk) 23:55, 15 September 2011 (UTC)

Entropy on closed timelike curves

Sorry, I think "On the other hand, the second law of thermodynamics is understood by modern physicists to be a statistical law rather than an absolute one, so spontaneous reversals of entropy or failure to increase in entropy are not impossible, just improbable (see for example the fluctuation theorem)." is improper to describe the abrasion of a watch, because in "real world" no law will predict different results from classical ones. Ltysdd (talk) 07:54, 29 September 2011 (UTC)

But spontaneous decreases in entropy have been observed, something which is predicted by statistical mechanics but not by classical (non-statistical) thermodynamics--see the fluctuation theorem, which is derived from statistical assumptions and which predicts the relative likelihood that entropy will increase or decrease in a given time, and which has been tested experimentally as discussed in the second-to-last paragraph of the section Fluctuation theorem#Statement of the fluctuation theorem. Hypnosifl (talk) 06:25, 30 September 2011 (UTC)

Not done for now: I am closing this edit request pending a response to Hypnosifl and a consensus to proceed. Monty₈₄₅ 19:02, 1 October 2011 (UTC)

additions to oldest time travel stories list .

18:09, 15 October 2011 (UTC) the Talmud relates that Among the Tannaim, the generations of rabbinic teachers whose work is recorded in the Mishnah, Rabbi Akiba is generally considered the towering personality. Approximately one hundred years after his death, a legend is reported by R. Judah bar Ezekiel (219-299 C.E.), in the name of his teacher and sometime traveling companion, Ray: - Rav Judah said in the name of Rav, - God translated Moses in time so that Moses himself attended a lecture given by Rabbi Akiva. story is told of Rabbi Akiva that when Moses ascended Mount Sinai, he saw that God was putting little taggim (the small 'crowns' on the top of the letters in the Torah scroll) on top of the Torah that was to be presented to the Jews at Mount Sinai. Moses asked God to explain the meaning of these taggim. God explained that in the future a man by the name of Akiva ben Joseph will reveal what these signs mean. - - Moses asked God to reveal to him this man and so God replied to Moses to turn around. When Moses turned around he saw a sage surrounded by many rows of students listening eagerly to this man's teaching. The greater students sat in front and the lesser in the rear. Moses, being a very humble man, took a seat in the eighth row and began listening. Rabbi Akiba taught a certain law and the students asked him what is the source, he replied that it came down to us from our great master Moses. At this lecture, Rabbi Akiva stated that all that he was teaching originated with Moses - yet Moses himself heard these matters for the first time! - Moses came back and questioned God, if there is such a great man like that why give the Torah through me? God answered, "Be silent, this is my will." (Menachoth 29b) - - One can then understand this in this way : Rabbi Akiva indeed originated the material, and then this fact allowed the material to become known to Moses via God prior to Rabbi Akiva's birth, at Mt. Sinai, creating again a non-causal loop.IF so this is the first time paradox story. - - The means by which at Sinai Moses was made aware of all the halachot which would eventually be developed is generally taken to be via direct transmission from God, as was the case with the rest of the Torah. However the means by which Moses is made aware of those matters discovered by Rabbi Akiva may have been by the bringing of Moses forward in time to participate in Rabbi Akiva's lectures. - ( see Rabbi Akiba's Crowns: Postmodern Discourse and the Cost of Rabbinic Reading - by Laurence L. Edwards http://findarticles.com/p/articles/mi_m0411/is_4_49/ai_68738707/) - eli eshed The earliest time travell to the past known is FAUST THE SECOND PART " by Johann Wolfgang von Goethe. Part of it was already published in 1827 and completly in 1832 . And in it there is what seems to Be a time travel of Faust to Ancient Greece. It is possible though that it is a journey to a parallel world of ancient Greece in which Greece gods still exist.The point in not very clear probably since the comcept of time travell to the past was not very clear to Goethe himself, he was just the inventor of it.... Time travell to te future on the other hand is ancient concept which exist from antiquity. eli eshed — Preceding unsigned comment added by 87.69.168.53 (talk)

Hans Christian Andersen Journey on Foot from Holmen's Canal to the East Point of Amager". a journey to the future of 2129.

MAY 1838 Hans Christian Andersen The Goloshes of Fortune - http://hca.gilead.org.il/goloshes.html - which is among other things about time travel to the past to Denemark of medieval times.After that the hero make a space journey. 1845 Hans Christian Andersen Lykkens Blomst (The flower of happiness). Magic comedy in two acts,C.A. Reitzel Publishers, 1845. this is a time travel play in which a 19 century dane is taken by magick to inhabite the bodies of a ,medival prince and a 18 century poet and eventuall returned to his time and place. eli eshed . — Preceding unsigned comment added by 87.69.168.53 (talk) 01:15, 16 October 2011 (UTC)

The story of Moses and the rabbi (spelled Akiva on most pages) could simply be a vision of the future, since Moses doesn't interact with anyone. And the time loop theory seems dubious, since according to this page Moses didn't understand any of the exposition in the lecture, and furthermore the page says that:

You see, when Moses heard Rabbi Akiva teaching, he was concerned. How could he know for certain that this was the Torah as he had received it? But once he heard that this man was not one to take credit for himself, but rather to quote in Moses’ name, he understood that Rabbi Akiva’s teachings were pure, unadulterated Torah, the same Torah Moses was to receive, only unfolded and unpacked.

So, Moses was just getting in advance a preview of the Torah that he was to receive as a revelation from God, it would only be a time loop if that vision was the only place he got the idea for these teachings (but since he didn't understand what he was hearing in the story, that doesn't particularly make sense).

The ones by Goethe and Hans Christian Andersen would be good to discuss, though. From the description here it sounds like Faust is taken to ancient Sparta to meet Helen of Troy by a homonculus that his assistant Wagner has created, and The Galoshes of Fortune definitely seems to involve magical transport to medieval times (someone has already added these to the list of stories but there could be a few sentences about them in the writeup). Hypnosifl (talk)

Also, I don't know if we really need to include Hans Christian Andersen's "Journey on Foot from Holmen's Canal to the East Point of Amager" since there had already been stories of magical travel to the future written before that, as detailed in that section. Did the main character return to his own time in that story? Hypnosifl (talk) 23:37, 16 October 2011 (UTC)

Light travels in time, per definition

This article is a bit ridiculous. Rather than focusing on the hypothetical, why not focus on the physical realities we know about first?! Nowhere in the article do I see it mentioned that light actually travels in time. Per definition, light travels in time. For the light, no time passes, and only the surrounds seems to speed up, which is in fact what I understand to be time travel. Albeit, only in one direction: forward. And with a speed limit. Doesn't anybody else think this relevant to mention in the first or second paragraph even?! — Preceding unsigned comment added by 196.215.115.224 (talk) 09:11, 28 November 2011 (UTC)

Moved this comment to the bottom, per wp:TPG.

Please sign your talk page messages with four tildes (~~~~)? Thanks.

I have removed the phrase you added ("much the same way that light travels"), as it is not sourced, and it isn't clear to which part of the sentence "much the same way" refers. The expression "much the same" is also quite vague — see, sort of, wp:WEASEL. - DVdm (talk) 09:22, 28 November 2011 (UTC)

There isn't really a meaningful sense that one can say "for the light, no time passes" since light does not have its own reference frame in relativity, nor can we construct any sort of physical clock that travels at precisely the speed of light. If you made a trip away from the Earth and back at a large fraction of the speed of light, hundreds/thousands/millions/etc. of years might have passed on Earth while the time experienced by you (as measured by a clock you carried, for example) could be arbitrarily small, but this is already discussed in the article, it's what's known as "time dilation". Hypnosifl (talk) 04:15, 7 December 2011 (UTC)

The ICARUS result just reproduced OPERA's results on the energy front

They found nothing new. If Cohen/Glashow is right, then the OPERA results are inconsistent; the ICARUS replication, despite the media hoopla a month after the paper came out, means nothing. Check the one line on ICARUS in the OPERA neutrino anomaly.

How is this relevant to the line you want to cut from the article? It didn't even mention the ICARUS argument, it was just talking about the analysis by Cohen/Glashow, which shows that if you consider the mainstream ideas about how the Standard Model could be extended to allow Lorentz-violation (as mentioned in this post and this one written by physicists, for example) you would get a prediction about Cerenkov radiation that was not observed. You could be correct that the results at OPERA are already demonstrably inconsistent with this prediction without the need for the additional ICARUS experiment (although I'd like to see a source for that, since the "one line on ICARUS in the OPERA neutrino anomaly" doesn't make clear whether the inconsistency with the Cohen/Glashow prediction was equally clear in the data from both experiments), but that doesn't mean that the Cohen/Glashow argument is "not credible" on a theoretical level, and that theoretical level is what the line you cut from the article was talking about. Hypnosifl (talk) 01:23, 20 December 2011 (UTC)

The line I cut mentions "and another group of experimenters argue that the lack of Cherenkov radiation indicates the neutrinos cannot have really been traveling faster than light, and the OPERA group must have just made a mistake in timekeeping." The "another group of experimenters" is the ICARUS experiment (I assume you are not debating that?) The line says nothing of Cohen-Glashow (the link to Cherenkov radiation does not mention C-G emissions, since, well, they are hypothetical). As to your other question, check ref 2 in the article: "Indeed, Dario Autiero, a physicist at the Institute of Nuclear Physics in Lyons, France, and OPERA's physics coordinator, says that measurements of the neutrino energies by OPERA, reported in a February 2011 paper, already failed to show signs of the effect later predicted by Cohen and Glashow. 'It is very well known, and it has been presented in tens of OPERA talks at conferences,' he says, 'it is not something that we learn today because of ICARUS.' Blogs are not reliable secondary sources, see WP:SPS. Nature News is. Ajoykt (talk) 03:33, 20 December 2011 (UTC)

The line says nothing of Cohen-Glashow (the link to Cherenkov radiation does not mention C-G emissions, since, well, they are hypothetical)

True, the line you cut doesn't go into details about how the prediction of Cherenkov radiation was derived, it's not necessary to go into those sorts of details in a short statement which contains a reference to an article. The reference itself does mention that the ICARUS analysis was based on Cohen-Glashow:

"Immediately after OPERA first announced its result two months ago, physicists Andrew Cohen and Sheldon Glashow at Boston University argued that radiation analogous to Cherenkov radiation, but tailored to the case of neutrinos rather than charged particles, ought to have been emanating from the neutrino beam studied at LNGS. As the neutrinos emitted this radiation, they ought to have been losing a commensurate amount of energy."

"David Cline, a physicist at the University of California, Los Angeles, and a member of the ICARUS team, said Cherenkov-like radiation in the form of photons and electron-positron pairs would have spewed from what physicists call "virtual particles" hovering around the neutrinos."

As to your other question, check ref 2 in the article

Yes, I only said that the OPERA neutrino anomaly article itself doesn't make clear whether the OPERA data was sufficient, but that reference does clear it up. The fact remains that this is not a good reason to call the article "not credible", since the article doesn't claim that the ICARUS data was necessary to the argument, it just discusses the analysis by the ICARUS group, which did use their own data but presumably could have just as easily used the OPERA data.

Blogs are not reliable secondary sources

No blogs were cited in the section you cut, I just cited some blogs by professional physicists for the purposes of discussion on this talk page. Do you debate the point they make that the argument of Cohen-Glashow and the ICARUS group was based on mainstream ideas about how one might incorporate Lorentz-violation into the Standard Model? I'm sure some more academic sources could be tracked down for this point, but it isn't even discussed in the section you cut so I don't see how it's necessary.

Meanwhile, I don't see that you've cited any secondary sources for the claim that the analysis by the ICARUS group is not viewed as "credible" by mainstream physicists. Just pointing to sources that say the assumptions they make could perhaps be wrong is not the same thing as saying their analysis is not credible--it is only intended to be a credible demonstration that the most straightforward way of incorporating FTL into known physics would yield to predictions inconsistent with the OPERA/ICARUS results, no claim is being made that it would be absolutely impossible to imagine new laws of physics that would allow FTL without Cherenkov radiation. Hypnosifl (talk) 03:55, 20 December 2011 (UTC)

The analysis by ICARUS is correct. The question is whether it is relevant and notable. In other words, whether your line there is an accurate and unbiased summary of the two paragraphs in the OPERA neutrino anomaly on the Cohen-Glashow effect. It is not - the issue is debated, and you have presented just one side. You should either leave the whole thing out, or present both sides succinctly. Since the whole issue is peripheral to the article, I think the first approach is right.Ajoykt (talk) 04:19, 20 December 2011 (UTC)

It's not intended to be a summary of the OPERA neutrino anomaly article, but to be a quick summary of what's said in the article given as a reference. While there may be no factual errors in the OPERA article, I think the amount of weight it gives to arguments supporting the possibility that the effect is real vs. arguments that it's probably not is actually pretty far from what the majority of particle physicists would see as the relative likelihood of those two possibilities. Virtually all the physicists I have seen interviewed on the subject (or discussing it in blogs) who were not directly involved with the experiment have expressed strong skepticism that the results will hold up, and the Cohen-Glashow analysis is often cited as one of the reasons to be skeptical. But in any case, the section you wanted to remove makes no claims about how strong the argument is, it simply mentions the analysis of a "group of experiments" and gives a reference to a reliable source, which is a sufficient justification under wikipedia's policies for putting a sentence like that in an article. If your reason for removing it is that you think it is "not credible", then you should find a reliable source that says words to that effect; but the sources in the OPERA neutrino anomaly article don't go that far, they just say that the assumptions might be wrong since they are based on current mainstream ideas in physics, and current physics itself might turn out to be badly wrong (if you want to add a sentence to that effect to the article, citing one of those sources, that'd be fine with me). Hypnosifl (talk) 04:53, 20 December 2011 (UTC)