Talk:Tachyonic antitelephone

Physics: Relativity Start‑class Low‑importance

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Rename?

Should this article be renamed to Tolman's paradox? This is the name used in the article by G. A. Benford, D. L. Book, and W. A. Newcomb. Albmont (talk) 15:13, 4 January 2009 (UTC)[reply]

New material on tachyons

The following paragraph was recently added to the lead:

For a tachyonic antitelephone to exist, it would be necessary for faster-than-light particles, known as tachyons, to exist. However, such particles do not exist even theoretically in the standard model of particle physics, due to tachyon condensation, and there is no experimental evidence that suggests that they might exist. The problem of detecting tachyons via causal contradictions is considered in Ref.<ref>[[G. A. Benford]], [[D. L. Book]], and [[W. A. Newcomb]], [http://link.aps.org/abstract/PRD/v2/p263 ''The Tachyonic Antitelephone''], ''[[Physical Review]]'' D '''2''', 263-5 (1970)</ref>''

I think this rather misses the point. I don't believe Tolman was saying, "here's a neat invention to send signals back in time; it's just missing the tachyons". Rather, the point is that if (i) you can send information faster than light, and (ii) the means of doing so is frame-independent, then you can also send signals back in time, and get the grandfather paradox and all the other stuff that comes with it. It's completely agnostic as to how you accomplish (i) and (ii); the particle physics surrounding tachyons is a red herring.

I'm not sure just what to do with the material; it is interesting, and not unconnected. But I don't think it should have such a prominent place in the lead. --Trovatore (talk) 03:53, 9 November 2009 (UTC)[reply]

The way I see it, before the additional text was added, the article had a rather severe problem in that it basically sounded like a tachyonic antitelephone was generally believed to be theoretically possible to build, and the only reason one didn't exist yet is because engineers haven't yet gotten around to building one. At least, I could see nothing in the article that would prevent a reader from coming away from the article with that mistaken impression. The previous text called it a "hypothetical device", but said nothing at all about why the device is only hypothetical. Maybe an advanced reader would know without being told that the device is basically just a thought experiment, and that most physicists would consider it impossible to ever build such a device. But most readers are not going to automatically know that, unless they've had a strong background in physics.

It isn't true that before the additional information was added, the article only cared about sending information faster than the speed of light, and didn't say anything about particles. Even before the change was made to the article, the article used the word "tachyon" twelve times, and even had the word "tachyonic" (which means pertaining to tachyons) in the title. Tachyons are a type of hypothetical particle. The article was already referring to particles repeatedly, and it unfortunately didn't give any clue that the type of particle it was talking about was a type of particle that doesn't actually exist.

I think the additional information was placed in an appropriate location within the article. It's a short article; the only thing after the new information is the section on the mathematical details of how such a device would work. The new information doesn't really belong within that mathematical section, because it just doesn't fit into that topic well. And I don't think the new information would belong in a new section after the mathematical section, either, since I think it's more important to first establish the device as (probably) only being a thought experiment, before getting into a nitty-gritty mathematical description. So the new information's current location is the latest place in the article that it could sensibly go. Red Act (talk) 14:50, 9 November 2009 (UTC)[reply]

I agree that you could get a false impression reading this article. Now, when I wrote ths article, I did so with the intention of simply writing up the argument that faster than light signals will lead to causal paradoxes, explicitely in a wiki article. Then I linked to this article from some pages, like special relativity. This to make sure that the readers can understand the fundamenal difficulty with faster thanlight signals. Previously, the special relativity article only said that in some frames cause and effect would be reversed, but that statement is not as strong as it should be. See also detailed discussion in this article.

I would therefore not object to rewrite this article so that it reads more as an explaination why faster than light signals are problematic. Count Iblis (talk) 15:06, 9 November 2009 (UTC)[reply]

I've reworked the second paragraph a little bit to emphasize the hypothetical nature of the device while not being too specific to tachyons in particular. 67.87.115.207 (talk) 08:32, 24 November 2009 (UTC)[reply]

Definitely an improvement conceptually; thanks very much. Someone probably needs to go back to the refs and make sure everything is still supported from them without much elaboration. --Trovatore (talk) 22:27, 24 November 2009 (UTC)[reply]

In a lovely little bit of synchronicity, check out today's xkcd. --Trovatore (talk) 21:44, 9 November 2009 (UTC)[reply]

conceirning L

Just a question, shouldn't L be the sum of the distance the tachyon travels in Alice' reference frame and the distance Alice travels during that time? —Preceding unsigned comment added by 188.174.9.223 (talk) 14:09, 26 December 2010 (UTC)[reply]

More explanation of the math

Line by line explanation of all the variables.

Math error?

It looks to me like Alice's velocity should be vc, not v. But I'm more hesitant to Be Bold when fiddling with someone's equations.--MillingMachine (talk) 11:18, 24 September 2011 (UTC)[reply]

We're using natural units here, so c = 1. Count Iblis (talk) 19:45, 24 September 2011 (UTC)[reply]

Einstein

The article is (historically) misleading. Here are some corrections:

Tolman was writing about a one-way path.
The same thought experiment was already described by Einstein in 1907 ("telegraph into the past").
The "Alice-Bob" experiment is a two-way example.

Based on that, I rewrote the article. --D.H (talk) 16:55, 5 October 2011 (UTC)[reply]

Possible typo

The Benford quote at the bottom of the page says that A sends a message at three o'clock and B receives it at two o'clock. Is this intentional, or just a typo? 220.253.51.244 (talk) 05:07, 17 November 2011 (UTC)[reply]

Yet it's correct, because the superluminal signal (in A's frame) is sent at three o'clock, and travels back in time (in B's frame) so that it arrives at B at two o'clock. Now, a superluminal signal (in B's frame) is sent to A, which travels back in time in A's frame, and thus arrives at A at one o'clock. --D.H (talk) 09:00, 17 November 2011 (UTC)[reply]

Possible Mistake in Picture Caption

In the caption for the picture, it says "Nearly infinite velocity". Is this intentional, or is it a typo? Because, as far as I know, you can't have "nearly infinite" anything, because no quantity can be more than halfway to infinity except infinity. Sorry if I'm wrong, and this is not a mistake. — Preceding unsigned comment added by PeterBrownlee (talk • contribs) 00:14, 22 February 2012 (UTC)[reply]

Wrong time calculation

Sorry people, but I think that the message Alice sent to Bob, doesn't take a time of L / a to reach him, at least for two reasons: 1) at the moment Alice sends Bob her message, the distance AB is less than L (or else the reply isn't received by Alice when x' = L); 2) this is a time calculated in S frame, so it should be converted to S' time before being properly added to the time t' needed for Bob's reply. --79.25.59.178 (talk) 19:45, 7 April 2012 (UTC)[reply]

1) We don't have to know the distance when Alice originally sends her signal. We only need the distance when Bob receives the message from Alice (measured in Alice's frame S'), which is L. So this is the actual distance traversed by the signal in S', and thus the time in this frame is L/a. Second, also the reply is of course received by Alice when x'=L, because this is the position of Alice in her rest frame, at which she always stays.

2) As explained above, L/a is calculated in S' (Alice's frame), so both values are measured in the same frame. --D.H (talk) 08:58, 8 April 2012 (UTC)[reply]

The whole math is not necessary, and is actually invalid.

Because the very heart of the theory of special relativity: the Lorens factor does not allow superluminal stuff at all, it will give an imaginary value and the whole other calculations which relay on Lorens transformation is nonsense. Superluminal transformation invalids the the theory of special relativity, not the other way around. — Preceding unsigned comment added by 84.159.22.189 (talk) 17:53, 3 February 2013 (UTC)[reply]

This article wasn't just made up by Wikipedia editors, it is based on published reliable sources, listed under "References". If a formula for a property of a superluminal particle comes out imaginary, that doesn't necessarily prove the particle can't exist, it just means the particle cannot possess that property. -- Dr Greg talk 18:46, 3 February 2013 (UTC)[reply]

Well, what I mean is that the so called "Tolman's paradox" does not actually exist. Because the theory is broken at beginning when trying to introduce such particle, thus it can not logically lead to such conclusion.