Talk:Tachyonic antitelephone

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)

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)

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)

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)

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)

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)

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

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)

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)

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

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)

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)

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)

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)

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)

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)

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)

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)

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. — Preceding unsigned comment added by 84.159.22.189 (talk) 20:42, 3 February 2013 (UTC)

There is no need to calculate the Lorentz factor for a tachyon, because a tachyon would not have its own inertial reference frame, only slower-than-light observers have one. Similarly a photon does not have its own frame (and the Lorentz factor blows up if you plug in v=c too), but that doesn't stop them from existing and having an influence on slower-than-light entities such as ourselves. Hypnosifl (talk) 03:17, 12 December 2013 (UTC)

Uncited source at the end of "One-Way Example" section

I'd like to see a source for the following statement: "Einstein and Tolman concluded that this result contains in their view no logical contradiction; they said, however, it contradicts the totality of our experience so that the impossibility of a > c is sufficiently proven." Proof requires test after test after test with the exact same result 100% of the time. Experience does not constitute proof. In my experience skydiving, all parachutes open perfectly every time. But, my experience in no way "proves" that parachutes never fail. We all know they do. As scientists, and with no possible way of scientific testing, I find it questionable that either Einstein or Tolman would dare say anything is "sufficiently proven" based simply on experience. — Preceding unsigned comment added by 66.69.153.164 (talk) 08:49, 28 November 2013 (UTC)

It's the same source given in the article: Einstein (1907). For instance, Einstein wrote:

Hence, if, as we have assumed, w > c, one can always choose v such that T < 0. This result means that we would have to consider as possible a transfer mechanism whereby the achieved effect would precede the cause. Even though this result, in my opinion, does not contain any contradiction from a purely logical point of view, it conflicts with the character of all our experience to such an extent that this seems sufficient to prove the impossibility of the assumption w > c. (Translation by Anna Beck, "Collected Papers of Albert Einstein", vol. 2, p. 265).

Note that Einstein's velocity "w" is the same as velocity "a" in our article. --D.H (talk) 09:22, 28 November 2013 (UTC)

Uncertain sentence meaning

I'm a layman, and not sure what to make of this sentence in the second paragraph (on Benford): "The problem of detecting tachyons via causal contradictions was treated scientifically." It's not clear, in reading, how this is related to the discussion of Benford. Is this (a) part of the argument that his description is physically improbable, or (b) a statement that this particular aspect of his description ("detecting tachyons via causal contradiction") was scientifically sound?76.105.130.39 (talk) 02:41, 9 December 2013 (UTC)

Question about Numerical example with two-way communication

Just for future readers that wonder the same as me, what if in this example it is not assumed that one person is at 0 and one at 0.8c but that they both travel at 0.4c away from each other. Do they then not experience the same time delitation and is therefore the paradox not avoided? The math probably shows the same results and my thought experiment is probably wrong but perhaps someone with a better understanding can point out my error in understanding this.

There is no single "time dilation" to be experienced. Time dilation depends on speed, and speed is always relative to something else; thus, time dilation is always relative to something as well. This is hard for us to wrap our heads around at first, because the amount of time dilation is so tiny when the relative speeds are low. We therefore have grown up assuming that everyone shares the same time. But in fact modern atomic clocks are sensitive enough to show the differences in the passage of time even for things moving at "ordinary" speeds relative to each other. Relative eric (talk) 15:13, 6 September 2021 (UTC)

Somebody remove the "too-technical" banner, please

It's absurd to require something this abstruse to be comprehensible to 3-year-olds. Nothing in the first two paragraphs is so badly written that it fails to get the point across to anyone who knows how to use a dictionary. Anyone else got here by mistake. 143.111.84.104 (talk) 17:47, 29 June 2016 (UTC)

Done. --Trovatore (talk) 17:52, 29 June 2016 (UTC)

Math in "Numerical example with two-way communication" seems dodgy

Also, I'd have called it a "Tachyonic Antetelephone" ;)

The analysis in the "numeric example" is very suspicious and reminds me a lot of the various "1=2" proofs, and is strongly reminiscent of the Twin paradox#Role_of_acceleration. In particular, it jumps around between reference frames, and compares times in Alice's frame directly to times in Bob's frame.

If we stay in a single reference frame (Alice's) the entire time, this is what comes out:

Time (days)	Event	Distance (light-days)	Tachyon travel time	Alice's prediction of Bob's clock
0	Bob passes by at 0.8c	0	0	0
300	"I ate bad shrimp"	240	150	180
450	Bob gets message (predicted)	360	225	270
675	Bob sends reply (predicted)	540	337.5	405
1012.5	Alice receives reply	810		607.5

The original text was unclear, so I assumed that it the 135-day delay was calculated by Alice and was actually 225 days from her perspective. Either way, Alice receives her reply well after she sent her original comment.

Stevie-O (talk) 21:13, 18 July 2016 (UTC)

A few flaws here:

(1) In the article as currently written, Bob immediately sends his reply, so "Bob sends reply (predicted)" should be the same as "Bob gets message (predicted)"

(2) The calculation for "Alice receives reply" depends on the speed of Bob's message. Bob is moving relative to Alice, so it is not correct to assume that Bob's message has the same speed as Alice's message. In fact you need to calculate what Bob's message speed will be, given his relative velocity. If you do this carefully, you'll find that Bob's message towards Alice has a negative speed, i.e. is traveling back in time in Alice's frame of reference. Relative eric (talk) 15:42, 6 September 2021 (UTC)

Is all the math really necessary?

The section "Numerical example with two-way communication" demonstrates the communication paradox using a long and complicated example that is difficult to digest. The primary reason that the example is difficult to digest is because there a lot of math related to signal travel time - Alice sends a signal that takes 150 days to arrive, according to her reference frame, so we have to do an entire paragraph of calculations to determine when and where this message reaches Bob. Likewise, his message takes 135 days to arrive, according to his reference frame, so we have another paragraph of calculations.

The example would be much shorter and easier to digest if the tachyon particles had no travel time in the reference frame of the individual sending the message:

Alice sends the message at day 100 in her reference frame, and it arrives instantly. In her reference frame, Bob is aging more slowly (at the rate of .6), so in her reference frame he receives the message at day 60. From Bob's perspective, he responds to the message 10 seconds after receiving it, and his response arrives instantly. He is 60 days into his journey, but from his reference frame, Alice is aging more slowly (at a rate of .6), so his response reaches Alice 36 days into her journey - 64 days before she sent the message he was responding to.

By saying the tachyon particles have no travel time relative to the sender, we can vastly simplify the example while still demonstrating the paradox. Some guy (talk) 21:26, 2 January 2019 (UTC)

One can go one step further: Eliminate the FTL.

Alice considers at day 100 in her reference frame. In her reference frame, Bob is aging more slowly (at the rate of .6), so in her reference frame he is at day 60. Bob considers at day 60, and from his reference frame, Alice is aging more slowly (at a rate of .6), so Alice is on day 36 day into her journey. Thus, Alice is 64 days before herself.

I think what this actually proves is that anything involving switching viewpoints is fundamentally flawed. David Garfield (talk) 16:34, 8 August 2021 (UTC)

No, what this actually proves is the Relativity_of_simultaneity. There is no absolute answer to the question of "when" things are or whether they are "at the same time". Relative eric (talk) 15:20, 6 September 2021 (UTC)

Whom is trying to remove paradoxes

In recent decades, there have been various proposed ways[by whom?] to possibly remove such paradoxes

http://mens.ly/backintime maybe get in touch with this guy and get him to actually publish a paper

<Siegfried voice>This is Wikipedia. We don't fun here.</voice>. --Trovatore (talk) 17:53, 3 December 2021 (UTC)

I don't know, but I don't see any paradoxes. They don't understand that they are just talking about tail recursion within the context of quantum mechanics, from the perspective of the wave function, it ether happens or it doesn't, which is to say, if the probability of something happening is greater than zero then it can, and eventually will likely, happen, and if the probability of it happening is less than or equal to zero, then it can't ever happen. There is no reason you can't interact with your past, present, and future self as long as the probability of you still existing in the future remains greater than zero, this is a core principle of the many-worlds interpretation of quantum mechanics. — Preceding unsigned comment added by 2600:1700:130:6200:994A:288:FDDF:4C50 (talk) 13:20, 27 May 2022 (UTC)

"Relativity of simultaneity"

This sentence should be entirely removed. There is no serious physicist who calls STR "nonclassical" It's entirely colloquial statement. In fact you can derive it from a simple Doppler shift (and corresponding article about Doppler effect also needs thic cleanup - it even starts a derivation). I think we should always talk about of relativistic CORRECTIONS, because effect is the same. 77.254.39.85 (talk) 06:56, 27 January 2024 (UTC)

Thrice Upon A Time

In terms of “…in fiction”. The book “Thrice Upon a Time” by James P. Hogan should be added. It details the workings of a Tachyonic antitelephone as a device that can be used to send and receive messages to and from itself at any time past or present. Gold333 (talk) 03:23, 29 March 2024 (UTC)