Talk:Rietdijk–Putnam argument

contradictory

"If one of the people were walking towards the Andromeda Galaxy, then events in this galaxy might be hours or even days advanced of the events on Andromeda for the person walking in the other direction.[contradictory]"

It is not contradictory. It is the whole point of the argument! This is what special relativity tells us. Betaneptune (talk) 07:28, 9 September 2021 (UTC)

Indeed, the “contradictory” label was certainly placed with a great deal of confusion. Allow me to attempt some clarification: the paradox does not refer to events that are “really” simultaneous, and certainly not in any transitive sense (there is no contradiction because if person A right here right now thinks an invasion is happening “now” in the Andromeda galaxy and person B thinks only planning is underway, while both A and B are necessarily simultaneous because they are at the same point in spacetime, it does not imply that the invasion and the planning that causally precedes it somehow both take place simultaneously). It states that either one of two causally related events (planning and then invading) could be “taken to be” simultaneous by observers crossing paths a long distance away (one deducing that Andromeda is planning as we speak, the other deducing that Andromeda is invading as we speak). Neither observer is currently seeing any such events while looking through a telescope at the event in which they cross paths: on the contrary, by definition they would be receiving the same photons from Andromeda at that point in spacetime. But if each of them later sees the planning-invading events unfold millions of years later as light finally reaches them, and compute using the constant speed of light what the timing of these events would have been in their reference frame, (assuming that both observers continued to fall free in straight lines and are now possibly light years away from each other) then they could conclude that 1. Andromeda was planning at the “time” of the two observers meeting or 2. Andromeda was invading at the “time” of the two observers meeting. And it is possible that the two observers disagree.

Ultimately, as Betaneptune pointed out this September, the inevitable conclusion is that the very idea of simultaneity can not be well defined in absolute. (Though, come to think of it, we could get fairly close to some weak cosmic frame of rest by ruling out those observers that are so fast that they can measure a significative directional redshift in the microwave background, or taking the average speed of galaxy groups as an approximate local frame of rest. But once that background and eventually most galaxies themselves vanish into the cosmic horizon, we are back to square one. If you are close to the supermassive black hole of a galaxy, you can still bury your head in the sand and tell yourself this is the local frame of rest. But in case of a galactic collision in the far future, or the ultimate evaporation of the massive body before the heat death of the universe, you must sooner or later face the fact that your empirical frame of rest is nonetheless arbitrary.) In principle, for a very flat spacetime and observers free falling on straight lines, each could define their own simultaneity by slicing the spacetime with coordinate hyperplanes. In practice, the slicing becomes ill defined very quickly: a spaceship orbiting close to a black hole will not “see” Andromeda age back and forth thousands of years each second, instead the spaceship itself already “sees” everything outside age much faster than normal, and the rest of the universe would also appear to be heavily contracted in the direction of motion. Orbiting close enough to a massive enough body, simultaneity so utterly breaks down that it is much more useful to talk about the signals being exchanged between distant events and the timing of their reception. For they, fortunately, really do exist independently from the choice of frame.

Edit: This supports the argument presented in the article, it is the same to state that simultaneity is valid but relative therefore vastly more points of spacetime have to be considered as “real” than a mere three dimensional slice, than to say simultaneity is invalid and only coinciding objects (such as a signal reaching an observer) are “real” therefore the whole four dimensional landscape must be considered at once with local three dimensional hyperplane slicing consisting of mental pictures that help us make sense of things but that are not physical, much like coordinate systems are helpful but not physical 85.31.132.219 (talk) 10:51, 12 November 2021 (UTC)

The picture is misleading. In a spacetime rotation, the time and space axes should "scissor" towards/away from each other. I assume this is what motivated the "contradictory" label: the text contradicts the picture. Aseyhe (talk) 07:25, 5 August 2022 (UTC)

Questions

• Penrose Quote? Where does the Penrose quote:

"....How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability." Come from?? This part quoted as coming from Penrose in the main Wiki article does not show up in the 1989 Book, see page 201, Referance given as source ref #4. —Preceding unsigned comment added by 208.119.135.29 (talk) 23:54, 20 April 2009 (UTC)

• Both names, Rietdijk-Putnam Argument and Andromeda paradox, seem not be used in scholarly writing about relativity. Can we get more evidence, that this is not a neologism we should avoid gibing an article. Isn't Relativity of simultaneity the right place for this stuff?
• Can we have other references than Vesselin Petkov, who is essentially advancing a one-person's theory of relativity, partially add odds with standard textbook results?
• What's the paradoxical about the "Andromeda paradox"?

Pjacobi 09:26, 28 September 2006 (UTC)

About the only reference: No representation is made by the archive on the accuracy or quality of preprints posted. The views and opinions expressed are those of the authors and not the Archive sponsors. With respect to the documents available from this server, the Archive sponsors make no warranty, expressed or implied, including warrants of merchantability, fitness for a particular purpose and non-infringement, and the sponsors do not assume any liability or responsibility for the accuracy, completeness, or usefulness of any information or material.

Probably that that settles it. If no good reference can be given that shows at least its notability in some way, then this article should be deleted. Harald88 19:54, 2 October 2006 (UTC)

Answers

1. The name "Rietdijk-Putnam Argument" is commonly used in the philosophy of time. I have included a reference to the Stanford Encyclopedia of Philosophy where the extended name "Rietdijk-Putnam-Penrose Argument" is used.

2. Is relativity of simultaneity the right place for this article? I think the philosophical implications of the Argument entitle it to a separate entry although it does sit below relativity of simultaneity in the hierarchy of knowledge. Should relativity of simultaneity be put in the section Special relativity? If it should then this article should be put in the relativity of simultaneity article.

3. Other references have now been provided.

4. The Andromeda paradox is a form of the argument in which two people at the same place and instant have two different universes attached to them. It is paradoxical that for one of these people a space-admiral might be discussing an invasion whilst at the same instant, for the other person, an invasion is under way.

I hope this clears up the misgivings of Pjacobi and Harald88. These users seem to be new to the philosophy of relativity. The following links might be useful:

Persistence and spacetime http://www.phil.uga.edu/faculty/balashov/papers/polenbarn.pdf

Four dimensionalism http://fas-philosophy.rutgers.edu/sider/papers/4d.pdf

Oxford handbook of metaphysics http://www.nd.edu/~mrea/Online%20Papers/Four%20Dimensionalism.pdf

Physics in the real universe http://arxiv.org/abs/gr-qc/0605049

3D/4D equivalence, the twins paradox and absolute time http://www.nottingham.ac.uk/journals/analysis/preprints/McCALL.pdf

Rietdijk's work: http://www.xs4all.nl/~bcb/rietdijk30.html

Geometer 10:26, 13 October 2006 (UTC)

I've read my Reichenbach and -- just IMHO and not quotable for any Wikipedia article -- must judge, that anybody who sees a paradox in this paradox is utterly confused. The foliation of spacetime into spacelike planes and time line is completely conventional. And no point in the elsewhere (neither in fwd nor in bwd light cone) Minkowski diagram can claim to be more simultaneous than another. --Pjacobi 10:30, 13 October 2006 (UTC)

Hi there. It was Penrose who called it a paradox. I also think it is bog standard SR but the consequences are philosophically profound which is why it has been discussed in a couple of hundred philosophical papers and deserves an entry. The alternative is Presentism which is the normal idea of time amongst people who are unaware of relativity. Geometer 10:34, 13 October 2006 (UTC)

As much as I otherwise like the Stanford Encyclopedia of Philosophy, that article is rather bad:

This is an odd situation indeed. An event in Bob's future seems in some way to become fixed or inevitable by being in Alice's past.

Ouch, everything outside your forward lightcone (which is independent of your velolicity, so it conincedes for Alice and Bob), is fixed and inevitable because nothing you can do now or in the future can have any effect there.

Pjacobi 10:45, 13 October 2006 (UTC)

The problem here is that the Stanford Encyclopedia is an encyclopedia of philosophy. Philosophers think that the relativity of simultaneity is "odd" because it suggests a block universe where concepts such as free will are questionable. But this is not really part of Rietdjik-Putnam where it is just pointed out that relativity implies that the universe really is four dimensional. ie: if special relativity is true then the universe is 4D. Geometer 15:23, 13 October 2006 (UTC)

The question whether the physics imply determinism and realizing that no spacelike hypersurface is more "present" than any other is orthogonal. --Pjacobi 21:47, 15 October 2006 (UTC)

ANDROMEDA PARADOX --Pgb23 20:58, 9 February 2007 (UTC) With respect to the Andromeda paradox, although it does emphasise a somewhat disconcerting aspect of time, it is also important to clarify a possible misconception arising from the way it is conveyed. Although not explicitly intended, it is implied that an ordinary car driver can observe the future in Andromeda, compared to a stationary observer on Earth (the car would therefore be able to convey this information to Earth by an electromagnetic signal). In actual fact the car would have to have been travelling for as long as the light signal from Andromeda, so that both arrive at Earth at about the same time. It would be no good briefly increasing one’s speed, just in time to intercept the information from Andromeda, since General Relativity puts a limit on how fast any transformation of space-time (e.g. gravity waves) can propagate over a given distance (viz. the speed of light). This is possibly overlooked in many Special Relativity discussions and indeed even in the Twin Paradox it is necessary for the rocket to return back to Earth in order to complete/fulfil the ’jump in the time’ that elapses as a result of the rocket reversing its direction. When the car in the Andromeda, paradox increases its speed from being stationary, the Lorentz transformation of its time coordinates only extend beyond him at a rate equal to the speed of propagation of electromagnetism and he would therefore need to cover a large distance/time for it to be valid all the way to Andromeda. In General Relativity there is a significant paradigm shift, since instead of employing the Special Relativity attitude of extended space-time, we use Einstein’s field equations, in which space-time itself, is malleable and personalised. Any such localised transformations in the space-time fabric can itself only propagate at the speed of gravity waves. The phase difference in time that becomes significant (even at slow speeds) over a great distance (Andromeda), does itself need a great deal of time to reach us and become observable data and the car must remain in motion throughout this time period.

Name

The names Rietdijk and Putnam occur only in the title. At the very least, to be in accordance with the Manual of Style, the article should begin in a fashion like this: "In late-night discussions on special relativity, the Rietdijk-Putnam argument is the argument that ...". That is, we need to see the name of the article back in some context. Then the name needs to be explained: "The argument is named after Penrose's two dogs, Rietdijk and Putname, who happened to be walking in opposite directions when he conceived of the argument." I'm not sure the references given meet the criterium of being reliable sources. -- --Lambiam^Talk 15:56, 28 September 2006 (UTC)

Your problem of sources is now answered. The problem of whether or not the world is a nD manifold is central to physics. Why are you insulting such eminent scientists and philosophers as Penrose, Putnam and Rietdijk? Geometer 10:26, 13 October 2006 (UTC)

Facts Disputed flag

I removed this because the facts were not disputed, Pjacobi wanted more explanation. Please provide it. The article agrees with what Pjacobi has said:

"Ouch, everything outside your forward lightcone (which is independent of your velolicity, so it conincedes for Alice and Bob), is fixed and inevitable because nothing you can do now or in the future can have any effect there."

But remember that this article is a description of the existence of the argument, it is not an attempt to argue for Stein's counter argument or even to argue for Putnam etc. Please explain what is factually incorrect about the article. If the article does not summarise Rietdijk, Putnam and Penrose's positions correctly then it should be put right. Geometer 15:16, 13 October 2006 (UTC)

If you want to handle this more specifically, we can do it sentence by sentence. The first sentence:

• If special relativity is true then each observer will have their own plane of simultaneity that contains a unique set of events that constitute the observer's present moment.

contains one misunderstanding and implies another. If it can be sourced, it must be attributed to someone specific. --Pjacobi 21:47, 15 October 2006 (UTC)

If, as you suggest, you know how to correct this sentence, please do so. Please edit rather than placing a "factual inaccuracy" flag on the article! I am removing the flag in anticipation of your edit. Geometer 08:14, 16 October 2006 (UTC)

Sorry, I don't have read the original articles giving this argument. All I can say, is that the exposition of their ideas in this article indicate a misunderstanding or a heterodox interpretetation of relativity. The best I can do, would be either to expose the orthodox view, but this would duplicate the article Relativity of simultaneity or propose it for deletion -- or ask you to address the raised the concerns. --Pjacobi 08:20, 16 October 2006 (UTC)

'This article is about the Rietdijk-Putnam argument, not about whether the argument is true or false.' Opinions on the truth or falsity of the argument would be original research. Geometer 08:26, 16 October 2006 (UTC)

For this reason, statements that contradict orthodox interpretations of SRT have to be attributed. --Pjacobi 09:00, 16 October 2006 (UTC)

I think the whole Andromeda Paradox is incredibly misleadingly described. If two people are standing in the street, and one person starts walking away from the other, then he absolutely will not view events in the Andromeda galaxy which are two days apart. That's a nonsense. As an example, let's imagine you are viewing a supernova through your telescope. If you decide to move your telescope to the left by a couple of foot you don't suddenly see the supernova as it existed two days ago - before it exploded. Move your telescope - the supernova explodes; stop moving your telescope - the supernova doesn't explode. Obviously not the case.

The paradox is about what events observers consider to be on their x-axes, no-one can observe such events, they can only observe things transmitted from the events like light. This does not mean that the events on the x-axes do not happen and, if you use SR to allow for the transmission of light and the effects of relative velocities you get the Andromeda paradox. (ie: Penrose is not stupid!).

If that was that case then the speed of rotation of Earth would dominate any relative velocity difference of two people walking in the street - people on one side of Earth would see the supernova exploding, while people on the other side of Earth would see the supernova two days earlier before it exploded. We'd see total confusion in space! Roger Penrose actually describes it misleadingly in his original description in his book: "Even with quite slow relative velocities, significant differences in time-ordering will occur for events at great distances" - well, that's absolutely not the case if the two observers are not spatially separated (see my earlier example about viewing a supernova).

The paradox is about what events observers consider to be on their x-axes at any instant, not about the order in which photons are received. Penrose is not stupid.

But the order in which photons are received defines what events the observers consider to be simultaneous. Remember the old special relativity thought experiment of an observer in a moving train sending two light pulses out to the back and the front of the train? He says the rays arrive simultaneously precisely because he sees the photons arrive at the same time (of course, it's not just limited to light - the speed of light provides an upper limit for any information). It's that speed of light which defines that x-axis to which you refer. I know Penrose is not stupid, but this Andromeda Paradox makes the mistake of overestimating the effect of special relativity on two walking observers. As I say, the rotation of the planet would have a far greater effect than any walking velocity! Penrose has made a mistake here.

The debate is about the geometrical rather than the dynamical interpretation of SR. Both observers get the light arriving in the right order and the car receives the same light signals as the man on the street while they coincide. The issue is a philosophical enquiry into the nature of the geometrical interpretation of SR - if the world is a (3+1)D manifold then, although the x axes of the man and the car coincide where they meet they diverge with distance by $vx/c^{2}$ seconds with distance along the man's x axis. The x-axis is on the hyperplane of things that are simultaneously present so, according to the geometrical interpretation, the car has different events on Andromeda in its present moment from those on Andromeda in the man's present moment.

If two people in the street are not spatially separated then they WILL agree about simultaneity (though one might experience time dilation - his clock might be running slower). And if the two observers are not spatially separated AND their relative velocities are small then there will be absolutely NO difference of opinion about simultaneity - in the Andromeda galaxy on anywhere else - OR time dilation. Basically, their experiences will be identical. I think the whole presentation of this "paradox" is desperately flawed and is misleading and basically incorrect in its current form as presented here. The only way that small relative velocities would have an impact (as suggested by Roger Penrose) is if the two observers are also separated by a great distance (i.e., the combination of small relative velocity IN COMBINATION with a long time for light to reach the observers from a distant galaxy has resulted in the observers being far apart when the two signals reach them - so they disagree about simultaneity - this agrees with the comments of Pgb23 above: "In actual fact the car would have to have been travelling for as long as the light signal from Andromeda ... It would be no good briefly increasing one’s speed."). The Rietdijk-Putnam argument is fascinating and fundamental, but - with the greatest respect for Roger Penrose - the "Andromeda Paradox" is just plain misleading in this form and should be rewitten or (preferably) deleted.Andrewthomas10 22:59, 7 February 2007 (UTC)

This discussion shows that:

(1) The paradox is an interesting, documented problem in physics that deserves a place in Wikipedia.

(2) That it leads to some stimulating debate about the problem of simultaneity.

The interpretation given in the article is the standard interpretation - the two observers are considered to be simultaneously at the same place but moving relative to each other. Their relative velocity gives them differing planes of simultaneity. Incidently, Penrose hated this standard interpretation and considers the conclusion although implied by SR to be unacceptable! That said, the article is just standard SR and the objections are philosophical. It might be interesting to extend the article with the various philosophical dissenting views...Geometer 13:55, 20 April 2007 (UTC)

No, I think the Andromeda Paradox is badly flawed for the reasons I give above and does not deserve a place in Wikipedia. However, the Rietdijk-Putnam argument is generally accepted and correct. Andrewthomas10 13:05, 30 May 2007 (UTC) Can I just say I have changed my mind about this, and I now think the article is correct. It's not well described, though. Andrewthomas10 (talk) 15:33, 23 January 2014 (UTC)

Reversions - Edit war by Pjacobi

Nastyyy.. I only wanted to add an article to explain an interesting area of philosophy. I hate this edit warring business. I am out of here. Bye Geometer 08:26, 16 October 2006 (UTC)

For heavens sake, I've removed the tag, without my concerns answered. But you're the best candidate to enhance this article. --Pjacobi 09:07, 16 October 2006 (UTC)

Criticise for irrelevance all you like, but I feel the need to point out that this article contains the most awesome picture on the entirety of Wikipedia.

I'd give you a barnstar for humour if there was one, lol Cls14 17:41, 21 August 2007 (UTC)

The relativity of simultaneity and the Andromeda paradox.

The illustration of the Andromeda Paradox is explained only by reference to the Wiki article on Special Relativity and to a Wiki book. The article on Special Relativity veers between the simplest example of relativity and mathematical theorems. There is nothing apparent there (without studying the subject in depth) that provides the necessary reference to the example given in this article. Could somone please either add a specific and detailed reference or provide a concise explanation for the example given?

LookingGlass (talk) 20:30, 18 October 2012 (UTC)

Diagram is wrong

The explanation in the diagram is backwards. In SR, the more one moves through space, the less one can move through time. Therefore it is Monday on Andromeda for the moving observer, and Tuesday for the stationary observer. — Preceding unsigned comment added by Irthomasthomas (talk • contribs) 10:18, 13 July 2019 (UTC)

It is even worse. The concept of an entity "moving in time" or "moving in space" is conceptually flawed, since the time as well as the space coordinates depend on the relative motion states of observers. There is no way to define "time" or "space" without fixing one observer and relating her coordinate system to another observer through a Lorentz transformation. Also, there is no such thing as a "stationary" observer. — Preceding unsigned comment added by Tigelriegel (talk • contribs) 21:17, 17 January 2020 (UTC)

No, if the car is moving towards Andromeda, then it is later, Tuesday in this example, for the driver than for somebody standing next to the passing car (for whom it is Monday on Andromeda). [Btw, if the car is driving away from Andromeda, it's Sunday for the driver.] - The "in SR, the more one moves spatially, the less one moves through time" has nothing to do with it (the latter means that for the standing person, time is dilated in the car (and btw also the other way around)). But the diagram is still (and utterly) wrong because the coordinate system for the car is rotated to the left, instead of changed according to the Lorentz-transformation. Also, on Andromeda, Tuesday (with the decision made) comes before Monday (before the decision) in the diagram, which is obviously wrong. Relativity (and reality) doesn't work that way. --Felix Tritschler (talk) 16:13, 5 October 2020 (UTC)

The catch in Roger Penrose's argument

In his presentation of the Andromeda Paradox, Roger Penrose made a fatal mistake when he states: "Then they can hark back to that chance encounter, and come to the conclusion that at that time, according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past."

At the time of their encounter both "see" -- if at all that would be possible -- and agree upon what has happened about 2.5 Mio years ago in the Andromeda Galaxy (which is about 2.5 Mio light years away from Earth) and not what happens there at the same time as both meet.

Thereby Penrose contradicts the two preceding sentences with the correct statements: "In fact neither of the people can yet know of the launching of the space fleet. They can know only later, when telescopic observations from Earth reveal that the fleet is indeed on its way."

This will be 2.5 Mio years after their encounter and then the two persons will disagree on what happens in Andromeda, if they keep their state of motion for the whole time, because then they will be several light days apart (about 4 light days, if their relative velocity is constantly 5 km/h for 2.5 Mio years in the direction of Andromeda). In this situation, however, they cannot communicate instantaneously anymore.

If the one closer to Andromeda, say Bob, would in the moment he sees the invasion fleet launching from Andromeda take his phone and call the second person, say John, to tell him about it, the phone signal would need under best conditions 4 light days to travel to John. So, when John hears his phone ring and picks up, the light of the invasion fleet launching towards the Earth would reach him in this moment, too, and he would agree to Bob: "Yes, you are right, I also see the invasion fleet leaving Andromeda". So, what?! Where is the paradox?

The fault in Roger Penrose's argument is to mix the two instances: the encounter of Bob and John in the park and the actual observation of the fleet launching from Andromeda by Bob 2.5 Mio years later at a location which is 4 light days away from the park where Bob and John had their encounter.

There is no relatevistic effect in this scenario other than the finite speed of light. No time dilatation or space contraction has to be taken into account, because the relative velocity for all protagonists (Bob, John, and the fleet commanders in the Andromeda Galaxy) is very small.

The formula to calculate the distance in light days between Bob and John after 2.5 Mio years can very easily be derived like this:

L (distance to Andromeda): 2.5 Mio light years

t (time for light to travel L): L/c = 2.5 Mio years

d (distance between Bob and John after t for a relative velocity of v = 5 km/h): vL/c = 2,500,000 x 365.24 x 24 x 5 km

Δt (time light needs to travel d): (vL/c)/c = (2,500,000 x 365.24 x 24 x 5) km / (300,000 x 3,600 km/h) ≈ 0.8 x 24 x 5 light hours = 4 light days.Rolf108 (talk) 10:47, 18 November 2021 (UTC)