Is the Andromeda Paradox accepted in physics?

[TheQuestionGuy14]

https://en.m.wikipedia.org/wiki/Rietdijk–Putnam_argument

Is the Andromeda Paradox accepted in physics? It seems to be based on the concept of relative simultaneity, but it seems quiet nonsensical. Wouldn't every observer seeing a different time period violate causality?

 

[PeterDonis]

Please read my Insights article on this:

https://www.physicsforums.com/insights/block-universe-refuting-common-argument/

Wouldn't every observer seeing a different time period violate causality?

I don't know what you mean by "every observer seeing a different time period". The argument assumes that SR is true--more precisely, that it makes correct predictions--so it is consistent with causality, since SR itself is.

 

[PeroK]

https://en.m.wikipedia.org/wiki/Rietdijk–Putnam_argument

Is the Andromeda Paradox accepted in physics? It seems to be based on the concept of relative simultaneity, but it seems quiet nonsensical. Wouldn't every observer seeing a different time period violate causality?

I was thinking about this recently. I imagined one of the observers sitting at a telescope looking at a distant galaxy, while the other observer cycles backwards and forwards. According to this paradox, the "present" in that distant galaxy is shifting backwards and forwards in time to the cyclist as he wheels about.

But, it makes no difference to what is seen through the telescope, so it makes no difference to anything.

 

[A.T.]

Wouldn't every observer seeing a different time period...

Did you read your own link? It explicitly says that there is no difference in what they see.

 

[Ibix]

https://en.m.wikipedia.org/wiki/Rietdijk–Putnam_argument

Is the Andromeda Paradox accepted in physics? It seems to be based on the concept of relative simultaneity, but it seems quiet nonsensical. Wouldn't every observer seeing a different time period violate causality?

SR doesn't talk about what you see, but rather about how you interpret what you see.

I can never see what's happening "now", because light takes time to get here. Everyone here, now, sees the same thing I do, but how they interpret it can differ depending on their state of motion, including the meaning of "now". That's the basis of the Andromeda paradox.

 

[Dale]

Wouldn't every observer seeing a different time period violate causality?

No. Causes preceed effects in all reference frames. The only things which can have their temporal order changed are things which can not be causes or effects of each other.

 

[pervect]

There is some physical content to the Andromeda paradox, but also some philosophical content too. The philosophical part is of course philosophy, not physics.

I think that frequently the physicist's notion of causality is not well understood by people interested in the paradox. Causality in special relativity is all about light cones. An understanding of the "paradox" that is incomplete because the understanding of the physicists notion of causality is likely to be incomplete at best, and downright wrong at worst.

The wiki article on light cones would be a good place to start in understanding light cones. If causality in SR is based on light cones, light cones must be understood first to undersand causality. . Once light cones are understood, given two events, one of which has the lablel P, and one which has the label F, then causality can be understood as the P event being in the past light cone of the F event, or, equivalently , the F event being in the future light cone of the P event. When this condition is met, P is in the past of F, and F is in the future of P.

 

[TheQuestionGuy14]

No. Causes preceed effects in all reference frames. The only things which can have their temporal order changed are things which can not be causes or effects of each other.

If someone here on Earth cycles toward Andromeda while another cycles away, if they both had telescopes strong enough to see the surface of a planet on Andromeda, would they see it differently? Or would the light coming from Andromeda still be the same?

 

[Ibix]

Or would the light coming from Andromeda still be the same?

Everyone here, now, sees the same thing I do

How could it be otherwise? How would light know that different light has to reach you just because you are on a bicycle?

 

[haushofer]

A bit offtopic, maybe, but am I the only one who reads "paradox" as "logically inconsistent" instead of "logically inconsistent at first sight"? I find the wording of "paradox" misleading. Dito for the twin "paradox". There is no paradox, just a warning that relativity dictates to be very precise in your description of events.

 

[Ibix]

There is no paradox

There are no paradoxes in relativity, true. There are paradoxes in common misunderstandings of relativity, so I take "resolving the twin paradox" to mean "replacing my flawed understanding with a less flawed one".

 

[A.T.]

Or would the light coming from Andromeda still be the same?

See link in post #1.

 

[Dale]

if they both had telescopes strong enough to see the surface of a planet on Andromeda, would they see it differently? Or would the light coming from Andromeda still be the same?

As @Ibix already said in post 5 they will see the same light. They will calculate that the light came from different distances and therefore different times in the past.

 

[PeroK]

If someone here on Earth cycles toward Andromeda while another cycles away, if they both had telescopes strong enough to see the surface of a planet on Andromeda, would they see it differently? Or would the light coming from Andromeda still be the same?

We already know that simultaneity is relative: if two events are simultaneous in one reference frame, they are not simultaneous in any frame moving with respect to the first (assuming the events are separated in the direction of the relative motion). This teaches us that simultaneity is not an inherent property of the two events.

The Andromeda paradox re-emphasises this: what is considered "now" at a distant location is not an inherent property of the two events: "here now" and "there now". And you mustn't read too much phsyical significance into it.

In a way, the whole paradox is a trick. You can fall for the trick if you don't understand SR, of course. But, to those who know SR it says:

You know how you learned that simultaneity is relative and how to calculate simultaneity across large distances? Well if you forget for a moment that this makes simultaneity a less important idea (especially across large distances) and pretend that simultaneity is still critcal to causality (as it would be in Newtonian mechanics, where you might theoretically have FTL signals) then you have a problem.

In other words, it says that if you selectively take parts of SR, but ignore others, then you have a paradox.

This paradox I particularly dislike, because it demands you focus on one aspect of SR and forget the others.

 

[TheQuestionGuy14]

As @Ibix already said in post 5 they will see the same light. They will calculate that the light came from different distances and therefore different times in the past.

I'm sorry but what do you mean by saying 'The light comes from different distances and therefore different times in the past? Doesn't that mean the light is different?

 

[Ibix]

I'm sorry but what do you mean by saying 'The light comes from different distances and therefore different times in the past? Doesn't that mean the light is different?

No. Light comes from the same event, but different frames assign different times and positions to a given event.

We might agree that we stand in the same place and be able to point to a particular landmark and see the same thing. But if my map uses magnetic north while yours uses grid north, we will disagree on the bearing to the landmark. This doesn't mean that there are two landmarks, just that we're describing the location in different coordinate systems. The same goes for events described in different frames. The distance and time may be different, but that doesn't mean there are two events, just that we're using different coordinate systems.

 

[Herman Trivilino]

A bit offtopic, maybe, but am I the only one who reads "paradox" as "logically inconsistent"

I used to think that, but changed my mind when I saw definitions like this:

Paradox: a seemingly absurd or self-contradictory statement or proposition that when investigated or explained may prove to be well founded or true.

 

[Dale]

I'm sorry but what do you mean by saying 'The light comes from different distances and therefore different times in the past? Doesn't that mean the light is different?

The comment above from @Ibix is spot on. I recommend that you read it carefully and that you simply work out the Lorentz transform to see how the different frames describe this situation.

 

[TheQuestionGuy14]

No. Light comes from the same event, but different frames assign different times and positions to a given event.

We might agree that we stand in the same place and be able to point to a particular landmark and see the same thing. But if my map uses magnetic north while yours uses grid north, we will disagree on the bearing to the landmark. This doesn't mean that there are two landmarks, just that we're describing the location in different coordinate systems. The same goes for events described in different frames. The distance and time may be different, but that doesn't mean there are two events, just that we're using different coordinate systems.

Ok, I understand.

[Moderator's note: The rest of this post, and the subthread it spawned, have been moved to a separate thread: https://www.physicsforums.com/threads/einsteins-train-and-simultaneity.960934/]

 

[Herman Trivilino]

I'm sorry but what do you mean by saying 'The light comes from different distances and therefore different times in the past? Doesn't that mean the light is different?

Certainly light that comes to us from further away takes more time to reach us. But that is not the only way that it can be said that light can take different amounts of time to reach us.

The issue is this. Let's say we have a source of a light that's some distance away from us. We can look at our wrist watch to determine the time here, but how do we know what time it is way over there at the location of that source?

 

[TheQuestionGuy14]

No. Light comes from the same event, but different frames assign different times and positions to a given event.

We might agree that we stand in the same place and be able to point to a particular landmark and see the same thing. But if my map uses magnetic north while yours uses grid north, we will disagree on the bearing to the landmark. This doesn't mean that there are two landmarks, just that we're describing the location in different coordinate systems. The same goes for events described in different frames. The distance and time may be different, but that doesn't mean there are two events, just that we're using different coordinate systems.

Does this different coordinate systems thing also apply to the train thought experiment?

 

[Nugatory]

Does this different coordinate systems thing also apply to the train thought experiment?

Yes. Using coordinates in which one observer is at rest, the two strikes are simultaneous; using coordinates in which the other observer is at rest they are not.

You can draw a space-time diagram of the thought experiment and draw in both observers' coordinate axes. If you do, you will see how the same five events (strike in front, strike in back, light from front and back strikes reaches one observer, light from front strike reaches the other observer, light from back strike reaches the other observer) line up with the different coordinate axes in different ways.

 

[Ibix]

Does this different coordinate systems thing also apply to the train thought experiment?

A frame is literally a choice of coordinates. So, yes.

 

[Herman Trivilino]

Does this different coordinate systems thing also apply to the train thought experiment?

It is the essence of the thought experiment! Each observer is using a different coordinate system to analyze the situation, and the comparison of their findings is what we're focused on. In one coordinate system the flashes of light are emitted at the same time, whereas in the other coordinate system they are not.

 

[TheQuestionGuy14]

A frame is literally a choice of coordinates. So, yes.

Is this effect actually visable in reality? My physics teacher stated that relative simultaneity is just a mathematical abstraction, and thus when someone uses different coordinate systems, it is just a mathematical abstraction. Is this correct?

 

[Ibix]

Is this effect actually visable in reality? My physics teacher stated that relative simultaneity is just a mathematical abstraction, and thus when someone uses different coordinate systems, it is just a mathematical abstraction. Is this correct?

It's the mathematical abstraction you use to interpret your clock readings.

Ultimately it's like using a map. Does the world actually look like a map? No. We use maps instead of photographs to navigate because they remove the observation point dependence of the photo, and maps are an incredibly useful way to describe and communicate about terrain.

The relativity of simultaneity is analogous to your freedom to draw a map with north pointing in any direction you like. Length contraction and time dilation are consequences of that choice - analogous to a building being square or diamond-shaped, depending on your choice of north.

None of the relativistic effects are directly visible for the same reason that the world doesn't look like Google Maps. But they are very real in the same sense that a building that is square on one map is a diamond on another.

 

[TheQuestionGuy14]

It's the mathematical abstraction you use to interpret your clock readings.

Ultimately it's like using a map. Does the world actually look like a map? No. We use maps instead of photographs to navigate because they remove the observation point dependence of the photo, and maps are an incredibly useful way to describe and communicate about terrain.

The relativity of simultaneity is analogous to your freedom to draw a map with north pointing in any direction you like. Length contraction and time dilation are consequences of that choice - analogous to a building being square or diamond-shaped, depending on your choice of north.

None of the relativistic effects are directly visible for the same reason that the world doesn't look like Google Maps. But they are very real in the same sense that a building that is square on one map is a diamond on another.

That's a good analogy, thanks. Is this video a good description?

 

[Herman Trivilino]

Is this effect actually visable in reality?

Yes.

My physics teacher stated that relative simultaneity is just a mathematical abstraction, and thus when someone uses different coordinate systems, it is just a mathematical abstraction. Is this correct?

No. It's a mathematical description. Is it just a mathematical description, or is there more to it than that? Does the mathematical description match Nature's behavior? If it does, then in at least that sense it's more than just a mathematical description. In this case it does match Nature's behavior, so to me it's more than just a mathematical description.

When I'm teaching physics I would never describe it the way your teacher did. And even when I'm not teaching physics I would never say it that way.

Simultaneity is a mathematical abstraction in the sense that it's a convention, it's not physical. That may be the notion your teacher was describing and perhaps you misunderstood it. It's pretty safe to say that almost everything you hear about special relativity is at first misunderstood. It's only when you study it yourself that you begin to understand it.

 

[TheQuestionGuy14]

Yes.
No. It's a mathematical description. Is it just a mathematical description, or is there more to it than that? Does the mathematical description match Nature's behavior? If it does, then in at least that sense it's more than just a mathematical description. In this case it does match Nature's behavior, so to me it's more than just a mathematical description.

When I'm teaching physics I would never describe it the way your teacher did. And even when I'm not teaching physics I would never say it that way.

Simultaneity is a mathematical abstraction in the sense that it's a convention, it's not physical. That may be the notion your teacher was describing and perhaps you misunderstood it. It's pretty safe to say that almost everything you hear about special relativity is at first misunderstood. It's only when you study it yourself that you begin to understand it.

I know it's a small bit off topic, but is it possible that your 'now' is simultaneous with your own past? Say if you went very far away from earth, on a far away planet, and moved so it made it so you were simultaneous with Earth 5 years ago, back when you were on earth, would you be simultaneous with your own past?

 

[PeroK]

I know it's a small bit off topic, but is it possible that your 'now' is simultaneous with your own past? Say if you went very far away from earth, on a far away planet, and moved so it made it so you were simultaneous with Earth 5 years ago, back when you were on earth, would you be simultaneous with your own past?

No, that's not possible.

Techically, two events can only be simultaneous in any frame if they are separated by a spacelike interval. Your worldline is timelike.