[varsigma (OP)]

From my window I can see a large barn. As the photons carry no history, I have to assume it was created spontaneously a few picoseconds ago, but common sense and context tell me it was built about 150 years ago from wood that was already at least 50 years old at the time.

Photons are visual information. The memories you cite (that common sense, and context--where does it come from) are not a thing the light you see now can reference in any way.

Since it takes picoseconds, as you say, to be transmitted in your direction.

We rely on the precision of photon history to locate tumors by positron emission tomography.

Yes, but we know what to look for. We construct the history--see Hawking and Penrose.

[varsigma (OP)]

Inverting the paradigm, you imply that all photons are the same age. Which means that every electron transition in the universe occurred simultaneously. Thus time, the dimension that separates sequential events, has no meaning. Somewhat counterintuitive.

I don't imply all photons have the same age, instead photons don't have an age.

How old is any quantum particle? How can I tell if two electrons have an age gap?
The question of age and getting older, is a thing that does seem to depend on context. 

[alancalverd]

Electrons are created by pair production or beta decay, so some, at least, have a birthday, and my detector says they don't all arrive together, so I can create an age gap by sending some around a magnetic loop

.

Yes, but we know what to look for. We construct the history--see Hawking and Penrose.

What we know is that if two 511 keV photons are detected simultaneously (OK, within femtoseconds, for the pedants among us!) in opposed detectors, they originated from a positron annihilation at point on the line connecting them. That is, we know their history (because there is no other possibility)  and (eventually) that tells us where the positron source is.

[Eternal Student]

Hi.

   I said I would look into the writing by Penrose.   Sorry, that's not going to happen for a while.   We have severe weather warnings where I am.   I won't be getting to a library for a few days.
   If anyone has an online reference I'll check that  (if the phone and internet lines stay up).

Best Wishes.

[varsigma (OP)]

What we know is that if two 511 keV photons are detected simultaneously (OK, within femtoseconds, for the pedants among us!) in opposed detectors, they originated from a positron annihilation at point on the line connecting them. That is, we know their history (because there is no other possibility)  and (eventually) that tells us where the positron source is.

Again, it's because you know about the positions of particles which emit photons.  This information is not "carried" by them, it's something you know before any photons are detected.

[varsigma (OP)]

Hi.

   I said I would look into the writing by Penrose.   Sorry, that's not going to happen for a while.   We have severe weather warnings where I am.   I won't be getting to a library for a few days.
   If anyone has an online reference I'll check that  (if the phone and internet lines stay up).

Best Wishes.

    Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made.

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. 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.

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. Was there then any uncertainty about that future? Or was the future of both people already "fixed"?

    Roger Penrose, The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics

from Wikipedia: https://en.wikipedia.org/wiki/Rietdijk%E2%80%93Putnam_argument

. . . and the explanation as to why this uncertainty about the future, or lack of certainty about the past exists, is that each person is on their own unique Poisson disc. Walking or any velocity relative to any other velocity, means the discs are tilted. So many many light years away, the "paradox" of Andromedeans having launched, or who will launch, a fleet, is well beyond an observational limit. Hence, the paradox unravels when observations of an Andromodean fleet occur.

[Halc]

Two people pass each other on the street; and according to one of the two people

This is a nit, I know, but this sort of language has set public understanding of modern science back by decades since the wording seems to suggests that physical reality is dependent on human observation, which is true of neither relativity nor quantum mechanics. That rant said, keep in mind that "according to one of the two people" is a misleading shorthand for "relative to the inertial frame in which one of the two people is stationary".

Humans choose different frames all the time, and not often that one except when checking one's pockets or something.

Secondly, another nit is that using special relativity presumes flat spacetime, and the spacetime between here and Andromeda is not flat.  I'm also willing to ignore that since it is besides the point being illustrated by the example.

an Andromedean space fleet has already set off on its journey,

Third nit is that this statement is a counterfactual, a statement of the state of something unmeasured. Such assumptions are typically assumed in special relativity exercises, but not in quantum mechanics where few consider the principle to be sound.  I am fine with this assumption for this exercise.

None of these nits are fatal to the point, or make it wrong.  Yet.

How can there still be some uncertainty as to the outcome of that decision?

Exactly because it has not been measured. That leaves it uncertain to the measurer, and in fact it will never be measured because by the time the light gets here, the telescopes will long since have ceased to function.

But Penrose assumes both a 3D spatial universe (presentism) and a 4D spacetime universe, and the paradox seems to revolve around these two incompatible models being used at the same time.
In the 4D block model, the entire history of the universe has the same ontology, meaning the invasion is objective fact and is not in any way a function of a reference frame. Presentism is not compatible with relativity of simultaneity. Simultaneity across the universe is objective and not observer or frame dependent. He needs to pick one model and stick with it. This is more than just a nit. It crosses the line into being self-inconsistent.

If to either person the decision has already been made, then surely there cannot be any uncertainty.

Under the block model, all events have equal ontology and the decision very much is part of it. The only thing that the two observers are doing is comparing different pairs of objective events. There is no 'has already been' of anything since that is a reference to the present.

according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past.

Here is a blatant reference to presentism, something totally incompatible with the 4D spacetime model his two people are leveraging. Thus Penrose sows the seeds of confusion rather than adding clarification to what SR actually says. Bad form...

So many many light years away, the "paradox" of Andromedeans having launched, or who will launch, a fleet, is well beyond an observational limit.

The observational limit is one's past light cone, and that cone is identical for both observers (well, a couple meters apart actually), but way over at Andromeda, those two different light cones are still only a couple meters apart.


A more correct wording of the Andromeda scenario is that we have these two different inertial frames differing by a couple m/sec in velocity, and we have one event of these two people passing by each other, and another event of the Andromedan fleet launching. Relative to one of those frames, the people passing event occurs first, and relative to the other frame, the fleet launch event occurs first.  Same two events, but ordered differently in one frame as compared to the other. That is a far better wording of the scenario that presumes SR and it utilizes only B-series language.

[Eternal Student]

Hi.

Thanks for posting that text from Penrose,   @varsigma .

I think many of us will have seen the Andromeda paradox in some form, even if we haven't read that exact text.

@Halc has already said quite a lot about it, which is perfectly good.
Of course, it's always possible to put different interpretations on SR.   Usually you'll have the same final results, just different ways of interpreting it   (similar to all the interpretations of Quantum Mechanics).

What is it that changes when an observer changes their own motion?
For example, an observer fires a rocket for a while and has some acceleration.

Option 1:     Is it something about the observer?   Something acted on them and they can know that it did because they may have an accelerometer with them.   Most things in the universe were not acted on and would not have recorded anything like an acceleration.    So it seems reasonable that the only thing that could have been changed is something about the observer. 

Option 2:   The observer wasn't changed, the rest of the universe was changed.   The observer still finds that their height is 1.5 metres, their width is 0.4 metres and their wrist watch still ticks away 1 second for every "one mississippi" that they count out to themselves.   All the properties and behaviours that they had before the acceleration are unchanged, it's everything else in the universe that has been changed.  Distances between various things in the universe are different, some clocks tick slower or faster etc.    Something acted on the observer and only on the observer but nothing about them was changed.  It's been the universe around them that has had some properties and behaviours changed.

   Does changing your motion change the universe   OR   do we assume it's only something about the observer that has changed?    

    That's how you ( @varsigma ) could have introduced your earlier comments.   Try to hook the audience and get them to do some thinking, before you reel them in a bit.
    Of course, I need to catch up a bit with what was discussed several posts ago, so I can't just end the post here.  We've got to do "the reeling in" straight away.

- - - - - - - - - -

   You can find ways to make either interpretation work.    Option 1 is commonly taken, that's easier when you have a block universe model for 4D spacetime.   When an observer changes their motion, the block universe is still the same block universe, it's just that the observer is taking a different path through it and their natural choice of time and space axis have become twisted (or rotated into each other in a complicated way).   We don't change the block universe, just some properties of the observer (e.g. how they would naturally measure things and how their time and space axis are orientated).

    However, option 2 is also quite interesting and can be given a suitable mathematical framework.    When you change your motion, you change the universe.   In the Andromeda paradox, the person walking the wrong way just had to stop, turn around and then keep accelerating away from Andromeda.  If they did this, then they can live in a universe where the decision to invade will never be made.   Of course people who walk the other way are in trouble, their universe is different.
    This is what I think @varsigma was talking about back in reply #7:

...Have you seen Penrose's description of two observers walking past each other? That this means they are literally in different 'spaces'.....   

    As I mentioned earlier, Penrose is very good at discussing the Physics and the understanding or philosophy that goes with that physics.   He can also provide the mathematical framework to go along with these other ways of thinking about these things.    It doesn't change the overall final results but it can be of some interest that we can imagine things in diffferent ways.

But Penrose assumes both a 3D spatial universe (presentism) and a 4D spacetime universe, and the paradox seems to revolve around these two incompatible models being used at the same time.

   Yes, it does seem that way but I think he was just leading into various different ways in which we can imagine what is happening.
    You can make SR compatible with a form of presentism if you assume that changing your motion has the effect of changing the universe that you're in.

Best Wishes.