quantum teleportation and time reversal

[Chris Jacobs]

In popular descriptions of quantum teleportation I see time and again
that it is explained by spooky action at a distance which is also
simultaneous.
Now I know from relativity that if the action is simultaneous in one
frame the effect appears _before_ the cause in other frames.
If I am going to believe in time reversal for one frame it is no big
step for me to believe in time reversal for all frames, the effect
occurring in the past light cone of the cause.
If you explain it that way you can get away with spooky action at a
distance.
any comments?

Chris

[Sylvia Else]

Post by Chris Jacobs
In popular descriptions of quantum teleportation I see time and again
that it is explained by spooky action at a distance which is also
simultaneous.
Now I know from relativity that if the action is simultaneous in one
frame the effect appears _before_ the cause in other frames.
If I am going to believe in time reversal for one frame it is no big
step for me to believe in time reversal for all frames, the effect
occurring in the past light cone of the cause.
If you explain it that way you can get away with spooky action at a
distance.
any comments?
Chris

The very fact that we can see this time reversal in the theory, together
with the fact that these processes cannot be used to send information
faster than light, is a good reason to think that there is no action at
a distance, spooky or otherwise. In particular, the time reversal
immediately raises the question of who is the actor, and who the actee[*]

Quantum mechanics tells us what results we'll get. It doesn't say
anything about how it works, and popular science descriptions are
usually just misleading.

Sylvia.

[*] Spell check says that this isn't a word, but what does it know?

[Richard Livingston]

Post by Sylvia Else
The very fact that we can see this time reversal in the theory, together
with the fact that these processes cannot be used to send information
faster than light, is a good reason to think that there is no action at
a distance, spooky or otherwise. In particular, the time reversal
immediately raises the question of who is the actor, and who the actee[*]

I don't like the term "spooky action at a distance". It implies something
non-sensical, yet there is all this evidence of something happening
that is very different from our normal macroscopic based intuition.

Post by Sylvia Else
Quantum mechanics tells us what results we'll get. It doesn't say
anything about how it works, and popular science descriptions are
usually just misleading.

You are correct, of course, that QM does not offer any mechanism
for this behavior, and I agree that the popular descriptions are
little or no help. But I do think that this is something worth thinking
about in order to get a deeper understanding, perhaps beyond QM.
I've been studying QM since the early 1970's, and it is only in the last
decade or so that there has been much interest in the "interpretation"
of QM and what it implies about the mechanisms of particle
propagation. I think this is long over due.

Post by Sylvia Else
Sylvia.
[*] Spell check says that this isn't a word, but what does it know?

BTW, I like "actee" as a word, even if it isn't in the dictionary!

Rich L.

[Sylvia Else]

Post by Richard Livingston
You are correct, of course, that QM does not offer any mechanism
for this behavior, and I agree that the popular descriptions are
little or no help. But I do think that this is something worth thinking
about in order to get a deeper understanding, perhaps beyond QM.
I've been studying QM since the early 1970's, and it is only in the last
decade or so that there has been much interest in the "interpretation"
of QM and what it implies about the mechanisms of particle
propagation. I think this is long over due.

Part of the problem is that the thinking can not go much beyond pure
speculation until and unless we get experimental results that are either
inconsistent with QM, or are consistent with it, but not fully described
by it, in the latter case indicating that QM is incomplete.

Sylvia.

[Richard Livingston]

On Thursday, June 16, 2022 at 2:27:54 AM UTC-5, Sylvia Else wrote:
...

Post by Sylvia Else
Part of the problem is that the thinking can not go much beyond pure
speculation until and unless we get experimental results that are either
inconsistent with QM, or are consistent with it, but not fully described
by it, in the latter case indicating that QM is incomplete.
Sylvia.

Agreed, but part of the problem has been that there has been little thinking
(except by a very few) about what quantum mechanics implies about our
concepts of time and causality, nor how the Born Rule works. There are
two ways to discover deeper theories about such things. One is by an
experiment that gives an unexpected result (e.g. the Lamb shift) that
inspires a theory. The other is a speculation that permits an experimental
test. (And then there are the speculations that are inherently untestable,
e.g. the so called many worlds interpretation of QM.)

I suspect out current impasse in physics is due to an unexamined assumption
in our theory. While it would be nice for an experiment to lead us in that
direction, until then we can speculate and try to test those ideas. But first
we have to admit that there is something that needs a deeper understanding.

Rich L.

[Sylvia Else]

Post by Richard Livingston
...

Post by Sylvia Else
Part of the problem is that the thinking can not go much beyond pure
speculation until and unless we get experimental results that are either
inconsistent with QM, or are consistent with it, but not fully described
by it, in the latter case indicating that QM is incomplete.
Sylvia.

Agreed, but part of the problem has been that there has been little thinking
(except by a very few) about what quantum mechanics implies about our
concepts of time and causality, nor how the Born Rule works. There are
two ways to discover deeper theories about such things. One is by an
experiment that gives an unexpected result (e.g. the Lamb shift) that
inspires a theory. The other is a speculation that permits an experimental
test. (And then there are the speculations that are inherently untestable,
e.g. the so called many worlds interpretation of QM.)
I suspect out current impasse in physics is due to an unexamined assumption
in our theory. While it would be nice for an experiment to lead us in that
direction, until then we can speculate and try to test those ideas. But first
we have to admit that there is something that needs a deeper understanding.
Rich L.

Still, we don't actually know that there is anything underneath quantum
mechanics (QM) that exists. Unless there are infinite layers of
mechanism, there must be a point at which it just does what it does,
with no more explanation being possible.

Perhaps QM has reached the bottom level, and that's just how the
universe is.

Sylvia.

[Richard Livingston]

Post by Sylvia Else
...
Still, we don't actually know that there is anything underneath quantum
mechanics (QM) that exists. Unless there are infinite layers of
mechanism, there must be a point at which it just does what it does,
with no more explanation being possible.
Perhaps QM has reached the bottom level, and that's just how the
universe is.
Sylvia.

That is certainly a possibility, but I'd argue that there must still be
some deeper layers that we should be curious about. For example, in
something like the two slit experiment, how does the photon end up in
only one spot always, as opposed to only statistically on average one
spot? For a given photon we calculate a wave function that can be used
to predict the probability distribution of photon destinations. If the
wave function represents some aspect of something real, then why don't
we sometimes see two photons arriving in different places? (NOTE: I'm
not suggesting that the wave function IS a real thing, it is a
mathematical object that we use to calculate probabilities. Rather I'm
suggesting that the wave function appears to capture some aspect of
something that is real.)

Now one way around that question is to say that the photon DOES follow
one path, and thus does end up in one location. But that suggests that
there is some mechanism that ensures that the photon is guided so that
it ends up, after many trials, reproducing the probability distribution.
This is the pilot wave interpretation of QM.

But even this has a problem with entanglement experiments and Bells
Inequality. It appears that the results of a measurement at one
location depends on a measurement at another spatially separated
location. This coordination of results is very hard to understand if we
still believe special relativity and locality. This reasoning is why I
strongly suspect that there is a deeper mechanism that we would like to
understand.

Of course this deeper mechanism, if it exists, might be too subtle for
us to discern experimentally. In that case, as I believe you've
indicated, this level is not scientifically accessible. But there are
enough hints that our physics is not complete yet that I'm hopeful that
we can still dig a little bit deeper.

Rich L.

[Nicolaas Vroom]

Post by Richard Livingston
For example, in
something like the two slit experiment, how does the photon end up in
only one spot always, as opposed to only statistically on average one
spot?

For the best description of this problem see this link:
https://physicsworld.com/a/do-atoms-going-through-a-double-slit-know-if-they-are-being-observed/#:~:text=In%20the%20famous%20double%2Dslit,no%20interference%20will%20be%20seen.

Google search: double slit experiment with photons

In the text we can read:
In the famous double-slit experiment, single particles, such as photons,
pass one at a time through a screen containing two slits. If either path
is monitored, a photon seemingly passes through one slit or the other,
and no interference will be seen.

My understanding is that the double-slit experiment is a physical
experiment and the explanation belongs to the physical realm. Secondly
in order to understand you should try to make small modifications to
this experiment. Thirdly if you 'fully' understand the experiment you
can also try to describe certain details mathematically. Fourth this
type of experiments can never be done with a thought experiment.

In the double-slit experiment the 'interference' patern changes
drastically if both slits are used or are open versus if only slit is
used. It is very important that as part of each experiment nothing
should be changed with the photon emitting device. It is very important
to perform the following experiment. Start with a double slit experiment
with both slits open and observe an interference pattern. Next keep the
left slit at position and move the right slit slightly towards the
right. This is technical maybe only possible with a new device were the
distance between the two slits is slightly larger. Next repeat this,
each time making the distance slightly larger untill there is no
interference pattern and what is observed resembles the one slit
experiment. What that means is that the observer is no active part of
the outcome of the experiment. What that also means that the one photon
has a certain size, and that when the distance between the two slits is
small, a part of the one photon goes through one slit and the other part
through the other slit and thereafter both parts interfer with each
other.

Post by Richard Livingston
For a given photon we calculate a wave function that can be used
to predict the probability distribution of photon destinations.

The concept of a wave function is not necessary to explain this
behaviour.

Post by Richard Livingston
But even this has a problem with entanglement experiments and Bells
Inequality.

Also to explain entanglement lies in the physical realm. Entanglement is
the fact that there exists a certain correlation between the outcome of
a certain experiment. For example: As part of a collision experiment two
photons are detected at two detectors and the outcome is correlated.
That means when the 'polarization direction' of one photon at detector 1
is +x the 'polarization direction' of the other photon at detector 2 is
-x. It is very important that this experiment is performed 1000 times.
The outcome of detector 1 is random. That means the outcome of detector
1 can be : +x, +x, -x, +x, -x, -x, -x, +x In that case the outcome of
detector 2 is ..: -x, -x, +x, -x, +x, +x, +x, -x By preference the
distance between the detectors and the point of the collision should be
the same, that means the measurements are performed simultaneous.

What does this physical means. It means that the direction of
polarization is already established at the point of collision and has
nothing to do where and when the polarization is measured. Specific at
the point of collision the correlation is established. What is true that
you know if one detector detects a +x that the other detector will
detect a -x. However and that is important there exists no physical link
between both measurements. It's like placing two of your shoes, in one
box each. Next you give these two boxes to a friend, with two stickers
to get packaged and shipped to two family members. When one of the
family members calls you and tells that she has received your left shoe,
than you know that the other family member sooner or later will receive
your right shoe. That is all. That is all what entanglement physical
means.

Post by Richard Livingston
But there are enough hints that our physics is not complete yet that I'm
hopeful that we can still dig a little bit deeper.

I'm pretty sure that we don't understand all the details of the physical
reality. The main step to increase our understanding is by performing
more accurate experiments.

Nicolaas Vroom
https://www.nicvroom.be/

[Julio Di Egidio]

Post by Chris Jacobs
In popular descriptions of quantum teleportation I see time and again
that it is explained by spooky action at a distance which is also
simultaneous.

There is something "spooky" about entanglement for other (intrinsic)
reasons, but it does not violate relativity since no information is
transferred. Indeed, quantum teleportation, beside entangled particles,
needs a classical channel to transmit classical information from the
origin site to the destination site, where that information is needed to
reconstruct the state. As WP puts it:

<< However, in addition to the quantum channel, a traditional channel
must also be used to accompany a qubit to "preserve" the quantum
information. When the change measurement between the original qubit
and the entangled particle is made, the measurement result must be
carried by a traditional channel so that the quantum information can be
reconstructed and the receiver can get the original information. >>
<https://en.wikipedia.org/wiki/Quantum_teleportation>

Julio

[Richard Livingston]

Post by Chris Jacobs
...
Now I know from relativity that if the action is simultaneous in one
frame the effect appears _before_ the cause in other frames.
...
Chris

Chris,

I don't believe that is quite correct. Events that are on each others light
cone are effectively simultaneous (the metric separation is zero), and
that is true for all observers. It is only spatially separated events that
have an ambiguous time sequence, and temporally separated events
have an ambiguous spatial sequence.

Rich L.

[Tom Roberts]

Events that are on each others light cone are effectively
simultaneous (the metric separation is zero), and that is true for
all observers.

Yes, for such pairs of events the metric separation is zero, and that
applies for all observers (it is really independent of observer and
coordinates).

But that is not at all what "simultaneous" means. Rather, that is a
lightlike interval.

Simultaneous means at the same time, and that is a coordinate-dependent
concept (because a coordinate system defines what one means by "time")
-- for a given coordinate system all events with the same value of the
time coordinate are simultaneous with each other. In an inertial
coordinate system, for each such value the locus of events is
necessarily a 3-D spacelike hypersurface. In general, for different
coordinate systems these loci are completely different.

Tom Roberts

[Julio Di Egidio]

[Cross-posted: please follow-up as appropriate.]

Post by Tom Roberts
Simultaneous means at the same time, and that is a coordinate-dependent
concept (because a coordinate system defines what one means by "time")
-- for a given coordinate system all events with the same value of the
time coordinate are simultaneous with each other.

No, that is fundamentally upside down:

The "time" we experience is *proper* time, which is the (physical) time
clocks and everything tick *locally* (i.e. at and per themselves), and
is *universally* (an invariant of the geometry and) one and the same for
every observer, clocks and everything, i.e. not even restricted to
inertial motion or observers. So, it is *proper* time that is
(physical) "time" and, not per chance, modulo more speculative research
on time itself, it is the primary parameter of any dynamical system.

OTOH, coordinate-time is an artefact of coordinate systems, i.e. of a
choice of reference frame, and it is a *relative* notion that introduces
distortions such as time dilation and length contraction for the
trajectory of any particle that is not simply at rest in that frame. It
is indeed overall "just" related to *simultaneity*, i.e. a class of
procedures to *measure* things (distances, speeds, etc.) that are not
local. But, indeed, as we should very well know and never forget: time
dilation is not actually happening on board that ship...

Julio

[Tom Roberts]

Post by Julio Di Egidio

Post by Tom Roberts
Simultaneous means at the same time, and that is a
coordinate-dependent concept (because a coordinate system defines
what one means by "time") -- for a given coordinate system all
events with the same value of the time coordinate are simultaneous
with each other.

No, it is not. It is what these words mean, and what is required for
determining simultaneity for spatially-separated objects. That is what
this (sub-)thread was about.

Post by Julio Di Egidio
The "time" we experience is *proper* time, which is the (physical)
time clocks and everything tick *locally* (i.e. at and per
themselves), and is *universally* (an invariant of the geometry and)

Sure, though you use an unusual and useless meaning for the word
"universal". Yes, the proper time of a given observer or object is
independent of coordinates (aka invariant). But it is not, and cannot
be, "universal" in the usual sense of the word: applying to everything
-- it only applies to the specific observer, object, of clock in
question.

Post by Julio Di Egidio
one and the same for every observer, clocks and everything,

No. Each observer, object, or clock's proper time is completely
independent of the proper time for other observers, objects, or clocks.
Proper time is a property of a worldline, separate for each worldline.

Post by Julio Di Egidio
i.e. not even restricted to inertial motion or observers.

Yes.

Post by Julio Di Egidio
So, it is *proper* time that is (physical) "time" and, not per
chance, modulo more speculative research on time itself, it is the
primary parameter of any dynamical system.

Hmmm. This depends on what one wants to do. Certainly the standard
relativistic equation of motion for a given object refers to its proper
time. The ticking of any clock displays its proper time.

But the subject was simultaneity, for objects separated spatially. For
that, the proper time of any object is useless -- a given observer,
object, or clock can only apply its proper time to events located along
its worldline, which does not include objects separated spatially.

For spatially separated objects, one must DEFINE what one means by
"simultaneous", and that means setting up a coordinate system. As I
said.

Post by Julio Di Egidio
OTOH, coordinate-time is an artefact of coordinate systems, i.e. of
a choice of reference frame, and it is a *relative* notion

Yes. Necessarily so. "Simultaneous" is a relative concept. (Simultaneity
is of course only one aspect of a coordinate system.)

Post by Julio Di Egidio
that introduces distortions such as time dilation and length
contraction for the trajectory of any particle that is not simply at
rest in that frame.

Hmmm. This is garbled. The coordinate system does not induce "time
dilation and length contraction" -- they are generated by certain
measurement procedures applied to objects moving relative to an inertial
coordinate system.

Tom Roberts

[Nicolaas Vroom]

Post by Tom Roberts

Post by Julio Di Egidio

Post by Tom Roberts
Simultaneous means at the same time, and that is a
coordinate-dependent concept (because a coordinate system defines
what one means by "time") -- for a given coordinate system all
events with the same value of the time coordinate are simultaneous
with each other.

No, it is not. It is what these words mean, and what is required for
determining simultaneity for spatially-separated objects.

IMO simultaneity means that at any moment in the universe millions
of events take place and that all these events are happening simultaneous.
Each of these events can be linked to a spatially-separated object.
This physical implies that none of these events can influence any
of the other events directly.

You can only claim that the primary cause of these events lies in the past,
and that the results of these events (what happens next) can only influence
each other in the future.

The reason why I use this approach is that this is the easiest way
to simulate the evolution of our galaxy. That means one coordinate system
and one clock.

Post by Tom Roberts

Post by Julio Di Egidio
one and the same for every observer, clocks and everything,

No. Each observer, object, or clock's proper time is completely
independent of the proper time for other observers, objects, or clocks.

Suppose there exists a same type of civilisation, like ours, near another
star at a planet which resembles ours. Suppose we both want to predict
the future of our galaxy. Is that possible?
IMO the biggest problem is time keeping and to try to synchronise our clocks
with their clocks.

Post by Tom Roberts
But the subject was simultaneity, for objects separated spatially. For
that, the proper time of any object is useless -- a given observer,
object, or clock can only apply its proper time to events located along
its worldline, which does not include objects separated spatially.

IMO to have two have observers, each with their own coordinate system
each considering their system at rest, creates havoc.

Post by Tom Roberts

Post by Julio Di Egidio
OTOH, coordinate-time is an artefact of coordinate systems, i.e. of
a choice of reference frame, and it is a *relative* notion

Yes. Necessarily so. "Simultaneous" is a relative concept. (Simultaneity
is of course only one aspect of a coordinate system.)

I don't understand what you mean with relative concept.
My understanding is that each observer, at the origin of his coordination
system, with his clock, based on his observations of 10 events, will have
different ideas which events are simultaneous or not.
There could also be a third observer. Which one is correct?

Post by Tom Roberts
The coordinate system does not induce "time
dilation and length contraction" -- they are generated by certain
measurement procedures applied to objects moving relative to an inertial
coordinate system.

The problem starts when objects are linked to different coordinate systems
moving relative to each other and using their own clocks.

Nicolaas Vroom
https://www.nicvroom.be

[Richard Livingston]

Post by Tom Roberts

Events that are on each others light cone are effectively
simultaneous (the metric separation is zero), and that is true for
all observers.

Yes, for such pairs of events the metric separation is zero, and that
applies for all observers (it is really independent of observer and
coordinates).
But that is not at all what "simultaneous" means. Rather, that is a
lightlike interval.
Simultaneous means at the same time, and that is a coordinate-dependent
concept (because a coordinate system defines what one means by "time")
-- for a given coordinate system all events with the same value of the
time coordinate are simultaneous with each other. In an inertial
coordinate system, for each such value the locus of events is
necessarily a 3-D spacelike hypersurface. In general, for different
coordinate systems these loci are completely different.
Tom Roberts

I agree, that is what "simultaneous" means, and I shouldn't have used
that word. The idea I did not state clearly enough is that *perhaps*
from the point of view of the physics of causality, that two events with
zero metric separation should be considered simultaneous and co-located.
I realize that this is not a standard concept in current physics, and I
still have some doubts about it, but applying this concept to
entanglement experiments would relieve much of the confusion about how
such entangled results can happen without invoking faster than light
communication.

Rich L.