# Teaching SR without simultaneity

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De-emphasizing simultaneity in SR curriculum. Thoughts? Experiences?
This thread, some earlier ones, and my signature got me thinking a bit about whether it would be a good idea to de-emphasize simultaneity when teaching SR since it is a frame dependent concept. I did not do much thinking myself yet but would like to hear people’s opinions and thoughts on how possible it would be to do and if anyone has tried it. I believe Lorentz transformations would still be of high relevance, but was thinking of removing focus from the meaning of events being simultaneous, time dilation, and length contraction and instead focus on spacelike separation between events, differential ageing and clocks measuring worldlines, and the geometry of spacetime. The other concepts would probably still need to be covered, but in a less prominent fashion.

Any thoughts? Did anyone try something similar?

Demystifier, AndreasC, vanhees71 and 1 other person

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I have not tried it before, but I also thought about it. The relativity of simultaneity is the most difficult concept for students to learn, and the source of most so-called paradoxes. So on one hand it would be easier to teach SR without it. But on the other hand the students would probably retain the concept of absolute simultaneity and would not know how to resolve the standard paradoxes.

Demystifier, PhDeezNutz and vanhees71
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But on the other hand the students would probably retain the concept of absolute simultaneity and would not know how to resolve the standard paradoxes.
Sure, this is why I think it cannot be skipped completely, but rather would like to de-emphasize it and relegate it to perhaps a single lecture after covering the other concepts.

vanhees71
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Sure, this is why I think it cannot be skipped completely, but rather would like to de-emphasize it and relegate it to perhaps a single lecture after covering the other concepts.
That sounds like an interesting compromise. If you teach it in a predominantly spacetime/covariant approach then it may not be such a stretch to convince people that simultaneity is relative since it is just a coordinate line.

Demystifier, PhDeezNutz and vanhees71
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IMO, it does much more than explaining paradoxes. It is the fundamental problem with frame-dependent measurements of distance and elapsed time. For beginners, it seems to be the most often overlooked aspect of relativity and, if anything, needs to be emphasized more.

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IMO, it does much more than explaining paradoxes.
If you do not start by introducing and discussing the simultaneity idea there are not many paradoxes to explain.
It is the fundamental problem with frame-dependent measurements of distance and elapsed time.
As above, those frame dependent measurements would not be there to behin with because they are, well, frame dependent. The discourse would be more focused on the geometry of spacetime and its geometrical structure.

For beginners, it seems to be the most often overlooked aspect of relativity and, if anything, needs to be emphasized more.
We draw different conclusions then. I would say it needs to be less emphasized precisely because it is prone to misunderstandings rather than being in any way fundamental.

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Summary:: De-emphasizing simultaneity in SR curriculum. Thoughts? Experiences?

This thread, some earlier ones, and my signature got me thinking a bit about whether it would be a good idea to de-emphasize simultaneity when teaching SR since it is a frame dependent concept. I did not do much thinking myself yet but would like to hear people’s opinions and thoughts on how possible it would be to do and if anyone has tried it. I believe Lorentz transformations would still be of high relevance, but was thinking of removing focus from the meaning of events being simultaneous, time dilation, and length contraction and instead focus on spacelike separation between events, differential ageing and clocks measuring worldlines, and the geometry of spacetime. The other concepts would probably still need to be covered, but in a less prominent fashion.

Any thoughts? Did anyone try something similar?
I think the problem is that to understand relativity you need to discuss the transformations between inertial reference frames, and then all these "kinematical effects", which are in principle not very interesting from a physical point of view, automatically become an issue. I thought a lot about to avoid all these socalled paradoxes as much as you can, but then came to the conclusion that this would imply to just introduce the Minkowski spacetime manifold as a postulate, but I found this not very attractive, i.e., you'd loose all the thinking in terms of symmetries, which has been explicitly introduced into physics by Einstein in his famous paper of 1905.

That's not so much the case with the standard way we teach and learn about Newtonian mechanics. There's no problem to completely ignore Galilei transformations without loosing much content, although of course to miss Noether's theorem is a missed opportunity to convey one of the most aesthetical aspects of physics.

PeroK
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That sounds like an interesting compromise. If you teach it in a predominantly spacetime/covariant approach then it may not be such a stretch to convince people that simultaneity is relative since it is just a coordinate line.
That's what I also thought, but then I had no idea, how to convincingly introduce Minkowski spacetime. So I start with the special principle of relativity (existence of inertial frames as in Newton's Lex I) and the independence of the speed of light from the velocity of the light source, as in Einstein's paper.

There are two approaches from these postulates: One is to just use the gedanken experiment with a short flash of light from a point-like light source leading to the invariance of the equation ##(c t)^2-\vec{x}^2=0## in all IFs, i.e., in another IF it looks the same ##(c t')^2-\vec{x}^{\prime 2}=0##. From this you get to the Lorentz transformations for general spacetime four-vectors, ##(t,\vec{x})## very easily, and almost immediately you only work in a manifestly covariant way. Then the apparent paradoxes are no paradoxes anymore to begin with, because everything is covariant (or even invariant, if you work in a basis-independent formalism). This is, however also somewhat abstract for a first encounter with SR.

The other approach is the one by Einstein in his original paper of 1905, i.e., using light signals to synchronize clocks and then derive also the measurement of spatial distances. Then of course you emphasize more the (1+3)-dimensional formalism and from the very beginning the relativity of simultaneity.

Dale and PeroK
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In my opinion, the effectiveness or ineffectiveness of the
highlighted use of various concepts (like "simultaneity" or "time-dilation")
depends on the primary mode of reasoning,
which can be crudely classified as
• verbal, with necessarily technical-definitions for previously-everyday words
in very-carefully crafted sentences
• formulaic and symbolic (what's primed and unprimed?)
• spatial-geometric (frame-dependent, using moving boxcars with clocks)
• spacetime-geometric (frame-independent with a spacetime viewpoint,
possibly relying on some analogs of one's Euclidean-intuition)
Ideally, one would like to easily translate among these modes.

In some modes, various concepts are easier to recognize.

For beginners, I think the Bondi k-calculus (which is lightly algebra-based and is operational [using radar-methods]) is the best approach. The spacetime view is developed (secretly) with the Doppler effect [that's why it's called "k"-calculus] (and, even more secretly, the eigenbasis of the Lorentz transformation). Traditional formulas (Lorentz transformations and velocity-composition) come later.

Bondi k-calculus
https://en.wikipedia.org/wiki/Bondi_k-calculus
https://archive.org/details/relativitycommon0000bond
and, yes,
there's an Insight for that: https://www.physicsforums.com/insights/relativity-using-bondi-k-calculus/

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berkeman, vanhees71 and Dale
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If you do not start by introducing and discussing the simultaneity idea there are not many paradoxes to explain.
I think that people will still come up with the same confusing scenarios, but will be less prepared to see the solution in any convincing way.
As above, those frame dependent measurements would not be there to behin with because they are, well, frame dependent. The discourse would be more focused on the geometry of spacetime and its geometrical structure.
That may be true as long as a short class lasts, but eventually, people will try to understand SR in familiar reference frames and will be just as confused and less prepared.

vanhees71
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I need to moderate my statements. I think that it is essential to point out how a person's familiar concepts of distance and elapsed time depend on simultaneity. They just need to be convinced that their long-held reference-frame concepts have problems in SR. That should allow them to more easily accept the geometry of SR. IMO, it is difficult to completely accept the new geometry until one is convinced that the old one will not work. Beyond that, I do not think that the "relativity of simultaneity" needs to be emphasized.

vanhees71
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I always say this, but Minkowski diagrams were revolutionary for me. Being able to see the transforms acting closely analogous to Euclidean rotations, and to see that the time axis and simultaneity plane are as arbitrary as the x and y axis, yet that there's a reason to pick my own rest frame for my own work, was what made the whole thing click. Then you can state and resolve things like the twin paradox as analogous to travel on a Euclidean plane, and see the change of simultaneity just like the change of definition of "normal to my path" when I turn a corner.

I think you probably do need something like radar coordinates (aka k-calculus) to derive the Lorentz transforms. Drawing light cones on a Minkowski plane is straightforward, and leads where you want to go. Actually, I think part of the problem is how conceptually trivial all this is - all of the SR "weirdness" is just a minor variation on rotation matrices.

I would definitely want to link this to electromagnetism in terms of the history of it, but I'd be almost inclined to present Maxwell -> Einstein -> Minkowski as a short historical introduction, then draw a Minkowski diagram, derive the boosted axes using radar, derive the transforms from geometry, and then maybe look at resolving some "paradoxes" (because they will come up, like it or not).

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PeterDonis, vanhees71 and robphy
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Not entirely relevant, but David Tong wrote this comment in his electromagnetism lectures.
Remember all those wonderful things that you first learned about in special relativity: time dilation and length contraction and twins and spaceships so on. You’ll never have to worry about those again. From now on, you can guarantee that you’re working with a theory consistent with relativity by ensuring two simple things
• That you only deal with tensors.
• That the indices match up on both sides of the equation
It’s sad, but true. It’s all part of growing up and not having fun anymore.

weirdoguy, Ibix, Orodruin and 1 other person
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As I teach the high-school-teacher students, it's easy to discuss such didactical issues with them, and after giving the intro to SR for the 3rd time now, the way to construct Minkowski diagrams seems indeed to help them a lot. May description, after having derived the Lorentz transformations via Einstein's "light-clock method" (which one can use also in high-school lessons and which is also nicely described in a few good high-school textbooks, though it's hard to find the; the best one I know of is the newest edition of "Metzler Physik" (of course in German)) and having established the "Minkowski pseudo-metric" is as follows:

Draw the ##x^1##-axis to the right and the ##x^0=ct##-axis upwards (i.e., as a usual Cartesian coordinate system in the plane, but then forget about anything related to Euclidean geometry of the plane, and that's the most difficult point, at least for me). The "tic marks" in these axes are of course arbitrary, defining units of lengths and "times ##\times c##". Then draw the light cone and the world line of the origin of another inertial frame, which is the ##x^{\prime 0}=c t'## axis of the new frame. According to Einstein's 2nd postulate ("constancy of ##c##") the ##x^{\prime 1}## axis is then the ##x^{\prime 0}## axis such that the light cone is the bisecting line. To get the correct scales for the tic marks on the new axes draw the hyperbolae ##(x^0)^2-(x^1)^2=\pm n^2## with ##n \in \mathbb{N}##. The intersections with the constructed ##x^{\prime 0}## (hyperbolae with ##+n^2##) and ##x'## (hyperbolae with ##-n^2##) then give the correct tick marks for these new axes. This clearly demonstrates again the importance to forget about "Euclidean thinking" when looking at a Minkowski diagram.

sysprog
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This is a good (non-mathematical) HS-level 'intro to the landscape' book:

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... To get the correct scales for the tic marks on the new axes draw the hyperbolae ##(x^0)^2-(x^1)^2=\pm n^2## with ##n \in \mathbb{N}##. The intersections with the constructed ##x^{\prime 0}## (hyperbolae with ##+n^2##) and ##x'## (hyperbolae with ##-n^2##) then give the correct tick marks for these new axes. This clearly demonstrates again the importance to forget about "Euclidean thinking" when looking at a Minkowski diagram.
While what you say is true,
it might be more useful to say
The Minkowskian hyperbolas suggest that the notion of "circle" is based on a similar looking quadratic form.

The direction orthogonal to a timelike-vector
is defined by the tangent line where the vector meets the quadratic form.
(This is associated with the notion of "simultaneity".)
• This was true for Euclidean geometry with its circle.
• This is true for Minkowskian geometry with its [generalized] or [pseudo-]circle (the hyperbola).
• This is true for Galilean geometry with its [generalized] or [pseudo-]circle (the "t=1" hyperplane, which is associated with the so-called "[degnerate] temporal metric of the Galilean geometry" ).
"Angles" can be defined using the areas of sectors, which could define arc-lengths along spacelike curves and define associated trig-functions. etc.. etc.. etc...

So, it might be useful to not-completely-forget Euclidean thinking...
...but, instead, think about how we start from a circle and get to (say) the rotation matrices to develop, by analogy, the Minkowskian and Galilean spacetime geometry and their associated constructions and formulas.

Orodruin and sysprog
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I am currently on my phone so unfortunately have not had the time to sit down to really go through the replies. So just let me make a couple of comments and remarks.

This is, however also somewhat abstract for a first encounter with SR.
The target level I had in mind was students in 4th year taking a specialized SR course. They have already been somewhat exposed to relativity in the typical modern physics course in year 2. Of course, the idea and its applicability to other levels is interesting as well.

While what you say is true,
it might be more useful to say
The Minkowskian hyperbolas suggest that the notion of "circle" is based on a similar looking quadratic form.

The direction orthogonal to a timelike-vector
is defined by the tangent line where the vector meets the quadratic form.
(This is associated with the notion of "simultaneity".)
• This was true for Euclidean geometry with its circle.
• This is true for Minkowskian geometry with its [generalized] or [pseudo-]circle (the hyperbola).
• This is true for Galilean geometry with its [generalized] or [pseudo-]circle (the "t=1" hyperplane, which is associated with the so-called "[degnerate] temporal metric of the Galilean geometry" ).
"Angles" can be defined using the areas of sectors, which could define arc-lengths along spacelike curves and define associated trig-functions. etc.. etc.. etc...

So, it might be useful to not-completely-forget Euclidean thinking...
...but, instead, think about how we start from a circle and get to (say) the rotation matrices to develop, by analogy, the Minkowskian and Galilean spacetime geometry and their associated constructions and formulas.
This is very close to what I had in mind for discussing the geometry of the theory. Of course, one also needs to arrive to Minkowski space, which I usually do through considering the invariance of ##dx = \pm dt## for a light signal and through there arguing for the invariance of the line element ##ds^2##. From there geometry takes over.

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This is very close to what I had in mind for discussing the geometry of the theory. Of course, one also needs to arrive to Minkowski space, which I usually do through considering the invariance of ##dx = \pm dt## for a light signal and through there arguing for the invariance of the line element ##ds^2##. From there geometry takes over.
Here's an old poster where some of these ideas are being developed
from the poster section of https://www.aapt.org/doorway/TGRU/
and slides from a talk

I use something I call the "Trilogy of the Surveyors" (inspired by Taylor & Wheeler's "Parable of the Surveyors") to motivate the "circle"... the light cone in (1+1) form the asymptotes of the Minkowski circle.

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I think the most important thing to get across is that simultaneity is not physics at all, but convention, and this is the radical departure from Newtonian physics. Instead of simultaneity, you have that events can be causally related or not, and nothing else matters. Relativity of simultaneity, as often taught, seems to lead to misleading notions there is physical meaning to planes of simultaneity, which must be unlearned for GR.

However, it is worth teaching that if you adopt any reasonable convention for synchronizing clocks, and apply it to two pairs of clocks in relative motion (each pair at mutual rest), then each pair will not be synchronized according to the other pair.

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Ibix, martinbn and Orodruin
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I think the most important thing to get across is that simultaneity is not physics at all, but convention, and this is the radical departure from Newtonian physics. Instead of simultaneity, you have that events can be causally related or not, and nothing else matters. Relativity of simultaneity, as often taught seems to lead to misleading notions there is physical meaning to planes of simultaneity, which must be unlearned for GR.

However, it is worth teaching that if you adopt any reasonable convention for synchronizing clocks, and apply it to two pairs of clocks in relative motion (each pair at mutual rest), then each pair will not be synchronized according to the other pair.
Pedagogically...
(akin to my reluctance to banish "Euclidean thinking" so soon),
I'm not sure if I'd go that far with demoting simultaneity like that [so soon]
to those just trying to get a handle on relativity...
unless you are prepared to explain many things in terms of causal relations alone [since nothing else matters].

While distant simultaneity may be a convention,
the related notion of orthogonality [of 4-vectors] [in the tangent space] at an event is not a convention.

I think it is more important to first develop spacetime intuition
while connecting (via the Correspondence principle) to what we think we know with our Galilean intuition.
Work on gradually reshaping their Galilean intuition into special-relativistic intuition,
then possibly to general-relativistic intuition and beyond (quantum-gravity intuition?).

In my opinion, it's too drastic to discard various concepts so soon.
It's better to peel away layers of Galilean thinking and
use the remaining structure to lead them to special-relativity and byeond.

My \$0.02.

PeroK and FactChecker
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While what you say is true,
it might be more useful to say
The Minkowskian hyperbolas suggest that the notion of "circle" is based on a similar looking quadratic form.
It's not suggested by but following from the introduction of the indefinite fundamental form. As I said, the spacetime geometry is derived before the Minkowski diagram is introduced. This is much better to understand than if you try it the other way around, because then this subtlety with how to define the "tic marks" on the new axes is more cumbersome to derive, and it becomes less clear.
The direction orthogonal to a timelike-vector
is defined by the tangent line where the vector meets the quadratic form.
(This is associated with the notion of "simultaneity".)
• This was true for Euclidean geometry with its circle.
• This is true for Minkowskian geometry with its [generalized] or [pseudo-]circle (the hyperbola).
• This is true for Galilean geometry with its [generalized] or [pseudo-]circle (the "t=1" hyperplane, which is associated with the so-called "[degnerate] temporal metric of the Galilean geometry" ).
"Angles" can be defined using the areas of sectors, which could define arc-lengths along spacelike curves and define associated trig-functions. etc.. etc.. etc...
There are of course analogies. You still have Euclidean geometry for the standard time slices for any inertial observer, and there you can use the Euclidean 3D geometry, which is of course important to operationally define the observables known from Newtonian physics. Also the rapidity is a very natural parameter to describe the hyperbolae, ##x^0= n \cosh \eta##, ##x^1=n \sinh \eta##. They are also the natural parameters for the Lorentz boosts. This is in very close analogy to defining circles in the Euclidean plane with an angle and rotations around an axis with a rotation angle.
So, it might be useful to not-completely-forget Euclidean thinking...
...but, instead, think about how we start from a circle and get to (say) the rotation matrices to develop, by analogy, the Minkowskian and Galilean spacetime geometry and their associated constructions and formulas.
You must forget Eulcidean thinking to correctly read a Minkowski diagram though! That's the greatest challenge the Minkowski diagram introduces.

I was surprised when in discussions with my students it came out that for the Minkowski diagrams are very helpful, even given the difficulty with the non-Euclidicity necessary for their interpretation. Before I avoided Minkowski diagrams at all costs and only introduced them very briefly with the excuse that they occur in many high-school textbooks, and sometimes in a very weird form, where they even work with Euclidean angles instead of the more appropriate notion of rapidity.

Somewhat artificially you can also use Loedel diagrams to stay within Euclidean geometry, but these only work if you deal with only two inertial reference frames. For me they are utmost confusing and never convinced me enough to try them out in teaching.

Of course, also your "rotated-graph-paper approach" is a viable alternative. The idea behind this is of course just to use light-cone coordinates and the fact that hyperbolae are described as ##l_1 l_2=\text{const}## with them. I've not tried yet this approach in the classroom either, because of lack of time, and this approach is not used in the high-school context at all.

robphy
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Relativity of simultaneity, as often taught, seems to lead to misleading notions there is physical meaning to planes of simultaneity, which must be unlearned for GR.
Certainly happened to me. I had real trouble initially with there not being a meaningful "time when something crosses the event horizon according to a distant observer" because I kind of believed that SR frame changes had to be physically meaningful because they had physical effects like length contraction (I am aware that's not accurate nor necessarily totally consistent, but it was a viewpoint I did hold). Where did all that meaningfulness go in the transition to GR? Once I got the 4d view of SR and the analogy to Euclidean space and could see what a frame change was then it was much easier to accept GR's near-total abandonment of simultaneity as a concept.

Orodruin and vanhees71
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I was surprised when in discussions with my students it came out that for the Minkowski diagrams are very helpful, even given the difficulty with the non-Euclidicity necessary for their interpretation.
Personally, it was when I finally discovered Minkowski diagrams and four-vectors that SR "clicked" in my mind.

vanhees71 and robphy
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For me it was the opposite. I learned about SR in high school, where heavy use was made of Minkowski diagrams, and I never knew, how this issue with the unit tic marks on the axes really works. For me it made click, when I first learned about the algebraic foundations in terms of the "Minkowski product" and got the idea with the hyperbolae ;-).

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Personally, it was when I finally discovered Minkowski diagrams and four-vectors that SR "clicked" in my mind.
For me, I also needed
the dot-product (for efficient coordinate-free linear-algebra calculations)
and radar-methods (for operational definitions to define space and time components)
and how they are related.

Dale and vanhees71
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People try to use their intuition for everything until they know it's wrong and sometimes even after that. As long as relativity of simultaneity is not introduced many students will assume there is universal simultaneity, and might fall back to that assumption even later if they don't understand it. It's probably one of the most unintuitive concepts of relativity. It's leading to all the threads where the answer is in Orodruin's signature. I would be surprised if relativity can be taught in a way that avoids this completely without being overly technical.

I like Minkowski diagrams. Relativity of simultaneity is very natural in them, and you can answer most relativity questions by putting the process into that diagram.

vanhees71, robphy and FactChecker
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Summary:: De-emphasizing simultaneity in SR curriculum. Thoughts? Experiences?
I think William Burke did a massive amount of work on this. His approach is concentrated around clocks, light signals, and free particles, de-emphasizing simultaneity and re-emphasizing spacetime diagrams and geometry right from the start. The first chapter of his famous book "Spacetime, Geometry, Cosmology" deals with special relativity in precisely this way. Also, his recently published lecture notes co-authored by Peter Scott seem to follow a similar approach, although I have not had time to read them carefully.

vanhees71 and robphy
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I think William Burke did a massive amount of work on this. His approach is concentrated around clocks, light signals, and free particles, de-emphasizing simultaneity and re-emphasizing spacetime diagrams and geometry right from the start. The first chapter of his famous book "Spacetime, Geometry, Cosmology" deals with special relativity in precisely this way. Also, his recently published lecture notes co-authored by Peter Scott seem to follow a similar approach, although I have not had time to read them carefully.
Here is a link to Peter Scott's page https://scott.physics.ucsc.edu/
and the Primer draft: https://scott.physics.ucsc.edu/pdf/primer.pdf
and StackExchange discussion where Prof. Scott contributed
https://physics.stackexchange.com/q...for-burke-and-scott-special-relativity-primer

Bill Burke's approach was influenced by JL Synge, who strongly advocated the use of spacetime diagrams.
There is some interesting stuff on Burke's webpage and webpage (which has been preserved),
like references to his unpublished Div, Grad, Curl are Dead , which influenced my attempts to visualize tensors (especially differential forms).

vanhees71, Sagittarius A-Star, Orodruin and 1 other person
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@robphy, thanks for listing those. I vaguely remember Burke mentioning Synge's book somewhere in the beginning chapters of "Spacetime, Geometry, Cosmology", but I have never read Synge's work. I first read Burke's book in the Russian translation, which appeared in 1985 and is quite popular in Eastern Europe (yet not many physics departments adopt his style of teaching relativity).

His approach is concentrated around clocks, light signals, and free particles, de-emphasizing simultaneity and re-emphasizing spacetime diagrams and geometry right from the start.

But the spacetime diagrams are based on a standard inertial coordinate system. It's time-coordinate is defined by Einstein's definition of simultaneity. So it isn't really "de-emphasizing simultaneity".
W. Burke said:
I. Spacetime and Spacetime Diagrams
In our geometrical approach to SRT, we shall emphasize the concept of spacetime, customarily represented by a spacetime diagram. The idea is a simple one, meant to draw attention to the intimate connection between the space and time coordinates that we use to describe the motion of particles in the universe.3
Source:
https://scott.physics.ucsc.edu/pdf/primer.pdf

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PeroK
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So it isn't really "de-emphasizing simultaneity".
It is. Of course, one doesn't completely get rid of it, because it is a useful concept afterall, but de-emphasizing its importance is exactly what Burke did in his book. In his own words:
As attention shifts from time to clocks, the concept of simultaneity plays a less important role in special relativity than it does in Newtonian physics. This shift is surprising, and leads to lengthy discussions of simultaneity in most textbooks. I had considered the radical step of leaving it out entirely, just to emphasize that one can do without it, but its discussion will bring in a number of useful ideas, and provide more examples of the kind of geometric thinking I find so useful.
We will discuss simultaneity not as another primitive notion, but as a concept defined in terms of the theory already given. There is no physical process that intrinsically defines simultaneity---no natural simultaneity meter---except one built out of clocks and light signals. For us simultaneity will be only a defined notion. A striking feature of special
relativity is that the most reasonable definition of simultaneity is not consistent. This failure of intuition leads to what is usually called the twin paradox.

vanhees71
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It seems to me that failing to mention the relativity of simultaneity doesn't mean that it is no longer a major pitfall for the unwary student.

Every student will at some point invoke the natural concept of simultaneous events unless they know that explicitly absolute simultaneity is gone.

It's like failing to mention there is a crocodile in the river. And if the students stay in the boat then no harm may come to them. But, it only takes the unwary student to unwittingly dip their toe in the water for disaster to strike.

vanhees71, FactChecker, mfb and 1 other person
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It seems to me that failing to mention the relativity of simultaneity doesn't mean that it is no longer a major pitfall for the unwary student.
It is not so much failing to mention it as it is to de-emphasize simultaneity as an overly relevant concept. It would still be mentioned but not hold such a central role. Effects such as time dilation and length contraction would be correspondingly de-emphasized.

Edit: More like saying ”look, there is a crocodile in the river and it will tear you apart” instead of going into excruciating detail on how it will painfully disembowel you.

vanhees71 and physicsworks
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It seems to me that failing to mention the relativity of simultaneity doesn't mean that it is no longer a major pitfall for the unwary student.
At some point one has to introduce it and discuss it properly, of course. And by how much the introduction of simultaneity is delayed is another pedagogical choice one has to make (in addition to demoting its central role, if one so chooses). For instance, Burke delays it until section 8 of his chapter 1 (consisting of 14 sections), and I cannot recall any other text that delays it for that long. Whether or not it's a good pedagogical choice is a whole different question, but in general having more approaches on top of the standard ones is always refreshing.

vanhees71, robphy and Orodruin