# Insights Relativity on Rotated Graph Paper - Comments

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1. Jul 17, 2016

### robphy

2. Jul 17, 2016

### Staff: Mentor

Do you know if anyone has used this approach in an introductory relativity class?

3. Jul 17, 2016

### robphy

As far as I know, I am the only one who has tried it in a relativity class or in an intro class (calc-based and algebra-based) discussing the topic of relativity. I have used the clock diamonds... But not the fancier causal diamond methods (yet).

4. Jul 17, 2016

### Staff: Mentor

How well did you feel that it worked? Did it help the students gain an intuition for SR?

5. Jul 18, 2016

### robphy

While I haven't done any detailed surveys or pre-/post-exams...,

I feel that this method does a better job of motivating and explaining relativity
over approaches that rely heavily on the Lorentz transformation formulas and other formulas (time-dilation, length-contraction,...).
(In a formulaic approach, the problem [for the students] seems to boil down to determining which quantities are primed and which are unprimed.)

In my opinion, constructing the diagram emphasizes the operational interpretation of relativistic quantities... with minimal mathematical requirements for the end user.
I like to think of it as a "physics first, [Lorentz Transformation] formulas later".
Then, having a completed diagram encodes many "tangeable" features that could be used for further discussion and elaboration [reinterpreted with standard formulas, if desired].

Hopefully, this helps make it easier to draw and interpret spacetime diagrams.

6. Jul 18, 2016

### pervect

Staff Emeritus
I've noticed many PF posters seem to have difficulties in drawing space-time diagrams (or perhaps it's just reluctance?). I'm not sure how to interpret this. I often suggest drawing time lines of events as a warm up exercise, hopefully that is a familiar exercise that will get across the abstract notion that one not only can draw a diagram that represents time, one has probably already done so in the past. I'm not sure if anyone has ever taken me up on my suggestion though, or if it's helped any.

It doesn't seem like much of a leap to go from drawing time-lines to drawing space-time diagrams, but I suspect the later seems harder than the former.

The end goal (as I see it) is to lead the reader to the realization that every event in space-time, aka "reality", is represented by one point on the space-time diagram, and that this is true no matter whose perspective the diagram is drawn from. This doesn't seem too outlandish, or hard, but it does involve the notion of one-one correspondences between infinite sets. Maybe it's the lingering (and perhaps somwhat justified) fear that there are non-intuitive aspects to infinite sets that causes the difficulties? I really don't know for sure, I suppose that it's something that would take a lot of 1:1 private converstations to get an appreciation of the issues - it's just too difficult to get anyone to talk about what they don't understand in a public forum.

7. Jul 18, 2016

### robphy

It seems that students generally don't draw useful diagrams: position-vs-time graphs or free body diagrams. Rather they try to reason with words, with possibly flawed intuition (especially on non-intuitive situations), and with formulas awaiting input. If a diagram is drawn, it's usually more of a cartoon rather than a diagram approximating the geometry of the situation.

For Relativity, I think not knowing where the tickmarks adds an additional obstacle. This is what motivated my approach.

8. Jul 20, 2016

### Greg Bernhardt

9. Jul 20, 2016

### hamideh

your approach is very beautiful and motivates me to think of using it for other areas
thanks

10. Jul 22, 2016

### vanhees71

I can draw Minkowski diagrams, but I never found them very helpful compared to just using the formulae, based on the covariant Minkowski-space formalism. My main trouble with Minkowski diagrams is that I have to forget the intuition we are used to from elementary school on interpreting the "paper plane" as a Euclidean plane.

11. Jul 22, 2016

### robphy

In my opinion, one has to refine (or relax) one's intuition since
some features (e.g. incidence, parallelism, and scaling) are common to Euclidean and Minkowski geometries...
and some features (e.g., "circles", tangency to "circles" as "perpendicular" to radii, "angle"="arclength"/"radius") are analogous
and some features (e.g., sum of the angles in a triangle = 180 degrees) have to be discarded.

Do you find it useful to draw position-vs-time graphs (Galiliean space-time diagrams) when doing PHY 101 kinematics problems?

12. Jul 22, 2016

### vanhees71

It's useful to draw such graphs. Also Minkowski diagrams can be useful to visualize the results of algebraic calculations, but indeed as you say, you have to get used to them.

13. Jul 26, 2016

### houlahound

14. Jul 26, 2016

### robphy

Last edited: Jul 27, 2016
15. Jul 27, 2016

### houlahound

thanks, i think i will to open an account at the site and buy the full paper.

16. Aug 23, 2016