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Discover the Ladder Paradox, relativity's greatest thought experiment (io9.com)
31 points by jonbaer 1427 days ago | hide | past | web | 19 comments | favorite

A picture would be worth a thousand words here. An interactive illustration would be worth a million. Instead, the article has two totally irrelevant images and a bunch of hard-to-follow prose.

Why, in 2013, are we still forcing people to construct images in their head based on words instead of showing them the full picture on their computer?

It's essentially linkbait spam. Just google 'ladder paradox' or 'barn-pole paradox' for a zillion detailed, often illustrated explanations. "Relativity's greatest thought experiment" itself is a tip off. Says who? Einstein and popularizers like, say, Carl Sagan were fond of

"[...] a paradox upon which I had already hit at the age of sixteen: If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest though spatially oscillating. There seems to be no such thing, however, neither on the basis of experience nor according to Maxwell's equations. [...]"

You can also make a reasonable argument that this is relativity's greatest thought experiment:



In an internet long past, when it was small enough to have such a thing as a physics FAQ, this was among them.


The first chapter of the referenced Taylor & Wheeler 'Spacetime Physics' can be found here:


> "Although we can think of reality as objective, there are some special cases that prove it's nothing of the kind."

This paradox does not say anything about the objectiveness of reality. Rather it just points out an interesting scenario in which individual perceptions of objective reality won't match up.

Not quite, assuming both observers had an accurate model of physics physics they would construct identical models of what happened. The difference is simply which coordinate system was used. So, they could for example predict what an observer on the other object saw.

I prefer the train going through a tunnel version. Here it is better explained:

Relativity Paradox - Sixty Symbols http://www.youtube.com/watch?v=kGsbBw1I0Rg

I don't understand, where's the paradox?

From the reference frame of the garage, the ladder is moving with a relativistic velocity and therefore experiences length contraction. From the equally valid reference frame of the ladder, it is the garage that is moving with a relativistic velocity, making it even smaller (so the ladder wouldn't fit).

There's a paradox so long as you're still thinking time is some sort of constant and not just another compressible dimension like the others. Its hard to do this at first. Most people end up with a half-muddled "time 'goes slower' when" without fully getting the implications of time contracting as well. The ladder "paradox" is designed to illustrate this to beginners.

To reduce this article to one sentence: The answer to the question "Which happened first, Event A or Event B?" depends on where you are.

It doesn't depend on where you are. The "explanation" at the end of the article is misleading. If you are sited next to a door you will "see" that your door closes before the other. But as you know the distance between the doors you can make the correction and calculate the exact time when the other door closed. After the correction you will realize that both doors closed at the same time in spite you don't "see" them closing at the same time.

You will have the same effect sitting at the other door. If you are sitting in between you must apply the corrections to both doors, and then you will realize that they really closed at the same time. The only point were you "see" at the same time that both doors close is in the middle, and obviously after the corrections you will realize that they really closed at the same time.

The important thing is your velocity. Let's suppose that you are traveling (very) fast with the ladder, holding it at the middle point. You will first "see" that the back door closes (and reopen) before the top of the ladder get to that point. Then you will see that the front door closes (and reopen) after the bottom of the ladder passed that point. But you still have to make the corrections because both doors are far away. Even after the corrections the back door close and reopens before the front door close and reopens.

If you hold the ladder form one extreme you will "see" essentially the same things, and after the corrections you will be sure that never were both doors closed at the simultaneously. An important detail is that the difference of time between the reopening of the back door and the closing of the front door doesn't depend on where are you holding the ladder. After the corrections you will get the same time interval.

And how fast you're moving? Or is that a non-factor?

That was unenlightening.

Basic, well-known undergraduate first (or perhaps second) year physics topic making the front page of HN ?? Is this turning into a forum for high school students?

Not everyone studies this level of physics at all, and it certainly "gratifies one's intellectual curiosity".

Football "gratifies my intellectual curiosity". Shall I start spamming hn with football threads then? Retard.

From the HN Guidelines [1]:

> On-Topic: Anything that good hackers would find interesting. That includes more than hacking and startups. If you had to reduce it to a sentence, the answer might be: anything that gratifies one's intellectual curiosity.

> Off-Topic: Most stories about politics, or crime, or sports, unless they're evidence of some interesting new phenomenon. Videos of pratfalls or disasters, or cute animal pictures. If they'd cover it on TV news, it's probably off-topic.

[1]: http://ycombinator.com/newsguidelines.html

Relativity was taught in Physics 3 when I was an undergraduate (Computer Science Student). However, no mention of that particular thought experiment was used in reference. If there's something to complain about, it's the source for the article, but not the scenario/thought experiment itself.

Wikipedia has a great explanation of it though: http://en.wikipedia.org/wiki/Ladder_paradox

It is just about September. Let's hope it's not an eternal one.

schrodinger's ladder?

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