"[...] it's utterly inaccurate and incorrect. There is no such thing as "moving through space"."

I think you're missing the point here. She's not trying to teach people the perfect semantics, she's trying to progressively teach them concepts that are tricky to understand and assimilate, step by step. And in a way they can relate to.

By the way, she addresses what you brought up in her first followup (with yet another compelling and easy to understand explanation).

This is not a matter of semantics, if you ever want to understand relativity the FIRST thing you need to do is get rid of the concept of absolute motion. As long as you think in those terms you will never be able to understand it.

And addressing something in a followup is no good - fix your initial post to be correct in the first place. Reddit allows you to edit your post.

If she had started by explaining relativity her post would have been twice the size and she would have lost half the audience. Doesn't it matter to you that people were actually interested in the topic and participative?

And why is a follow up no good? She didn't follow up to fix anything. She followed up to follow up. (And mentioned it by, yes, editing her article).

The problem here is that when explaining things improperly to be "good enough" you have to be very very careful to make it clear what you're doing.

In my opinion, she failed to properly do that and crossed the line between "helpfully simplifying" and "incorrect".

I went through your comments and you mentioned that "in a nutshell" the fact that there's no such thing as a full stop in space is why "this explanation is actually doing a disservice". But that's confusing to me because she clearly stated - relatively to motion - "Those things are true, but we're ignoring that kind of stuff right now."

Can you point out exactly where she crossed the line in your opinion?

The confusion being expressed among commenters here is what I'm basing my assertion off of. Simplification is fine, but I think in this case it pretty clearly set a lot of people down the wrong track.

Some confusion is to be expected. After all, a lot of the concepts are pretty tough to comprehend cleanly.

The important part of relativity is that there is no absolute reference frame, all motion is relative, thus the name. The mathematics behind time dilation is an aspect of that. Teaching someone about time dilation without teaching about the fundamental nature of relativity is a disservice, in my opinion.

Well what if the coordinate system is in reference to one of the objects. That is what I had imagined in my head, I never supposed an absolute frame of reference. The faster the two objects accelerate from each other, the more the time vector (that she mentions in her explanation) rotates for both of them relative to each other so the slower the relative passage of time.

Yes, you can say that. (But it isn't what you said before.)

And be careful saying "accelerate from each other", once you start including acceleration things become much more complicated.

I'm pretty sure he explained that he was talking in terms of general relativity and his simplifying assumption of absolute directions and a stationary viewpoint. As far as I can tell, you haven't explained why that's wrong — merely asserted that there's another viewpoint that can describe the same thing.

If you don't understand that there is no such thing as absolute direction or motion then you don't understand relativity at all.

And for him/her to explain things in such a way is a MASSIVE disservice to you and makes it very hard to learn it correctly later.

And it's not even your clock that ticks slower - it's theirs.

If you're moving with a speed near the speed of light, time-passing for you will be slowed down compared to a person that is standing still.

And if you would stay, say for a minute in that speed and go check on the standing person, you will perceive that she is aging very fast.

No, you would see her aging very slow. Since you are standing still, and it's she who is moving near the speed of light.

And if she checked you YOU would be the one aging very slow, since she is standing still and you are moving fast.

You are still thinking in terms of absolute motion "you're moving". You are not moving, you can only compare your speed with someone else's.

Hmm so if someone moves near the speed of light, time basically stops for everyone?

Remember - he isn't moving near the speed of light THEY are. He is standing still.

But if he accelerates then things are quite different, and much more complicated.

From which reference-frame are you asking?

I'm just trying to wrap my head around the concepts discussed here.

I used to think about it as I explained in my previous comment. But now that ars is making these comments, it seems like I'm grasping something new.

I'll do my best to help.

This FIRST thing to remember is that you can only measure speed by comparing with something else. There is no such thing as the concept of "I am moving", but rather "relative to them I (or they, it's interchangeable) are/am moving".

Now when you compare speeds you can also compare clocks, and the same principle applies - you can only measure time by comparing to someone else.

If that's true, how come the mythical astronaut is young when he gets back to earth while everyone else has aged? Why not the other way around?

The astronaut accelerated, the people on earth did not.

They accelerated away from him though. If there's no absolute frame of reference how can you say who accelerated?

Velocity is relative, but acceleration is not. The earth-dwellers and the astronaut will both agree that the astronaut accelerated and the Earth didn't.

Similarly, rotation rate isn't relative. Acceleration and rotation rate can both be measured against absolute states of no acceleration and no rotation (respectively). This is why our phones have accelerometers and gyroscopes which are entirely internal, but determining velocity, position, and direction require the external references provided by GPS and compass.

How can acceleration not be relative if velocity is? Isn't it just the derivative of velocity? How do you escape the relativeness? Or is that what all those theories of relativity are about?

It's possible to absolutely measure the change in a quantity without knowing the absolute value of the quantity itself. Think of a hole in the ground of unknown depth but known radius, filled with water. If the water level changes you can calculate the absolute change in water volume without knowing the total volume of water in the hole. Similarly, you can measure an absolute value for acceleration (change in velocity) even though an absolute value of velocity doesn't exist.

If you have a fancy sports car, you can feel acceleration. It slams you back in your seat. (check out this: https://secure.wikimedia.org/wikipedia/en/wiki/Equivalence_p...) You cannot however feel speed.

Therefore, you can measure acceleration with nothing but equipment (a simple spring-scale will suffice) in your car (reference frame), but you can only measure your speed relative to something outside of your car (traditionally, the road).

That might be a deeper question about the nature of the Universe.

Newton's first law (inertia): an object in motion tends to stay in motion. This gives rise to the idea of an "inertial reference frame" which is the idealized reference of a hypothetical object in motion. In simplistic terms, if you pretend that the Universe is merely Newtonian in nature and contains a preferred reference frame then you could imagine that an inertial reference frame can be described by a velocity vector (a speed and a direction). If an object has zero speed relative to the inertial reference frame that will tend to continue to be true so long as no forces intervene.

If you induce an acceleration on an object you will change it's velocity. In it's initial inertial reference frame it will appear to be accelerating. In any other inertial reference frame it will also appear to be accelerating.

There is no such thing as an accelerating inertial reference frame precisely because such a frame is not inertial, it would imply that forces are being applied to the objects involved. The laws of physics make sense if you apply them within any inertial reference frame, but they don't make sense in accelerating reference frames (mass, energy, and forces become inconsistent and confused).

Does that make sense?

I don't think acceleration is a torsor, quite, but if you understand a torsor, you understand how acceleration can be absolute even if velocity is not: http://math.ucr.edu/home/baez/torsors.html

In this example, the earth isn't in an accelerating reference frame, only the astronaut is. There isn't a "master reference frame", but that doesn't mean that acceleration is relative.

Despite the upvotes on reddit, this really is a topic that can only be done justice by good book or instructor. Any self-respecting library should have a copy of Relativity: The Special and General Theory by Einstein, it's written specifically for the layman. Come to think of it,... I should probably pick it up again myself.

Because unlike speed acceleration is not relative.

Acceleration can be measured without reference to anything else. If you are in a box and accelerating you can measure it - it feels exactly like gravity.

He feels that gravity, they don't.

> Maybe it helped you, but it's utterly inaccurate and > incorrect.

Utterly inaccurate and incorrect as the Newton's second law? :)