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  #1  
Old 05-03-2012, 08:09 AM
Candyman74 Candyman74 is offline
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*My* Problem with Relativity

No, don't worry - I don't think I've figured out why Einstein was wrong.

Nope, my problem is more simple. I understand it in broad strokes conceptually (as in no math, but plenty of popular science books). Well, kinda. I came to that point by reading - over the last 20 years - many similar attempts by scientists to explain relativity in layman's terms; and I think a lot of the do a pretty good job at it. I think I stared with A Brief History of Time when it came out all those years ago, and worked my way through many, many books, TV shows, and so on.

My problem is that I'm not *quite* there. I've gotten to the point that I simply can't face readng yet another book crammed with descriptions of trains passing through stations and spaceships shining flashlights, or twins in rocket ships, and so on. There seems to be a group of commonly used analogies and they now bore me senseless to the point that I just can't read them any more.

So my question is this: are there any explanations out there which look different, narrative-wise? Ones which use totally different analogies, perhaps (but still no math)? Anyone you'd recommend who writes well, has a a fresh approach to how they explain it, and doesn't once mention a train passing through a station?
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  #2  
Old 05-03-2012, 08:17 AM
iamnotbatman iamnotbatman is offline
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The problem is that relativity is specifically about how locations in space-time are related in different reference frames. No matter how you cut it, you are going to have to talk about something moving relative to something else (be it train/station or spaceship/planet, etc) and how measurements of position and time and velocity compare for different observers.
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  #3  
Old 05-03-2012, 08:48 AM
Keeve Keeve is offline
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Originally Posted by Candyman74 View Post
My problem is that I'm not *quite* there.
What are your remaining difficulties? Are there specific questions you still have, or specific details you're trying to learn more about?

If you just want a new perspective on the whole thing, then, yeah, what iamnotbatman said. But if you can be more specific, maybe we can help.
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  #4  
Old 05-03-2012, 09:12 AM
Half Man Half Wit Half Man Half Wit is offline
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You could try the classic by Gamow, Mr Tompkins in Wonderland, which explores the effects of relativity in every day life if the speed of light were merely 30 mph... I don't think there's trains or stations, but there are cyclists and such -- plus it's got the advantage of being available for free online (pdf link).
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  #5  
Old 05-03-2012, 12:14 PM
Candyman74 Candyman74 is offline
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Originally Posted by Half Man Half Wit View Post
You could try the classic by Gamow, Mr Tompkins in Wonderland, which explores the effects of relativity in every day life if the speed of light were merely 30 mph... I don't think there's trains or stations, but there are cyclists and such -- plus it's got the advantage of being available for free online (pdf link).
Interesting. Thanks - I'll give that a try!
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  #6  
Old 05-03-2012, 12:15 PM
Candyman74 Candyman74 is offline
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Originally Posted by Keeve View Post
What are your remaining difficulties? Are there specific questions you still have, or specific details you're trying to learn more about?

If you just want a new perspective on the whole thing, then, yeah, what iamnotbatman said. But if you can be more specific, maybe we can help.
I think it really is just a case of a fresh perspective. Just an author who presents it slightly differently.
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  #7  
Old 05-03-2012, 01:24 PM
Yumblie Yumblie is offline
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Well one different approach is to imagine going fast as a rotation.

http://upload.wikimedia.org/wikibook.../1c/Coord6.gif

In a simple one dimensional space-time diagram (one dimension of space, and time), a moving reference frame will have a squished coordinate system with respect to your frame. A stationary object moves straight up; it has no motion in space but still continues to "move" in time. A moving object will have its trajectory tilted so that it is now moving in space, and slightly less in time. This is because an object's total space-time velocity is always a constant. So if they gain velocity in space, they lose some in time (hence time dilation). Light itself has zero velocity in time, and c velocity in space. And so you sitting in your chair right now are going the same speed, c, but in the time direction instead of the space direction!
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  #8  
Old 05-03-2012, 02:23 PM
Sicks Ate Sicks Ate is online now
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Quote:
Originally Posted by Half Man Half Wit View Post
You could try the classic by Gamow, Mr Tompkins in Wonderland, which explores the effects of relativity in every day life if the speed of light were merely 30 mph... I don't think there's trains or stations, but there are cyclists and such -- plus it's got the advantage of being available for free online (pdf link).
I hoped this would help me as well, and I'm sure it would be helpful to many people, but I just still can't get my head wrapped around it.

I read the words, they go into sentences, but they don't mean anything.
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  #9  
Old 05-03-2012, 03:19 PM
PacifistPorcupine PacifistPorcupine is offline
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I think there was a decent explanation on Brian Greene's The Elegant Universe. What must clung to me is the idea that everything moves at the speed of light, the difference being that for me and you most that is spent traveling through time, while photons only travel in space.
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  #10  
Old 05-03-2012, 03:20 PM
TonySinclair TonySinclair is offline
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Originally Posted by Candyman74 View Post
My problem is that I'm not *quite* there.
You may never get there if you keep insisting on explanations that have no math. The ability to solve quantitative problems in General Relativity requires a pretty strong math background, but the ability to understand what's going on in Special Relativity, at a good conceptual level, requires no more than high school math.

It's been a while since I've read it, so there may be newer and better books, but one I can recommend is Mermin, "It's About Time: Understanding Einstein's Relativity." Give it a try, you may surprise yourself.

Last edited by TonySinclair; 05-03-2012 at 03:25 PM..
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  #11  
Old 05-04-2012, 01:39 AM
naita naita is offline
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I've found this resource useful: http://www.phys.unsw.edu.au/einsteinlight/

It has a lot of non-math explanation, and the math bits are clearly labled.
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  #12  
Old 05-04-2012, 05:46 AM
Keeve Keeve is offline
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Originally Posted by TonySinclair View Post
You may never get there if you keep insisting on explanations that have no math. ... the ability to understand what's going on in Special Relativity, at a good conceptual level, requires no more than high school math.
Ditto.

I read something interesting once:

Biology is just applied chemistry.
Chemistry is just applied physics.
Physics is just applied math.

In other words, when you study [biology/chemistry/physics] on its most basic level, you'll find that you're not really looking at [living things / chemicals / objects] any more, and that what you're really doing is learning about [how organic molecules interact / how subatomic particles interact / how to make the formulas work].

So, if you wantto really understand relativity,
Quote:
Originally Posted by TonySinclair View Post
You may never get there if you keep insisting on explanations that have no math.
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  #13  
Old 05-04-2012, 01:03 PM
Ruminator Ruminator is offline
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Originally Posted by Candyman74 View Post
So my question is this: are there any explanations out there which look different, narrative-wise? Ones which use totally different analogies, perhaps (but still no math)?
Quote:
Originally Posted by TonySinclair View Post
You may never get there if you keep insisting on explanations that have no math.
I think that's ultimately the main stumbling block. Relativity is math. Relativity is a mathematical model -- albeit one that describes certain physical phenomena very accurately. It still doesn't explain everything, or one could say that there are flaws in it that math model that physicists don't completely understand yet. For example, Relativity doesn't explain anomalies with Dark Energy, Dark Matter, and quantum effects.

Anyways to try a different narrative without trains and lightbulbs...

I think one of the conceptual difficulties with Relativity has to do with everyday human experience. If we consider other concepts such as "gravity", "heat", or "electricity", we come into contact with it right away before having to learn any math equations relevant to each concept.

To continue specifically with a comparison to "gravity"...

From the day you're born, you "feel" its effects. As a child, you throw a ball up in the air, and you notice it comes right back down. You also "feel" or "experience" your feet touching the floor and your butt firmly planted in the chair you're sitting in. Unless you're one of the lucky few to travel into space or ride the Vomit Comet, you've been dealing with the reality of "gravity" non-stop.

It's only later in life (approximately 15 years) at high school where you'll first encounter some of the "mathematics" of gravity. You'll see numbers like 9.8 ft per second per second. Or you'll see Newton's mass distance equations. These math formulas aren't so crazy looking. To you, they're just fancy algebra problems to quantify the phenomena of gravity you've been dealing with all your life.

The sequence of exposure for gravity is: #1 life experiences --> #2 mathematics

Relativity doesn't have a "life experience" component for the typical person to "feel an intuition" before the mathematics. To "understand" relativity is to jump right into the math. There lies a huge conceptual hurdle.

Because the life experiences aren't there, popularizers of science have to resort to contrived analogies to avoid the math. Like you said, you've seen them all... traveling twins, lightbulb flashes on trains, etc.

Since you as a human have no way of traveling near or at the speed light, you have no way of "seeing" or "feeling" relativity as a tangible concept like you did with gravity. Therefore, we still have to talk about via some kind of contrived example. We'll try a different one that involve GPS satellites.

Suppose you're a multi-billionaire and you'd like to launch your own private network of satellites to report your position. You want your personal private Global Positioning System because you're paranoid that the US government will shut theirs down or whatever. Ok, after you spend millions launching all your satellites, you test out the accuracy with your personal GPS receiver. Your initial calculations for satellite triangulation (high school trigonometry) showed that you should get accuracy within plus/minus 30 feet. But you're shocked to find out that you're way off by about 6 miles! And worse, the triangulation error is accumulating every day. Pretty soon it's 100 miles from your actual position. You double check all the components and notice nothing wrong. The tangible concept here is that your multi-million dollar satellite system doesn't work. Finally you mention your problems to a scientist and he says, "Oh, I've got some math formulas that will counteract the errors you're seeing and adjust them correctly." And, if you ask him if there's a "name" for those math formulas, he'll say "relativity." That's really the punchline... "relativity is math."

The amazing thing is that Einstein formulated the math of relativity before GPS satellites were invented. The "errors" that he was trying to correct were the observations made by other scientists he heard about. For example, the mathematics of Newton didn't accurately model the motion of Mercury. And the experiments carried out by others to detect and quantify luminiferous aether didn't give results they expected.

Last edited by Ruminator; 05-04-2012 at 01:08 PM..
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  #14  
Old 05-04-2012, 09:01 PM
Rysdad Rysdad is offline
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Originally Posted by Ruminator View Post
You'll see numbers like 9.8 ft per second per second.
Didn't they do that with one of the Mars exploration vehicles or something?

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  #15  
Old 05-04-2012, 09:27 PM
Ruminator Ruminator is offline
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Didn't they do that with one of the Mars exploration vehicles or something?

Correction, that should have said 9.8 meters per second squared. Funny how the eyeballs catch a mistake when it's isolated in a quote.

Mars acceleration due to gravity is 3.2 m/s^2 so not sure how the 9.8 number would come up in relation to the Mars rovers.
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Old 05-05-2012, 09:20 PM
zombywoof zombywoof is offline
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Originally Posted by Ruminator View Post
Correction, that should have said 9.8 meters per second squared.

...not sure how the 9.8 number would come up in relation to the Mars rovers.
I think Rysdad's joke is referring to the Mars Climate Oribiter, a mission that failed (to the tune of $327 million) due to a mixup between English and metric units being used for some critical calculations.
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  #17  
Old 05-06-2012, 12:05 PM
Rysdad Rysdad is offline
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Originally Posted by zombywoof View Post
I think Rysdad's joke is referring to the Mars Climate Oribiter, a mission that failed (to the tune of $327 million) due to a mixup between English and metric units being used for some critical calculations.
Exactly. And, Mar's gravity is pretty close to 9.8 feet/sec2...so you weren't far off anyway.

Last edited by Rysdad; 05-06-2012 at 12:08 PM..
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  #18  
Old 05-06-2012, 12:22 PM
ralph124c ralph124c is offline
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Of the four dimensions, why is time asymmetric? You can move forward or back in the three spatial dimensions, but not in time. If spacetime were truly symmetric, wouldn't time be a constant (with the speed of light varying)?
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  #19  
Old 05-06-2012, 03:13 PM
Candyman74 Candyman74 is offline
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Originally Posted by ralph124c View Post
Of the four dimensions, why is time asymmetric? You can move forward or back in the three spatial dimensions, but not in time. If spacetime were truly symmetric, wouldn't time be a constant (with the speed of light varying)?
Isn't that to do with increasing entropy and the laws of thermodynamics?
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  #20  
Old 05-07-2012, 06:46 AM
iamnotbatman iamnotbatman is offline
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Time is just not like a spatial dimension. If all we had was four spatial dimensions, there would be no time, and nothing would change. The "time" dimension is what gives freedom to locations in the spatial dimensions to change relative to one-another. Why shouldn't we be able to move freely backward and forward in time? Well, what if I decide to move backward in time? How does that work if the rest of the universe keeps going forward in time? You have to think carefully about what this all means and how it would work. How do I "decide" to move in time, anyways? Presumably there would be some kind of particle interaction that causes different particles to move faster, slower, backward, forward, in time, similarly to how particle interactions cause different particles to move faster, slower, backward, forward, in space. But what does it mean for a particle to go backward in time, while another particle goes forward in time? Does the particle going forward in time still "see" the particle going backward in time? If you think about it carefully, the only way any of it makes sense is if you equate "going backward in time" with "reversing your momentum, charge, and parity." This is why antiparticles are sometimes described as particles "going backward in time." But what about just slowing down or increasing your movement through time? Again, if you think about this carefully, it is equivalent to simply changing the energy of a particle. When a particle gains energy, it moves more quickly through time (it's debroglie frequency increases, it's speed increases, etc). For example if you give a particle a little more kinetic energy than another particle, it can get to their destination before it, ie earlier in time. And this connects us again to antiparticles, which were originally seen as "negative energy" solutions (later reinterpreted as positive energy solutions because the negative energy is moving backwards in time). So in a certain sense, things are moving around in time, just like they do in space, you just have to realize that time is different from space! It is not a spatial dimension!

So along comes relativity, and it says, look, time and space are different, but they are connected. If I move quickly through space, I move more slowly through time, and vice-versa (as seen by an inertial observer), in exactly such a way so that the speed of light is the same in all inertial reference frames. Going back to the last paragraph, in this unusual way of looking at energy and time, what relativity says is basically that you can keep increasing the energy of a particle and going faster through time, but you can never go so fast through time to catch up with a ray of light.

The asymmetry (the minus sign for the time-component) in the Minkowski metric reflect this. If it were symmetric, it would not be relativistically invariant: it would be saying that sitting still and going through time and not space is just as valid as moving through space and not moving through time. But we know that it is impossible to move through space in zero amount of time. Time and space need to be treated differently! Going through time is easy -- you just sit still, but going through space is hard, and you can't exceed the speed of light. With the minus sign in the Minkowski metric, moving through time gives you a different answer from moving through space. If the inner product is zero, two events are separated by a light ray, if the inner product is negative, two events are separated by a rocket ship, and if the inner product is positive, the two events are causally separated.

Last edited by iamnotbatman; 05-07-2012 at 06:46 AM..
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