Relativity

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Well, i hope to ask experts in physics to tell me what is General Relativity? Relativity? and what is the ether?

Erm, i tried finding it out but all i get is some mathematical equations which i don't have a clue about.

Thanks

P.S. I have no knowledge or whatsoever in physics.

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sup gene!

I will try to give you a brief heads up about these things, but i don't really know how easy it will be, as they are samewhat difficult subjects. If you really want to have a good introduction for the layman, try reading Mr. Thopmkins in Paperback by george gamow.

Now for your questions...

As far at aether goes, well, it doesn't exist. it was once hypothesized to exist. It was supposed to be the medium through wich light travelled. Why? Well, people needed an expliniation for how light traveled throught the vacume of space. YOu see, light has a weavelength. If it has a wavelength, then it must be a wave (seems reasonable enough, right?) Well, waves need meduims to travel through. Like ocean waves travel through water, sound waves travel through air, ect. Without a medium to travel through, there is no wave. (Incidentally, this is why there is no sound in space -- nothing for the sound to travel through).

Ok, so it didn't really look like there was anything in the depths of space, but we know that light can travel through space, since the light from the sun and all the other stars gets to us. So in order to rectify this with the idea that light is a wave, scientists hypothesized that there was medium throught wich light traveled, but htat we couludn't detect. This was called teh aether.

Now however, we do not belive there is an aether. Rather, we think that light can travel thorugh vacumes. But, waves can't travel throught vacumes, so what is light then. WEll, particles can travel through vacumes. SO light must be a particle then. But remember that light has a wavelength, so we know that it is also a wave.

So we are left with the idea that light is both a particle and a wave. This is the famous particle-wave duality. There are other other ways that light makes this strange duality manifest, but this is definiately one of them.

Anyways, the take home message is that scientists thought there was an aether, but now they don't.

Now as to relativity...

there are two types of realtivity, general relativity and special realtivity.

Special relativity is based on the hypothesis that all frames with uniform motion are physically identical. That is to say that if you have two frames of refference wich are moving, but not accelerating, then they are both equally valid. For instance, let us say that you are riding on a bus at a constant speed, right? WEll, are you moving through a stationary world? Or perhaps you are stationary and the world is moving past you? Or perhaps both you and the world are moving, relative to someone wallking down the street?

Special realtivity says that all three of these "viewpoints" are valid. that the pysics of the situation can be accpetably described in all three of these frames. Cool.

Now there was another idea, that when teamed with this first gives some strange results. This other idea is that the speed of light is constant for all observers. That is to say this; lets say that i was standing on a platform and shined a light at you. Now you are in a ship moving towards me at half the speed of light. Then you measure the speed of light, light's speed would appear to be the same as it would be for me, who is standing still! It is quite bizzar.

So given that light always has the same speed for all observers, we find that htere are a few strange consiquences that occur. The first is called "length contraction." This is the idea that objects that are moving close to the speed of are shorter (have less length) in the direction of the motion. that is to say, if you were standing around and a a meter stick (lengthwise) flew by at close to the speed of light, it would be shorter than a meter to you!

The second consequence is called "time dialation," this is the idea that, for a stationary observer, time appears to slow down for things moving the speed of light. That is to say that if you saw someone moving past you at close to the speed of light, that person would be aging more slowly than you do.

The final consequence (that is commonly talked about) is called "realtive mass" or "momentum gain." THis states that, relative to a stationary observer, things that are going close to the speed of light have more mass than they do at reast. That is to say that if you were standing still and a 1kg weight wizzed by at close to the speed of light, then it would be heavier than 1kg to you.

ok, so those are the basic concequeces and ideas of specail relativity. It is some wierd and wakey stuff, but cool. I should say that these effects occur even at noraml, everyday speeds, but the effects are so small as to be unmeasureable by our equipement today. The effects of special realativity do not become very noticable until you obtain a considerable fraction of hte speed of light. So, we do not really notice them ever. Of course that is why the ideas of special realaitivy do not really make alot of sense to us. They are not observed on a reagular baises.

So now we come to general realtivity. I am afraid that i can only give you a cursory introduction to this. Both becuase the math behind it is quite intense and becuase i myself do not udnerstand it that well.

General relativity is biult on a statement similare to that of special relativity -- with on important addition. IN general relativity is is stated that all frames that are in uniform acceration are equally valid. Prior to this is was just frames that were not acperating (called "reast" frames).

Cool, so we can see this works by other simple thought experiment. Imagine you are in a box without window. And you are sitting on the floor. YOu are expreienceing a force pulling you down wich is the same as the force you experiece from earths gravity. now is there anyway that you can determine wether you are in a box sitting on earth or in a box in the middle of space that is accerlerating at a constant 9.8 m/s2 (this is the same acceleration you expreince from earths gravity)? General realativty says that there is no way to tell these two situations apart, if you are in the box. So we are left with the idea that all constantly accelerating frames are equally valid.

Ok, so then you can start to unravel the concequences of general realitivity. The simplest of these is that space is curved. Space is curved in three dimentions by mass. That means that every bit of mass "bends" space. The earth does, you do, a baseball does, ect. Large masses bend space more than do small masses, as would be expected. SO there is consequence one.

There are others, but it would take me a great amount of time to tell them and i have already typed for a while. Perhaps later i iwill talk about htem.

As for now, i am leaving. I have to go to a funeral so i won't be around to answer questions about this for a few days. however, i am sure there many others here can talk about this knowledgably (some more than I). So have fun leraning

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VEnding, THANK YOU SO MUCH for your effort and time.

but what do u mean by "bends space" that part about general relativity i am not sure.

I understand our explanation abt ether. But, special relativity is quite hard to understand. But, i 've some idea what you mean.

Thanks again.

Well, i read some where about Relativity and Quantum Mechanics.

how related are they and what is quantum mechanicas btw.

Sorry about the funeral thing. Have you lost a loved one or what?

Take care

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From what I've read, space and time are supposedly linked together to create a continuum called "space-time." So when you have masses on it, it bends it... A good example would be if you have a plate of jello. When you put a ball on it, the jello dips in. In this analogy the jello would be space-time and the ball would be any mass, planets, stars, people, etc.

I still do not understand how, if everything is 3 dimensional, space-time is explained as a 2 dimensional plane. This leads me to believe that there is possibly more dimensions than we can percieve. Maybe there are 5 dimensions...

I read something that explained this very well. Say there is a universe with 2 dimensional shapes walking around (or, sliding around). From the top all they look like are shapes; squares, triangles, circles, etc. All they see when they look at each other is just a flat shape. But one day the 2 dimensional square walks in his 2 dimensional house, then later comes out and there is a 3 dimensional apple standing in front of him. The apple sees the square as a square on the ground, but the square sees the apple as just another one of his buddy circles, because all the square can percieve is 2 dimensional objects.

So, what if there is a 4th directional dimension, but we can only percieve 3. We would have no idea it was there. It's interesting to think about it that all directions are perpendicular to each other. 1st dim. you have only a line, 2nd dim. you have a line perpendicular to the 1st dim., 3rd dim. you have a line perpendicular to both the 1st and 2nd dimensions. What would a fourth look like?

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Man, that is so profound. I got confused among the post.

Geez, is there like a simple and easy to understand definition (not in mathematical form)

about Special Relativity and General Relativity?

Cause even given the examples and explanation, i still am quite far from understanding it as a whole. So booper... what is that dimension thing about?

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Because from what i understand, relativity is about different perspective of things... and how each of the perspective is acceptable... IS it?

Ah, this is hard......

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that dimention thing was not about special relativity. it was about space-time, which is linked to general relativity, but is mostly seperate.

and about that dimention thing, the jell-o (or sheet, or whatever) is just a simplification designed to help beginners understand (and it doesn't work because it just brings up more questions about that subject). space time isn't actually bent like that, and there isn't actually a "downward" force that would pull down the sheet at that point, and let "planets" roll in.

a better example would probably be if you had the same jell-o, but in a closed, airtight container so that the jell-o is touching all walls. then you could stick the "planet" (a tennis ball or something) inside the jell-o (still inside the airtight container). at this point, the jell-o will be thicker around the "planet", and will thin out the farther away from the "planet" you get (ending up at about the same thin-nes as before the planet was there). i don't know exactly how that relates to gravity, but it is better than using a 2D sheet.

oh, and just like the book said, you would not be able to see another dimention, you would only be able to see in your 3 dimentions. is the thing you read flatland? if it is, then it was wrong in the sense that, at the end of the book, the square was pulled out of his 2D plane by the sphere and could automatically see another dimention. he would, in reality, still only be able to see in 2D. also, he would never be able to go back to his 2D plane, since it is supposed to be infinitely thin, allowing for an infinite possibility of error when putting him back.

but yes, it is quite interesting to think about it and try to understand what another dimention would be like, or if there is one that we just don't know about.

P.S. i can't do any better at explaining special relativity (VM was the one who i learned from too), you'll just have to think about it and try to connect it to other scenarios, and think about the examples VM gave of special relativity (and ask questions of course).

P.P.S "acceration" and "acperating" are supposed to be "acceleration" and "accelerating", as you probably figured out from reading the rest of VM's post

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gene said in post #6 :

Because from what i understand, relativity is about different perspective of things... and how each of the perspective is acceptable... IS it?

yes! try to think, "i am god, everyone else is wrong." then switch perspectives and think the same thing.

try to imagine what each individual consequence would look like from one perspective, affecting everything else. then switch perspectives, make everything back to normal, and imagine the same thing on the first perspective.

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I like the aether theory better! I'm sticking with that one! Light is a particle, that rides a wave in aether like a surfer.

Pincho.

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iglak said in post #7 :

oh, and just like the book said, you would not be able to see another dimention, you would only be able to see in your 3 dimentions. is the thing you read flatland ?

No, I read that from a book called Cosmos .

I guess I am still sort of a beginner to this as well, which is why I use that example. : )

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Is the theory of General Relativity true?

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Hey gene *waves*

On the Ether:

In 1887, a crucial experiment was performed by Michelson <http://scienceworld.wolfram.com/biography/Michelson.html> and Edward Morley <http://scienceworld.wolfram.com/biography/MorleyEdward.html> in an attempt to detect the existence of the ether. The experiment, named the Michelson-Morley experiment <Michelson-MorleyExperiment.html> in honor of its authors, shocked the scientific community by yielding results which implied the non-existence of ether. This result was later on used by Einstein to refute the existence of the ether and allowed him to develop special relativity <SpecialRelativity.html> without this artificial (and non-existent) constraint.

However this appeared in a paper published by Einstein on May 5th 1920:

>Thus the endeavour toward a unified view of the nature of forces leads to the hypothesis of an ether.

>This theory also called the theory of the stationary luminiferous ether moreover found a strong support in an experiment which is also of fundamental importance in the special theory of relativity, the experiment of Fizeau, from which one was obliged to infer that the luminiferous ether does not take part in the movements of bodies. The phenomenon of aberration also favoured the theory of the quasi-rigid ether.

>Since such fields also occur in vacuo i.e. in free ether the ether also appears as bearer of electromagnetic fields. The ether appears indistinguishable in its functions from ordinary matter. Within matter it takes part in the motion of matter and in empty space it has everywhere a velocity; so that the ether has a definitely assigned velocity throughout the whole of space.

>According to the general theory of relativity space without ether is unthinkable; for in such space there not only wonld be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense.

Thus the Special Theory of Relativity, developed in the absence of the ether, subsequently again allowed the inferrance of its existence by experimentation and further, the General Theory of Relativity is dependant upon it.

These days however it would, in my opinion be far more contemporary to refer to "space" (space-time, the vacuo), as a plasma.

"Space is not a vacuum but rather a diffuse gas of relict photons left over from the Big Bang."

-fourmilab.ch

About the Theory of Relativity (SR/GR, short version):

Special Relativity is about light.

General Relativity is about gravitation.

Either are absolutely integral among the foundations of modern physics, however still find themselves regularly within heated debate by and between scientists, hobbyists and pseudoscientists alike.

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Once again to keep things memorably simple and easy both for me to keep track of and yourself to understand:

Einstein's Special Relativity (published 1905 under the title: on the Electrodynamics of Moving Bodies)

Contemporary, Newtonian era physics theorum (ie. without Special Relativity), does not hold true with physical observation with relation to the relative motion of objects.

Thus the frame of reference is inaccurate and time itself cannot be a universal constant.

However the Speed of Light is an observed constant in nature.

Hence the frame of referance for developed physics is altered to be relative to c: the Speed of Light in a vacuo.

Naturally the implications to physics were far reaching.

In 1915 Einstein published his General Theory of Relativity.

This (was dependant upon and) described the contemporary properties of the ether/vacuo, in terms of gravitation and redefined Newton's postulative "space-time" conceptualisation, this definition of which with it is integral and includes the proposed envisionment of "curved space," of which you have read renditions above.

In simple essence you could say there are the three dimensions of height, length and breadth and a fourth dimension which is combined space-time. There is no "empty space" as such cosmologically, it's a big plasma. And further that the concept of "time" is integral to that of "space" and vice versa.

The implications to physics of this adjacent theory were also far reaching.

Both compositions have been well established and heavily corroberated, however continue to become contraversial even today. I've been arguing (good naturedly) with some PhD/MSc's on it recently.

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Oh god, not more ether freaks.

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I don't understand why you call it a plasma? Perhaps because I don't know exactly what it is, but I thought it was just the 4th state of matter, and is the hottest of the 4.

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2.7 Kelvin, for something which is supposed to be a vacuum is toasting. In fact, it's impossible.

Moreover:

" fallacy lies in assuming the (a, space-) ship is moving inertially through empty space. The ship is in fact traversing a diffuse photon gas (the CBR), which creates a drag force counter to the direction of motion."

-source: fourmilab.ch

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fafalone said in post #14 :

Oh god, not more ether freaks.

Quoted for effect

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elfin vampire said in post #12 :

Thus the Special Theory of Relativity, developed in the absence of the ether, subsequently again allowed the inferrance of its existence by experimentation and further, the General Theory of Relativity is dependant upon it.

Special Relativity specifically denies the possibility of the existance of an absolute rest frame duder. No ether possible. GR doesn't rely on an ether either.

elfin vampire said in post #12 :

These days however it would, in my opinion be far more contemporary to refer to "space" (space-time, the vacuo), as a plasma.

No it isn't, because it's not a plasma.

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Quoted from Einstein's 1920 paper (http://www.tu-harburg.de/rzt/rzt/it/Ether.html) entitled "General Relativity and the Ether," this is the conclusion paragraph:

>According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense.

No it isn't, because it's not a plasma.

I've just had this argument on another science forum. Yes it is. GR is dependant upon the "ether." The contemporary term, or description of "space-time" which has replaced "space" over the course of development in physics would be "plasma."

Agreed that it may not be immediately prevailant as a plasma in the traditional sense of supposition of quantifiable mass within a "vacuum of space" however this is precisely the traditional perception of physics, which although as with Greek preposition is instrumental in learning is nevertheless precisely that which was developed, in a working sense by Relativity.

"Space is not a vacuum but rather a diffuse gas of relict photons left over from the Big Bang..."

"...fallacy lies in assuming the (a, space-) ship is moving inertially through empty space. The ship is in fact traversing a diffuse photon gas (the CBR), which creates a drag force counter to the direction of motion."

-source: fourmilab.ch

Plas-ma: n.

Physics. An electrically neutral, highly ionized gas composed of ions, electrons, and neutral particles. It is a phase of matter distinct from solids, liquids, and normal gases.

-source: The American Heritage (r.) Dictionary of the English Language, 4th Edition, Copyright@2000 by Houghton Mifflin Company.

Unorganized material; elementary matter

-source: Webster's Revised Unabridged Dictionary, Copyright@1996

1: an electrically neutral ionized gas in an electric discharge; distinctly different from solids and liquids and normal gases

4: (physical chemistry) the gaseous state of hot ionized material consisting of ions and electrons and present in the stars and fusion reactors: sometimes regarded as a fourth state of matter distinct from normal gasses

-source: Wordnet, Copyright@1997 Princeton University

As was arrived at upon the other (said) science forum, I am increasingly becoming disposed to in fact referring to the vacuo as "ether" however the practise I am certain, of referring to it as a plasma shall be continued by New Scientist journal and published astrophysicists therein.

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New Scientist isn't a journal dude. At best it's a pop science political rag.

ps.

You speak craziness, earth boy.

space isn't.

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MrL_JaKiri said in post #20 :

New Scientist isn't a journal dude. At best it's a pop science political rag.

ps.

You speak craziness, earth boy.

Excuse me? Speak for yourself! It is a journal! I have about 200 issues of it! And it IS a good journal! I learned a lot from it.

Boy, you hit a nerve.

Oh, and "ether" has been proven not to exist. I think the theory was nuts. :bs: I'll explain later.

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is it a journal or a magazine. I am not quite familiar with it. I was always junder the impression, though, that it was a magazine (much like scientific american) and not a joural (like Nature).

Am i incorrrect?

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It's a pop science magazine.

That doesn't stop it being a decent read (I subscribed myself for years, although now it's more of a biology-y thing, and the physics is at a too low level anyway. I just read feedback atm). But it being a decent read doesn't mean it's a journal, which has a very specific definition, involving peer review and the like.

And, to be honest, it's clear that some of the tosh in New Scientist doesn't have peer review.

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Why pop?

Yeah, feedback and The Last Word is good.

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