Relativity

[Radical Edward]

pop = popular. New Scientist is just like an extended version of the Times science column.

[gene]

Vending,

Can you tell me about "Time dialation" and "Length contraction" in your 2nd post? because when i read it a second time, i got confused and lost.

 

So, am i right to say that Special Relativity (i'll now relate to an example.) Is like a tall person and a short person. The tall person sees that most of the things are lower or at the same height at he is. But, a short person sees things at the same height or higher than he is. And they are basically seeing the image of the same thing, let's say an apple place on a shelf.

So, is this example acceptable?

 

Btw, if i am right about SR, it is more like perception... of the mind... and how to think or look in a different perspective... it would be more on the psychology side, don't you think?

[gene]

Btw, i find News Scientist like a Sience newspaper. It keeps you up to date of the latest science advances. P.S. I only visited the webpage (the main page) and not the magazine. And, i have not seen Scientific American or Nature.

[Radical Edward]

please check the announcement at the top of the relativity forum.

[gene]

Sorry, i didn't get the mathematical equation. Iwas too hard to understand? what can those equations tell abt SR?

[VendingMenace]

Gene

 

It is kinda is a matter of perspective, and kinda not.

 

WHere your analogy is good is in this; how you see an object depends on your frame of reference. If you are in a frame of reference such that the observed object is moving close to the speed of light with repect to you, then you will observe that the object appears contracted (shortened) and more massive (has more mass) than it would if you observed the same object in a frame of reference such that the object was not moving with repsect to you. Thus, we see that your view of an object does in fact depend on your perspective, so to speak. That is somewhat the point of SR. That your view of objects can change depending on the frame from wich you are observing them.

 

However, your alalogy does bread down. Specifically, it breaks down with this concept;

IN your alalogy, the view of the apple was different depending on a persons perspective. right? However, despite their differing views, you could make an absolute statement about the reality of the apple. That is to say that it is a red apple, it is 10 cm long, and it has a mass of 0.5 kg. These facts would be absolute, in your example. that is, no one would contest that these are the "true" facts about the apple.

 

However, in SR, all views from an inertial frame (one in which you are not accelerating) are equally valid. So, you cannot really make absolute statements about the apple'

s color, lenght, or mass.

 

Imagine that you were in an inertial frame in which the apple was stationary to you (not moving realitive to you). You would observe the apple to be red , 10 cm long, andhaving a mass of 0.5 kg.

 

Now imagine that i am in an inertial frame for wich the apple is travelling near the spead of light realive to me. For the sake of argument, let's say that the apple is moving towards me. I would observe a doppler shift in the light, a lenght contraction, and a mass gain, for the apple. That is to say, i could see the apple as blue, 2 cm in leghth, and having a mass of 4 kg. ( i have not used the equation, becuase it is not nessesary for a qualititive explilation. I htink you get the idea, thought the numbers may be of somewhat).

 

Now what do we have? WE have two people observing the same apple and getting quite different results! NOw the fact that we got different results rests on the fact that we had different perspectives on the apple (ie. we were in different inertial frames). HOwever, (and this is the important part) since all inertail frames are valid, there is no way to say whose measurements are correct. Both of our observations are correct. The apple really is both read and blue, 10 cm and 2 cm, 0.5 kg and 4 kg. It just depends on where you are when you meausre (which intertial frame). And both results are valid! Crazy!

 

 

Anyways, that is the crux of SR. You can get different results from different interial frames, however, these results are not more or less correct, they are just different. All the results that you get from different interial frames are all equally valid. It is quite cool.

 

Well, i hope that answers some questoins, ask more if you wish

[gene]

CAn you give some examples of different inertial frames?

 

I think i'm getting the gist of it but there are still some science jargons(I think) that i don't quite get.

 

So what actually are inertial frames?

 

Thanks for your time

[VendingMenace]

A reference frame is just a frame from wich you can make observations. It is an abstract concept, and may be somewhat hard to grasp. Perhaps this will help.

 

Think about riding home on the bus, ok? From wher you sit, you are in a reference frame. You are in a frame that is moving exaclty as the bus is moving. So we can say that your refference frame is the bus's reference frame too. From this point of view of this reference frame, the bus is stationary and the world is sliding by. Right?

 

NOw lets assume that as you are looking out the window, you see a person standing on the street looking at you. From your perseptive, this person is sliding by you. However, from their perspective, you are moving down the street and they are standing still. Their refference frame is that of the street.

 

SO now we have two reference frames, that of the bus and that of the street. It turns out that we could have an infinite number of refference frames. WE could assign one to the car that is driving down the street the oposite direction of the bus. From its refference frame the bus is moving past it and so is the person standing on the sidewalk. WE can keep assigning frames ad noseum.

 

Refference frames to not need to have an "object" associated with them. IN the above examples our three reference frames had a bus, a person and a car associated with them, respectively. However, we could assign a reference frame to just a random space. It can be a completely arbitrary, abstract, construct.

 

Once we have a reference frame picked out, we can start to describe it. We can say things like how fast it is moving, in what direction it is moving, whether or not it is accelerating, what sort of feilds it is exposed to, ect.

 

As far as SR is concered, we need only consider two aspects of the reference frame -- how fast is it moving and is it accelerating. The speed at wich it is moving is quite important, it tells us how extreme the relativistic effects will be for that object (relativistic effects gain magnituted as you approach the speed of light).

 

the other thing wich we must answer is whether or not it is accelerating. In special realtivity, we claim that all frames that are not undergoing accleleration are equally valid. That is to say that for two frames that are not accelerating, measurements taken from these frames are equally valid (see the my above post).

 

However, should one of the frames be accelerating, this does not hold. Measurements made from both frames are not longer equally valid. The acceleration "breaks" the symmetry, so to speak, and fixes one of the frames as the "absolute" frame -- the frame whose measurements are acutally correct.

 

So we see that the idea of non-accelerating frames is quite important for SR. In frames are accelerating, then they are not equally valid. So, we must know when frames are accelereating and when they are not. Rather than writting every time "this frame is not accereating" (which would be rather lengthy) we instead give reference frames wich are not undergoing acceleration a speacial name. WE call such frames inertial frames.

 

THus, when you read that a particulare frame of reference is a inertial frame, that means that that reference frame is not accelerating.

 

 

Hope that helps out some, again, feel free to ask more questions

[Tom Mattson]

gene said in post # :

Btw, if i am right about SR, it is more like perception... of the mind... and how to think or look in a different perspective... it would be more on the psychology side, don't you think?

 

The predictions of SR (via the Lorentz transformation, hereafter LT) are not illusions of the mind, but rather how events actually happen. The numbers that you calculate from the LT are the spacetime coordinates of events as measured by rods and clocks local to the event. The numbers that you calculate do not describe the events as you see them. In order to make that prediction, you have to take into account the signal delays due to the finite propagation speed of light.

[Guest swift00]

I was trying to learn about relativity and found this thread very informative, a few questions tho. Is the ether a type of medium if it really exists, and is there such thing has empty space like vacumes. Im use to the idea of space being a vast expanse of nothing with very small amounts of scattered matter and energy. Also if light is photons and they are travelling at the speed of light, wouldnt there mass be absurd, and wouldnt it change in its amount of energy has it reaches diffrent mediums? and does that tie in with the doppler shift thing (which i know nothing about) , like by changing your speed (a very high speed) to or from the light traveling at you, would that affect the lights percieved color?

[Dave]

I was trying to learn about relativity and found this thread very informative, a few questions tho. Is the ether a type of medium if it really exists, and is there such thing has empty space like vacumes. Im use to the idea of space being a vast expanse of nothing with very small amounts of scattered matter and energy.

 

Scientists at the time believed that light waves had to permeate (sp?) through a substances (e.g. water waves travel through water, sound through air, etc). They invented the ether as a substance for light to travel through. Don't know whether this explains your question, but hope it helps.

 

Also if light is photons and they are travelling at the speed of light, wouldnt there mass be absurd, and wouldnt it change in its amount of energy has it reaches diffrent mediums? and does that tie in with the doppler shift thing (which i know nothing about) , like by changing your speed (a very high speed) to or from the light traveling at you, would that affect the lights percieved color?

 

As you may know, light when passed from one medium to another refracts. There is a way to explain this that I'm not completely familiar with, so I'll refer you to this that I found:

 

http://theory.uwinnipeg.ca/mod_tech/node123.html

 

As regards to Doppler shift, it applies for light at high (i.e. relativistic) speeds, but the formula changes somewhat because relative velocities are calculated differently for extremely high speeds.

 

Hope this helps.

[swansont]

As regards to Doppler shift' date=' it applies for light at high (i.e. relativistic) speeds, but the formula changes somewhat because relative velocities are calculated differently for extremely high speeds.

[/quote']

 

The Doppler shift applies at all speeds. One example - atoms moving only a few m/s can be shifted out of resonance with a laser and basically stop absorbing the light.

[geistkiesel]

CAn you give some examples of different inertial frames?

 

I think i'm getting the gist of it but there are still some science jargons(I think) that i don't quite get.

 

So what actually are inertial frames?

 

Thanks for your time

I'll try it too. SR says we have to discard the concept of simultaneity. If you are on a mioving platform and just pass through the midpoint of two photon sources just as the sources emit some photons you will see the photon from the source you are heading to before the photon from the rear catches up. What was simultaneious in the stationary frame becomes nonsimultaneous in the moving frame. See Einsteins "relativity" regarding simultaneity, I wasn't convicned but that is the theory.