Special relativity question. (1 Viewer)

[echelon4]

HI i have a little question about special relativity. I've been reading about that example of a train going at the speed of light, a person using a mirror inside the train and a person standing outside the train. I realise that since the speed of light stays constant, only time and distance can change. But my question is, why is light constant? I'm just wondering why the light on the train couldn't travel twice the speed of light??

Thanks in advanced

 

[+:: $i[Q]u3 ::+]

To the observer on the platform, light travels at c.
If the observer is looking into a train travelling at c, then by definition the train and light would be moving at the same speed.
It would not make sense for light to just "speed up" whenever it felt like it. The observer outside cannot simultanously observe light behaving normally, and light speeding up inside the train. (What makes the train different from the rest of the observer's environment?)
Hope that helps with your understanding.

 

[rnitya_25]

HI i have a little question about special relativity. I've been reading about that example of a train going at the speed of light, a person using a mirror inside the train and a person standing outside the train. I realise that since the speed of light stays constant, only time and distance can change. But my question is, why is light constant? I'm just wondering why the light on the train couldn't travel twice the speed of light??

Thanks in advanced

basically, nothing can travel faster than the speed of light. its about the fastest thing that exists. light just is constant, because its not affected by anything, remember with the michelson-morley expt, it showed that the so called 'aerther wind' didn't exist because it didn't affect the way light travelled? this shows that light is at a constant speed, mainly because it is affected by nothing to slow it down or speed it up. hope that helps and more so, hope that made sense.

 

[acullen]

It is simply a physical constraint of nature. An object can travel the speed of light if and only if it has a zero rest mass, and the only thing that has this property is energy (i.e. a photon).

There are 3 formulas you are likely to encounter during the HSC for this phenomenon:

Time Dialation
t = t₀ / √(1-v²/c²)

Length Contraction
L = L₀•√(1-v²/c²)

Relativistic Mass
m = m₀ / √(1-v²/c²)

Where:

t, L, or m = time, length or mass from the perspective of someone on the moving body
t₀, L₀ or m₀ = time, length or mass from the perspective of an outside observer
v = the relative velocity of the body
c = the speed of light (299,792,458 m/s)​

 

[jenorater]

I have a question and was wondering if anyone could answer it:
How long would a metre rule be if it travelled past you at a speed of 0.99c?

Okay, this is how I've looked at it:

Lv = Lo (Square root) 1- {0.99c } all squared
--------
c


Because Lv is the moving frame and Lo is the still frame, and because the question refers to the ruler passing you, hence you're in the still frame. Substituting Lv for 1 results in
Lo approx. = 7.09 (2dp)
So the rule would be 7.09m long.
Howevver i have done this wrong, if anyone could help me out with this question, it would be much appreciated.

 

[Mono!]

Lv=lo square root of lv squared divided by c squared= 1 times square root of .99c squared divide by c squared = 5.74 times 10 to the power of negitve 5 any else agree with this answer?

 

[kurt.physics]

BASIC answer for first question by echelon4

I have herd that it is due to Einsteins principal of physics that states;

There is no test to tell you if you are still or moving with a constant velocity unless reference to an outside point

And aparently, from what i have herd, if there was light in the 'train' and light didnt travel at the same velocity, then it would be easy to tell if you were moving because the colour of the light would change because it was chaning velocity.
But if that was true, then that would defy Einsteins principal. So then Einstein deduced that time and lenght was relative.

 

[kurt.physics]

Lv=lo square root of lv squared divided by c squared= 1 times square root of .99c squared divide by c squared = 5.74 times 10 to the power of negitve 5 any else agree with this answer?

Sorry, i dont. Now i could be wrong, but i dont think i am, here is how i looked at it

So, we have a 1 meter ruler and it is travelling at 0.99c. What is its length seen by an outside viewer.

So length contracts, or more simply, length gets smaller. So any thing smaller than 1m would be a decimal, so it would be best if we use cm.

So 100cm ruler goes 0.99c
The formula is;

lv= lo times √1- (v^2/c^2) or, in words lv= lo time root of 1- (v squared divided by c squared)

so i like to say that length seen by out side observer is L (capital L) and normal lenght is l (little l)
So,

L= l times √1-(v^2/c^2)
so,

L= 100cm times √1- ([0.99c]^2 / c^2)
or

100cm times √1- [(0.99 times 300,000) squared] / 300,000^2]

= 100 times 0.1410673598

= 14.10673598 cm
= 14.107 cm

So, lets say that a meter ruler was on a ship going 0.99c. The ruler (even illogical and stupid) measures its self to be 1m, but, a man looking at the ship decides that he wants to measure the ruler, he cant come aboard so he looks through the window and measures the ruler to be only 14cm long.


And thats is basically it

 

[Kirjava]

I think that the point of this thread has been forgone somewhat. echelon64 asked "why is [the speed of] light constant"?. It's a good question really, and a somewhat simplified answer can be derived from the discovery that mass and energy are essentially equivalent (by E=mc^2 - just about everybody has heard that one). It turns out that as a result of this, objects in motion are actually heavier then when they are at rest, due to the fact that their kinetic energy actually 'weighs' . Anyone who's done elementary physics should recall Newton's second law (F=ma, or more appropriately, a=F/m). From these two equations we can postulate that if we were to accelerate an object to greater and greater speeds, the energy required to accelerate it the next say 1km/h would become greater than the last (since the mass of the object increases with speed). If you plot this trend on a graph, it turns out to resemble an exponential curve which asymptotes at "c" (the speed of light).

If that alone isn't convincing, then you're probably forgetting that it's a fundamental postulate in physics (that's turned out to work rather well) that the laws of physics should be the same in all frames of reference. If that's the case, then you're buddy in the train travelling at c cannot be observing the other train travelling in the opposite direction also at c because we've just shown that that should be an impossible event otherwise (this is just one reason among many).

Hope that helps.

EDIT: Woah, just realised that the this thread was started over a year ago, guess that was kinda pointless... meh.