Two Relativity Questions (1 Viewer)

[IBEKEVINN]

I came across a problem that I can't seem to fully understand:

If, for example, a person was travelling at 0.9c relative to the Earth (or a person on Earth), then according to relativity, the person moving would experience a "slower" time then the person on Earth.

However, in the reference frame of the travelling person - they are stationary whilst the person on Earth is moving towards them at 0.9c - in this reference frame, it would mean that the person on Earth is experiencing less time than they are.

How does this all work out? How do both references view the other as having slower time?


Secondly, for the relativity formulas e.g. t_v= t_o / sqrt (1 - v²/c²)
I used to consider the time experienced WITHIN the spaceship (or whatever) to be t_o which would therefore make t_v the larger number, making sense.

I came across a HSC question:
http://puu.sh/gx0pB/a7989bfe7b.png

As above, I would consider L_o to be the apparent length as seen by the electron, however, by using this method, it would indicate that L_o is a larger number - which isn't the case. How do I determine which values to use for L_v or L_o? i.e. How do you think about it?

 

[Kaido]

simply put, you are confused about special relativity
lets first clear up that 'slow time' misconception: when a person is in a ship moving at 0.9c and measures 1s in their frame, an observer outside (stationary) would measure this as 1/sqrt(1-0.9^2); the 1 on top is t(0), known as 'proper' time (research this yourself, im not great at explaining this concept to others, just confuses them more)
the converse is true, 1s passes on earth will be measured as 1/sqrt(1-0.9^2) on the ship (no this not paradoxical, this is simply what happens)

anything denoted as a (v) is relativistic, i.e. the mass/length/time RELATIVE to US
(0) means 'proper' length/mass/time; and is the measurement taken in SOMEONE's frame of reference

(i was initially clueless and confused, but through trial and error as well as many questions, i've devised this concept myself, lol)

hope this helps, or doesn't.

 

[IBEKEVINN]

simply put, you are confused about special relativity
lets first clear up that 'slow time' misconception: when a person is in a ship moving at 0.9c and measures 1s in their frame, an observer outside (stationary) would measure this as 1/sqrt(1-0.9^2); the 1 on top is t(0), known as 'proper' time (research this yourself, im not great at explaining this concept to others, just confuses them more)

So this would mean that if the person in the ship experiences 1s, an outside observer would experience roughly 2.3 seconds.

the converse is true, 1s passes on earth will be measured as 1/sqrt(1-0.9^2) on the ship (no this not paradoxical, this is simply what happens)

Scaled onto this example, the person on Earth would be experiencing less time then before (i.e. from 2.3 seconds -> 1 second) BUT NOW rather than the travelling person ALSO experiencing less time (which would make sense ), the travelling person would experience MORE TIME THAN BEFORE (2.3 seconds). I don't know but this definitely seems paradoxical.

 

[iStudent]

Yea, it appears paradoxical but remember - all frames are equal and none is more correct than the other. Both happens.
If you ask what happens when the spacecraft returns back to earth - i.e. what happens to the time then? Well what happens is that SR doesn't apply anymore, as you've decelerated in turning back to Earth so it makes sense.

 

[IBEKEVINN]

Yea, it appears paradoxical but remember - all frames are equal and none is more correct than the other. Both happens.
If you ask what happens when the spacecraft returns back to earth - i.e. what happens to the time then? Well what happens is that SR doesn't apply anymore, as you've decelerated in turning back to Earth so it makes sense.

Sorry to be that guy that picks out every little thing, I just really want to understand this XD

Well obviously everything doesn't return back to normal when a spaceship returns back to Earth - the people who have travelled will have aged less (e.g. growing a small moustache compared to a person on Earth with a large moustache). But then, as before, the people on the spaceship will have viewed members on Earth to have aged less as well (growing a large moustache whilst the people on Earth grew a small moustache). And when they both come to rest back at the same inertial reference frame (Earth), one situation seems to prevail which is the travellers having smaller moustaches.

Relativity just seems so impossible to understand - or I'm just understanding relativity completely wrong.

 

[Kaido]

Sorry to be that guy that picks out every little thing, I just really want to understand this XD

Well obviously everything doesn't return back to normal when a spaceship returns back to Earth - the people who have travelled will have aged less (e.g. growing a small moustache compared to a person on Earth with a large moustache). But then, as before, the people on the spaceship will have viewed members on Earth to have aged less as well (growing a large moustache whilst the people on Earth grew a small moustache). And when they both come to rest back at the same inertial reference frame (Earth), one situation seems to prevail which is the travellers having smaller moustaches.

Relativity just seems so impossible to understand - or I'm just understanding relativity completely wrong.

This is a common perception and is proof that you have grasped the principles of special relativity

You are referring to the 'twin paradox' scenario (which I recommend googling and reading the wiki)

Come back and let me know what you thought

 

[IBEKEVINN]

This is a common perception and is proof that you have grasped the principles of special relativity

You are referring to the 'twin paradox' scenario (which I recommend googling and reading the wiki)

Come back and let me know what you thought

The twin paradox does seem to make sense for the reference frame for a person Earth, as they would view the person on the spacecraft as aging less. However, as you mentioned before, the person on the spacecraft would see a person on Earth age less, i.e. from the reference of the person on the spaceship, they would be older than a person on Earth. However when they return to Earth, the person Earth would now be older.

^ This unfortunately, is the thing that confuses me - in both their respective frames, each person views themselves as being "older" but when the traveler returns to Earth, ONLY the people on Earth are older.

 

[wrenos]

The twin paradox does seem to make sense for the reference frame for a person Earth, as they would view the person on the spacecraft as aging less. However, as you mentioned before, the person on the spacecraft would see a person on Earth age less, i.e. from the reference of the person on the spaceship, they would be older than a person on Earth. However when they return to Earth, the person Earth would now be older.

^ This unfortunately, is the thing that confuses me - in both their respective frames, each person views themselves as being "older" but when the traveler returns to Earth, ONLY the people on Earth are older.

Hi, this is what i've been told:
The laws of relativity does not apply for the twin in the spacecraft because it doesn't always remain in an inertial frame of reference. It accelerates from Earth, changes direction at the midpoint of the trip then decelerates back to Earth.
On the other hand, the twin that remains on Earth is always in an inertial frame of reference.

 

[IBEKEVINN]

Hi, this is what i've been told:
The laws of relativity does not apply for the twin in the spacecraft because it doesn't always remain in an inertial frame of reference. It accelerates from Earth, changes direction at the midpoint of the trip then decelerates back to Earth.
On the other hand, the twin that remains on Earth is always in an inertial frame of reference.

Thanks for this and everyone's answer! Upon further research about this, I think I grasp the idea now - that acceleration factors into how SR works.

Whilst browsing I found a website that definitely helped in any questions I had about SR:
http://www.astro.virginia.edu/~jh8h/Foundations/quest7.html

and more specifically the part that referred to this exact scenario:
http://puu.sh/gykz4/6bd08cc595.png