concept i cant grasp..need help

[theANTIcraig]

for a while now theres been something regarding relativity that i just cant seem to understand...i asked my physics professor and it looked like he had a reasonable explanation but he doesnt speak english very well and it was hard for me to understand his explanasion....

 

as we all know relativity states that there is no ABSOLUTE velocity or speed...but it is only measured relative to its surroundings or environment...

 

so when talking about time dilation...for example...a rocketship versus the earth...the rocketship is moving near the speed of light...and the earth is still...so time is passing slower on the rocketship than on earth...therefore when the rocketship returns...the brother on the rocketship will be younger than his twin on earth b/c lesss time has passed for him...

 

what i dont understand is how relativity distinguishes WHICH object was the one traveling near the speed of light...b/c the way i see it...its paradoxical to think that the spaceship should be the one experiencing the time dilation....b/c who is to say that the rest of the universe isnt moving away from the spaceship...thus causing the REST of the universe to experience the time dilation...

 

im pretty sure my professor was telling me something about acceleration...and i still cant see why that would be relevant...b/c the way i see it...theres 2 different entities being compared...the universe and the spaceship....

 

how do u find out which is experiencing the slowed down time?? which entity is traveling at the speed of light and which is considered stationary?

[doublejab]

the brother in the spaceship experiences a change in direction and acceleration whereas the other on the earth did not and is always in constant uniform motion, eventho frm the spaceship it seems otherwise. i think tt determines the frames of references in special relativity.

[CPL.Luke]

I'm pretty sure that if you have a space ship and the earth, from the perspective of the ship time slows down on the earth. but then from the earths perspective time slows down on the ship

[swansont]

I'm pretty sure that if you have a space ship and the earth, from the perspective of the ship time slows down on the earth. but then from the earths perspective time slows down on the ship

 

Right. The effect is symmetric, since you can't say which one is moving, until one of them undergoes an acceleration.

[J.C.MacSwell]

I'm pretty sure that if you have a space ship and the earth, from the perspective of the ship time slows down on the earth. but then from the earths perspective time slows down on the ship

 

Also if the Earth was somehow accelerated and caught up to the spaceship it would be in the spaceship's inertial frame and it (the Earth) would contain the younger twin.

[Saint]

Right. The effect is symmetric, since you can't say which one is moving, until one of them undergoes an acceleration.

 

When you say that once something undergoes an acceleration, it can be determined to be moving, wrt what is it "moving". It already has a velocity wrt the other something. What does acceleration do for you? It sounds like you're trying to define absolute motion in a round about way.

[J.C.MacSwell]

When you say that once something undergoes an acceleration, it can be determined to be moving, wrt what is it "moving". It already has a velocity wrt the other something. What does acceleration do for yo[/b']u? It sounds like you're trying to define absolute motion in a round about way.

 

When you accelerate you are then moving wrt the rest frame you started in.

 

You have also changed your rest frame and, with it, changed:

 

Distances to various objects

 

Time lapses to past and future events

 

The accepted chronological order of events at various distances

[MetaFrizzics]

To try to understand motion and acceleration and measurement of speed, and how 'time' elapses in Special Relativity Theory (SRT), it really helps to first have a bit of a background in Newtonian Mechanics first. There are two reasons for this:

 

(1) SRT reduces to Newtonian Mechanics for large relatively slow moving objects.

 

(2) Many of the modified concepts required must first be learned in their unmodified forms through Newtonian Mechanics, to have a real understanding of what is going on.

Essential Background

In the real world, all ordinary measurements of an object's movement, velocity, and direction are done relative to some other object. This other object (the ground, the observer etc.) becomes the 'frame of reference' for all the measurements. A car or train is given a 'speed' relative to the ground, or a fixed radar trap. Anything not moving compared to this starting point is said to be in the same 'frame of reference'. The motion and direction of all these objects (trees, parked cars etc.) will be ZERO with respect to each other and the frame of reference we have chosen to measure from.

Not All Frames of Reference are Equal

Physicists would like all the laws of motion to take a standard form, a simple form, regardless of the 'frame of reference' chosen (A true case of Relativity of Motion). However, it turns out that this is not the case. There actually is a special set of reference frames in which the laws of motion are indeed simple: In Newtonian Mechanics, these turn out to be the Frame of Absolute Rest (relative to Absolute Space), and all frames moving at a constant speed in any direction relative to this same Frame of Absolute Rest. In all these frames of reference, the Laws of Motion turn out to be rather simple in form and easy to use.

 

Appearance of Forces

For instance, if we are travelling at a constant speed in a single absolute direction, then when we observe other (non-accelerating) objects in motion, they will appear to obey a simple set of laws that seem to support the Law of Conservation of Momentum. Objects which are NOT moving at constant speeds or directions from the observer's point of view (accelerating) will NOT appear to be following these laws, and so we assume that forces are acting upon them causing them to behave as if they had a life of their own.

 

Preferred Frames

We prefer the set of reference Frames which explains motion and forces in the simplest manner. If we try to make measurements and calculations of forces from a reference frame that is itself 'accelerating' relative to Absolute Rest or a frame of Constant Speed, all kinds of things seem to behave weirdly. We have to propose all kinds of 'phantom' forces to explain behaviour that would in another frame of reference appear simple and need no extra forces.

 

Constant Speed versus Acceleration

As it turns out, in BOTH theories, (Newtonian Mechanics and Special Relativity) you have preferred frames in which motion is more simply described, and other frames (accelerated frames) in which motion is complex and difficult to describe simply.

 

Whats Special about Special Relativity?

However, in SRT, you have an added level of complexity and diifficulty, since in this theory it is proposed that all objects will obey physical laws that resemble Maxwell's Equations for Electromagnetic Waves, modified by the Lorentz Transforms. Here, in some interpretations of SRT at least, time itself is not an 'absolute' dimension of a linear Cartesian type, but rather a 'local' variable that can only be measured and defined locally and relatively. In Einstein's original interpretation, time is said to be stretched or compressed as needed by the Lorentz Transform, and the object or particle experiences 'time' to slow down or speed up as necessary.

 

Other Interpretations of SRT

Other interpretations propose that it is only 'measured' time which is changed. That is, clocks slow down, (including atomic clocks and quantum interactions) but Proper Time itself remains a kind of involute dimensional frame. Since all time is measured locally and relatively, SRT merely predicts clock behaviour and mechanics under acceleration, rather than 'Time' distortion.

[CPL.Luke]

Other Interpretations of SRT

Other interpretations propose that it is only 'measured' time which is changed. That is' date=' clocks slow down, (including atomic clocks and quantum interactions) but Proper Time itself remains a kind of involute dimensional frame. Since all time is measured locally and relatively, SRT merely predicts clock behaviour and mechanics under acceleration, rather than 'Time' distortion.

[/quote']

 

if you can't observe something, and there it is more efficient to just call it something else than it doesn't exist. Otherwise its like trying to argue that God exists (its impossible)

[swansont]

When you say that once something undergoes an acceleration, it can be determined to be moving, wrt what is it "moving". It already has a velocity wrt the other something. What does acceleration do for you? It sounds like you're trying to define absolute motion in a round about way.

 

No, because even if I can tell that I accelerated, I can't tell if that slowed me to a stop, or sped me up from rest, or whatever. All I know is my velocity with respect to some reference has changed.

[starbug1]

I'm going to try an explain this as briefly as I can.

I will try to use the car example of inertia. Traveling 55 mph in a car you would say you are also traveling at 55 mph. If there is an abrubt stop, you are propeled forward at 55mph until you are stopped, hopefully with a seat belt. Earth is a fixed reference in space, and in our solar system. Time moves the same for all inhabitants of earth, but maybe not for the inhabitants of another planet. If our average age is 75, and on another planet it is 105, it is all because of the velocity at which the planet moves in its frame of reference. The ship is the same way. the inhabitants of a space ship are the same inhabitants of a car. They would move (and age) according to the movements of the ship and its surroundings, which are much faster than that of earth's. Therefore, because the entire universe is propelled outward at a fixed rate, and galaxies, solar systems, and planets are at fixed rates, a space ship is its own accelerator because it moves independently to its surroundings.

[theANTIcraig]

Well, it can be said that, disregarding all frames of reference, that for the two objects relative to each other, one of them will experience time dilation (the one traveling faster) and the other will not.

 

What if you treat the traveler as one entity, and the rest of the universe as the other entity. For whom will the time be slowed down for? Is the traveler traveling at near speed of light relative to the universe, thus slowing his own time down compared to the rest of the universe? Or is the rest of the universe traveling at the near speed of light, thus leaving the traveler the only one experiencing the "actual" sense of time?