Relativity - Oh dear, here we go again!

[Tumbleweed]

Please don't use the word reality here. It looks horrible. Even if we remove that word, that sentence still looks strange. The traveling twin changed his frame of reference by changing his velocity, not by changing his position.


No, time dilation is symmetrical here. If both of the twins say "my brother's aging slower than me", they're both right. The reason why this symmetry doesn't cause a paradox is that something else entirely happens when the rocket turns around. It's not a physical effect. It's just a change of coordinates. I explained these things earlier in the thread (starting with post #239, near the end of page 6).

(And it's just as correct to say that the Earth is speeding away from the rocket. That's just the same set of events described from a different inertial frame).

I do not understand , if one twin is obviously older than the other, how both can claim "my brother is aging slower than me". That would be the expected outcome if symmetry ruled, but the reality of both being together again, floating through space at the same velocity here on earth, is that only one has aged slower. Now they will age at the same rate.
When they did that experiment flying an atomic clock around to test relativity, did both clocks claim the other one was slow? Or was one now ahead of the other

 

[69dodge]

No, time dilation is symmetrical here. If both of the twins say "my brother's aging slower than me", they're both right. The reason why this symmetry doesn't cause a paradox is that something else entirely happens when the rocket turns around. It's not a physical effect. It's just a change of coordinates. I explained these things earlier in the thread (starting with post #239, near the end of page 6).

I don't get how you're thinking about this. The final difference in age is surely real. Yet all relevant "physical effects" are symmetric? What causes the age difference, then?

I tend to think of the slow aging of each twin, as perceived by the other, as also not quite a physical effect. It too is related to coordinates---not a change in coordinates, but a choice of coordinates. So, nothing is "physical", and I give up the whole idea of searching for the "physical effect" that causes the final age difference. An age difference didn't exist initially, and one does exist when when the twins finally meet again. But it is hopeless to try to pin down, any more precisely than that, exactly "when" this age difference accrued. Doing the calculation using different coordinate systems gives different answers to the question, so none of them should be taken too seriously.

 

[robinson]

Fascinating little story about relativity and time. (TOTH to the SC)

The year 2005 was the widely publicized 100th anniversary of Einstein's first paper on relativity and the lesser-known 50th anniversary of Louis Essen's first cesium clock. To celebrate, I created Project GREAT (General Relativity Einstein/Essen Anniversary Test), perhaps the first "kitchen science" relativity experiment.

As a collector of vintage and modern atomic clocks, I discovered it was possible, using gear found at home, to convert our family minivan into a mobile high-precision time laboratory, complete with batteries, power converters, time interval counters, three children, and three cesium clocks (see photograph). We drove as high as we could up Mount Rainier, the volcano near Seattle, Washington, and parked there for two days. The trip was continuously logged with the global positioning system; the net altitude gain was +1340 meters.

http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_60/iss_3/16_1.shtml

 

[Paulhoff]

So if no one can agree on how far away something is, except for those in the same frame of reference, velocity-wise, Einstein did indeed screw up the whole universe

How does one screw up the whole universe, was Einstein a so-called god or something, or only someone who learned and/or found a truth.

Paul



and, if you read what I wrote, some do agree.

 

[Paulhoff]

I do not understand , if one twin is obviously older than the other, how both can claim "my brother is aging slower than me". That would be the expected outcome if symmetry ruled, but the reality of both being together again, floating through space at the same velocity here on earth, is that only one has aged slower. Now they will age at the same rate.
When they did that experiment flying an atomic clock around to test relativity, did both clocks claim the other one was slow? Or was one now ahead of the other

Because both can say that the other is slower as the one travels away. But that is only if the two can only see each other, then they both could say that the other's time has slowed down. That is until, UNTIL, they meet again, than it will be revealed who did the traveling. But it is only a thought experiment and not reality. Remember the one who traveled will not have the same experience as the one who stayed behind. The one who traveled will see the earth, moon and sun change in size, see changes in the stars and planets positions and at some point he must come back and his experience will be different on the return trip too. Get your head out of the thought experiment that is use to understand the concept and back to reality when it comes to what really happens when they meet again.

Paul



As the one travels back, he will go thru all the light that has come from the one who stayed behind and will see him quickly age.

 

[Fredrik]

Tumbleweed: I explained all of this earlier in the thread, so it wouldn't make sense to write another explanation here. I suggest that you read the post with the space-time diagram and my posts on the page after that.

69dodge: You're right. It's not right to think of time dilation as a physical effect and what happens when the rocket turns around as just a coordinate change.

The age difference when the twins are reunited after taking different paths through space-time reflects a property of space-time and has nothing to do with coordinates. So this is definitely a physical effect.

The different descriptions of what happens during the trip is a result of using a specific set of coordinate systems. So it might seem that time dilation is not a physical effect, but just a result of a choice of coordinates. The truth is that it is a result of the choice of coordinates, but the coordinates weren't really chosen. They were more or less forced upon us by the physics.

OK, the physics doesn't really force a coordinate system upon us. It's more of a suggestion, but if we ignore it, there's no way to describe anything from an observer's point of view. The only reason why the concept of "one observer's point of view" even makes sense is that the properties of space-time suggest a coordinate system to associate with each inertial observer (i.e. with each straight line through space-time).

So time dilation can be thought of both as a coordinate effect (because it is) and as a physical effect (because the coordinates were chosen by the physics). The same thing is of course also true about what happens when the rocket turns around.

In general relativity, and even in special relativity when we consider non-inertial observers (i.e. observers on a path through space-time that isn't a straight line), there's (in general) no natural way to associate a coordinate system with a path, and therefore no such thing as a "point of view", or a way to compare the ages of two twins that have been separated until their paths cross again.

 

[Tumbleweed]

Hey, I'm not arguing about the mechanics of time dialation because I certainly don't have the background or the inclination to do so. I'm just interested, as a lay person, in the results. I accept it as real due to the experiments conducted and simply like to do thought experiments as to the consequences such as the twins. I particularily like musing about the extremes of space time dialation, because it suggests to me that one can, using his own clock, heartbeat, or whatever else you want to use to show passage of time on your travelling spaceship frame of reference, go a great deal of distance in a very very small amount of time. Again, using the extremes of time/space dialation. The old Wormhole theory.
And I just read an article that says they are experimenting to try and detect wormholes in space, postulating that they may be disguised as black holes
Arthur C. Clark and his 2001 a Space Odyessy pretty much suns it up, concept-wise

 

[Tumbleweed]

Another question. Assume there is an odometer on the spaceship. The mission is to go x miles out into space really really fast to Planet X and report the mileage on the odometer back to Earth. It then returns.
Will the odometer read 2x miles?
Will the reading sent back to Earth read x miles?
Or will the captain of the ship claim that the engineers on Earth are all screwed up because Planet X was a lot closer, and therefore took much less time to travel to, than those pinheads calculated
Will the scientists on earth say, what in the hell took you so long to get back? Took a detour to Venus, did we? And why did you roll back the odometer??
Just wondering if I am visualizing relativity in the right way

 

[Fredrik]

The distance measured would be less than x. If the velocity is v the whole trip (which can't be exactly true), the distance would be

[latex]$$x\cdot\sqrt{1-\frac{v^2}{c^2}}$$[/latex]​

For example, v=0.8c:

[latex]$$x\cdot\sqrt{1-\frac{(0.8c)^2}{c^2}}= x\cdot\sqrt{1-0.64}=x\cdot\sqrt{0.36}=0.6x$$[/latex]​

 

[Tumbleweed]

So the question arises: what is the fastest object, moving towards any fixed point in the universe? Excluding photons. And if something else is moving toward that same fixed point at close to the same speed from exactly the opposite direction, what is the maximum combined or relative speed attainable? Am I missing something here and the answer is still not faster than the speed of light? And if a relative speed of c is attainable, by the above equation you get 0 miles travelled, assuming the square root of 0 is 0

 

[Fredrik]

As Sol told you in #313, velocity addition in special relativity isn't just u+v, it's (u+v)/(1+uv/c²). Play around with it and you'll see that the "combined" speed never exceeds c.

 

[Paulhoff]

As Sol told you in #313, velocity addition in special relativity isn't just u+v, it's (u+v)/(1+uv/c²). Play around with it and you'll see that the "combined" speed never exceeds c.

Yes, the word is NEVER.

Paul

 

[ynot]

Hey ynot, did you ever sort out your dilemma regarding this issue?

I had been pondering relativity lately, found this thread and started reading it with a completely compatible opinion of your first post.
Midway through the first page it suddenly struck me, and i think i understand the concept now.

The problem comes with viewing an event as happening with one true timeframe. When in reality, time does not work like this, time is created by the relative viewer, in relation the the event in space.

Once you abandon the idea that there is one true measure for a single event, you see how everyone's relative perspective is the only thing that matters, and this means their perspectives have to be adjusted for, if you are relying in information to be synchronized between each perspective.

(please anyone, correct me if i am wrong here)

Unfortunately I’m just as confused as ever.

Observing a thing from different reference frames results in different perceptions of the thing, but I don’t see how this in any way creates different realities of the thing. A thing can only exist how it is, where it is, and it can only have one reality. There is an absolute “is” when it comes to the actual reality of a thing. It still seems to me that relativity is based on anomalies of perception and not on actual reality.

I don’t mean to pick on Fredrick but will use one of his posts as an example.

Fredrick (post #13)- “The clock is running slower in the other frame, but this doesn't mean that we're talking about two different "realities". We're just talking about two different points of view”.

How can you say “The clock is running slower in the other frame” when the clock is never actually in the other frame? It’s only perceived to be running slower when observed from the other frame. How does an anomaly of perception change the actual reality of the clock as it is, where it is?

 

[RecoveringYuppy]

How can you say “The clock is running slower in the other frame” when the clock is never actually in the other frame?

All things exist in all frames of reference.

A frame of reference is just an arbitrary grid chosen to be at rest compared to some point(s). The choice of point(s) and frame is arbitrary and usually chosen for convenience in solving some particular problem.

 

[ynot]

All things exist in all frames of reference.

A frame of reference is just an arbitrary grid chosen to be at rest compared to some point(s). The choice of point(s) and frame is arbitrary and usually chosen for convenience in solving some particular problem.

I don’t agree. All things can be observed from all frames of reference, but they can only exist in their own frame of reference. Existence has nothing to do with observation.

 

[RecoveringYuppy]

Well, if you want to talk to physicists you're going to have a problem then.

 

[ynot]

Well, if you want to talk to physicist you're going to have a problem then.

Not sure if you’re a physicists, but lets say you’re in a rocket speeding past me and my clock.

You’re in the frame of the rocket (correct?).

The clock is in my frame (correct?).

There are two different and distinct frames (correct?).

The clock is only with me and always in my frame, never in the rocket and in your frame(correct?).

The clock is never therefore actually in your frame (correct?).

You merely observe the clock that’s in my frame from your frame (correct?).

The light from the clock, that has travelled from my frame to your frame, has been affected by movement and distance, so you observe the clock to be different than it actually is (correct?).

 

[RecoveringYuppy]

First you have to understand that there is really no such thing as "your frame" and "my frame" although it would be assumed in conversation to be a shorthand for "a frame of reference at rest with respect to me/you"

Not sure if you’re a physicists, but lets say you’re in a rocket speeding past me and my clock.

You’re in the frame of the rocket (correct?).

Yes, and I'm also in the frame of the clock. I'm just at rest with respect to one and moving with respect to the other.

The clock is in my frame (correct?).

Yep. And it's in the other frame too.

There are two different and distinct frames (correct?).

Yes, but everything in the universe can be measured against either of them.

The clock is only with me and always in my frame, never in the rocket and in your frame(correct?).

Everything is in every reference frame. Reference frames are arbitrarily chosen points of reference.

The clock is never therefore actually in your frame (correct?).

Everything is in every reference frame. Reference frames are arbitrarily chosen points of reference.

You merely observe the clock that’s in my frame from your frame (correct?).

I observe whatever I look at. I can interpret those observation from any and all reference frames.

The light from the clock, that has travelled from my frame to your frame, has been affected by movement and distance, so you observe the clock to be different than it actually is (correct?).

Frames do not "end". They are infinite in extent since they are just points of reference. You do not leave one frame and enter another. There is no privileged frame so there is no single definition for what "it actually is". It is actually all those things.

 

[ynot]

First you have to understand that there is really no such thing as "your frame" and "my frame" although it would be assumed in conversation to be a shorthand for "a frame of reference at rest with respect to me/you"

Yes, and I'm also in the frame of the clock. I'm just at rest with respect to one and moving with respect to the other.
Yep. And it's in the other frame too.

Yes, but everything in the universe can be measured against either of them.

Everything is in every reference frame. Reference frames are arbitrarily chosen points of reference.

Everything is in every reference frame. Reference frames are arbitrarily chosen points of reference.

I observe whatever I look at. I can interpret those observation from any and all reference frames.

Frames do not "end". They are infinite in extent since they are just points of reference. You do not leave one frame and enter another. There is no privileged frame so there is no single definition for what "it actually is". It is actually all those things.

Don’t see how you can be both “in a frame” and “moving in respect to it”. Surely to be “in a frame” you have to be “moving with it”.

ETA -
There are an infinite number of possible reference frames, but only one can be used at any one time.

 

[Fredrik]

Think of it this way: Imagine a blank piece of paper, and think of that as a representation of space-time (with only one spatial dimension). Each point represents an event. An object moving from one location to another, e.g. a ball falling to the floor, would be represented by a curve. Note that we still haven't associated any numbers with any of this.

To a physicist, a "frame of reference" is not something physical. It's just a function that assigns numbers (coordinates) to the points in the plane. The existence of a curve that represents a ball falling to the floor clearly doesn't depend on what coordinates we assign to the points on the curve.

When we describe a set of events like a ball falling to the floor, we would usually do it by talking about the coordinates that have been assigned to points on the curve, rather than about the actual points on the curve. When we say that it took the ball 0.5 seconds to reach the floor in a certain frame, we really just mean that when we use that particular assignment of coordinates to events, the difference between the time coordinates assigned to the endpoints of the curve is 0.5 seconds.

The same thing goes for the statement "the clock is running slower in the other frame". There is a natural way to associate an inertial frame with a moving clock, so it does make sense to speak of "its" frame of reference. But the clock's frame of reference is still just an assignment of coordinates to events, and we can use another if we'd like. What my statement means is that when we use this "other" coordinate assignment, the time displayed by the clock increases by less than 1 second when we go from an event with time coordinate t to an event with time coordinate t+1.

I think I suggested to you earlier in the thread that you learn about space-time diagrams. I still think that's a good idea. It will be much easier for you to understand these things if you study space-time diagrams first.