# Question about Brian green book THE FABRIC OF THE COSMOS

• lnsanity
In summary, the conversation discusses a train paradox where two people have clocks and there are two observers, one on the train and one outside. They agree to set their clocks at 12:00 when they see a light open between them. From the outside observer's perspective, the clocks may show different times due to the relative motion of the train. However, when the train stops, the clocks will show the same time for all frames of reference. The concept of non-inertial frames is also mentioned, which can complicate the relationship between velocity and the tick rate of a clock.

#### lnsanity

On page 56 last paragraph Brian give example of a train paradox where two person have clock and there is two observer on on the train the other outside. They agree to set there clock at 12:00 when they see the light open witch is placed between them.

From there perspective each clock will be set at 12:00 but from the outside observer perspective it may be different if say 12:00 and 12:02 my question is what append if we stop the train and the outside observer keep watching both clock at the same time ? Will he see them synchronize 1 slow down the other speed up ? or only the one being will speed up ? or none of that they stay both desynchronized ? Think carefully because I have more dificult question coming up ! (by the way I am asking the question because I don't know the answer...)

Last edited:
Depends on the details of how they decelerate, but if both ends of the train instantaneously decelerate from v to 0 in the outside observer's frame, and they do so simultaneously in the outside observer's frame, then after they come to rest both clocks will be ticking at the same rate as the outside observer's clock, but the two clocks at either end will remain out-of-sync by the same amount as they were when the train was moving, since neither one jumps forward or backward at the moment of the deceleration.

JesseM said:
Depends on the details of how they decelerate, but if both ends of the train instantaneously decelerate from v to 0 in the outside observer's frame, and they do so simultaneously in the outside observer's frame, then after they come to rest both clocks will be ticking at the same rate as the outside observer's clock, but the two clocks at either end will remain out-of-sync by the same amount as they were when the train was moving, since neither one jumps forward or backward at the moment of the deceleration.

But from the inside perspective view they never desynchronize? So when they stop they always stay in the same frame of reference so from there perspective the clock should stay synchronize ?

Imagine its a digital clock and instead of setting there clock at 12:00 they stop the clock at whatever time it is imagine when they stop its exactly 13:00 so they are inside the train they can even take a picture but the outside viewer take a picture too, on its picture its 13:00:01 on one of the clock but if you stop the train the clock and picture will still be at 13:00 for the people inside the train since they stopped it... how could it be ?? when they stop they become on the same frame of reference as the outside viewer... Once the train stop they are on the same frame of reference they can't see different things ?

lnsanity said:
But from the inside perspective view they never desynchronize?
What do you mean by "inside perspective"? Do you mean the inertial frame where the train was originally at rest (which continues to move inertially when the the two ends of the train change velocity, so it sees the train start to move), or do you mean some kind of non-inertial frame where each part of the train remains at rest at all times? If the latter, non-inertial frames are tricky, there's a variety of ways to construct a coordinate system where an accelerating object is at rest, and you also can't assume that normal SR rules (like the relation between velocity and the tick rate of a clock) still apply in non-inertial frames.
lnsanity said:
Imagine its a digital clock and instead of setting there clock at 12:00 they stop the clock at whatever time it is imagine when they stop its exactly 13:00 so they are inside the train they can even take a picture but the outside viewer take a picture too, on its picture its 13:00:01 on one of the clock but if you stop the train the clock and picture will still be at 13:00 for the people inside the train since they stopped it...
Why do you think it'd be different? If the clocks each stop when their end of the train decelerates, all frames will agree about what times show on the clocks when they stop (different frames can't disagree about local facts like what a clock reads at the moment it changes velocity). Keep in mind that in the inertial frame where the train was at rest before any part decelerated, the two ends decelerate at different times (rather than simultaneously as in the outside observer's frame) because of the relativity of simultaneity.

JesseM said:
What do you mean by "inside perspective"? Do you mean the inertial frame where the train was originally at rest (which continues to move inertially when the the two ends of the train change velocity, so it sees the train start to move), or do you mean some kind of non-inertial frame where each part of the train remains at rest at all times? If the latter, non-inertial frames are tricky, there's a variety of ways to construct a coordinate system where an accelerating object is at rest, and you also can't assume that normal SR rules (like the relation between velocity and the tick rate of a clock) still apply in non-inertial frames.

Why do you think it'd be different? If the clocks each stop when their end of the train decelerates, all frames will agree about what times show on the clocks when they stop (different frames can't disagree about local facts like what a clock reads at the moment it changes velocity). Keep in mind that in the inertial frame where the train was at rest before any part decelerated, the two ends decelerate at different times (rather than simultaneously as in the outside observer's frame) because of the relativity of simultaneity.

The inside perspective is the perspective of the 3 people inside the train while it is moving, when the train stop there clock from there perspective is always synchronized. But the outside viewer saw something else, so now if the train stop what will see the outside viewer ?? since the clock as always be synchronized from the point of view of people inside the train...

lnsanity said:
The inside perspective is the perspective of the 3 people inside the train while it is moving, when the train stop there clock from there perspective is always synchronized.
In relativity when you talk about someone's "perspective" about when events happen or whether some clocks are synchronized, you're talking about a coordinate system (reference frame) that assigns time-coordinates to different events. For a person moving inertially there is a unique inertial reference frame where they're at rest so we can call that their "perspective", but for a person who moves non-inertially there's an infinite variety of different non-inertial coordinate systems where they're at rest, and none is more deserving of being called their "perspective" than any other. So for any statement about the "inside perspective" to be meaningful, you need to specify if you're talking about an inertial frame (in which case the train can only be at rest before the decelerations, after that it'll be moving in this frame) or a non-inertial frame, and if it's a non-inertial frame there need to be details about how it's constructed.
lnsanity said:
But the outside viewer saw something else, so now if the train stop what will see the outside viewer ??
Again it depends on how the acceleration happens. Do the two ends accelerate simultaneously in the outside observer's rest frame, or do they each accelerate when the train's clock at that end read a certain time (simultaneously in the inertial frame where the train was originally at rest), or in some other way?

JesseM said:
In relativity when you talk about someone's "perspective" about when events happen or whether some clocks are synchronized, you're talking about a coordinate system (reference frame) that assigns time-coordinates to different events. For a person moving inertially there is a unique inertial reference frame where they're at rest so we can call that their "perspective", but for a person who moves non-inertially there's an infinite variety of different non-inertial coordinate systems where they're at rest, and none is more deserving of being called their "perspective" than any other. So for any statement about the "inside perspective" to be meaningful, you need to specify if you're talking about an inertial frame (in which case the train can only be at rest before the decelerations, after that it'll be moving in this frame) or a non-inertial frame, and if it's a non-inertial frame there need to be details about how it's constructed.

Again it depends on how the acceleration happens. Do the two ends accelerate simultaneously in the outside observer's rest frame, or do they each accelerate when the train's clock at that end read a certain time (simultaneously in the inertial frame where the train was originally at rest), or in some other way?

OK you don't have is book I get it... they are on the same platform same train they move together like in any example of that classic experiment I don't get why you are asking question like this and you do not mention you did not read the book it would have been simpler if you told me before so I can't recopy the pages of the book but they do everything simultaneous they are in the same time frame relative to each other all the time.

lnsanity said:
OK you don't have is book I get it... they are on the same platform same train they move together like in any example of that classic experiment I don't get why you are asking question like this and you do not mention you did not read the book it would have been simpler if you told me before so I can't recopy the pages of the book but they do everything simultaneous they are in the same time frame relative to each other all the time.
No, I read the book, but I don't recall him talking about the "perspective" of a non-inertial observer the way you're doing. The example on p. 56-57 only deals with a train moving at constant velocity relative to the platform, not one that decelerates.

## 1. What is "The Fabric of the Cosmos" about?

"The Fabric of the Cosmos" is a book written by physicist Brian Greene that explores the fundamental nature of space, time, and the universe as a whole. It discusses concepts such as relativity, quantum mechanics, and string theory in an accessible and engaging way.

## 2. Is "The Fabric of the Cosmos" suitable for non-scientists?

Yes, "The Fabric of the Cosmos" is written for a general audience and does not assume any prior knowledge of physics. However, readers may benefit from having a basic understanding of scientific concepts.

## 3. How is "The Fabric of the Cosmos" different from other popular science books?

"The Fabric of the Cosmos" stands out from other popular science books because it not only explains complex concepts, but also delves into the philosophical implications of these ideas. It also includes vivid analogies and thought experiments to help readers grasp the concepts.

## 4. Does "The Fabric of the Cosmos" present any controversial theories?

Yes, the book explores the controversial theory of string theory, which suggests that all matter and energy in the universe is made up of tiny strings vibrating at different frequencies. This theory is still being researched and debated in the scientific community.

## 5. Can "The Fabric of the Cosmos" change my understanding of the universe?

Yes, "The Fabric of the Cosmos" presents cutting-edge ideas and theories that challenge our traditional understanding of the universe. It may inspire readers to think differently about the nature of reality and our place in the cosmos.