# Understanding Spacetime Diagrams from "Reality is not What it Seems

• B
• fee_de
In summary, Rovelli is explaining the concept of simultaneity in terms of a train journey, and how the observer can determine when two events have taken place. He argues that the observer uses a convention, which is relative to the motion of the observer, to determine when two events are simultaneous.
fee_de
TL;DR Summary
I don't understand a spacetime diagram about relativty and simultaneity that I found in a book and its explaination by the author
I'm reading a book called "Reality is not what it seems: the journey to quantum gravity" by Carlo Rovelli and I'm struggling to understand this diagram that is part of the chapter about special relativity.

https://i.stack.imgur.com/JgBDJ.jpg

Before this image Rovelli writes:

"It's not possible to have a satisfying conversation from Earth to Mars. If I'm on Mars and you are here, I ask you a question, you answer as soon as you hear what I told you and I receive your answer 15 minutes after I asked you the question. This quarter of an hour of mine is time that is neither past nor future as compared to the moment when you answered. This implies that you can't say that an event on Mars is happening "now" because "now" does not exist".

I understand this concept, but then the author adds a footnote:

"The astute reader will object that the moment in the middle of my quarter of an hour can be considered simultaneus to your answer. The reader who studied physics may recognize that this is the "Einstein convention" to define simultaneity. However, this definition depends on the way I move, so it doesn't define simultaneity directly between two events, but only a simultaneity that is relative to the motion of specific bodies. In picture 3.3 (the diagram I attached), a dot is halfway between a and b, the points where I exit the past of the observer and I enter his future. the other dot is halfway between c and d, the points where I exit the past of the observer and I enter his future if I follow a different path. Both the dots are simultaneous to the reader, following this definition of simultaneity; however, they happen one after the other. The two dots are both simultaneous to the reader, but relatively to two different motions."

(I'm reading this book in Italian so I had to translate the paragraph, I'm sorry if it's not faultless).

First of all I don't understand what it means that "the moment in the middle of my quarter of an hour can be considered simultaneus to your answer". I tried to draw diagrams, I'm really struggling but I can't grab the concept. Talking about the diagram, why can we say that the observer perceives the two dots as simultaneous? They don't exit his past and enter his future at the same time, do they? In these diagrams souldn't we find simultaneous events on the same horizontal line?

I'm completely stuck, I would be very grateful if you could help me.

[Mentor Note -- Image attached to post from the OP's link]

Last edited by a moderator:
Delta2 and PeroK
fee_de said:
First of all I don't understand what it means that "the moment in the middle of my quarter of an hour can be considered simultaneus to your answer".
This just states that, in our everyday experience, if we know something takes a given time to go to some destination and back again, then it must have arrived at the destination after half this time.
Think of something more tangible than a beam of light - like a train. I write a letter on day 1, give it to the conductor, and wait for your answer. The conductor says the train will be back in two weeks, so I can conclude that you will receive the letter and write your answer after one week. If I'm writing a week before your birthday, I could put a 'happy birthday' in the letter, and be certain that you'll read it on that day.
I.e. I can assume that the moment of your answer will happen after half the round trip time.

It's same here. A beam of light is the equivalent of the letter traveling on the train. You take the half-way moment between emission and reception to conclude when the answer was composed and sent.
It takes 15 mins for a round trip between Mars and Earth in this example, so it should take half of that until you get to answer, right?

fee_de said:
Talking about the diagram, why can we say that the observer perceives the two dots as simultaneous? They don't exit his past and enter his future at the same time, do they? In these diagrams souldn't we find simultaneous events on the same horizontal line?
The observer ('the reader') has no way of knowing whether any distant events happen on the horizontal line or not. He has to exchange signals and try to use some convention for deducing when that thing happened. On Earth, pretty much everything is relatively stationary (any arrow drawn on the diagram would be vertical), which let's us use the convention described above - every event half-way between reception and emission does happen on the same horizontal line as the reader's position.
The argument here is that if we tried using the same convention when the communicating objects are not nailed to the ground, we'd arrive at different answers depending on how the objects move. Which makes the aforementioned convention pretty useless.

Just to clarify what's on the diagram:
The sender ('Me') is traveling along one of the two arrows relative to 'the reader' (from a to b in one case, and from c to d in the other case), while the reader is considered stationary. The movement is shown in space-time, so in spatial-only terms the sender is moving away from the reader in one case (a-b), with some speed, and towards the reader (c-d) with a different speed.
The long lines show the path of the signal from emission (points a and c) to the reader, who responds and sends the signal back to sender to be received where and when the line intersects the arrow again.

In the first case the sender sends the signal at point (in space-time it's called an 'event') a. In the second case, it's at event c.
The signal travels to the reader along the line shown. It is reflected back to the sender, and is received by him at event b or d, respectively.
The key is, the differently angled arrows intersect with the path of the signal in such a way, that the half-way point between emission and reception (the middle of the a-b and the c-d lines) is in different places. These are those dots marked on the arrows - in particular, as you notice, they're not on the same horizontal line (not the same time coordinate).
So if you were to try and apply the same convention for assuming when the answer was composed as was tried before, by halving the signal travel time, you'll arrive at different answers depending on the relative motions between the sender and the reader.

PeroK and fee_de
Bandersnatch said:
This just states that, in our everyday experience, if we know something takes a given time to go to some destination and back again, then it must have arrived at the destination after half this time.
Think of something more tangible than a beam of light - like a train. I write a letter on day 1, give it to the conductor, and wait for your answer. The conductor says the train will be back in two weeks, so I can conclude that you will receive the letter and write your answer after one week. If I'm writing a week before your birthday, I could put a 'happy birthday' in the letter, and be certain that you'll read it on that day.
I.e. I can assume that the moment of your answer will happen after half the round trip time.

It's same here. A beam of light is the equivalent of the letter traveling on the train. You take the half-way moment between emission and reception to conclude when the answer was composed and sent.
It takes 15 mins for a round trip between Mars and Earth in this example, so it should take half of that until you get to answer, right?The observer ('the reader') has no way of knowing whether any distant events happen on the horizontal line or not. He has to exchange signals and try to use some convention for deducing when that thing happened. On Earth, pretty much everything is relatively stationary (any arrow drawn on the diagram would be vertical), which let's us use the convention described above - every event half-way between reception and emission does happen on the same horizontal line as the reader's position.
The argument here is that if we tried using the same convention when the communicating objects are not nailed to the ground, we'd arrive at different answers depending on how the objects move. Which makes the aforementioned convention pretty useless.

Just to clarify what's on the diagram:
The sender ('Me') is traveling along one of the two arrows relative to 'the reader' (from a to b in one case, and from c to d in the other case), while the reader is considered stationary. The movement is shown in space-time, so in spatial-only terms the sender is moving away from the reader in one case (a-b), with some speed, and towards the reader (c-d) with a different speed.
The long lines show the path of the signal from emission (points a and c) to the reader, who responds and sends the signal back to sender to be received where and when the line intersects the arrow again.

In the first case the sender sends the signal at point (in space-time it's called an 'event') a. In the second case, it's at event c.
The signal travels to the reader along the line shown. It is reflected back to the sender, and is received by him at event b or d, respectively.
The key is, the differently angled arrows intersect with the path of the signal in such a way, that the half-way point between emission and reception (the middle of the a-b and the c-d lines) is in different places. These are those dots marked on the arrows - in particular, as you notice, they're not on the same horizontal line (not the same time coordinate).
So if you were to try and apply the same convention for assuming when the answer was composed as was tried before, by halving the signal travel time, you'll arrive at different answers depending on the relative motions between the sender and the reader.
Thank you for your explanation, it was really clear!

fee_de said:
Summary:: I don't understand a spacetime diagram about relativity and simultaneity that I found in a book and its explanation by the author

I'm reading a book called "Reality is not what it seems: the journey to quantum gravity" by Carlo Rovelli and I'm struggling to understand this diagram that is part of the chapter about special relativity.

https://i.stack.imgur.com/JgBDJ.jpg

Before this image Rovelli writes:

"It's not possible to have a satisfying conversation from Earth to Mars. If I'm on Mars and you are here, I ask you a question, you answer as soon as you hear what I told you and I receive your answer 15 minutes after I asked you the question. This quarter of an hour of mine is time that is neither past nor future as compared to the moment when you answered. This implies that you can't say that an event on Mars is happening "now" because "now" does not exist".

I understand this concept, but then the author adds a footnote:

"The astute reader will object that the moment in the middle of my quarter of an hour can be considered simultaneus to your answer. The reader who studied physics may recognize that this is the "Einstein convention" to define simultaneity. However, this definition depends on the way I move, so it doesn't define simultaneity directly between two events, but only a simultaneity that is relative to the motion of specific bodies. In picture 3.3 (the diagram I attached), a dot is halfway between a and b, the points where I exit the past of the observer and I enter his future. the other dot is halfway between c and d, the points where I exit the past of the observer and I enter his future if I follow a different path. Both the dots are simultaneous to the reader, following this definition of simultaneity; however, they happen one after the other. The two dots are both simultaneous to the reader, but relatively to two different motions."

(I'm reading this book in Italian so I had to translate the paragraph, I'm sorry if it's not faultless).

First of all I don't understand what it means that "the moment in the middle of my quarter of an hour can be considered simultaneus to your answer". I tried to draw diagrams, I'm really struggling but I can't grab the concept. Talking about the diagram, why can we say that the observer perceives the two dots as simultaneous? They don't exit his past and enter his future at the same time, do they? In these diagrams souldn't we find simultaneous events on the same horizontal line?

I'm completely stuck, I would be very grateful if you could help me.
That all seems very confusing to me (and I'm not sure it's caused by poor translation). I don't like complicated wordy descriptions like, as I find them hard to follow.

It may be that Rovelli wants to say more than just that simultaneity is relative, but I've seen much simpler presentations of the basic idea.

fee_de and vanhees71
Also a real Minowski diagram may be much clearer. Looking at Rovelli's picture alone, I don't understand it too.

PeroK said:
That all seems very confusing to me (and I'm not sure it's caused by poor translation). I don't like complicated wordy descriptions like, as I find them hard to follow.

It may be that Rovelli wants to say more than just that simultaneity is relative, but I've seen much simpler presentations of the basic idea.
Honeslty I think you are right. I generally like him as a science writer but in this book he made general relativity easier to understand than special relativity ^^"

vanhees71 and PeroK
fee_de said:
Honeslty I think you are right. I generally like him as a science writer but in this book he made general relativity easier to understand than special relativity ^^"
I think there are two aspects to the relativity of simultaneity. The first is the (relatively simple) kinematic calculations to show that universal simultaneity and the invariance of light speed are incompatible.

The second, which is perhaps more difficult to grasp, is that there is no global "now". If we are watching a live event on TV which is happening on the other side of the world, then we are able to imagine that happening "now" and our concept of "now" across the planet is well established. So, it's difficult to realize that that idea does not extend to all circumstances. What does "now" in the Andromeda galaxy, which is 2 million light years away, really mean? Even without relativity, you can ask what "now" at such great distances might mean.

And, of course, once you have relative motion the whole idea of a global, shared simultaneity vanishes. Even if you establish a convention for now on Earth and now in Andromeda, that doesn't extend to reference frames where the Earth is moving.

## What are spacetime diagrams?

Spacetime diagrams are graphical representations of the relationship between space and time. They show how objects move and interact in a specific reference frame.

## Why are spacetime diagrams important?

Spacetime diagrams are important because they help us understand the fundamental principles of relativity and how the universe works. They also allow us to visualize complex concepts and make predictions about the behavior of objects in space and time.

## What is the significance of the light cone in spacetime diagrams?

The light cone in spacetime diagrams represents the maximum distance that light can travel in a given amount of time. It is used to define the boundaries of causally connected events and plays a crucial role in understanding the concept of causality in relativity.

## How do spacetime diagrams relate to Einstein's theory of relativity?

Spacetime diagrams are a visual representation of Einstein's theory of relativity, which states that the laws of physics are the same for all observers in uniform motion. They illustrate the effects of time dilation and length contraction, which are key principles of relativity.

## Can spacetime diagrams be used to explain other phenomena in physics?

Yes, spacetime diagrams can be used to explain a wide range of phenomena in physics, including gravitational waves, black holes, and the expansion of the universe. They are a powerful tool for understanding the complex interactions between space and time in our universe.

• Special and General Relativity
Replies
6
Views
1K
• Special and General Relativity
Replies
14
Views
819
• Special and General Relativity
Replies
37
Views
5K
• Special and General Relativity
Replies
1
Views
2K
• Special and General Relativity
Replies
8
Views
2K
• Special and General Relativity
Replies
20
Views
854
• Special and General Relativity
Replies
51
Views
2K
• Special and General Relativity
Replies
29
Views
516
• Introductory Physics Homework Help
Replies
8
Views
944
• Special and General Relativity
Replies
16
Views
746