# What is Relativity of simultaneity: Definition and 105 Discussions

In physics, the relativity of simultaneity is the concept that distant simultaneity – whether two spatially separated events occur at the same time – is not absolute, but depends on the observer's reference frame.

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1. ### I Why doesn't relativity of simultaneity lead to a contradiction?

I cannot get the following out of my head. Suppose this situation. Three frames, with varying velocities, simultaneously intersect their origins at the same time and place, making this point and time x0=0 and t0=0. These frames... let's call them observers. These observers have an agreed upon...
2. ### B A question about relativity of simultaneity

Hello. I'm new here and very much afraid of breaking rules. I would gladly post this question in the Homework section, because it's homework, but my question doesn't fit the template, it's a theory question. I hoped to find it in Relativity FAQ's, but it's not there. I can tell you I grasped...
3. ### I Understanding Simultaneity Through Male & Female Clocks Meeting

This is not a homework question, just a scenario I've come up with. Imagine I have a male and a female clock moving towards each other. If they're in sync, one will fit inside the other and they'll continue on their way. If not, they'll collide. (Apologies for the crude drawings.) I place these...
4. ### Can the number 1 be squared infinitely many times?

When calculating relativity of simultaneity I am confused by one thing. How is the math = to the answer below? ##\left( \frac {LC+LV } {2C} \right) - \left( \frac {LC -LV} {2C} \right) = \left( \frac {LV} {C^2} \right) ## The part that throwing me off is ## C^2 ## . I assume mathematically...
5. ### I am a little confused by the quoted text is worth mentioning.

Hi I am reading a book and I am confused why some text is mentioned. Could someone help explain. https://scholar.harvard.edu/files/david-morin/files/relativity_chap_1.pdf page 14 " There is always a difference between the time that an event happens and the time that someone sees the event...
6. ### B Is the relativity of simultaneity more than just a fudge factor?

<Moderator’s note: a reference to an article published in a predatory journal has been removed. References on PF should be from the professional scientific literature or from other sources consistent with the professional scientific literature> I read through many posting on various threads in...
7. ### Relativity and Simultaneity: A Cosmic Perspective

Posed question is that a comment plunges into Jupiter. Same time as my physics class starts on Earth. My friend is traveling towards Jupiter at a significant percentage of c. Would the person in spaceship say that the comment collision occurred before, after or as class started? My answer: same...
8. ### B No way to catch up with galaxies currently receding at >c?

Consider a far-away galaxy that is considered to be currently receding from Earth at 2x the speed of light. (With this 2x c recession velocity, we are speaking of the Vnow, the imputed relative velocity of the galaxy compared to Earth now, not the velocity of the galaxy relative to Earth at the...
9. ### I Relativity of simultaneity doubt

(excuse me for my english, but I'm studying physics and I am not a native English speaker) One observer OE, is on the ground, we take him as the fixed frame of reference. The other OT is on the train that is moving relatively to the OE at a costant velocity ( they are both inertial frame of...
10. ### I Relativity of Simultaneity Questions

Hi All, I've been doing some reading on the above but having some problems understanding certain parts of it (maybe it's wrong from Wikipedia!) To simplify it for me I will first pose a simple scenario where we are not factoring in speed yet, then go from there. Say we have two planets A and B...
11. ### B Doubt about the relativity of simultaneity

(I hope my English is understood). Hello, I have a question regarding the relativity of simultaneity. The mean temperature of the cosmic microwave background tells us how big the universe is with respect to the recombination epoch. If I now measure that the mean temperature of the cosmic...
12. ### I How Does Relativity of Simultaneity Clash w/Thermodynamics?

In special relativity, observers can disagree on the order of events - if Alice thinks events A, B and C are simultaneous, Bob can think A happened before B which happened before C, and Carlos thinks C happened before B which happened before A - provided A, B and C are not causally connected, of...
13. ### B Calculating Relativity of Simultaneity

The question is in reference to calculating relativity of simultaneity. I am on the step where I take the time in Alice's frame from the front and from the rear clock and minus it to the get the total time. I end up with gamma squared etc (For more details see the picture below) I have...
14. ### B Bell's theorem, QFT, and the Relativity of Simultaneity

I've been slowly grinding away with what I can about quantum mechanics and QFT. I'm not sure how far I've gotten but I've come up against a bit of a roadblock concerning how the relativity of simultaneity applies in QFT with specific reference to the outcome of Bell tests. My misunderstanding...
15. ### I Relativity of Simultaneity: What Einstein's Contribution Was

Something that I consider very silly, yet I try to solve relates to the relativity of simultaneity. According to SR , two evevnts taking place in different positions along the line of relative motion of two inertial observers are not simultaneous in both frames. Now, I wanted to see how this...
16. ### B Refined Interpretation of Relativity of Simultaneity?

I have an image of the block universe that is probably similar to how many people view it, as a 4D non dynamical construct where the "present" is a slice of this block and this slice moves forward from the past to the future. Furthermore, depending on your relative velocity, your slice may be...
17. ### B Quantum entanglement and special relativity

There are a pair of entangled particles moving in opposite directions. A measurement is done on particle A, the wavefunction collapses randomly, you observe either spin up or spin down, A does an action at a distance on B, particle B instantly collapses to the opposite spin state, a measurement...
18. ### Special Relativity - Relativity of Simultaneity?

Homework Statement A thin rod of proper length 4a is traveling along the x-axis of a frame S with a speed ##{\frac {\sqrt 3} 2}c## in the positive x-direction. A hollow cylinder CD of proper length 2a is placed with its axis along the x-axis, so that when the ends of the cylinder are open the...
19. ### I Tangible example of relativity of simultaneity

I always liked the idea of starting relativity discussions with the idea that your father could travel at a substantial fraction of the speed of light and return back at roughly the same age as you. This makes time dilation seem interesting, and drives curiosity in students. But I'm struggling...
20. ### B Resolving the Relativity of Simultaneity: A Geometric Approach

Hi, I read through einsteins popular book on relativity translated into english around 1922 and subsequently read the original 1905 paper on the electrodynamics of moving bodies. So we have a fixed observer seeing flashes happening at the same time and we have a moving observer seeing one...
21. ### I Relativity of Simultaneity: Analyzing Wavelengths

Am i right in thinking that relativistic simultaneity explanations tend not to mention the fact that one of the ‘simultaneous’ events may well be red shifted and the other blue shifted and by analysing wavelengths you could presumably work out whether the event was in reality simultanous or not.
22. ### I Exploring Simultaneity in Spacetime Diagrams - A.P. French, 1968

The images have been taken from page no.74,75 and 76, special relativity , A.P.French,1968 I understood that A1' and C1' are not simultaneous in S -frame. But I don't understand the principle on whose basis it is claimed that A1' and C1' are simultaneous in S' -frame. Here, the simultaneity is...
23. ### B Relativity of Simultaneity: Proving Lightning Strikes A & B Same Time?

My question is based around the popular thought experiment regarding Einstein's relativity of simultaneity. That is, the one regarding two lightning strikes and two observers. Observer 1 is stationary relative to the ground, and is located equidistant between lightning strikes A and B. Observer...
24. ### I Confusion with relativity of simultaneity

I know variations of these have probably been asked numerous times before, but I'm having trouble with this specific scenario. Imagine the classic Train Paradox, except instead of lighting strikes we have an observer at the centre of the train shooting laser pulses towards the rear (Event E1)...
25. ### I Velocity Addition & Relativity of Simultaneity

I have some issues understanding the following thought experiment: Suppose you are standing still, and two balls are moving towards you from opposite direction. From your own reference frame, Ball A is ##10^5## m away from you, moving towards you from the left with speed ##0.8c##, and Ball B is...
26. ### I Relativity of simultaneity, time dilation misunderstanding

Hi everybody, I'm new on this forum so I apologize in advance if I don't respect some formalities (and sorry for my English). It is known that in the perspective of the stationary observer the events in the back and in the front of the moving spaceship are not simultaneous. If I understand well...
27. ### Relativity of Simultaneity and lightening

Homework Statement You are standing at x = 9.0 km and your assistant is standing at x = 3.0 km. Lightning bolt 1 strikes at x = 0 and lighting Bolt #2 strikes at x = 12.0 km. You see the flash from Bolt #2 at t = 10 μs and the flash from Bolt #1 at t = 50 μs. According to your assistant, were...
28. ### I Relativity of Simultaneity and frames

Hello everyone, sorry if this is a noob question; I'm just starting out with special relativity. I was wondering whether relativity of simultaneity is a direct consequence of our ability to "know" being dependent on sight (light reaching a point). If, for example, we could only judge an event...
29. ### I Relativity of simultaneity -- an example in D.Morin's book

Ok, so here's an example from David Morin's book that I seriously don't understand: Two clocks are positioned at the ends of a train of length L (as measured in its own frame). They are synchronized in the train frame. The train travels past you at speed v. It turns out that if you observe the...
30. ### B Einstein's relativity of simultaneity & quantum paradox.

Einstein's relativity of simultaneity & quantum measurement paradox. Suppose a rocket traveling close to the velocity of light which emits a single photon from its midpoint at point A, illustrated below. The rocket is equipped with a single detector drawn in green at the front of the rocket...
31. ### I Why can't you use Simultaneity when doing Length Contraction

Suppose that frame O' moves at speed v = 0.6c relative to frame O. A rod with two balls is attached to its ends is 10 meters long in its rest frame, O'. Length contraction will tell you that in frame O, the rod is 8 meters long. But aren't the two balls at the ends "events"? They are clearly...
32. ### The Classic Pole in Barn Relativity Question

Homework Statement A pole-vaulter holds a 5.0 m pole. A barn has doors at both ends, 3.0 m apart. The pole-vaulter on the outside of the barn begins running toward one of the open doors, holding the pole level in the direction he is running. When passing through the barn, the pole just fits...
33. ### Relativistic Time Dilation and Length Contraction in Overtaking Rockets

Homework Statement Two rockets are each 1000m long in their rest frames. Rocket A, traveling at 0.800c relative to the earth, is overtaking Rocket B, which is moving in the same direction at 0.600c. (i) According to the crew on B, how long does A take to completely pass? I.e. how long is it...
34. ### I Relativity of Simultaneity: An Overview

I recently started studying Special Relativity an my book discusses the following: Say I have synchronized two separated clocks in a reference frame S, if then an observer in another reference frame S' for whom the clocks are moving sees the clocks he would say those clocks are out of...
35. ### B Relativity of Simultaneity - Missing Event

Obviously I have to learn how to start a topic, but I believed that in a scientific forum, what matters is the idea, not the presentation. I'll try harder this time. Since every scientist knows very well the ladder paradox, I believe I won't be wrong, by missing something in the set. --- A...
36. ### B How Does Relativity Deal with Simultaneity as a Requirement?

OK, guys and galls, I'm back I admit that the ladder paradox topic that I started was put quite stupidly, and I wouldn't have the chance to make my point, even if Peter didn't lock it. But... we had quite a long conversation in private with him, I learned quite few things, and I believe I made...
37. ### B Relativity of simultaneity and the balance of the systems

Hi everybody, Glad to be with you, in the most popular physics forums on the Internet. First of all, I'm not a physicist and the problem I hope to get the answer from you, is part of the work I'm writing in another field. To cut it short, I don't quite understand how the relativity of...
38. ### Understanding Relativity of Simultaneity

I am having a bit of trouble with understanding a basic point re the concept of relativity of simultaneity. I get the point of observers seeing events at different times, due to the passage of time of light traveling to them from the event. But I don't understand why this precludes events being...
39. ### Relativity of Simultaneity and Time

Hey guys, i am not a science guy by any means so any answer written for the layman would be much appreciated. i may not be understanding this right but here it goes: In einstein's relativity of simultaneity we talk about how events A,B, and C are simultaneous for one observer (lets call him...

41. ### I Black holes and the relativity of simultaneity

Over the years I've watched Science try to deal with the Information Paradox regarding black holes. http://news.sciencemag.org/physics/2015/12/physicists-figure-out-how-retrieve-information-black-hole I've always been curious how we got to the point where we see this as a problem in need of a...
42. ### Relativity of simultaneity, understood, now not so sure.

Hello PF. I have no problem with the embankment observer. He sees the flashes as simultaneous being equidistant. Likewise the train observer who sees the flashes as separate given he is closer to one flash. What I don't follow is what is the significance of this? Thanks.
43. ### Question about relativity of simultaneity

In the train example that supposedly shows the relativity of simultaneity, why, in the frame of the observer on the ground, the "midpoint" of the train in the train's frame is still the midpoint? Does it have anything to do with Lorentz contraction? If so, there might be some circular reasoning...
44. ### What is the relativity of simultaneity measuring?

Hello PF. Prior to the train/embankment thought experiment, Einstein argues that in order to determine if events are simultaneous, an equidistant observer is required. The embankment observer is equidistant, and when the light from the two flashes reaches him he sees them as simultaneous. The...
45. ### Doubt in Relativity of Simultaneity

Hey everyone, I have this doubt for quite some time now. So could somebody please help me and explain where I am going wrong with this. According to the relativity of simultaneity, it is impossible to say in an absolute sense whether two distinct events occur at the same time if those events...
46. ### Understanding the Relativity of Simultaneity in Special Relativity

Good morning, Yesterday I was reading a book about special relativity, It focused a lot about consequences of the theory, but there's only one thing I couldn't understand, which is as the title suggests, Relativity of simultaneity, that two different observers can't agree if two event are...
47. ### Motion vs Rest: How Does Relativistic Time Differ?

There are two separate clocks, each set in a plane and on the ground. Assuming inertial reference frames, how can this be? Perspective of the observer in motion: The observer in motion on the plane will have recorded some time duration. Since the the world outside the plane is moving at a...
48. ### Clock synchronization and relativity of simultaneity

I addressed already recently in this thread the issue of defining the synchronicity of clocks moving relatively to each other (considering that the synchronization by Einstein's method implies clocks at rest), but it occurred to me now that even for clocks at rest relatively to each other there...
49. ### Symmetrical time dilation implies relativity of simultaneity

This is something I've been meaning to write for a while, I finally got the time to do it, though the topic isn't currently "hot". I'm sure it will pop up again, though. Imagine we have two observers, moving at a constant velocity relative to each other far away from any objects that might...
50. ### Interpretation of Relativity of Simultaneity

If relative simultaneity implies that one event that could be already measured w.r.t. one reference frame not yet measured w.r.t. another reference frame: then could one find an reference frame at which none of the events were measured? If such reference frame exists, could one observe the...