Timeline for How can two events which are not causally related be perceived to be simultaneous by an moving observer?
Current License: CC BY-SA 4.0
14 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Mar 9, 2025 at 18:13 | comment | added | Amit | @SyntaxJunkie Also to defend Feynman just a little bit (not that he needs it), note that in the linked lecture he makes some preceding remarks before giving that explanation, which essentially may be what you find lacking from my answer. If you or anyone else are interested, the relevant part starts at about 16:55 in the above link. | |
| Mar 9, 2025 at 18:05 | comment | added | Amit | @SyntaxJunkie Well, it is implied by the setup (but I agree perhaps not emphasized enough), that we don't care how the spaceship is moving relative to space. So the motion of the spaceship does not add to the velocity of the "left flash" nor does it subtract from the velocity of the "right flash". Secondly I want to generally remark that the fact that the speed of light is finite (apart from constant) actually does play a central part in relativity. Here, the OP asked for an intuitive explanation, so I didn't get into how that is tied up with synchronization of clocks, etc... | |
| Mar 9, 2025 at 17:59 | comment | added | Syntax Junkie | @Amit. I agree with you, but the Feynman example doesn't seem to depend on the constant speed of light to explain anything. When the person isn't at the center of the ship, the Feynman example merely has to do with light traveling a smaller distance on one side of the ship (from the closer flashlight to the person) and a longer distance on the other side of the ship (from the farther flashlight to the person). The person, knowing the constant speed of light and her position, could easily calculate the true time (in her reference frame) and deduce the events were simultaneous. | |
| Mar 9, 2025 at 17:49 | comment | added | Amit | @SyntaxJunkie If this was sound or any other kind of subluminal signal, it wouldn't have the same velocity in all reference frames, I agree that's important to emphasize. If the ship was moving through air, then sound may generally propagate in different speeds in different directions, even if the observer stays right in the middle. So to know about when the sound signals really were emitted, we would have to station separate observers at each end, but since the speed of light is universal, it is good enough to have a single observer in the middle. | |
| Mar 9, 2025 at 17:49 | comment | added | Syntax Junkie | Feynman is such a revered teacher that I'm worried to bring this up, but here goes. Examples like this one have never sat well with me. They don't seem to explain anything about relativity. Isn't this just a speed of light issue that would happen in a Galilean reference frame? Or if we were talking speed of sound instead of speed off light? If you want to know the true time of occurrence of an event (in your reference frame) you always need to account for the amount of time it takes the signal to reach you. This example doesn't seem to elucidate any relativistic principles. | |
| Mar 9, 2025 at 5:25 | vote | accept | Soham Pine Std 9 A Roll no 31 | ||
| Mar 8, 2025 at 18:45 | comment | added | Soham Pine Std 9 A Roll no 31 | I am trying but it is not allowing me to, says I have low reputation, I am new maybe that's why. | |
| Mar 8, 2025 at 18:44 | comment | added | Amit | @SohamPineStd9ARollno31 consider accepting & upvoting if you've found this answer helpful, cheers. | |
| Mar 8, 2025 at 18:43 | comment | added | Soham Pine Std 9 A Roll no 31 | Brother you are the best, thanks a lot. Are you from India by any chance, because your name seems Indian. | |
| Mar 8, 2025 at 18:31 | comment | added | Amit | @SohamPineStd9ARollno31 The $x'$ axis in what you're mentioning is a hyperplane of simultaneity, meaning that lines parallel to it define what is "now" for the inertial observer, they are lines of constant $t'$. Just like in my answer above, when the observer started moving left/right, his "now" evidently changed relative to the spaceship (recall, relative to the spaceship the lights were emitted at the same time, but not for the moving observer), so his $x'$ is tilted if we choose to draw the spaceship's axes as the "stationary frame" (though there is no preferred stationary frame). | |
| Mar 8, 2025 at 18:28 | comment | added | Soham Pine Std 9 A Roll no 31 | Brother this "aha" moment was gorgeous, thanks a lot, but one more slightly unrelated question, can you give me an intuitive reason for why the x'-axis seems to be tilted at an angle from the x-axis, most explanations include that a light ray's world line has to be symmetric and at a 45 degree angle but it doesn't feel intuitive enough, like the ct or time axis being tilted feels perfectly valid as it represents the world line of the moving observer because it is always at t'=0, but the x' axis being tilted seems to be a little weird. | |
| Mar 8, 2025 at 18:20 | comment | added | Amit | @SohamPineStd9ARollno31 imagine instead of one of the flashlights we had a mirror. Can you perceive the mirror to be reflecting the light coming from the flashlight, before the flashlight emitted this light, by moving to some other inertial frame? | |
| Mar 8, 2025 at 18:18 | comment | added | Soham Pine Std 9 A Roll no 31 | It is great point but why only non causally related events, why can't two causally related events be perceived in a similar fashion. | |
| Mar 8, 2025 at 18:09 | history | answered | Amit | CC BY-SA 4.0 |