# Speed of Light question **URL:** \ **Category:** Factual Questions\ **Created:** [July 12, 2025, 1:07pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771 "2025-07-12T13:07:06Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [July 13, 2025, 8:19pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/21 "2025-07-13T20:19:26Z")
The discussion about two observers misses the point of the OP I believe. I interpreted the question as one observer as in #1 I send light to a reflector 30 light-seconds away. It is impossible for me to see it before 1 minute has elapsed. #2 I send light to a reflector 30 light-seconds away. It is impossible for me to see it before I sent it. Are these impossible for the same reason? And my question #3 I send out a tachyon burst. Will that result in #1 or #2? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [July 13, 2025, 9:27pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/22 "2025-07-13T21:27:25Z")
> [@Biffster](#): > > Actually it would take about four seconds, because it’s two light-seconds going in one direction. It’s also not something that grade-school students can measure. Even laser beams spread out some, enough that over those distances, you need to start with a really big laser and then use a very good telescope to see the reflection. But these are irrelevant details to this thread. ---
**Author:** ![LSLGuy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lslguy/32/5813_2.png) [@LSLGuy](https://boards.straightdope.com/u/LSLGuy)\ **Post date:** [July 13, 2025, 9:38pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/23 "2025-07-13T21:38:56Z")
Aiming laser beams at the moon has a long and storied history of going badly awry: **[Laser Pointer - What If - xkcd](https://what-if.xkcd.com/13/)**. ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [July 13, 2025, 10:05pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/24 "2025-07-13T22:05:06Z")
For the tachyon case, every observer traveling slower than light will see 1) or 2). In special relativity, the combined distance (in the Minkowski metric) makes it so that any two events (an event is “something that happens at a particular time and place”), like "X emits a particle at point a and time a’ " or "Y detects the particle at point b and time b’ " can stand in three possible relationships: 1) the two events can have a light-like separation (light emitted at point a at time a’ would get to point b at time b’ (or vice versa)), 2) the two events can have a time-like separation (light emitted at point a at time a’ arrives at at point b _before_ time b’, or 3) the two events can have a space-like separation (light emitted at point a at time a’ arrives at point b _after_ time b’). **And according to special relativity, all observers traveling slower than light will classify the relationship between the two events the same way**. So 1) and 2) are really the same - in either case, a signal is getting between two space-like separated events - which is highly unusual ---
**Author:** ![hogarth](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hogarth/32/1773_2.png) [@hogarth](https://boards.straightdope.com/u/hogarth)\ **Post date:** [July 13, 2025, 11:19pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/25 "2025-07-13T23:19:12Z")
> [@markn\_1](#): > > Both scenarios are equally impossible because they are essentially the same. If a signal could be sent faster than light, some observers (although not the sender) would see the signal received before it was sent. Yes, I don’t remember much from my Special Relativity class in college, but I do remember one thing: _simultaneity is relative_. ---
**Author:** ![Saint\_Cad](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/saint_cad/32/18907_2.png) [@Saint\_Cad](https://boards.straightdope.com/u/Saint_Cad)\ **Post date:** [July 13, 2025, 11:21pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/26 "2025-07-13T23:21:00Z")
> [@Andy\_L](#): > > For the tachyon case, every observer traveling slower than light will see 1) or 2). Again, not talking about **any** observer. Talking about the observer at rest relative to the tachyons at the source. Would that be #1 or #2? ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [July 13, 2025, 11:31pm UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/27 "2025-07-13T23:31:55Z")
Sorry about not answering before. I think #1 is the answer. You see the particles come back remarkably quickly, but not before you sent them. But it’s possible I’m missing something. ---
**Author:** ![The\_Other\_Waldo\_Pepper](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/the_other_waldo_pepper/32/12370_2.png) [@The\_Other\_Waldo\_Pepper](https://boards.straightdope.com/u/The_Other_Waldo_Pepper)\ **Post date:** [July 14, 2025, 12:04am UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/28 "2025-07-14T00:04:37Z")
> [@Andy\_L](#): > > But it’s possible I’m missing something. Speaking as the OP, I’m _definitely_ missing something: wouldn’t it be a tachyon if the ‘observer at the source’ hypothetically beams something at the moon that’s a _little_ faster than light, and so then sees the reflection a mere two seconds later or one second later instead of roughly two-and-a-half seconds later? And wouldn’t it _also_ be a tachyon — which is to say, wouldn’t it _equally_ be a tachyon — in a wacky hypothetical where it’s moving so fast that said observer sees the reflection one or two seconds before he beams it at the moon? ---
**Author:** ![Andy\_L](https://avatars.discourse-cdn.com/v4/letter/a/c67d28/32.png) [@Andy\_L](https://boards.straightdope.com/u/Andy_L)\ **Post date:** [July 14, 2025, 12:15am UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/29 "2025-07-14T00:15:28Z")
I think (but I’m not sure) that for this case, even an infinite speed only gets a zero delay, not a negative one, because the two points in space in question are the same spot in the observer’s frame. ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [July 14, 2025, 12:34am UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/30 "2025-07-14T00:34:00Z")
> [@hogarth](#): > > _simultaneity is relative_ Mostly. If two things happen in the same place and at the same time, then all observers will agree about this. Though that can really only happen if it’s actually the same event. If two events have _timelike_ separation–that is, they’re close enough that one can influence the other (B happens one second after A, but is less than 1 light-second away), then all observers will agree about which event happened _first_, though they won’t agree on the duration or distance. ---
**Author:** ![FinsToTheLeft](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/finstotheleft/32/2930_2.png) [@FinsToTheLeft](https://boards.straightdope.com/u/FinsToTheLeft)\ **Post date:** [July 14, 2025, 12:50am UTC](https://boards.straightdope.com/t/speed-of-light-question/1020771/31 "2025-07-14T00:50:58Z")
> [@Francis\_Vaughan](#): > > Use of [Thiotimoline](https://archive.org/details/Astounding_v41n01_1948-03_UnkSc-cape1736/page/n119/mode/2up?view=theater) has been suggested as a way of creating a cascading _telechronic battery_ that could create causality violations of even days in length This was exactly what I thought of when I read the OP. [Previous page](https://boards.straightdope.com/t/speed-of-light-question/1020771.md?page=1)