12 ms·

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we

Something I don't understand: when they say "X is 1 minute from happening", does that mean it's really 1 minute from happening or does that mean "in 1 minute we'll receive the signal that X has happened"? Maybe my question makes more sense in the case of "X is happening right now", because then I should either understand "we infer that X should have happened right about now" or "we have confirmed via signal that X has happened", and that's a big big difference. I know in some cases they explicitly say the latter, so I guess my real real question is, do they just keep the communication delay implied in all countdowns & references in discussion, to avoid confusion? (ETA: No need to let me know about simultaneity problems in relativity — earth and mars are, relative to c and to macroscopic time scales, essentially not moving relative to each other AFAIK, so that simultaneity is essentially well-defined. My question was about a much more boring classical-universe problem.)

Sharlin 6y ago
Yes, those are all Earth Receive Time, that is, when they were saying that eg. entry interface is two minutes away, in reality the rover was already sitting on the surface and we were just waiting for the radio signal to get here.
- unanswered 6y ago
  > Earth Receive Time Ah great, that's a great phrase to make everything clear and provide a kind of "frame of reference" to think & communicate in. Always need these abstractions.
- m4rtink 6y ago
  The local Czech stream (20k viewers!) I watched went over the events in real time and then commented how events were happening as signals were received on Earth - with the final confirmation of successful landing coming first via Twitter, no less! :) Still a very nice yet nerve wrecking idea to do it like this - you know the lander is on Mars now. But is it safely on the ground or is there a third Shapirelli crater now ? You don't know! A huge relief in the end. :)
- matt-attack 6y ago
  But aren’t those two events essentially simultaneous in the relativistic sense? That is by some definitions of “simultaneous”?
  - zwkrt 6y ago
    In one sense, light experiences no time during travel, so anytime you are hit by radiation (like from a star) there is frame of reference in which the event was instantaneous. On the other hand, if you were on Earth and I was in between Earth and Mars, I would receive the data more quickly than you, and I could even watch it whiz by me on its way to you. The thing about relativity is that it’s... relative!
OliverGilan 6y ago
Doesn’t relativity tell us it doesn’t matter?
- cjohnson318 6y ago
  There's a lag time in communication due to distance. I don't see what that has to do with relativity.
- runarberg 6y ago
  PBS Space Time recently explained what the present time means within general relativity[1]. As I understand it... it matters in this context. [1]: https://www.youtube.com/watch?v=EagNUvNfsUI https://www.youtube.com/watch?v=EagNUvNfsUI
  - scotty79 6y ago
    True fun begins when you consider General Relativity (which takes into account gravity and acceleration). From what I heard there is no definition of simultaneity and you can define it in different ways.
PeterisP 6y ago
It's the latter. The Earth-Mars latency at this time is something like 11 minutes, and the landing itself takes about 7 minutes, so when we on Earth first saw the craft entering atmosphere on Mars, by that time all the landing was already over, one way or another.
- interestica 6y ago
  NASA has a good breakdown of their expected miletones at 'earth receive time' - https://www.nasa.gov/feature/jpl/nasa-s-next-mars-rover-is-ready-for-the-most-precise-landing-yet https://www.nasa.gov/feature/jpl/nasa-s-next-mars-rover-is-r... Apparently the latency time is currently 11 mins 22 seconds -- which is somewhere near the average. It goes from under 4 mins to over 22mins depending on distance.
DiogenesKynikos 6y ago
It depends on what coordinate system you're using. Simultaneity is ill defined in relativity. There's only future, past and "spacelike-separated" (neither past nor future). When they say, "X is 1 minute from happening," it's actually neither in the past nor the future. It's currently spacelike-separated, but in 1 minute, it will be in our past.
- unanswered 6y ago
  Yes, yes, you have shown you know what relativity is. But the relative velocity of earth and mars — which I can't convince Wolfram Alpha to tell me, but it's got to be on the order of their orbital velocity so let's say 5x10^4 mph — is a tiny tiny fraction of c so their inertial reference frames are essentially identical. So sitting in our reference frame, we can make inferences about what's happening "now" on mars,such that these inferences are consistent (to within that tiny fraction of c) with all of our current and future observations in this reference frame; i.e., consistent with a classical(+ finite speed of light) model of the universe. Which is why I left this out of my question and only asked about the consequences of a finite speed of light. Put another way, simultaneity is perfectly well defined in a single inertial reference frame, and for purposes of my question, earth and mars can be considered to be relatively motionless.
  - johncolanduoni 6y ago
    Now is the part where someone brings up even with the low relative velocity, you haven’t accounted for the simultaneity issues we’d have if the earth collided with a black hole while the rover landing was happening.
    - unanswered 6y ago
      Or the fact that mars is not as deep in the gravity well of the Sun! I wonder if they have to account for that one when programming, like, antenna aiming or something.
      - 6y ago
colechristensen 6y ago
It looked like they were quoting time as it would appear for an earth-local observer (i.e. a million light-year away supernova that showed up five minutes ago happened "five minutes ago" not 1 million years and five minutes ago. With your personal light cone, it's fine to equate "now" with what you see in the moment. It just has to be clear what you mean for situations where communication might be ambiguous. If you have a person on mars, be sure to be precise what you mean when you tell them to do something in five minutes, when they receive the message they won't know if you mean five minutes after they receive the message or anywhere between 17 minutes before and 2 minutes after they receive the message. When you get into relativistic speeds (and especially very short time intervals), nobody can even agree on when something "actually" happened, different observers have different opinions about what happens when even after you account for light travel time.
- extropy 6y ago
  Relative time (in five minutes) without relativistic speeds is actually uniform. The is no observable difference to either participant. And there is no concept of now in a significantly distant location. Related video: https://youtu.be/pTn6Ewhb27k https://youtu.be/pTn6Ewhb27k
gfiorav 6y ago
Waiting for the physicist in the room to point out: there is no such thing as simultaneity! :)
- theNJR 6y ago
  Came here to suggest The Order of Time by C Rovelli, which explains this in such a captivating way.
  - amelius 6y ago
    I think this video explains the issue quite well in only two minutes: https://www.youtube.com/watch?v=wteiuxyqtoM https://www.youtube.com/watch?v=wteiuxyqtoM
    - melenaboija 6y ago
      Another one here, it is in Catalan but with subtitles in english. Minute 18 is where it is explained although I think is worth it to watch all of it https://www.ccma.cat/tv3/alacarta/quequicom/tempus-fugit-subtitulat-en-angles/video/5540737/ https://www.ccma.cat/tv3/alacarta/quequicom/tempus-fugit-sub...
    - Dylan16807 6y ago
      But that only happens with sufficiently high relative velocities. Earth and Mars are effectively in the same reference frame.
- deleted 6y ago
  [deleted]
- Daho0n 6y ago
  S/he already did. We just don't know yet.
  - extropy 6y ago
    She both did and did not until we observe :p. And even that is not correct. Events happening propagate with speed of light - the event horizon. We can predict we will receive information of something happening, bit that's just prediction about future events, regardless of the location.

Sharlin 6y ago

Yes, those are all Earth Receive Time, that is, when they were saying that eg. entry interface is two minutes away, in reality the rover was already sitting on the surface and we were just waiting for the radio signal to get here.

unanswered 6y ago
> Earth Receive Time Ah great, that's a great phrase to make everything clear and provide a kind of "frame of reference" to think & communicate in. Always need these abstractions.
m4rtink 6y ago
The local Czech stream (20k viewers!) I watched went over the events in real time and then commented how events were happening as signals were received on Earth - with the final confirmation of successful landing coming first via Twitter, no less! :) Still a very nice yet nerve wrecking idea to do it like this - you know the lander is on Mars now. But is it safely on the ground or is there a third Shapirelli crater now ? You don't know! A huge relief in the end. :)
matt-attack 6y ago
But aren’t those two events essentially simultaneous in the relativistic sense? That is by some definitions of “simultaneous”?
- zwkrt 6y ago
  In one sense, light experiences no time during travel, so anytime you are hit by radiation (like from a star) there is frame of reference in which the event was instantaneous. On the other hand, if you were on Earth and I was in between Earth and Mars, I would receive the data more quickly than you, and I could even watch it whiz by me on its way to you. The thing about relativity is that it’s... relative!

OliverGilan 6y ago

Doesn’t relativity tell us it doesn’t matter?

cjohnson318 6y ago
There's a lag time in communication due to distance. I don't see what that has to do with relativity.
runarberg 6y ago
PBS Space Time recently explained what the present time means within general relativity[1]. As I understand it... it matters in this context. [1]: https://www.youtube.com/watch?v=EagNUvNfsUI https://www.youtube.com/watch?v=EagNUvNfsUI
- scotty79 6y ago
  True fun begins when you consider General Relativity (which takes into account gravity and acceleration). From what I heard there is no definition of simultaneity and you can define it in different ways.

PeterisP 6y ago

It's the latter. The Earth-Mars latency at this time is something like 11 minutes, and the landing itself takes about 7 minutes, so when we on Earth first saw the craft entering atmosphere on Mars, by that time all the landing was already over, one way or another.

interestica 6y ago
NASA has a good breakdown of their expected miletones at 'earth receive time' - https://www.nasa.gov/feature/jpl/nasa-s-next-mars-rover-is-ready-for-the-most-precise-landing-yet https://www.nasa.gov/feature/jpl/nasa-s-next-mars-rover-is-r... Apparently the latency time is currently 11 mins 22 seconds -- which is somewhere near the average. It goes from under 4 mins to over 22mins depending on distance.

DiogenesKynikos 6y ago

It depends on what coordinate system you're using. Simultaneity is ill defined in relativity. There's only future, past and "spacelike-separated" (neither past nor future). When they say, "X is 1 minute from happening," it's actually neither in the past nor the future. It's currently spacelike-separated, but in 1 minute, it will be in our past.

unanswered 6y ago
Yes, yes, you have shown you know what relativity is. But the relative velocity of earth and mars — which I can't convince Wolfram Alpha to tell me, but it's got to be on the order of their orbital velocity so let's say 5x10^4 mph — is a tiny tiny fraction of c so their inertial reference frames are essentially identical. So sitting in our reference frame, we can make inferences about what's happening "now" on mars,such that these inferences are consistent (to within that tiny fraction of c) with all of our current and future observations in this reference frame; i.e., consistent with a classical(+ finite speed of light) model of the universe. Which is why I left this out of my question and only asked about the consequences of a finite speed of light. Put another way, simultaneity is perfectly well defined in a single inertial reference frame, and for purposes of my question, earth and mars can be considered to be relatively motionless.
- johncolanduoni 6y ago
  Now is the part where someone brings up even with the low relative velocity, you haven’t accounted for the simultaneity issues we’d have if the earth collided with a black hole while the rover landing was happening.
  - unanswered 6y ago
    Or the fact that mars is not as deep in the gravity well of the Sun! I wonder if they have to account for that one when programming, like, antenna aiming or something.
    - 6y ago

colechristensen 6y ago

It looked like they were quoting time as it would appear for an earth-local observer (i.e. a million light-year away supernova that showed up five minutes ago happened "five minutes ago" not 1 million years and five minutes ago. With your personal light cone, it's fine to equate "now" with what you see in the moment. It just has to be clear what you mean for situations where communication might be ambiguous. If you have a person on mars, be sure to be precise what you mean when you tell them to do something in five minutes, when they receive the message they won't know if you mean five minutes after they receive the message or anywhere between 17 minutes before and 2 minutes after they receive the message. When you get into relativistic speeds (and especially very short time intervals), nobody can even agree on when something "actually" happened, different observers have different opinions about what happens when even after you account for light travel time.

extropy 6y ago
Relative time (in five minutes) without relativistic speeds is actually uniform. The is no observable difference to either participant. And there is no concept of now in a significantly distant location. Related video: https://youtu.be/pTn6Ewhb27k https://youtu.be/pTn6Ewhb27k

gfiorav 6y ago

Waiting for the physicist in the room to point out: there is no such thing as simultaneity! :)

theNJR 6y ago
Came here to suggest The Order of Time by C Rovelli, which explains this in such a captivating way.
- amelius 6y ago
  I think this video explains the issue quite well in only two minutes: https://www.youtube.com/watch?v=wteiuxyqtoM https://www.youtube.com/watch?v=wteiuxyqtoM
  - melenaboija 6y ago
    Another one here, it is in Catalan but with subtitles in english. Minute 18 is where it is explained although I think is worth it to watch all of it https://www.ccma.cat/tv3/alacarta/quequicom/tempus-fugit-subtitulat-en-angles/video/5540737/ https://www.ccma.cat/tv3/alacarta/quequicom/tempus-fugit-sub...
  - Dylan16807 6y ago
    But that only happens with sufficiently high relative velocities. Earth and Mars are effectively in the same reference frame.
deleted 6y ago
[deleted]
Daho0n 6y ago
S/he already did. We just don't know yet.
- extropy 6y ago
  She both did and did not until we observe :p. And even that is not correct. Events happening propagate with speed of light - the event horizon. We can predict we will receive information of something happening, bit that's just prediction about future events, regardless of the location.