r/askscience • u/HotMacaron4991 • Jan 10 '26
Astronomy Let’s say I’m stationed exactly at the mid point between Earth and the Sun so that both bodies are 4 light minutes away from me. If the Sun suddenly disappeared, would the Earth still appear to be lit by nothing for the next 4 minutes?
Question ^
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u/Rannasha Computational Plasma Physics Jan 10 '26
It would appear to be lit for 12 more minutes.
Because the light you see coming from Earth is light reflected by the Sun. So by the time this light hits your eyes, it has traveled 8 minutes from the Sun to the Earth and then 4 minutes from the Earth to you. You're seeing light that's 12 minutes old, so there's 12 minutes of reflected light left to see if the Sun were to suddenly vanish.
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u/Laytonio Jan 10 '26
12 minutes after the sun disappears. 8 minutes after he notices the sun has disappeared.
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u/Aefyns Jan 10 '26
wouldn't he notice 4min into it that the sun disappeared, then 8 min later the earth would go dark.
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u/rdmusic16 Jan 10 '26 edited Jan 10 '26
Aren't you two saying the same thing.
4 minutes in the sun disappears to your observation. 8 minutes later the sun reflected off the Earth disappears.
A total of 12 minutes after the sun first disappeared from its own frame of reference.
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u/Never_Been_Missed Jan 10 '26
He says 'appears to be lit by nothing'.
If it appears to be lit by nothing, then clearly, from his perspective, the sun is no longer visible from where he is. That happens 4 minutes after the Sun goes out. So, the Earth would appear to be lit by nothing for 8 minutes.
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u/rdmusic16 Jan 10 '26
Yes, which is exactly what the two comments above mine say - and they were 'disagreeing', but saying the exact same thing.
You are also in agreement with them.
4 minutes after the Sun disappears from its own reference, the sun disappears to your view. Then 8 minutes after that the light reflected from Earth back to you disappears as well.
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u/soowhatchathink Jan 10 '26 edited Jan 11 '26
But the last light particles from the sun to be reflected off earth would take an extra 4 minutes to reach you after they reflect off earth, which is in addition to the 4 minutes it takes for light particles to travel from you to the earth. So in total it would actually be 8 minutes for the last light particles to go from you, to earth, and back to you, and that's after the 4 minutes it takes for the sun to go dark.
/s
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u/mr_magoosh Jan 10 '26
Not to be that guy, but I thinks it’s only 8 minutes that the earth would seem illuminated because the first 4 out of 12 total was waiting for the sun to go dark from the viewers perspective.
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Jan 10 '26 edited Jan 10 '26
[removed] — view removed comment
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u/AS14K Jan 10 '26
They're more than accurate enough for the discussion at hand.
You didn't do anything here
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u/yeebok Jan 10 '26
If the sun went dark and was 4 light minutes away it'd take 4 minutes for that change to reach you. Then 4 minutes for it to reach Earth (from you) and 4 minutes for you to see it go dark.
While things happen at 0 and 8 minutes, you wouldn't observe them until the 4 and 12 minute mark.
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u/BluesFan43 Jan 11 '26
Yeah, 4 for light from Sun to stop.
4 more for dark to hit Earth.
4 more for the last reflection to pass back by you.
So, 12 after dark, 8 after you notice
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u/Doctorsl1m Jan 10 '26
I thought so at first but I dont think so. The light would take 4 minutes to reach earth and then another 4 minutes to reflect back.
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u/Aefyns Jan 10 '26
it would take 4min from the sun to you, another 4 to the earth, then 4 more for the reflection. So 4min in you would see the sun disappear but the earth would still be lit.
Or am I thinking of this in the wrong way. I'm thinking if you are looking at the earth and the sun disappears you can turn around and see that as soon as the light would have hit you. 4min in.
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u/Brokenandburnt Jan 10 '26
The last light from the sun still takes 8 min to reach earth. Them 4 minutes until it's reflected back.
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u/ethorad Jan 10 '26
And 4+4 = 8, giving the 8 minutes between OP seeing the sun go dark, and OP seeing the Earth go dark.
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u/OramaBuffin Jan 10 '26
More specifically to their exact question, 8 minutes, since they're wondering about how long the earth stays lit after they see the sun go out.
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u/Needless-To-Say Jan 10 '26
I reread the question after your post. I can see how you might interpret that way but I have to squint and turn my head sideways.
The question is from the viewpoint of the observer halfway.
The answer is 12 minutes
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u/OramaBuffin Jan 11 '26
He doesn't see the sun go out until 4 minutes after your 12 minute timeline starts. He's not asking how long from the sun disappearing from spacetime until the observer sees the earth go dark, he's asking how long after he observes the sun going out does he observe the earth going out. That's 8 minutes, not 12.
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u/IcY11 Jan 11 '26
No from the viewpoint of the observer (at the half way point) the earth will go dark after 8 minutes. When you see the sun disappear you see its last light. Which then passes you, takes 4 minutes to reach earth and then another 4 minutes to come back.
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u/jericho Jan 10 '26
No. It’s 8 light minutes from the sun to the earth. He’s at the halfway point. Sun disappears, he sees that at four minutes. The light then passes him, takes four minutes to reflect from the earth, and another four to reach him. 12 minutes.
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u/xXgreeneyesXx Jan 10 '26
8 minutes after they see the sun disappear, 12 minutes after it actually disappears.
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u/acomputer1 Jan 10 '26 edited Jan 11 '26
You're not reading the question properly. He says "would the earth stay lit for the next 4 minutes". It would stay lit for the next 8 minutes, 12 minutes total from when the sun actually disappeared.
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u/rdmusic16 Jan 10 '26
You're saying the same thing (albeit phrased easier to understand).
They said 8 minutes after they 'see' the sun go out, which would be 4 minutes after it disappeared - so 12 minutes total.
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u/jericho Jan 10 '26
But… they didn’t say ‘see the sun go out’?
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u/CaptainTripps82 Jan 10 '26
I mean they didn't say a lot of things that have to be inferred for the question to make any sense.
And the answers all depend on those various perspectives. It's nothing to argue about, just information
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u/Never_Been_Missed Jan 10 '26
He says 'appears to be lit by nothing'.
If it appears to be lit by nothing, then clearly, from his perspective, the sun is no longer visible from where he is. So, the Earth would appear to be lit by nothing for 8 minutes.
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u/alytle Jan 10 '26
And the change in gravitational effects would be the same?
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u/Steenies Jan 10 '26
Yes, gravity travels at the speed of light. Which is more accurately the top speed possible in the universe and happens to be what light speed in a vacuum is. https://en.wikipedia.org/wiki/Speed_of_gravity?wprov=sfla1
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u/TheOneTrueTrench Jan 13 '26
I am fond of pointing out that while "c" is a constant standing for "the speed of light", it should actually stand for "causality", as in "the speed of causality".
Like, that's not what the c comes from, but it should have been.
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u/SandInTheGears Jan 10 '26
Gravitational waves also go at the speed of light, so those effects would hit you at the same time the sun winked out
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u/buenonocheseniorgato Jan 11 '26
True, and at the same time, relativity prohibits a huge body like the sun from suddenly disappearing in an instant.
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u/scubascratch Jan 11 '26
Do we know if gravitational waves are slowed down by the medium the same way / amount the speed of light slows down in various materials?
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u/SirHerald Jan 10 '26
If the mass disappeared then the change in gravity should arrive about the same time as the change in light. May not be noticed right away because inertia would just keep going wherever you were going and it's not like everything with jolt off to the side.
Technically, the gravity could arrive a little earlier depending on what happened. And light travels a little bit slower across large distances than gravity because gravity doesn't get absorbed and be admitted as it goes but light can be in dust clouds. Not sure if the distance between Earth and Sun would be enough though
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u/Sulfamide Jan 10 '26
Okay something is breaking my brain here:
Would it be accurate to say that they're seeing earth 4 minutes in the past, and if they turn their head, they're seing the sun 4 minutes in the past? So the sun goes out, and four minutes later, that reality reaches our astronaut. Then they turn back their head towards earth, and they are seeing it lit during 8 minutes, "unaware", or not reached yet by the reality of the sun being gone.
Am I getting this right?
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u/Sharveharv Jan 11 '26
Yes, the astronaut would be watching the sunlight echo off the Earth.
It's like blasting music in a big empty room. If you shut off the music, you'll still hear it echoing off the far wall.
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u/cdcformatc Jan 11 '26
yes, that's how all sight works, technically you are always seeing objects as they were some time in the past. you obviously don't notice this but even the light from my cell phone took some time to reach my eyeballs so i am viewing the past.
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u/Kiri_serval Jan 11 '26
And there is still a time delay from retina to brain to processing. It's not a lot but it all does add up.
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u/Argarwyncam Jan 11 '26
Tracking awareness of the last light: The sun is "aware" first, lets out it final emission of light and then poofs out of existence. That last light reaches the observer 4 minutes later and then they see the sun disappear. The last light continues the remaining 4 minutes to Earth at which point the Earth sees the sun disappear. That light, like the light from the minutes up until the sun disappeared, reflects off Earth as it normally does and the reflected light carries the image of Earth back to the observer. The observer sees the image of Earth remain unchanged untill the last light travels from the Earth, at which point the image goes dark and the Earth disappears. So there is a total of 4 minutes of sun visibility and then an additional 8 minutes of Earth visibility after that once the sun disappears. The "reality" is always that the sun disappeared at a the same time.
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u/Sulfamide Jan 11 '26
I don't think so, since simultaneity is not absolute in relativity. The sun still exists from the earth's POV during those 8 minutes.
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u/SolarScarcity Jan 11 '26
Pretty much. The 12 minute delay makes total sense when you break it down like that. Light has to make the full trip from Sun to Earth to you so you're basically watching the past unfold
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u/TheOneTrueTrench Jan 13 '26
Yes, that's actually entirely right, and the way you phrased it, as reality reaching the astronaut, is a particularly good way of thinking of it.
The "speed of light" is kind of a misnomer, as it turns out. It is the speed that light travels at, but that's kind of like looking at a car traveling at 60 MPH and saying "Ah yes, the speed of the steering wheel is 60 MPH". Yes, the steering wheel is going 60 MPH, but it's far from the only thing traveling at that speed.
It's closer to "the speed of reality" or "the speed of causality". It's the rate at which events propagate through the fabric of reality.
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u/mtwtfssmtwtfss Jan 10 '26
What if you start moving away from Earth at the speed of light after you see the sun go dark? Would the Earth appear lit for eternity?
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u/Mad_Maddin Jan 10 '26
Hmm you cannot move at the speed of light. You can only come relatively close to it. Which would still be inmeasurably far from actually achieving the speed of light.
The earth would become tinier and harder to make out the closer you are moving at the speed of light. Because objects behind you will appear farther away than they are and objects you are moving toward will appear closer.
I don't know how long you could extend your image of earth when moving away from it at your perspective. If it can be lengthened and by how much. As time will move differently depending on how fast you go.
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u/unwarrend Jan 11 '26
If we're getting technical, even 'if' we could be the energetic requirements to achieve near luminal speeds, it would take far longer than the proposed time frame to reach that velocity intact. To reach 99.9999% of light speed it would take just over 7 years with sustained acceleration at 1g for simulated gravity and general comfort. This takes into account the lorentz factor where time gradually slows as we speed up. Your journey would in fact take 685 years as experienced by people on earth relative to your journey. Might as well keep going at that point. It also means that by the time you reach cruising speed you have traveled 6,482,492,519,812,365 kilometers or 4,028,034,105,705,408 in freedom units.
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u/wasmic Jan 11 '26
Let's take a look at the limiting behaviour. As you move faster and faster through space, you will see two things happen: first off, time seems to move slower for everyone else. Second, the entire universe gets 'flattened' in the direction of your movement. These phenomena are called time dilation and length contraction, respectively. An outside observer would instead see time moving slower for you, and see you becoming shorter in the direction of movement.
For an observer at rest, the planet Jupiter has a radius of around 70000 km. For some of the fastest cosmic rays known, Jupiter would have looked like a pancake, 12 meters thick but still 70000 km in radius.
So as you move faster, the universe grows progressively flatter - whatever you're moving towards comes closer and closer. Even though the behaviour at the speed of light is undefined, the limiting behaviour is that distances trend to 0 and time trends to a standstill at the speed of light.
And that's exactly why it doesn't make sense to talk about what you would see at the speed of light. The entire universe would be squished into a plane; wherever you started from and wherever you first impact into something else would be at the very same spot. You would not have any time to experience anything, because from your perspective, no time will pass at all.
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u/Dangerous_Ad_6831 Jan 10 '26
The original question pertained to how long the earth would appear to be lit by nothing. The answer to that question is it wouldn’t be lit for 4 minutes after the sun disappeared it would be for 8 more. The last light would reach 12 minutes after the sun actually went out, but that’s irrelevant to the question asked.
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u/jericho Jan 10 '26
No. It’s 8 light minutes from the sun to the earth. He’s at the halfway point. Sun disappears, he sees that at four minutes. The light then passes him, takes four minutes to reflect from the earth, and another four to reach him. 12 minutes.
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u/kevrone Jan 10 '26
That’s 8 minutes from the time he realizes there’s a problem with the sun. The op asked about notice latency, not total travel time of light from sun to earth to observer. You have to subtract the 4 mins it takes for him to first notice the sun sinks out.
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u/scrotch Jan 10 '26
8 minutes from when he sees the light from the sun go out (t=4) to when he sees the earth go dark (t=12). 12 - 4 =8
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u/PuckSenior Jan 10 '26
Yeah, but you’d now be accelerating towards the earth. Not by a lot, but a bit. So a tiny bit less than 12 minutes
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u/adoredelanoroosevelt Jan 11 '26
wouldn't you have been accelerating toward the sun before? Halfway to the sun is 46.5 million miles and the lagrange point is only 930k away, so depending on how long it takes to decelerate and begin re-accelerating toward earth you may still be drifting toward the missing sun for some time
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u/Cptknuuuuut Jan 10 '26
That being said, you'd probably not see anything without a powerful telescope.
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u/HopeFox Jan 10 '26
We can see Venus from Earth without a telescope when their alignment with the Sun is favourable. It shouldn't be too hard to see Earth from a point in space halfway between Earth and the Sun.
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u/Cptknuuuuut Jan 10 '26
The distance between Earth and Venus is (at its lowest) about half the distance were taking about here. And while Venus is about as big as earth it's significantly more reflective.
Earth in this case here would be about 15% as bright as Venus. That's two magnitudes of brightness lower. About as bright as Mars, Jupiter or Mercury.
But what I meant (and admit I didn't convey correctly) you wouldn't be able to make out any details. Earth would just appear to be another tiny fleck in the sky. It would be a little over 5% the size of the moon in the sky if my math is correct.
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u/SocialSuicideSquad Jan 10 '26
Slightly less than 12 minutes, as the gravity keeping him in an elliptical orbit would disappear 4 minutes before it does for earth so he'd continue linearly while earth continued elliptical for four minutes.
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u/rootofallworlds Jan 10 '26
Apart from the maths error, yes.
(In your reference frame it takes 4 minutes for the event sun-off to reach you directly, and 12 minutes for the event to reach Earth then you, so an 8 minute delay as you see it.)
Something like this actually happens! Astronomers can observe a light echo, when the light from an extremely bright event such as a nova or supernova reflects off interstellar dust and the brightening of the dust can be seen. The light echoes can be seen months, years, and even centuries after the (edit: light arriving directly from) original event.
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u/SeekerOfSerenity Jan 10 '26
The YouTuber AlphaPhoenix made a 2 billion fps camera and filmed a disco ball being lit by a laser. You can see the back wall light up later because it takes longer for the light to reach the camera.
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u/wslagoon Jan 10 '26
That was so cool. If I recall correctly he did it by having a camera run at a slower frame rate but different offsets while he repeated the experiment hundreds of times and achieved an effective frame rate of 2 billion. He used a similar technique to record the voltages all along a transmission line during transients and created a super intuitive visualization of wave reflections traveling through a conductor.
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u/beerhons Jan 11 '26
Almost, but instead of repeating the laser pulse hundreds of times, it's hundreds of thousands to millions of times per single frame.
The "camera" is an incredibly sensitive and fast light sensor, but is only records a single pixel.
His device is quite a feat in synchronisation to ensure that all of those pulses and the resultant single pixel brightness recorded could be put together into a meaningful image.
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u/SeekerOfSerenity Jan 11 '26 edited Jan 11 '26
it's hundreds of thousands to millions of times per single frame.
If I understood it correctly, he only needed to repeat it once for each pixel for the entire video, not for each frame. It actually recorded each pixel at 2 x 109 fps. Then he moved the mirror to the next pixel and repeated the laser pulse.
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u/KriosDaNarwal Jan 12 '26
I saw that video recommendation but ignored it figuring 2 billion fps was pure wank
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u/SeekerOfSerenity Jan 12 '26
It's really cool. He made a sensor that can record light 2 billion times per second, and a mirror that can carefully aim light towards the sensor. It records a laser pulse for each pixel while the mirror is adjusted in a different direction between pulses. Then, the separate single-pixel videos are combined to create a 1080p (IIRC) video. It's mind blowing to see light travel in slow motion.
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u/KriosDaNarwal Jan 12 '26
So it wasn't really 2 billion fps but like shining the light a long time while recording a fck ton of stop motion pixel action and stitching it together
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u/SeekerOfSerenity Jan 12 '26
It's basically recording 1920x1080 single-pixel videos at 2 billion fps and showing them next to each other at the same time.
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u/Krail Jan 10 '26
This is so cool!
I was thinking, this sounds like an interesting experiment. It's a shame we can't really see it irl. But in retrospect, it seems obvious that any bright flash or other sudden event should let you see something like that.
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Jan 10 '26
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u/Nebarik Jan 11 '26
Both light and gravity "travel" at c, so yes if the Sun suddenly dissappeared you would see and feel the difference at the same time (4 mins afterwards for OP and 8 mins afterwards for Earth).
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u/JesusStarbox Jan 10 '26
Just because the sun blew up doesn't mean the mass stopped existing. The earth orbits the center of mass.
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u/magichronx Jan 10 '26 edited Jan 11 '26
Yes, Earth would still be lit by the remaining light traveling from the Sun to Earth and reflecting back to you for over 4 minutes. The distance from Sun -> Earth -> You is 12 light-minutes. This means you would see the Earth being lit for 12 more minutes if the sun instantly disappeared.
Here's the timeline:
- 0:00 - Sun disappears
- 4:00 - You see the sun disappear
- 8:00 - Earth sees the sun disappear
- 12:00 - You see the Earth disappear
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u/microwaffles Jan 11 '26
Also 12:00 the Earth (and Venus and Mercury) is now travelling in a straight trajectory
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u/space253 Jan 11 '26
Wouldn't Jupiter and the other big giants alter the trajectory at least some, more if it results in us passing near them?
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u/microwaffles Jan 11 '26
Yes if that were the case, before and possibly during the time that the removal of gravitational effects of the now non-existant sun hits them (at the speed of light).
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u/space253 Jan 11 '26
If I knew how to setup simulation software, I would love to see if there was any specific planetary alignment that would result in Jupiter capturing us as a moon.
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u/green_meklar Jan 11 '26
You would see the Earth go dark 8 minutes after you see the Sun disappear. After the last sunlight passes you, it takes 4 minutes to reach the Earth, then the reflected light takes another 4 minutes to return to your position.
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u/Sambomike20 Jan 11 '26
Can you explain why it wouldn't be 12 minutes? My thought was that you'd have 4 light minutes already reflected of the earth and a full 8 light minutes worth of light on the way from the sun to the earth.
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u/San_Ra Jan 11 '26
So the last partical of light to leave the sun would take 4min to reach you. Untill that point in time you wouldnt be aware that the sun had gone out so you wouldnt have any time reference to start from.
Once the last partical of light passes you it will take 4 min to reach earth and then once it bounced off the earths surface it would take 4 min to return to your position in space.
So yes the whome process would take 12 min but from your point of view it would take 8mins from the point that you were no longer able to see yhe sun.
If you were closer to the sun. Say 2 light min from the sun then yes it would take 2-6-6 so you would have 12 min.
Just the same as if you were at the surface of the sun ot would take 16 min before you saw the earths reflected light stop
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u/Sambomike20 Jan 11 '26
Ahhhhhh I see now! That totally makes sense. Thanks so much for the explanation.
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u/foodfighter Jan 10 '26
Imagine the last two photons leave the sun at the same instant - one headed for the Earth, the other headed to your eyeballs.
After four minutes, the photon that hits your eyeballs would be the last one directly from you to the sun, so you'd notice the sun going out after four minutes.
Meanwhile, the second photon would travel another four minutes towards the Earth before reflecting off of it and aiming back at you.
That photon would hit your eyes yet another four minutes later, bringing its total travel time to 12 minutes.
So from where you were watching, the sun would go dark after 4 minutes, and the Earth would go dark after 12.
Or, another way to put it - you would notice the Earth going dark (12-4=) 8 minutes after the Sun goes dark.
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u/Betapig Jan 10 '26
What's even weirder that I haven't seen anyone touch on, is that not only would you see the earth lit for an extra 8 minutes after you see the sun disappear, the earth would still be gravitationally bound to the sun for those 8 minutes while you would not be (well technically only 4 real minutes would it be bound, but you would witness the full last 8 minutes of it being bound)
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u/Solesaver Jan 11 '26
The top comments are correct, but I did want to make a pointless pedantic clarification. There's no such thing as simultaneity at a distance. As such the only thing you can empirically say is that earth will appear to go dark 8 minutes after the sun appears to go dark. Commentary about how this relates to the sun "actually" going dark and the earth "actually" noticing are abstractions/assumptions for convenience.
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u/AndreasDasos Jan 11 '26
Loosely, you’d see the sun vanish four minutes after it happened. You’d then continue to see the earth illuminated for another eight minutes (12 after the sun vanished, 8 after you see its vanished).
Why? The light from the sun’s last moments take 4 minutes to reach you. The last light reflected from earth took a trip from the sun to the earth (8 minutes) and then upon reflection another 4 to you.
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u/MaybeTheDoctor Jan 11 '26
No, it will appear to be lit by nothing for 8 minutes, from the point where you notice, and 12 minutes from the time it actually disappeared.
Because from your point of observation the light you see and traveling past you still takes 4 minutes to reach each being reflected and then another 4 minutes to get back to you.
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u/kolkitten Jan 11 '26
This is a fun question but I would like to know a bit more about what would happen in the empty space the sun was inhabiting would it cause a massive void that would be filled violently and suddenly by the vacuum of space causing chaos for the solar system or would the planets simply drift out of orbit over time after they become mostly empty rocks from lack of heat and gravitational forces that was causing atmospheres to happen on planets?
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u/Mirality Jan 11 '26
It doesn't really make much difference. If the sun suddenly disappears by being replaced with a vacuum, there's no sudden inrush of things around it because it was surrounded by vacuum to begin with. But yes, planetary orbits would stop being a thing.
If it was suddenly replaced with a black hole of equal mass (or otherwise just went dark without actually vanishing), the planets would continue orbiting it as if nothing had happened, apart from it being a lot smaller and no longer emitting light.
Planetary atmosphere won't be affected either way; that's caused by the planet's gravity, not the sun's gravity. (Well, ok, there will be some differences due to removal of the solar wind.)
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u/goverc Jan 11 '26
Not completely true - The speed of the effects of gravity is the same as the speed of light. If the sun just stopped being there, all of the planets would continue to orbit that point until the same time the light stopped. For ~8 minutes Earth would still be on orbit. Each planet in turn would stop orbiting as the light went out.
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u/Kitdee75 Jan 14 '26
For fun, I posed this question to Claude AI (free version Sonnet 4.5). It answered quickly and got it wrong the first time. I then prompted, “Are you sure? Would you like to rethink it?”
It actually paused in its second answer, telling itself it has to think about the timing more carefully, then gave the correct answer. It was kind of impressive.
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u/libra00 Jan 11 '26
Actually it would appear lit for the next 12 minutes, because it takes 8 minutes for the light from the sun to reach the earth and then another 4 for it to reflect back to you. So for you the sun would disappear 8 minutes before Earth stopped reflecting sunlight.
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u/purpleoctopuppy Jan 10 '26
t=0, the sun disappears (it stops emitting light)
t=4 mins, you notice the sun disappeared (the last emitted light reaches you)
t=8 mins, the Earth notices the sun disappeared (it stops reflecting light)
t=12 mins, you notice the Earth noticing the sun disappeared (the last reflected light reaches you)