If the Sun were to suddenly disappear...

Not collapse, but disappear, as in: “poof!” it’s magically gone.

Would we know it immediately, or would it take eight minutes?

If it happens by magic, then either could be true depending on the magic.

The physics answer is that the photons could continue along their way and stop arriving at Earth 8 minutes later unless they got magicked away as well. I’d assume there would be a gravitational wave (but magic) associated with the disappearance which theory says also propagates at the speed of light.

It’d take eight minutes for the light to stop shining on Earth.

Not completely sure what the deal with gravity would be (and it’s hard to posit this because enormous amounts of mass suddenly winking out of existence without warning or trace is not a thing that can happen) but I assume the eight-minute rule also applies.

[On preview] Or what OldGuy said.

Gravity also moves at the speed of light.

On a side note, how long before we are all dead?

Thanks guys, the gravity well is what I was wondering about.

The basic concept here is that causality moves at the speed of light, so any evidence of the sun’s existence – radiation, gravity, etc. – always takes 8.32 minutes to reach us.

Chronos addressed this issue in a 2003 staff report.
https://www.straightdope.com/columns/read/2095/what-is-the-speed-of-gravity/

Potentially a while before every last human is dead.

But most people are gone within weeks since temperatures worldwide will drop below freezing within days. Though life in some form could conceivably persist hundreds of thousands of years near those oceanic thermal vents.

Here’s a Popular Science article on it.

There is no new thing under the missing sun.

I believe there are some tidal effects from the sun that would disappear but I do not know how their absence would affect life on earth.

All the bodies orbiting the sun, including us, would leave our orbits at a tangent. That might not have any effect on us, unless we collide with something else. Not sure if the moon would stay in the same orbit around the earth. If not, that would affect tides.

A more interesting question (the answer to the OP, as noted above, is that gravity moves at the speed of light as well). I guess it depends. From what I recall, it would take about a century or so to cool off the globe to the point that the atmosphere starts to freeze and fall to the surface. Assuming that we don’t lose the sun at some very sub-optimal part of the orbit such that we head towards one of the other planets or something else to crash into us, I’d say we would probably continue as a species for a few decades, unless we did some rather major things. I can think of a few that we might be able to do if we were fast and draconian enough to keep the species going for a long time, if not indefinitely, but I seriously doubt we would. If you are including life in the ‘we’, then potentially billions of years, again assuming no unfortunate meetings with large bodies in space.

You’re not going to have much in the way of tides when the oceans freeze over in a few weeks.

But yes, the Moon would stay orbiting the Earth if the Sun vanished by magic, it’s orbit would be slightly changed but not enough to notice as everyone freezes to death.

From what I recall, it would take more than a few weeks until the oceans froze over solid (well, I guess the surface, lower down you’d still have liquid water for billions of years). More like a year or so. I wonder what would happen to the weather though? Would we not get rain or snow anymore?

You wouldn’t have neap and spring tides but the moon is the primary driver of the tides. Note we would still have tides as even the crust moves a little bit but the physical effects but until they completely were frozen ice would rise, fall and bend with the tides.

Considering it only takes a few months of less-than-optimal sunshine to get snow and iced-over lakes, considering in one night even the tropic desert can get close to freezing with no cloud cover - I’m guessing the effect would be a lot faster. Plus presumably cloud cover would disappear rather quickly with no evaporation happening. True, nuclear decay in the core would keep the Earth’s internal temperature pretty good for a while, but how many centuries before the crust solidifies thick enough to effectively cease plate tectonics and most volcanic activity? Until then, people living around active areas like Yellowstone and Iceland might find some comfort in geothermal heating, and people living near oil field (oil refineries?) would do well or until natural gas starts to liquify and lose pressure…

Pro tip. If the sun suddenly disappeared it would be bad. Just say no.

Pluto STILL has (well, probably) a liquid core, so I don’t think the entire Earth freezing over any time soon is in the cards…we have a massively larger core that is good for billions of years, IIRC. I think that, even without the sun the Earth’s insulation and tidal forces from the moon would be good to keep the oceans surfaces from completely freezing over for more than a few weeks.

With our luck it would be straight into another planet, then it’s adios muchachos…

ETA, though I wonder what would happen if the Earth somehow, but some very extremely low probability event we managed to go into orbit around something like Jupiter or Saturn without coming to a grisly end via high speed encounter with something along the way.

I wonder, would the teams in Antarctica last the longest? They’ve got a good amount of supplies and are already set up to handle the cold. Would the pole cool at a slower rate than the rest of the Earth? How quickly would we reach mostly-consistent temperatures across the Earth?