I plan to subject an entire earth-like planet to about 300m/s^s acceleration, what is the shortest duration I can get away with for non-scientists to realize something has happened, without everyone’s brain’s smashing to jelly against their skulls. For this scenario, we can imagine a sphere approximately 1 solar mass and 1 solar radius pops into and out of existence above the planet, such that the sphere and planet are exactly tangent to each other above the planet’s only city. Ignoring potential collisions with the sphere itself or radiation from what would otherwise be considered a stellar object, how short of a pulse can I get away with while still somehow registering to an average man on the street (maybe altered balance, proprioception, vertigo? A spike in blood pressure or drop in pressure on the spine? Maybe items visibly jerking, rippling or sloshing? Anything noticeably off by most people.)

For humans, I am aware of https://www.nasa.gov/sites/default/files/atoms/files/nasa-std-3001_vol_2_rev_b.pdf but the study itself does not cover microsecond to nanosecond durations; i’m tempted to continue the trend line and assume duration shorter than a few milliseconds are reliably survivable, my thinking being its only like a few micrometers displacement at a max delta v of a few millimeters per second. I’m not certain that would even be noticeable given the milliseconds range limits of human perception, though. Surely the brain has more clearance within the skull than the diameter of a microns before things really accelerate up to dangerous speeds

I don’t know if my narrative can support anything longer than a single microsecond duration, but if a longer period gets me something dramatic but initially survivable or potentially fully medically treatable i’m all ears.

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    $\begingroup$ I think you will need to edit your question to avoid failing the single question condition. I'm not sure of the details of that rule but it looks like a single answet could comfortably deal with 1/2/3, while 4 to 8 should be split off into a question dealing with the general case. $\endgroup$ Jun 26 at 1:40
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    $\begingroup$ VTC: You are allowed to ask one and only one question. But there are other things that I"m having trouble with. Gravity affects everything uniformly. We might be tempted to compare this to a gunshot (294 m/s is a low-velocity bullet speed) and suggest that 30ns would result in that "bullet" penetrating 2,000 microns of skin by 8.82 microns. But in reality, the effect would be universally felt across the entire body. Would that knock someone down? What direction is the gravity going? ... $\endgroup$
    – JBH
    Jun 26 at 5:04
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    $\begingroup$ .. Maybe my problem is that the question is worded badly for a site that expects questions to be specific, have only one "best answer," and be objective. Maybe it just needs to be rewritten to something like, "How long must a 30G force be applied to the Earth for an average building to show noticeable damage?" Of course, what's the definition of "noticeable damage?" What's an "average building?" There's a huge difference between a building floating on water and one built on bedrock. There's a huge difference between a 1-story building and a 100-story building. ... $\endgroup$
    – JBH
    Jun 26 at 5:07
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    $\begingroup$ ... And there's a MASSIVE difference between the side of the planet next to your "source" and the side away from it. As objective as this question seems, it feels massively subjective. You want a simple answer to a remarkably complex problem - and maybe that's the problem. Perhaps you don't realize that this simple question reflects a result so complex that it's outside of the scope of this Stack to give you a sensible answer. $\endgroup$
    – JBH
    Jun 26 at 5:08
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    $\begingroup$ I have retracted my close vote. That's a much better question. Please note that it would be improved if you deleted the last paragraph. If you think about it, the effect of a good answer on your plot is irrelevant to how this Stack works. Remember from the help center, "When asking questions keep in mind that the goal of the site is to help you build your world, not to tell your story." $\endgroup$
    – JBH
    Jun 27 at 3:18

2 Answers 2


There will be very little effect on tiny timescales as you describe the problem, but it won't take long for major destruction

Since you are ignoring the intersection on the 2 bodies, you can simple say that the solar equivalent is a point source, massing 1.98857E+30 kg located about 695700 kg from the surface of the earth or 702100 km center to center.

Ignoring tidal forces, you are simply in a free-fall situation. I.e., other that the fact the the Earth velocity relative to the solar system changes, there is no effect.

Of course, there will be tidal forces - but tidal effects will be negligible on small objects - small includes things like cities on this scale (a vertical height of perhaps a fraction of 1 km). As earth will be well inside the Roche limit of the solar mass, it is obvious that tidal forces will cause complete destruction if allowed to continue.

So, what is the tidal force on Earth? For Earth and Sol the tidal force is 5.05e-7 m/s^2, For Earth and the theoretical point source, I calculated 4.89 m/s^2, almost 10 million times as large, about 0.5 G - easily enough to destroy the planet. I believe a 1 seconds would no doubt result in quite dangerous widespread major quakes, major ocean tidal events, etc. I say this because 0.5 acceleration would lift/lower crust by 5 meter (2.5 meters during the tidal event plus another 2.5 meter via inertia after the tidal event) At 0.1 seconds the 0.5 meter effects could still could trigger large quakes on existing fault lines, but ocean tides would not be too bad - easily observed, but not normally harmful.

As you asked quite a few separate questions, I'm treating this hopefully in a way to give you a decent perspective on the expected outcome.


Most dramatic effect is at 42ms duration

This is the time it takes for a front of gravitional wave to reach from one side of earth to the another.

Then, after 42ms, it becomes free fall situation, as @Gary Walker correctly states, but then he starts talking about tidal stuff and destruction at seconds time scale and considering resulting distance it is not pointless, but time is too much, let's put it that way.

That being said, most problematic time is appearance of that new gravity field, because parts which are closer to that source of new gravity already accelerating while more distant parts still stay put, in essence it creates tension due that apperance of this new pulling force which later parts of planet do not know anything about, yet. For different reason, but a closest example is falling stream of water, when it breaks in droplets (this is example of gravity stretching an object)

Maximum elongation created in 42 ms at 30g is 0.26 m, and for the earth object which is ca 12'600 km diameter, it is not a lot, it is 1/1'000'000 and even a stone will behave like a rubber at this situation. So no ruptures, or anything, as a direct result of this situation gravity wave front.

However, due the mass, gravity and volume of the earth it may be a considerable energy as a total, and there will be some bouncing after that, some sort of waves in the matter, and it requires more efforts to say anything about that, now will it develop and dissipate. I guess some simulation could answer this question better than my fantasy about it, but it not necessarly some drastic events(most likely not, but it does not mean it won't trigger earth quakes which had potencial to happen anyway) but it may if waves will focus in one region eventually, and if speed of dissipation is not fast enough)

More problematic will be effects of a flash, radiation emitted by that star body in this time.

Distance to that star - we are talking in this case about 1/222 of the distance to the sun, or about 670'000km to the center of the star, while mean star radius is 695'000km so it well in the photosphere of the star, and thus getting the full flow from it(photosphere) of energy

1ms of radiation from it, at density 63MW/m, is 63 kJ/m2. It, as time and density of the flash goes it canbe compared to a flash of a nuke, and will be everywhere at all over the planet surface, and have similar effects. So people exposed to it may go blind, especially if it continues for 42 ms. They may not understand or notice, but it will be like switch off for them, a little bit later.

Burns as well, check nuke effects.

Athmosphere it also will get a kick (from surface) because of the energy dissipated by surface and consequencial heating up of the athmosphere(so as some energy dissipated in it due optical path). Those effects will be noticiable, but specifics of them is hard to say, but due of some uniformity(all over the surface) it won't be exactly like a nuke air blast situation, hard to tell, but it will get more windy for some time for sure.

Plants will get sun burns, and it may become an exctinction level event this way, at 1ms, but hard to say for sure, maybe it possible to say for some aspects but too much efforts.

30 ns

Your original 30 nanoseconds, brings about 1.9 J/m2 as a flash as well, something like nanosecond laser pulse, so it not necessarly a totaly harmless value even if it just 1.9 J/m2, if not the density of the pulse, if effects which happen due it being a dense pulse are minor then it not a big deal, so it needs to look up what bohazards a pulse laser poses, and do they apply in the case. This duration not necessarly extinction level event, but not necessarly small thing as well, and may be bad enough.

As about noticing that 30ns situation - time duration here really does not matter that much on capacity of humans to notice the event, due the star radiation(ligth), people won't be able to tell if it is a high intensive 30ns flash, or less intensive 1ms flash, or a 0.01sec flash - photoreceptors in himan eye will registers this event (due the big intensity, 63MW/m2, it mor eof a matter how much J lands on retina of human eye) proper chemistry rection will get enough energy and all that. Maybe at 30ns it will be harder to realise it happened, and not just ones fluke, will take time(discussiins over the internet), but for 1ms event it will be hard not to notice, I guess, but realising the scale also will take time.

All in all, even if gravity may have some effects, especially over time/eventually(change of orbit, waves in earth blob), more harm is expected from the flash associated with photosphere of the new star, and it also will create some bouncy bouncy effects due apthmosphere heating and interacting with crust, crating spherical wave in both.

For humans and plants animals - potencial for blindness, surface burns similar to nuke blast situation, in extreame ms's range, and it may hold to certain extenteven for 10's of ns range/territory, but it harder to tell.

For infrastructure, it mostly secondary effects of crust athmoslhere, and/or EMP for microelectronics but that depends on the luck to some extend - which spot is near by on star surface.

So there can be immediate direct effects, but if they pass safely, not bringing much harm, secondary effects will have some place in this situation, but it harder to talk about them, and discuss which and how. Triggring hidden crust tensions to act it may happen, or in contraty it may ease it a bit - hard to tell, what will we observe increase of earth quakes or reducing of them - most likely both, some are triggered, some are eased, so there will be some secondary effects, and even if plants won't get sun burn on extinction level, it does not mean that 1-2% degradation of them all over the surface won't have some longer lasting effect. Etc.


  • Even if the intruder pops back out in less time the gravitational pulse will continue to cross the planet. 42ms isn't special. – @Loren Pechtel

Yes, it will continue to cross the planet, and will pass it in 42ms plus that lesser time, and stop having any effect on the planet, front and back side of the wave, in that sense in my case I should have talked about 84ms, but back front is probably less dramatic.

@Gary talked about tidal force, I understand it as value of gradient of gravity field(how uneven it is), and in that sense in this time frame, 42ms, uneveness of that field is at its max(front of the wave propagating trough the planet), and max amount of energy transfered each ms.

In that sense this timeframe is distincive enough, which has to be investigated separately from longer time frame of established field. And for shorter pulses, it still a timeframe to consider, when front of the wave enters the planet and exits it, sure we have to consider backfront as well.

  • $\begingroup$ Even if the intruder pops back out in less time the gravitational pulse will continue to cross the planet. 42ms isn't special. $\endgroup$ Jun 28 at 0:43

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