Gravitational blink

To end the week on an appropriately surreal note: no, the Earth will not "lose its gravity" for seven seconds on August 12.

I found out about this rumor, currently making the rounds on social media, from a friend and loyal reader of Skeptophilia.  The whole thing apparently started with a video posted on Instagram by user @mr_danya_of; the video was subsequently removed, but not before it was reshared thousands of times, downloaded, and posted all over the place.  The claim is that there were gravitational waves emitted from two different black holes equidistant from the Earth, and that they are 180 degrees out of phase with each other, so where they intersect -- here, evidently -- they'll undergo destructive interference.  The result is that it will "cancel Earth's gravity" for the seven seconds it takes them to pass by us, and we all need to, I dunno, make sure everything is tied down securely or something, because otherwise it's going to cause huge amounts of death and destruction.

Whoo.  Okay.  Where do I start?

First of all, the information was alleged to come from NASA (of course), from something called "Project Anchor."  Which doesn't exist.  Of course, over at NASA they would say that, wouldn't they?  So let's move on to a few other, harder-to-argue-with objections.

Second, according to the General Theory of Relativity, gravitational waves travel at the speed of light, whereof nothing travels faster, remember?  So if there were gravitational waves headed toward us from a black hole (let alone two of them), we wouldn't have any way of knowing about it ahead of time.  Now, you might be thinking, what about the gravitational waves that have been detected by the interferometer array LIGO?  Well, there, we knew there were two neutron stars that had been orbiting each other and were about to merge, so all we had to do was watch until it happened.  (Okay, I'm making it sound simple; in practice it was a lot more complicated than this, but the point is we did have some advance warning in that case.)  Here, we just supposedly have black holes out there emitting gravitational waves for some undisclosed reason, and we've somehow found out about this eight months ahead of their arrival, which Einstein says is impossible, and on the whole I'm inclined to side with Einstein over "mr_danya_of."

Third, what was immediately obvious is that whoever is taking this seriously has no idea how destructive interference actually works.  Simply put, destructive interference occurs where two waves in the same medium intersect in such a way that the crest of one wave overlaps the trough of the other.  At that point, their amplitudes will cancel.  Here, supposedly these two gravitational waves are exactly 180 degrees out of phase, so they'd cancel completely wherever they intersect.

But if that happened, what we'd see is... nothing.  If the two waves did completely cancel, the result at that point would be an amplitude of zero.  In other words, they'd be undetectable.  This would not somehow "erase Earth's gravity."

Fourth, the Earth's diameter is about 0.04 light seconds, so if a gravitational wave or two passed across us, that's how long the effect would last.  How this person came up with seven seconds as a plausible time duration for something traveling at the speed of light, I have no idea.

Fifth, the gravitational field of the Earth at a given distance is dependent on only one thing: its mass.  As long as the Earth's mass doesn't change, the strength of the field won't, either, regardless how it's jostled by gravitational waves (or anything else).

Sixth, what the actual fuck?

[Image licensed under the Creative Commons AllenMcC., GravityPotential, CC BY-SA 3.0]

I mean, it's a good thing the Earth's gravity isn't going to disappear, even for seven seconds.  If you, unlike the people posting this story, passed high school physics, you may recall that the reason we're all happily glued to the Earth's surface is the pull of gravity -- and without it, Newton's First Law (an object experiencing no unbalanced forces continues at rest or moving in a straight line at a constant velocity) takes over.  We're all right now moving at a good clip -- at the Equator, about 1,670 kilometers an hour -- but our tendency to fly off is counterbalanced by the centripetal (center-pointing) pull of gravity.  If gravity suddenly disappeared, we'd continue moving at our original speed, but tangent to the circle we're currently traveling in.  The Earth, presumably unperturbed, would continue to rotate out from underneath us, and when the gravity switched back on seven seconds later, we (and everything else not moored) would come crashing back down.

I did a quick back-of-the-envelope calculation for my own latitude, just shy of halfway between the Equator and the North Pole, and found that in seven seconds unsecured objects traveling tangent to the Earth's surface would end up about twenty centimeters up in the air.  Falling back to Earth from that height would be a bit of a jolt, and no doubt the sudden change in stress would damage some buildings, but it's far from the carnage mr_danya_of and others are claiming.

But to reassure you that you have no cause for concern, even in that regard... no, NASA isn't "94.7% certain" that the Earth's gravity is going to blink for seven seconds on August 12.  There is no such thing as Project Anchor.  Gravitational waves, and in fact waves in general, do not work this way.  We have far more important things to worry about right now, such as trying to figure out what country FIFA Peace Prize Winner Donald Trump is going to declare war on next.

If you see anyone posting hysterical nonsense about how NASA Admits We're All Gonna Die In August, you should definitely inform them that this is complete horseshit, and suggest that maybe at least reading the Wikipedia pages about the relevant physics concepts might be a good idea before publicly humiliating themselves by pretending they understand science.

So anyway, there you have it.  To the friend who sent me the link, thanks just bunches for further reducing my already-abysmal assessment of humanity's overall intelligence.  Me, I'm going to go back to fretting about real stuff.  Not that this is productive either, mind you.  But at least it's better than making shit up so you have additional imaginary stuff to fret about.

Even I am not that neurotic.

****************************************