Oh no, it's a Cosmic Tsunami!
July 29, 2012 7:51 PM   Subscribe

For a story's sake, let's say a series of enormous gravitational waves caused the fabric of space time to fluctuate. What would that be like for us here on Earth, experiencing the fallout?

I realize that gravitational waves as we know them only alter space-time by incredibly minute increments. But humor me, and let's say that two super-black holes collided and created massive gravitational waves headed towards Earth. I'm trying to wrap my mind around what that would mean for us Earthlings. Earthquakes and other catastrophic damage I imagine, but what about the time element?

I'm interested in both grounded scientific answers ("we'd be dead in an instant so we wouldn't notice anything, duh") as well as fun/silly theoretical spit-balling ("we'd experience brief hiccups of the past before being obliterated, perhaps?")

Apologies in advance for my damnable lack of an astrophysics background.
posted by MrHalfwit to Science & Nature (15 answers total) 7 users marked this as a favorite
Enormous gravitatational waves hold you fast to the Earth despite the fact that it is travelling many thousands of miles an hour in one direction while spinning quickly.

If two black holes coilided neary by, the tidal forces created would disintegrate the earth.
posted by Ironmouth at 8:09 PM on July 29, 2012

If you don't get an answer here, consider asking here: http://what-if.xkcd.com/.
posted by zanni at 8:47 PM on July 29, 2012 [2 favorites]

As I understand it, gravitational waves are transverse, and would manifest as alternating compression/expansion pseudoforces in directions at right angles to their direction of travel. I'm not sure what that would be like if they were strong enough to be perceived— maybe as an omnipresent sound? Collapsing-black-hole systems are supposed to get up into the audio-frequency range towards the end of the event.
posted by hattifattener at 9:02 PM on July 29, 2012

You would most likely feel some variation of spaghettification, nicely described in a fun interview of Neil Degrasse Tyson... with the difference being that you'd experience it as a moving wave rather than the Great Attractor of a black hole. (Visualize the local motion as some variation on a giant pasta shape, depending on the vector of the gravitational wave relative to your position and orientation on Earth... if the wave was perpendicular, you could look momentarily like the world's most exotic limbo dancer before being popped apart.) Your body already experiences a gravitational tides, at a very low level - the force of gravity at your feet is very slightly stronger than at your head, for example - and a gravitational wave would be an extreme variant of this effect.

As you point out, spacetime itself is being stretched in such a scenario, so it is possible that you'd have an extended awareness of this process, but I don't have the math to say how long. The greatest likelyhood is that the Earth would have been devastated by the gravity well of any object large enough to cause such distortions (think of the atmosphere being slowly ripped off) so I imagine that your primary experience would be feeling the effect as a strong wind before you suffocated.
posted by Bora Horza Gobuchul at 9:20 PM on July 29, 2012

If you don't get an answer here, consider asking here: http://what-if.xkcd.com/.

if you don't get a useful answer there, consider asking here: Ask a Mathematician / Ask a Physicist.
posted by russm at 9:24 PM on July 29, 2012

Enormous gravitatational waves hold you fast to the Earth...

No, that's not the phenomenon he's talking about -- that's just gravity. Gravitational waves are caused by giant masses accelerating quickly.

Apparently Feynman had a thought experiment explaining what the effect of large amplitude gravitational radiation would be: the sticky bead argument.
...a passing gravitational wave should in principle cause a bead on a stick (with the stick parallel to the wave velocity) to slide back and forth, thus heating the bead and the stick by friction. A gravitational wave pulse will stretch spacetime behind the bead, pushing the bead forward; after the wave passes through the bead the stretching will occur in front of the bead, accelerating the bead in the opposite direction. This heating, said Feynman, showed that the wave did indeed impart energy to the bead and stick system, so it must indeed transport energy.

So, one effect would be heat. Since gravity waves don't get attenuated by passing through matter (I think), you'd get more or less the same heating effect happening across the whole volume of the Earth, not an explosion at ground level.
posted by justsomebodythatyouusedtoknow at 9:25 PM on July 29, 2012 [1 favorite]

Visualize space-time and gravity using the usual 'rubber sheet' kind of analogy, as in this graphic.

Now visualize the gravity waves as waves on the surface of that rubber sheet, sort of like this graphic. But keep in mind that these are WAVES so they are not just static, they are moving, sort of like this video.

So a reasonably (sort of) good way to visualize the gravity waves is to think of the rubber sheet of space-time, with the sun, the earth, moon, whatever, in it as 'dimples' and then think of these waves moving on that same rubber sheet surface like waves in a lake or a pond.

Now just like waves on the surface of water, those gravity waves could be small ripples, medium, large, or any size. They can also be any *frequency* - that is, the time from one wave peak to another could be a few seconds, a second, a half second, a tenth of a second, a millionth of a second, or any other amount of time.

They can also regularly spaced, like we think of waves in the ocean that produce the surf--they are all about the same size and they come in at pretty regular intervals, and the wavefronts move along more or less parallel to each other.

OR you could have concentric waves moving out in ever-increasing circles from something like a rock thrown in the water or a point source that continually disturbs the water (say a fish thrashing around in one spot--waves will go out from that spot in circles).

AND you could have instantaneous/one-off type waves like those caused by a rock being thrown in the water or by an underwater earthquake or landslide.

Point is, all those types of waveforms can be overlaid on top of the basic rubber blanket of space time. And always think of them as moving, just as waves on the surface of water move.

And if you have a basic understand of what the rubber blanket means, you can start to figure out what different waveforms moving across it would mean in physical terms.

The basic interpretation of the rubber blanket is that points that are deeper have stronger gravity. The points that are high have weaker gravity. So as a wave trough moves through a point, that point will feel stronger gravity and then as the wave peak moves through the same point, it will feel weaker gravity. If there is a regular series of wave peaks & troughs coming through, the object will feel periodic stronger-weaker-stronger-weaker-stronger-weaker* gravity.

If there is just a single 'pulse' coming through, like a tsunami, it could be just one single strong-weak cycle. Or just a few, quickly tapering off.

Depending on how large the waves are, this could be extremely subtle (an effect one millionth or trillionth the strength of earth's gravity) or extremely strong (same size as earth's gravity or 10X larger) or anywhere in between.

And you have to think of the frequency--at relative fast frequencies, like hundreds to thousands of hertz, the main result might be the frictional heating mentioned in the sticky bead link. This could range from minor/undetectable heating of everything to a minor but noticeable warm-up to explosion. At slower frequencies you might experience it as a kind of vibration--anywhere from small to very large. At very high frequencies (like light waves, 10^15 hertz--not sure if gravity waves can have such a high frequency or not), I'm not sure what would happen

Also, you have to think about how fast these waves move. I think it is probably at the speed of light--see here. So you're not going to experience yourself feeling stronger gravity while your buddy in the next room feels weaker gravity. Everything in your immediate perceptual area will be feeling the strong-weak cycles simultaneously as far as you will be able to observe, though there will be these wave-like variations in intensity on a much larger scale.

*Technically it won't just be stronger-weaker because that is assuming gravity works on a two-dimensional axis, up/down, like earth's gravity. Really the gravity force is a three dimensional vector and you could visualize the vector spinning around and as it does the force goes first down, then forward, then up, then backward, etc. Or it could spin front-right-back-left-etc, or in any other direction. So the end result would be much like an unbalanced wheel spinning. It would sort of jerk you around in a regularly repeating fashion. If the frequency (speed of the wheel rotation) is very high it will be perceived as more of a vibration and if the frequency is low it will be more of a bump-bump type of effect as it jerks you around.

A similar but possibly helpful way of thinking of this is to think of the tides and how these are caused by the moon. The moon is basically creating a gravity wave over the entire earth with a period of one day. From your perspective at a point on the earth, the moon's gravity pulls everything up at one time, then straight west 6 hours later, then straight down 12 hours later, then straight east 18 hours later and finally straight up again 24 hours later.

Now imagine that tidal cycle happened once per hour instead of once per day. Now imagine one per minute, once per second, 10X per second, 1000X per second, and so on. This would be very similar to the effect of gravity waves with these different frequencies.

OK, that was a lot longer than I intended. Also I am no great expert in this, so take with some grains of salt.
posted by flug at 11:04 PM on July 29, 2012 [2 favorites]

Now imagine that tidal cycle happened once per hour instead of once per day. Now imagine one per minute, once per second, 10X per second, 1000X per second, and so on.

This brings up an interesting possibility--if the gravity waves have a frequency in the acoustical range (20-20,000 hertz, roughly) it is extremely probable that they would set up resonances in different physical objects and those resonances would have the same frequency as the gravity waves that drive them.

In short, objects would vibrate and we would be able to hear the results. These 'gravity wave induced sounds' could range from very soft/subtle to screamingly loud, depending on the strength of the gravity waves.

You could potentially analyze the audio spectrum in order to get a grip on the frequencies associated with the gravity waves.
posted by flug at 11:11 PM on July 29, 2012

At very high frequencies (like light waves, 10^15 hertz--not sure if gravity waves can have such a high frequency or not), I'm not sure what would happen.

I should say, my first thought is this very high frequency gravity wave, if it exists, would induce anything with mass and charge (ie, the various subatomic particles in every atom) to discharge electromagnetic radiation more or less of that same frequency. So light, x-rays, radio waves, microwaves, etc etc. But I'm not very sure about this.

This would be very bad (ie, potentially everything exploding all at once) and could potentially dissipate a whole lot of energy very fast.

Flip side is, it could only be very destructive if a whole lot of energy goes into it at this frequency--presumably from the black holes in your example. If only a little energy goes into it then the amplitude at these frequencies wouldn't be large and even though it would lead to electromagnetic discharge it would only be a little here and there. So you notice a slight increase in ambient microwave radiation, slight increase in background x-radiation, or a slight glow sort of emanating from everywhere--that type of thing.

On the other hand, if a LOT of energy goes into the wave at these frequencies it mean be a LOT OF electromagnetic energy discharged everywhere, basically everything the wave passes through immediately blows up spectacularly.
posted by flug at 11:34 PM on July 29, 2012

FYI a quick look at the wikipedia article section on gravity wave detection confirms a lot of the speculations I made above. For instance the simplest form of gravity wave detector just looks for the (very, very, very small) vibrations the gravity waves will induce in a metal bar. Another type carefully measures the distance between two masses, which will slightly vary when the gravity waves hit.

So, for very, very strong gravity waves, imagine everything being vibrated like crazy at whatever frequency your gravity waves are, and the vibrations being stronger or weaker depending on how large the amplitude of your gravity waves is.

Also the article mentions that gravitational waves can indeed vary from 10^-7 hertz to 10^10 hertz, depending on the source.
posted by flug at 12:02 AM on July 30, 2012

Also in the interest of pedantry, I must clarify that the idea that gravity waves can be visualized like ripples on the surface of the rubber sheet, and (related) that the gravity wave makes the gravity in a spot strong-weak-strong-weak are both very much oversimplifications.

The idea (which I mentioned above) that the gravity wave is like a rotating 3-D vector, or a rotating out-of-balance wheel or a moon rotating about the earth very, very quickly, is a bit closer but likewise a simplification.

The wikipedia article actually have a fairly nice explanation of how it really works in the section Effects of a passing gravitational wave, along with this helpful animated diagram that shows one example of how a gravity wave would affect particles and the distances between them.
posted by flug at 12:45 AM on July 30, 2012

Perhaps it's not as cinematic as temporal chaos, but a sizable, predictable gravity wave would be a good opportunity to manipulate the global financial markets. One effect of a gravity wave would be that sensors and measuring devices in different places relative to the wave would be slightly out of sync. So for example, a ton at sea level in Hong Kong would be slightly different than a ton at sea level in New York. Things like GPS and long distance navigation would also be affected. Per this previous thread, people are already talking about using the time it takes light to travel across the Atlantic to profit off the difference in trades between New York and London. Knowing where and when a measurable relativistic distortion would hit would allow a savvy investor to trade against that distortion, kind of like putting a cosmic thumb on the scales.
posted by chrisulonic at 7:42 AM on July 30, 2012

The waves themselves I can't speak to, but if space-time was fluctuating, I'm not entirely sure we'd notice since we are in it. We would be in the same frame of reference.

I guess it would depend on the frequency of the wave, wouldn't it? If spacetime is water in a fishtank and the Earth is a fish floating around in it, we wouldn't really notice if the whole tank was bouncing around. We'd only notice higher frequencies.
posted by gjc at 7:58 AM on July 30, 2012

It's a nitpick, but as nitpicks go it's fairly important. Gravity waves are something completely different in physics to gravitational waves.
posted by edd at 7:40 AM on July 31, 2012

ravity waves are something completely different in physics to gravitational waves.

Explanation here.
posted by flug at 12:18 AM on August 1, 2012

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