One of the delights of science is how captivating the results can be. About 1.3 billion years ago, and incredibly far away, two black holes spiraled together, collided at half the speed of light, coalesced into a single black hole, and in that joining three suns worth of mass were turned into energy in the form of gravitational waves. In September of last year the sound of that collision (literally a sound: you can hear it at https://www.youtube.com/watch?v=TWqhUANNFXw) was recorded by the Advanced Laser Interferometer Gravitational-Wave Observatory's (LIGO) parallel scientific instruments in Washington and Louisiana. The waves reached Louisiana 7 milliseconds before Washington, suggesting a location in the Southern hemisphere.
The result, predicted 100 years ago by Albert Einstein, simultaneously proves the existence of black holes and launches gravitational astronomy as a new field of scientific endeavor.
Gravity is both incredibly weak and impressively strong. It is the weakest of the four fundamental physical forces in nature, some 38 orders of magnitude weaker than the strong force, 36 orders of magnitude weaker than the electromagnetic force, and 29 orders of magnitude weaker than the weak force. But at the macroscopic scale it dominates, affecting the trajectory of heavenly bodies and of humans walking around here on earth. Try jumping into the sky and see how far you get
What we know about gravity results largely from the scientific work of the two smartest humans who ever lived, Isaac Newton and Albert Einstein. Newton discovered the inverse-square law of universal gravitation, summarized in this equation:
Put another way, Newton's law of universal gravitation states (and here I quote Wikipedia) that any two bodies in the universe attract each other with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
Before Newton, gravity meant something different: something serious. But the term derives from the Latin “gravitas,” which means weight, or heaviness, and this was the meaning Newton called upon. Newton's law does quite well for predicting most of what we experience, and its application to our solar system led to the discovery of Neptune in the 19th century.
This worked well enough for two centuries, until astronomers found it could not explain the orbit of Mercury. This discrepancy between theory and fact led Einstein to think about gravity, and to his field equations, principle among them:
I do not remember any of this from my sole semester of college physics in the early 1970s, but Wikipedia assures me these are true equations. I do remember the thought experiments that represented the beginning of the two great theories attached to these great discoveries: Newton's apple, Einstein's elevator.
One important consequence predicted by Einstein's equations is the existence of gravity waves, caused by distortions in the space-time continuum around large masses. Einstein himself was uncertain that such waves would ever be detected, or even whether they existed. But now, thanks to LIGO, we are certain they are real.
Gravity & Health
I don't usually think about gravity when I see my patients in clinic, though like many things I don't think about, gravity is always there in the background. It weighs us down.
Recently, astronaut Kelly Scott came back from a year spent at the International Space Station some two inches taller than when he had left. His spinal disks, not as weighed down, expanded, though they reverted as gravity reasserted itself. Scott is one of a pair of twins, allowing NASA scientists a mini-randomized controlled trial of low-G effects on human physiology.
Scott might end up shorter than when he left Earth, and certainly weaker. Astronauts lose muscle mass and bone density. Spend 6 months off-planet and you will lose 10 percent of your bone mass. Even the hearts shrink a bit while in orbit, leaving the returning astronaut faint with exertion. We evolved living at the bottom of a gravity well. Sit on top of the well and molecular adjustments occur. NASA worries about these things, as it would like to send astronauts to Mars some day.
Low gravity's molecular effects aren't perfectly well understood. That old biological workhorse (workworm?) Caenorhabditis elegans has been compared on ground and in space. One study saw down-regulation of numerous genes, whose cumulative effects prolong C. elegans lifespan. Another saw decreased accumulation of toxic protein aggregates in ageing worms' muscles. Space was made for worms searching for immortality.
There is a fascinating plastic surgery literature on gravity. Here's a fun little experiment reported by two groups (Aesthet Surg J 2014;34:809-822, and Int J Cosmet Sci 2015;37:291-7). Take patients who show signs of midface aging and have them either stand up or lie down. Take pictures while smiling and in repose. Compare “brow position, ear trough length and depth, steatoblepharon, cheek volume, malar bags/festoons, and nasolabial folds.” (Steatoblepharon, by the way, is “the prolapse of fat from the orbit of the eye below the eyelid.” A festoon is a chain or garland of flowers, hung in a curve as a decoration, unless you are a plastic surgeon, when it refers to significantly damaged skin in the lower eyelids.)
Then, because perception is as important as measurement if you are a plastic surgeon, have someone estimate their age by looking at the pictures. What happens?
The supine patients measure younger, and look younger. Indeed, if you are over the age of 55, and you want to look younger, never get out of bed. The simple act of lying down makes you look six years younger, on average. Gravity redistributes interstitial fluids, subtly contorting our faces. Remember that the next time you visit a funeral home and hear the magical words “doesn't he look natural?” No, he doesn't. It's just gravity. We sag when we stand. I'd rather sag, thank you very much, as long as I can keep standing.
This redistribution of interstitial fluids occurs in space as well. Fluids no longer pool in the legs, but in the upper body. The body's blood volume sensors are in the upper body, so red blood cell production declines. Anemia is yet another reason to faint when you get back to Earth.
Cancer in Space
Gravity is a boon to plastic surgeons. What about cancer?
When I treat a postmenopausal breast cancer patient with aromatase inhibitor therapy, I routinely discuss the bone loss that results from estrogen deprivation. How does one prevent bone loss? I tell my patients bone is laid down along lines of stress, and that weight-bearing exercise is among the best things they can do to prevent osteoporotic fractures. Though I never mention gravity, I silently invoke its inexorable force.
NASA has spent some small fortune performing experiments with cancer cells in space (examined in Nature Reviews Cancer 2013;13:315-27). Under microgravity conditions, cancer cells form spheroids, sometimes quite large: in one case, golf ball-size organoids of prostate cancer cells that far outpaced their earthbound cousins. Gliomas, in contrast, roll over and die in space, for reasons no one understands.
Virtually all cancer cell types undergo major structural changes in microgravity. Tubulin is assembled into microtubules by gravity-dependent reaction-diffusion processes: get rid of gravity, and the microtubules change their orientation. Would Taxol work differently in space?
So far, I've seen no rush to set up an orbital outpatient oncology clinic. Astronauts do not appear to have a higher rate of cancer incidence or mortality than the rest of us, though the numbers are small and the comparisons highly confounded from a statistical standpoint. But there is, apparently, a thriving trade in pharma-driven space experiments, with a start-up company called Space Tango contracting with around 50 customers this year to perform contract microgravity experiments in tissue-box-size automated labs.
Meanwhile, back on Earth, my ears are peeled for the next echo of two black holes colliding in some distant galaxy. Can you hear it? Let's listen together.