P1: Gravity
In the early 1980s, I was working for the Australian Government, in a statutory authority. It was a demanding time, usually working 12 hours a day or more, often with several long sessions during the weekends – some right through the night! My escape was music and theatre. I was a regular at concerts at the Melbourne Concert Hall, and at plays put on by either the Melbourne Theatre Company or the Playbox Theatre Company. Each year I would buy subscription tickets, and while work would get in the way, I did get to many events. The 1983 Playbox subscription series of plays included Insignificance, by Terry Johnson.
I didn’t know what the play was going to be like, but it had a fascinating description: “New York. 1953. High above the city, in a luxury hotel bedroom, on a hot summer’s night, four of America’s most famous legends—a beautiful film star, a Nobel Prize-winning scientist, a renowned baseball player and an infamous senator—meet for an extraordinary confrontation”. The four comprised Albert Einstein, Marilyn Monroe, Joe DiMaggio and Senator Joseph MacCarthy. There was no way they could have ever met, especially not in a hotel bedroom, but it made for a great fiction. Einstein was played by Carrillo Gantner, the then Artistic Director of Playbox, and Marilyn Munroe by Kate Fitzpatrick: both were to prove extraordinary in their roles.
There is a kind of compelling fascination that takes hold of you attending a really well produced play where you have no idea what is going to happen, except a reassuring belief that the experience would be engaging in some way, or at least you hoped so! However, if this play sounded odder than most it was captivating. Terry Johnson explained “the real question is why we make heroes for ourselves in the first place. Certainly most of them are extraordinary people, but do we enthrone them out of respect for their extraordinariness (and promptly dethrone them if they prove mere mortals after all) or is the whole business a feeble excuse for not exploring the extraordinary possibilities within ourselves?”
Insignificance was later made into a film, and I won’t spoil the experience in case you decide to watch it. Instead I’ll focus on one scene, where Marilyn Munroe explains relativity to Einstein, with the aid of two toy trains. It scene begins with her waking Einstein:
– Have I disturbed you?
– No. No.
– Shall I go?
– No. No.
– It’s late. I just spent the last four hours of my life having my skirt blown up around my ears. They rigged up this fan beneath the grating out on 53rd. Whoosh. All night long. Do you ever get the feeling it might be later than you think? Anyway, I just knew my only chance to see you before you fly away or I died of intimate exposure would be to wake you up in the middle of the night. So I said to myself, “Go ahead,” because if he doesn’t understand how you have to wake people up in the middle of the night sometimes then nobody will. So I thought, “what the hell?” …
[Munroe starts to explain things] – Now then we have to imagine a man driving in a car at 30 miles an hour and a hitchhiker standing by the road waiting for a lift. Now, the car’s traveling at 30 miles an hour. And the man inside the car throws a stone at the hitchhiker at another 30 miles an hour. Now, he’s a league pitcher. So the question is, if the car’s going 30 miles an hour and the stone is going at another how many miles an hour is the stone going when it hits the hiker? Answer?
– [Einstein replies] Sixty miles an hour. Right?
– [Munroe] Mmm. Pretty straightforward. But now let’s forget about the stone. Instead, we’ll imagine the car is traveling at 30 miles an hour, and he… Wait a minute. We have to put the hitchhiker back. All right. Imagine. The car is driving along and he’s flashing his headlights at the hitchhiker telling him to get the hell out of the road. Does the light travel 30 miles an hour faster? The answer… no. Why? Because the speed of light is always the same. Right? Did you ever prove that hypothesis?
– [Einstein] It’s never been disproved. Let’s hope it never is. Mm-hmm.
– [Munroe] You ready? Here we go. We have to imagine two locomotives speeding past each other at a hell of a speed. A red one and a green one. Now, the driver of each train – You’re the driver of the red train. Turn it on when I say go, okay? Has a flashlight which he turns on at the precise moment that they pass each other. Now remember, the light from the flashlight travels at the same speed regardless of the speed of the flashlights themselves. …
– I travel away from you at a hell of a speed. Say, one-fifth the speed of light. And I travel for five minutes, and it gets me here. Now, I look at my watch. It says 20 minutes past 8:00. But it’s not very reliable so I look across the universe to check with your watch. And what does your watch say?
– [Einstein] Twenty minutes past 8:00?
– [Munroe] Not to me it doesn’t. It says 19 minutes past 8:00 because 20 minutes past 8:00 hasn’t reached me yet. It takes a minute for me to see your watch because it takes a minute for the light to reach me. See? So your watch appears to be getting slower and slower. And now comes the thousand-dollar question. Remember, if you look at my watch it’s gonna take a minute for it to reach you too. So now what do you say my watch says?
– [Einstein] Nineteen minutes past 8:00.
– [Munroe] Which means you say I’m going more slowly than you while I say you’re going more slowly than me. …
Whoa! I was dazzled by Marilyn Monroe’s explanation, or perhaps I was dazzled by Kate Fitzpatrick’s portrayal of Marilyn Monroe. The play moved on, and I think Joe DiMaggio was about to appear. However, I was left thinking about trains. I had some kind of weak understanding about relativity, and Insignificance played on that. However, Einstein put all that within a broader framework about space-time dimensions, and my high school level of science wasn’t up to his analysis. I did notice we didn’t move on to an Einstein-Monroe analysis of gravity, but then the magic of theatre took over, and I forgot about it.
Forty years later, towards the end of 2023, I have begun to feel rather overwhelmed by what seem to be never-ending disasters. The war between Russia and Ukraine shows every sign of becoming a matter of long-term attrition, and the future of the Ukraine nation increasingly dire. Israel is in the middle of smashing Hamas to smithereens in Gaza, with horrendous civilian casualties (and very little likelihood of obliterating Hamas in the long term). Global warming seems to have reached a critical point, with average temperatures crossing the +1.5 degrees threshold. On top of all that, AI burst into our lives, along with dire threats about artificial intelligence taking over the world. Disasters everywhere.
Commentators were heard to report on the gravity of the events they were describing, an expression which highlighted the degree to which the situation was extremely serious, worrying or even solemn. I checked in the online dictionary and found “You don’t understand the gravity of the situation” is basically saying that “you are not taking it as seriously as you should”. These pundits were reinforcing the weightiness of the matters under discussion. Some reports even went back to source of the word, explaining that experts were talking with ‘gravitas’.
I am not clear as to why this happened, but in the face of all this horror, I retreated from trying to make sense of the senseless and found myself hoping to find sanctuary in good old cold, hard science. Marilyn Monroe hadn’t got around to helping me with understanding gravity forty years earlier, and I realised that among the many puzzles that still fascinate me, gravity is one of the most challenging. It’s not that I don’t understand there is a force, which we call gravity, one which ensures that when I let go of an object it falls towards the floor. Equally important, that same force stops me from floating up to the clouds. However, while I experience gravity, that does not mean I understand it. For a long time, I was happy to accept the definition of gravity which I was taught in high school physics, it is a ‘fundamental force’ which causes all physical objects with a mass to be attracted to one another. It took me a while, but I eventually got the point that ‘mass’ and weight’ are not the same thing: weight is mass proportionally affected by gravitational attraction. Once I saw astronauts bouncing about on the surface of the moon, there was the evidence I needed to see: those astronauts could do massive leaps because the gravitational attraction of the moon is much less than that of the earth: they weighed less while, obviously, their mass remained constant.
As one of those people who likes numbers and equations, I soon learnt the basic rule of physics, w = ma, (weight = mass x acceleration). I’m holding an object (let’s settle for a pencil): when I let go, its mass combined with the force of gravity ensures the pencil falls to the ground, its weight plunging it through the air. I could weigh the pencil by putting it on a scale where the mass and the force of acceleration acting on the scale would give a measure of weight. The pencil would weigh less (a minute amount less) if I weighed it on top of Everest, and more if I took it to the bottom of a deep-sea trench – wouldn’t it?
A quick check, and I think I have that right. The equation that measures gravitational force between two objects is (g x m1 x m2)/d2: g is some strange thing called the gravitational constant, m1 is the mass of one object, m2 the mass of the other, and those three items are then divided by the square of the distance between them. That last bit is important: gravity diminishes rapidly with distance. So, I can summarise that by saying the earth is a big mass, and I am a very light one: I and the earth attract each other, but the effect of the gravitational force on me is large, whereas I have a minute (possible unmeasurable) effect on the earth.
If only I could leave things alone, that would be it. But I can’t, so let’s go a little further. Gravity is defined as a fundamental force. There are four fundamental forces: gravity, electromagnetism, weak interaction and strong interaction. Fortunately weak and strong interactions are minute subatomic forces, and we can forget about them (as far as this discussion is concerned), and that leaves us with gravity and electromagnetism, both of which are long-range forces whose effects can be seen directly in everyday life.
The electromagnetic force is carried by the photon, which creates electric and magnetic fields which holds atoms together, as well as facilitating chemical bonding (all that stuff about positive and negative ions we learnt about at school), together with electromagnetic waves, which include visible light as well as shorter and longer wavelength phenomena and is the basis for electrical technology. There is something reassuringly solid about light and magnetism. You can almost see those waves. You can play with magnets. But gravity? You are unable to see it, and it is even harder to understand how something like gravity acts at a distance. For a long time now, scientists have hoped they would be able to identify some ‘gravitrons’ as part of a possible theory of ‘quantum gravity’ and put us out of our misery. Unfortunately, there hasn’t been any success.
Yes, it is almost time to return to Einstein, but just before we do let’s just summarise the understanding of the field for those of us who find some aspects of late 20th Century thinking tricky. Pity Marilyn Munroe isn’t around to help us! Gravity is an invisible force that acts through empty space. It is a force that attracts bodies based on their mass. So, the puzzle is ‘How does it attract?’ Is it the force that creates black holes, those supermassive vacuum cleaners in space that suck up everything around them creating an incredibly massive and incredibly small location? Action at a distance with no means of communication is always puzzling and gravity is a particularly vexatious example. As for those black holes …
When Einstein looked at gravity, it had been described as a fundamental interaction which causes mutual attraction between all things that have mass. Gravity is the weakest of those four fundamental interactions, between approximately 1038 to 1029 times weaker than the other three forces. As a result, it has no significant influence at the level of atomic particles, but it is the most significant form of interaction between objects on the large scale, and it determines the motion planets, stars, galaxies and even light. On Earth, as already noted, it gives ‘weight’ to physical objects, from keeping us on the ground to the Moon being responsible for ocean tides. Gravity underpins many of the large-scale structures in the universe, with infinite range, although its effects become weaker as objects get farther away.
However, although we didn’t get the details in Insignificance, Einstein’s general theory of relativity describes gravity not as a force, but as the “curvature of spacetime, caused by the uneven distribution of mass, and causing masses to move along geodesic lines” (which are the shortest distance between two points). The most extreme example of the curvature of spacetime is a black hole, from which nothing—not even light—can escape once past the black hole’s ‘event horizon’. For most applications, gravity is well approximated by Newton’s law, as I tried to outline it above.
Einstein proposed that spacetime is curved by matter, and that free-falling objects are moving along locally straight paths in curved spacetime, those geodesics. As in Newton’s first law of motion, Einstein believed that a force applied to an object would cause it to deviate from a geodesic. For instance, people standing on the surface of the Earth are prevented from following a geodesic path because the mechanical resistance of the Earth exerts an upward force on them. Moving along a geodesic without any force being applied in spacetime is what happens when we talk about inertia. Hmm, this getting tricky. I might be confused already!
Einstein’s description of gravity was quickly accepted by the majority of physicists, as it was able to explain a wide variety of previously baffling experimental results. A wide range of experiments have provided support for the theory of general relativity. Today, Einstein’s theory is used for all calculations where absolute precision is desired, although Newton’s inverse-square law continues to be a useful and fairly accurate approximation. While we’ve made significant progress in understanding gravity, it remains a fascinating and somewhat mysterious. Whether it’s the force that keeps us grounded, or ensure planets orbit the sun, gravity is a fundamental yet ‘invisible’ force. Despite forty years of progress, I think I need Marilyn Munroe, or at least the fictional version of her in Insignificance, to explain what we know about gravity, because I am clearly confused. Please explain: what is going on?
Here is the test data for the page. Here is the test data for the page. Here is the test data for the page.
