In physics you have four fundamental forces and four fundamental dimensions and two fundamental types of stuff with associated properties and fates. In each case you have something, the odd one out - the one that is not symmetrical - the jokers in the physics deck. What are they?
1) You have four fundamental forces of which three have symmetry.
*Electromagnetism or the Electromagnetic Force where symmetry abounds - magnetism can generate electricity; electricity can generate magnetism. Electricity/magnetism symmetry includes the positive vs. the negative; the northern pole vs. the southern pole of a magnet; attraction vs. repulsion; the negative electron vs. the antimatter twin, the positron.
*Strong Nuclear Force has symmetry in that within the nucleus of an atom, positively charged protons repel each other - protons push outwards, yet gluons keep them in their assigned place within the nucleus - gluons pull inwards: attraction vs. repulsion.
*Weak Nuclear Force has symmetry in that particle interactions can go in either direction. Weak interactions govern radioactivity. Radioactive nuclei can obviously be created; they also obviously can come apart at the seams (radioactive decay)!
*Gravity (the Joker): Gravity is unidirectional - it is attractive only. There is no equal and opposite antigravity except in the minds of science fiction writers.
2) There are four fundamental dimensions (ignoring unverified string theory) of which three have symmetry.
You need all four dimensions (space-time) in order to specify any particular event. You cannot have a happening in space without also having it happen in time; you cannot have an event that happens in time without it also happening in a three dimensional space. Yet only three of these dimensions are symmetrical.
*Left & Right is obviously symmetrical. These movements can be undone or reversed.
*Back & Forth is also obviously symmetrical. These movements can be undone or reversed.
*Up and Down are two directions that are obviously symmetrical. These movements can be undone or reversed.
*Time (the Joker): Time is unidirectional. Time flows in one direction only, from past to present to future. There is no equal and opposite arrow of time that extends from the future to the present and onto the past. Time cannot be undone or reversed. You cannot go back in time and change what has already happened. You remember the past; you do not remember the future.
3) There are two kinds of stuff plus the properties of stuff (like velocity, temperature, pressure and density) and the ultimate fate of stuff.
*Mass: There's a conservation law - the conservation of mass/matter - which states that matter can neither be created not destroyed, only changed in form. Matter (mass) can be converted to other forms of matter. You can go from a solid to a liquid to a gas and back again. You can go from hydrogen and oxygen to water and from water to hydrogen and oxygen. You can fuse hydrogen into helium (which powers the Sun) which is also an example of the equivalence between matter and energy (see below) since that fusion process releases a lot of energy at the expense of a tiny bit of mass. Matter (mass) also has another form of symmetry - antimatter. Antimatter is the same as matter only with opposite electric charge (like the negatively charged electron and its antimatter counterpart, the positively charged positron).
*Energy: There's another conservation law - the conservation of energy - which states that energy can neither be created nor destroyed, only changed in form. Energy can be converted to other forms of energy. The chemical energy inherent in petrol gets converted to the kinetic energy of motion and heat energy. The electromagnetic heat and light energy from the Sun powers up green plants, which in turn convert that solar radiation to chemical energy and ultimately your petrol as a fossil fuel, or fuel you directly as you munch on your salad.. Unlike matter however, there is no anti-energy since energy doesn't have any charge. But I hear an objection here. What about electrical energy? Surely electricity is the flow of electrons and electrons have negative charge.
There's another conservation law - the conservation of charge. An electron just cannot shed its negative charge and remain an electron. The electron in fact doesn't shed its charge after electrical energy has been converted to other forms of energy. Just think of your everyday household electrical appliances. Electrical energy gets converted to sound, heat and motion in your electric razor; ditto your electric tea kettle. Your TV set receives electrical energy which is converted to light, sound, and heat. Your electric radiator converts electrical energy to heat and light; your flashlight battery converts chemical energy to electrical energy hence to light (and some heat). But sound, heat, light, motion etc. doesn't not contain any charge. The electron's negative charge does not literally get converted to heat or light or motion or sound. So it's not the electron's charge itself that's the source of the energy in electrical energy.
There's one other broader conservation law which combines the conservation of matter and the conservation of energy. One of the forms matter can be changed into is energy; one of the forms energy can be changed into is matter. The symmetries between mass and energy relate as we all know from Einstein's most famous of equations, mass equals energy; energy equals mass. Mass has often been described as 'frozen' energy. So antimatter should also obey that relationship. Matter can be converted to energy; antimatter can be converted to energy. If matter of one charge and antimatter of the opposite charge meet, you also get energy - a 100% conversion to energy - but there's no longer any charge since energy isn't electrically charged. The positive charge and the negative charge cancel.
But there's an exception to that rule - we think. If you have a matter Black Hole, and an antimatter Black Hole, and they merge, you just get a bigger Black Hole without the ka-boom. The 'we think' bit is because we can't actually see inside a Black Hole so we don't really know what's happening inside. For all we know, all the hell of matter-antimatter annihilation has broken loose, but the resulting conversion of matter and antimatter to pure energy is energy that still can't escape the gravity of the Black Hole to let us know what transpired. For the sake of argument, I'll assume real events in real time can happen inside a Black Hole; real physics can happen inside a Black Hole.
If the merger of equal amounts of both matter and antimatter can be converted to 100% energy, then energy can create both matter and antimatter in equal amounts. And in fact the vacuum energy; quantum fluctuations, verify that virtual particle pairs - one matter, one antimatter can and are in fact created. However, these particle pairs usually then immediate annihilate again to pure energy, restoring the borrowed energy that created them back to the cosmos. More symmetry!
The properties of stuff (like temperature) can go up and they can go down (symmetry). There's no preferred direction.
The joker comes into play when you ask what eventually happens to stuff. Left to themselves, things go from order to disorder; things cool off; eggs don't unscramble; your automobile doesn't un-rust, an exploded firecracker doesn't revert back into an unexploded firecracker. That unidirectional fate of life, the universe and everything is termed entropy. Entropy is not symmetrical. The Universe ultimately 'dies' when everything that is in the Universe, is in the ultimate state of disorder it can achieve. Translated, that means when the Universe attains the same temperature everywhere, what's referred to as the 'Big Chill' or the 'Heat Death' of the Universe. Don't lose any sleep over that - it won't come to pass for trillions of years yet.
4) What are the relationships between the three jokers?
Recall that there's gravity (which refuses to be unified with the other three forces of nature); time (which carries you along for a wild and woolly one-way ride whether you like it or not); and entropy (which in the final analysis is going to make a real irreversible mess out of you).
*Gravity [+] Time: This is a lopsided, unidirectional relationship. Einstein's General Theory of Relativity tells us that the higher the gravitational field, the slower time passes. So, clocks at the surface of the Earth will tick-tock ever slightly more slowly than identical clocks that are on the Moon which has lesser gravity relative to the Earth. However, the passage of time influences gravity not a bit. Gravity gets neither stronger nor weaker with the passage of time.
*Gravity [+] Entropy: Gravity has no apparent broad-brush effect on the natural decay of the Universe from order to disorder. Entropy in turn can't affect gravity because gravity itself just is; it has no more or less order today than it did yesterday. However, when gravity becomes extreme, say when you form a Black Hole, what you get in the centre, the heart of a Black Hole, is a singularity which is about as uniform a bit of stuff as you are ever likely to encounter. Well, uniformity is in fact the highest state of entropy - maximum disorder - that you can obtain. You can't get more disordered than something which is 100% uniform, for example like the ultimate 'Heat Death' of the Universe referred to above. A singularity has no structure, no architecture, and no distinguishing features that can further degrade into something even more disordered and bland.
*Time [+] Entropy: Time flows in one direction; entropy flows in one direction. However, in some cases, one can change the direction of a small bit of entropy albeit while adding to its general unidirectional flow in a broader context. For example, one can assemble a jigsaw puzzle, going from an initial disordered state to a more ordered state. However, that's at the expense of energy expenditure on your part, ordered energy that has been transferred now into energy (of heat, movement, etc.) that's no longer available to you after-the-fact - an overall increase in the disorder of the Universe even if a small part of the Universe, the completed jigsaw puzzle, has locally reversed the general trend. That's not a trade-off that's possible with time. You can't grow a bit younger at the expense of making something else grow a bit older a bit faster.
5) So which one (if any) is the greatest joker of them all?
*Gravity is weird. Unlike say electromagnetism, you can't shield yourself off from gravity - there's no barrier you can put between yourself and gravity (and thus help reduce the daily wear and tear on your body or improve your athletic skills). Since its range is infinite, there's no escape. However, it does drop off in intensity via the inverse square law relationship. That is, if you go twice as far away from the source, the force is one quarter of what it was. If you go three times farther away, the force is but one-ninth; four times farther away, one-sixteenth, and so on. You can also 'cheat' gravity by living on a lower gravity object, like our Moon (one-sixth Earth's gravity) or on an asteroid. Even though gravity dominates the entire Universe; holds stars and solar systems and galaxies even clusters of galaxies together in its embrace, and even though there's no shield that cuts it off, if you hate gravity that much, there's a solution. You of course could just fall in gravity's well of attraction indefinitely, and as long as you're falling, you don't experience or feel any gravity. You are weightless. Translated, go into orbit and experience Zero-G. Orbiting is just indefinite freefall under gravity's unrelenting pull.
However, gravity probably isn't the ultimate joker in the physics deck. That's because there does appear to be at least a quasi-symmetrical counterpart, an 'antigravity' of sorts, called "dark energy" (or "funny energy"), also known or related to Einstein's Cosmological Constant (revisited); quintessence; or quantum fluctuations. Dark energy is a repulsive force that is apparently causing the expansion of the Universe, contrary to commonsense, to ever accelerate. Also, physicists are 100% convinced that gravity can be, ultimately must be, unified with the other three forces. There is no logical way, it is in fact unthinkable, that there are two sets of software running the cosmos, unless of course the cosmos we think of as real is just a simulated, virtual reality universe that resides on someone's (or something's) super-computer.
*Entropy is not quite the ultimate joker either since local pockets of entropy can be reversed at the expense of increasing entropy outside of that local pocket. Your life and activities are one constant battle trying to reverse local entropy, but all that is at the expense of increasing entropy (via expending your store of energy) in the broader arena called the cosmos. You'll win all the battles, except the last, but those local reversals were fun while they lasted.
*Time is the ultimate joker in that it's unidirectional everywhere for everyone and despite all the speculation about physics allowing time travel to the past, well there really does appear to be, in astrophysicist Stephen Hawking's phraseology, a "Chronology Protection Conjecture" making the Universe safe for historians. Apart from that, we just don't see time travellers from the future coming around to gawk at their primitive ancestors.
But even if you could go back in time, it achieves nothing for you personally. Time travel to the past, even if possible and when postulated in the popular or technical literature, you don't anti-age. If you time travel back to the year of your birth, you don't revert back to being a baby again. You still retain your before-the-fact actual chronological age, and you just keep on ageing, ever ageing at one second per second.
Now time, rate-of-change can slow down for someone undergoing high and constant acceleration and/or being in the presence of or experiencing a very high gravitational environment. That was postulated by Albert Einstein and has since been experimentally verified many times over. The fly in the ointment is that the slowing down of time, the slowing down of rate-of-change, is only from the point of view of an external observer. The person undergoing the acceleration, or experiencing a high gravitational field, still notes their own personal time ticking away at the usual rate of one second per second.
Time is absolutely the one thing in physics you have no control over. Nothing you do will ever alter your personal rate-of-change (ageing) at that rate of one second per second. And that's the unfortunate bottom line.
In conclusion from this little, admittedly layman's analysis, you can escape (even if not shield yourself from) gravity's joker; you can reverse your local entropy joker; but you are absolutely powerless against the ultimate joker, time.
I'm not entirely sure what the actual significance of any of this is, but if there is none, that still leaves behind some rather amazing asymmetrical facets that's part and parcel of our Universe. However, as we've seen, the trio of jokers have relationships between them (time and entropy are obviously connected; ditto gravity and time; and gravity can freeze entropy at say a singularity that lies at the heart of a Black Hole). Ultimately there's still a lot yet to be learned about the nature of gravity, entropy and especially time, and why these pieces of the cosmic jigsaw don't seem to fit all that neatly into the overall puzzle due to their asymmetrical relationships within that broader cosmic context.
