Relativity in brief... or in detail..
Why there would be no chemistry without relativity
Any chemist will tell you that with no chemistry, there would be no biology, and without biology there would be no psychology.... It's obvious that physics (especially electricity, mechanics and thermal physics) underlies chemistry. It may not be so obvious that chemistry as a whole is dependent on effects that involve relativity, and that the existence of different chemical elements can only be adequately described in terms of relativity. But to see how, we first need to know a little bit about:
The uncertainty principleIt took theoreticians quite a while to realise something that experimentalists have always known: you can't measure something exactly. Fourier was the first to admit that you have to compromise: if you want to measure frequency (how frequently something happens, or the number of occurrences per unit time) then you need to measure for a certain interval of time. And the longer you make that interval, the better your measurement. You don't have infinite time (your research grant runs out before then), so you don't measure frequency precisely. If you measure over a time Δt , you end up with an error in frequency that cannot be better than approximately
Now, in 1905, Einstein was not loafing around. As well as special relativity, he wrote a paper about the photoelectric effect: a key paper in the development of quantum mechanics. Energy transmitted by light of a certain colour is not continuous, but comes in lumps with energy
Werner Heisenberg applied Fourier's result to photons: an error in frequency (Δf) impies an error in energy (ΔE = hΔf). Multiply Fourier's result by h and you get
This constraint is not confined to electromagnetic radiation. Other ways of storing energy involve oscillations too, and this equation is general. Further, it is a statement about waves and oscillations, not (only) about our measurement of them. One poetic way of putting it is that Nature can't measure energy and time better than this either.
So Heisenberg's uncertainty principle has a strong implication for the law of conservation of energy. The variation in total energy with time is not necessarily exactly zero. Rather, the law of conservation of energy may be written that energy is conserved with a precision of ΔE, where
E = mc2 and virtual particlesWhen a particle and an antiparticle collide, such as the electrons and positrons do in modern particle accelerators, they annihilate and their proper energy 2mc2 is carried away by electromagnetic radiation (gamma rays, in this case). Reversing this, we could say that, if we had energy 2mc2, where m is the mass of an electron, we could create an electron and positron. And indeed this reaction often occurs in nuclear physics experiments, too.
Hey, but who needs energy? Taking advantage of the limited fluctuations discussed above, let's borrow 2mc2 and give it back in a time t that satisfies
The strong force, the Yukawa meson, gluons...Enjoy this story, because it is the heart of the work that won Japanese physicist Hideki Yukawa the 1949 Nobel prize for physics.
A typical atomic nucleus has many protons. They are all positively charged and, at typical nuclear distances of 10-15 metres or less, they repel each other really strongly. The neutrons are neutral, so they don't help with the electric force. It is obvious that, within the nucleus, there must be some really strong attractive force, one that involves neutrons and protons. Further, it probably has a finite range because, once a nucleus gets more than 200 or so nucleons, it is not very stable: above this separation, electrostatic repulsion wins out. (See also the discussion of binding energy.)
A force with finite range? Doesn't that suggest a force mediated by virtual particles? So, they only travel a few times 10-15 metres which, at the speed of light, takes 10-23 seconds. Substitute that into the equation above and you get a mass of about 10-28 kg. This is much more massive than an electron (a light particle or lepton) and rather less massive than a proton (a massive particle or hadron). Let's call such particles mesons, for medium mass particles.
Medium-mass particles were found and Yukawa received his Nobel prize.
Particle physics has moved on, and virtual particles are now also used to explain the forces within nuclear particles. Protons and neutrons are made of (electrically charged) quarks. The attractive force between quarks is called the colour force and is transmitted by virtual gluons (for a while, particle physicists competed in inventing whimsical names) and, in analogy with quantum electrodynamics for understanding the atom, the new theory is quantum chromodynamics.
But to return to the title: without E = mc2, there would be no virtual particles, so no strong forces, so no attraction to hold nuclei together. The periodic table would have only one entry, chemistry wouldn't exist, animals made of biochemicals wouldn't exist and you wouldn't be here reading obscure footnotes.