Jason Lisle. The Physics of Einstein. Biblical Science Institute, 2018.

In my younger days, I picked up a couple of books on Einstein’s theory of relativity. Honestly, I found them hard to understand, so I more or less gave up on learning about it. Having said that, I realize that over the years I have picked up various bits of knowledge concerning it. For example, I understand the famous observation of Mercury during a solar eclipse which showed that light is affected by gravity.

I also recall in high school physics that we derived the E=mc² formula from a formula that involved wavelength (λ). No, I no longer remember how it was derived, but I saw that “anyone” with knowledge of basic algebra could derive the formula from the basic physics. It would take Einstein to understand its significance. After reading and faced with teaching Stoppard’s Arcadia, I gradually read more about particle physics and related things. Some of the books I read have been reviewed on these pages.

Still, Lisle’s The Physics of Einstein explained relativity in a way that I could understand. Perhaps it is only that now I am older and have been exposed to the ideas longer. I do believe, though, that Lisle simply explains relativity in a very clear manner. The reader does need to recall high school algebra, square roots, and Cartesian geometry (the x-axis, y-axis grid). Lisle includes one formula in the appendix that has some calculus, but this is strictly optional for someone who knows calculus. The book is still understandable without that addition.

The Physics of Einstein notes that the theory of relativity could probably more clearly be called the theory of invariance. This is the idea that the speed of light in a vacuum does not vary. It is always a little over 186,000 miles per second. (The book actually has it out to four or five decimal places.)

A main theme of this book is that the theory of relativity was developed not by experimental observation, as, for example, the three Newtonian laws of motion. It was derived by looking at the mathematics of electromagnetic waves, especially light waves. Einstein showed that if the mathematics represents reality, then there are things about electromagnetic and gravitational forces that require us to look at things a little differently.

The book does a great job of illustrating and explaining such ideas as time dilation and showing how Einstein showed that time and space are connected. In our everyday lives, time appears separate from any phenomenon we observe. But we see that as objects approach the speed of light, the perception of time changes, and time itself slows down.

This is not merely because an item might be far away, say a star that is four light years away, so ostensibly its light would take four years to reach us. If that is so—and Lisle shows it is actually unproveable—then we would be seeing something from that star that happened four years ago. No, what Einstein showed is that approaching the speed of light, mass decreases and time slows down as the speed increases.

Some of this we can experience, but in very slight ways. For example, the atomic clock in Colorado loses microseconds over time compared to the atomic clock in London because the Colorado clock is about a mile higher in elevation. The tug of the earth’s gravity slightly less on the Colorado clock, so it “ticks” at a minutely slower rate. Because relativity involves such high speeds, only an atomic clock would be able to detect the difference.

The reason the theory is usually called relativity rather than invariance has to do with observations of objects or particles moving at high velocities. The speed we observe has to do with our relative speed as well. Some of this is obvious with Newtonian vectors. If I am in a car going thirty miles per hour and a car passes me in the opposite direction going at thirty, it would be going sixty miles per hour away from me.

That could mean that a light beam going in the opposite direction of another light beam would appear to a photon on that light beam is going twice the speed of light. Except nothing goes faster than c, the speed of light. So what happens? Light always travels at the same speed in the same medium—invariance.

Because space and time are actually related (people speak of the space-time continuum), then certain time concepts like simultaneity become problems. Technically, even a small movement involves a loss of mass which affects the time in which things happen in space.

We have all read that people on a spaceship approaching the speed of light would age at a significantly slower rate than people left on earth. Science fiction is full of such stories. That is what would happen. Of course, at a high rate of speed one’s mass would be reduced also. We cannot anticipate what that reduction of mass would do, but that space traveler might not survive at such high speeds.

Lisle even has a chapter entitled “How to Build a Time Machine.” At high speeds, it might be possible to make a machine that goes into the future a bit. (That might create other problems, according to Split Second). However, it would be mathematically impossible to go backwards in time. The formula would require something like dividing by zero.

We reviewed the book Starlight and Time here a while back. Starlight and Time understands the question of both time dilation and gravitational dilation. Applying that to the big bang and an expanding universe, time at the center of a small mass containing all the mass of the universe would be extremely slow. The outer fringes could be billions of years faster.

The Physics of Einstein takes a slightly different approach to the question of light from distant stars. Lisle notes that light going in a single direction could be considered instantaneous, going at an infinite speed. This is mathematically solid and, though even less intuitive than space-time, could explain why we see distant stars and perhaps even see them at the same time the events actually happen—whatever the term simultaneous may mean!

The Physics of Einstein also gets into black holes and models for those.

This book really delivers on its promise. I cannot say I get or remember it all. Nor can I say that I could teach it, but The Physics of Einstein is the most helpful introduction to relativity for the layman that this reviewer has read.