Part 2—Much heat, little light: Last November, Nova set out to make Einstein easy again.
Its hour-long program, Inside Einstein's Mind, was timed to mark an anniversary.
It had been a hundred years since 1915, when Albert Einstein brought forth the general theory of relativity. To commemorate this important event, Nova became the millionth professor, publisher or broadcast org which seemed to think that it knew how to make Einstein easy.
At about the nine-minute mark in its program, Nova began its first attempt at explaining the revolution in physics Einstein created. At roughly that point, it began to trace the first of "the crucial thought experiments that led to his great discoveries."
More precisely, Nova presented the "thought experiment" which led to Einstein's presentation of "special relativity" in 1905, when he was just 26. It would be ten more years before Einstein produced the paper describing general relativity. But in 1905, Einstein's "miracle year," special relativity constituted a huge major step on the way.
Yesterday afternoon, we presented the transcript from the chunk of the program in which Nova described the thought experiment which led to special relativity. To peruse that transcript, click here.
Today, we'll review the very elementary physics involved in Einstein's thought experiment. Tomorrow, though, we'll present an awkward fact:
Nova's presentation doesn't justify the "mind-blowing," semi-metaphysical statements the program derives from that famous thought experiment.
Nova's physics is very basic. Its "metaphysics" fails.
Starting around the nine-minute mark, Nova describes the thought experiment which led to special relativity. This thought experiment comes straight from Einstein himself. As Nova describes it, it involves a man standing on a railway platform; a woman passing by on a very fast train; and a pair of lightning strikes.
The physics involved in this thought experiment concerns the speed of light. That physics is extremely easy, even if Einstein isn't.
Here's how the physics plays:
Light travels at a very high rate of speed, but it does take time for light to travel from one place to another. For that reason:
If Person A is closer to a lightning strike, and Person B is farther from that lightning strike, the light from the strike will reach Person A before it reaches Person B.
That's extremely simple stuff. As presented by Nova, it turns into a major ball of confusion.
Below, you see the transcript from the part of the show where Nova discusses the thought experiment in question. For the fuller transcript, see yesterday afternoon's post. To help you picture what is occurring, you can watch the full Nova program (links below).
Having said that, let us also say this:
When we watched Nova in November, we thought what follows was perhaps the worst non-explanation explanation we had ever seen.
We haven't changed our mind about that. Having said that, here's the part of the transcript in which the thought experiment is described:
From Nova, Inside Einstein's Mind:For reasons we'll discuss tomorrow, that's a terrible non-explanation. In fairness, let's clarify a couple of points the Nova presentation obscures.
WALTER ISAACSON: [Einstein] realized that any statement about time is simply a question about what is simultaneous. For example, if you say the train arrives at 7, that simply means that it gets to the platform simultaneous with the clock going to 7.
NARRATOR: In a brilliant thought experiment, he questions what "simultaneous" actually means, and sees that the flow of time is different for an observer that is moving versus one that is standing still.
He imagines a man standing on a railway platform. Two bolts of lightning strike on either side of him.
The man is standing exactly halfway between them, and the light from each strike reaches his eyes at exactly the same moment. For him, the two strikes are simultaneous.
Then, Einstein imagines a woman on a fast-moving train traveling at close to the speed of light. What would she see?
As the light travels out from the strikes, the train is moving towards one and away from the other. Light from the front strike reaches her eyes first.
For the woman on the train, time elapses between the two strikes. For the man on the platform, there is no time between the strikes.
This simple thought has mind-blowing significance. Simultaneity, and the flow of time itself, depends on how you're moving.
CARROLL: If there's no such thing as simultaneity, then there's no such as absolute time everywhere throughout the universe, and Isaac Newton was wrong.
NARRATOR: This concept, that time and space as well are relative, became known as "special relativity." It led to remarkable results, such as the famous equation relating energy to mass.
First: It makes best sense to picture these two lightning strikes occurring at a great distance from the man on the platform.
(As specified by Nova, "the man is standing exactly halfway between them.")
It makes best sense to picture these two lightning strikes occurring at a substantial distance. Einstein articulates this point in Chapters 8 and 9 of his 1916 book, which Nova's presentation tracks. By way of contrast, Nova's graphics make it look like the lightning strikes hit very close to the man. This doesn't negate the basic logic of the thought experiment, but it's easier to grasp the logic if the strikes are farther away.
Second, and crucially important: The lady on the fast-moving train is immediately adjacent to the man on the platform when the lightning strikes occur.
(Einstein specifies this in his book. Nova doesn't articulate this point in its transcript, although this point is suggested by its visual presentation.)
Because her train is moving at extremely high speed, the lady quickly moves closer to the one lightning strike, and farther away from the other. But in the example, she is directly adjacent to the man on the platform when the strikes occur.
The physics of this homely example is extremely simple. To wit:
Because the man is standing exactly halfway between the two lightning strikes, light from the two lightning strikes will reach him at the same time. On the other hand:
Because the woman quickly moves toward the one lightning strike and away from the other, she will have a different experience. The light from the one lightning strike will reach her sooner than the light from the other strike.
For the man, there will be no lapse of time between the arrival of the light from the two lightning strikes. For the woman, there will be a lapse of time between the arrival of light from the one strike and the arrival of light from the other.
This is very basic stuff. It's related to our own (un-confusing) everyday experiences, even to our experiences concerning lightning strikes.
The basic physics is simple and clear. That said, Nova takes that simple, homely example and creates a ball of confusion.
In the transcript we've posted above, Nova derives a set of "mind-blowing" conclusions from this homely example. (We'd almost be inclined to describe them as "metaphysical" statements.) As stated by the narrator, one of them goes like this:
"Simultaneity, and the flow of time itself, depends on how you're moving."
We're sorry, but that homely example doesn't explain, support or justify that "mind-blowing" statement, which is extremely fuzzy.
Under even the simplest questioning, very few PBS viewers could explain what that statement means. Those viewers shouldn't feel bad about that. We doubt that the author of Nova's program could explain what that statement means, or how it derives from that homely example.
When we watched Nova's program in November, we thought the passage we have posted constituted one of the worst non-explanation explanations we had ever seen.
We've watched the program many times since then. It utterly fails to justify the many dramatic, "mind-blowing" statements it derives from the example of the man on the railway platform and the lady on the very fast train.
Tomorrow, we'll show you why we say that. In our view, our society's culture of incoherence emerges from that transcript in all directions, producing more heat than light.
To watch the Nova program: We've been linking you to Nova's site to watch last November's program.
You can still watch the program there. To do so, just click here.
Yesterday, we noticed that some extremely annoying ads have been inserted into the program at completely random places. One such ad interrupts the passage we've posted above.
To watch the program without interruption, you can just click here.
Einstein married a physicist, yet he still told his mistress, "My wife doesn't understand me."ReplyDelete
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I tried Pepitos Pulverized Pulga Powder on my husband. He came to me and apologized for the wrongs he did and promise never to do it again. Ever since then, everything has returned back to normal.
Our watches are synchronized. Unfortunately, when he get into bed he always miss by a foot and fall onto the floor. that is ok. Time is more improtant than space as long as he is not sharing space with someone else. Besides we have enough children now and nuns terrify them at school.
I am intrigued by the recent ongoing series of posts on Einstein and his theory of relativity. Full disclosure: I am a physicist by training. Special relativity is not my specialty (no pun intended), but I have a decent understanding of it. I have not seen the Nova episodes in question. I have read some of Einsteins’s book, but I have not looked at it in more than two years. Although I have never had full-time employment as a physics teacher or instructor, I like to think that I have good communication skills and can do an adequate job of explaining relativity to non-experts. Needless to say, it cannot be fully explained in a single short essay. However, I would like to try to shed some light on the issue of light signals (pun intended). So, here goes.ReplyDelete
Having read the most recent post (Wednesday, 30 March 2016), I think that what is missing or unclear is the substance of Einstein’s revolutionary postulate regarding the nature of light propagation. This postulate can be stated as follows: The speed of light is independent of the motion of its source. This was truly a novel proposal that violates all human intuition. If I send a light signal to you, and you are moving toward me at, say, half the speed of light, then you would expect the light signal to have a speed relative to you 50% greater than the speed that I would claim it has. This is not true. If you approach me at half the speed of light, and I send a light signal toward you, from your point of view the light is traveling at 300,000 km/s, the same speed that I ascribe to it. (There would be differences. You would see the light as a different color – blue shifted for you.) This is all counterintuitive, and the human mind tends to rebel at such a notion. It was Einstein’s genius to propose the constancy of the speed of light as an explanation to a long-standing problem in physics regarding the speed of light as measured relative to an earth that is moving rapidly around the sun, i.e., an earth that is in motion. (If you are curious about the history, look up the Michelson-Morley experiment.)
What does this have to do with the lady on the train? For her, the lightning strikes are symmetrical in space as they are for the man; they occur at an equal distance in front of her and behind her. Here is where Einstein’s postulate comes in. For the lady, the light signals from behind her and in front of her travel the same distance as considered in her frame of reference. Therefore, the time interval from the light signal origin to her reception of the light signal is the same for the two light signals. But we know that she receives the signals at different times; from the man’s perspective she is rushing toward one signal and rapidly retreating from the other. Since the travel time for the two signals is the same (Einstein’s revolutionary postulate) they must have originated at different times. What is simultaneous for the man on the ground is not simultaneous for the lady on the train. This is a consequence of the fact that the speed of light is the same for any observer.
If I just confused matters more, then tell me so. If this explanation was at all helpful, please comment favorably.
well in your last paragraph you say the light travels the same distance to the lady, however in the Nova example the light does not travel the same distance, she is closer to one than the other.Delete
From the point of view of the man, the woman is indeed moving toward the point of one of the lightning strikes and moving away from the other. In thinking about this from the woman’s point of view, one must imagine oneself in her frame of reference. From her point of view, she is not “moving.” Let me imaging the lightning strikes as occurring at specific points in space and at specific instants of time. From the woman’s point of view all that matters is that a light signal originates from a point in space in front of her and another behind her. Again, I emphasize, that is all that matters. One must not think of these points as moving in her frame of reference. (Admittedly, the atoms from which the light signals originate are moving with respect to the woman. Nevertheless, the emission of the light signal occurs at a point in space in her reference frame.) Once you abandon the notion that she is moving, the things of consequence are the points in space and time where the light signals originate (in her frame of reference!) and the distance the light must travel to reach her. Do not think of her as moving through space. In her reference frame she is not moving. There are simply points of time and space where events (e.g., emission of a light signal) occur.Delete
By the way, using reasonable human-scale numbers, such as a train traveling at 100 km/h and simultaneous lightning strikes one km in front of and behind the train, I calculate that the time difference between lightning strikes in the rest frame of the woman would be less than a picosecond. One picosecond is a trillionth of a second.
I admit that your question forced me to think about this a bit more deeply, and I don’t know if this explanation helps at all. I often wonder if earth-bound examples (like trains and train platforms) are really a good way to illustrate the counterintuitive aspects of relativity. One must abandon the concept of “absolute space.” (Right! Whatever that means!) Imagine yourself floating in interstellar space (better yet, intergalactic space), and you consider yourself at rest. Why? Because you feel no force. You are weightless. (Admittedly, few of us know what this really feels like.) Suddenly you see your friend, Mr. Somerby, whizzing by you at high speed – a constant speed and in a straight line. (He is no doubt typing one his verbose blog posts on his computer.) You think: “Wow! He is moving very fast.” Mr. Somerby, on the other hand, feels no forces; he is not accelerating – just moving in a straight line at a constant speed. From his point of view, he is at rest. You are moving. Who is really at rest? Einstein taught us the following. The very nature of space is such that to ask the question who is at rest, you or Mr. Somerby, makes no sense. The very question “Who is at rest?” cannot be asked. Such is the nature of space. This is what we mean when we say that there is no absolute space – no universal frame of reference that we can consider “at rest.”
"In our view, our society's culture of incoherence emerges from that transcript in all directions, producing more heat than light."ReplyDelete
More heat than light? This from the blogger who has spent how many columns now flogging the same what-is-just-ahead promises on this subject?
Get on with it, will you?
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