Einstein’s unfinished thought experiment

Mark Anderson

Mark Anderson

As you know, I am fascinated by many aspects of Albert Einstein, and one of them is his habit of doing thought experiments. We don’t do those enough!

In Einstein’s case, he mused (picture him day dreaming) about things such as elevators falling through space and painters inside of them, about two-dimensional beetles crawling around three-dimensional wires, and so on.

But his most famous thought experiment has always bothered me. So I was delighted that Mark Anderson, a physicist who writes the Strategic News Service, which offers trend-spotting analysis, echoes my frustration in a recent newsletter. Here he goes:

The most famous scientific anecdote of all time remains half-done, unfinished, although countless authors have told the story of Albert Einstein as though it makes sense. Here is how the “thought experiment” goes: when he was 16 or so, Einstein decided that he needed to travel alongside light to understand its nature. (Drum roll.) In this way, he came to understand Special Relativity, a bit later in life. Wow.

There’s only one problem with this apocryphal story: Special (and General) Relativity talk about time and space. They don’t say a word about light, except as it responds to gravitational force.

So, none of us knows what Einstein saw (or did not see) of the light itself, as he (illegally) screamed along at the speed of light, looking sideways…

Well, I have been doing this thought experiment for a while now, without success. (That is not surprising since I opted out of physics as soon as I could in high school.) Here, by the way, is a cool illustration of it.

It always seemed to me that if I were looking sideways at a wave-like quantum of light going at the same speed, it should not even “exist”. Mark seems to think the same thing:

Waves, at their deepest origins, are relative. If you stand at the shore, in they come. But if you fly along with one, like a seagull – say, at the crest of a traveling wave – there is no motion at all; there is no wave.

Having said that, I remind myself, through the haze of my confusion about such matters, that Einstein’s Relativity ended up being about time as much as space.

So perhaps what happens to a light-beam rider is that time … stops. Which is, ironically, exactly what happened when I opened my email inbox this morning.

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12 thoughts on “Einstein’s unfinished thought experiment

  1. “It always seemed to me that if I were looking sideways at a wave-like quantum of light going at the same speed, it should not even “exist”.” I think the photon would exist; the question is whether you would, since this implies you would also be moving at the speed of light (which is impossible). If you could get your mass moving at the speed of light, I think you would achieve enlightenment – Maslow’s express lane; skipping stages one through five. A guru once said to me, “Remember, Weedhopper, as you sit in Indra’s waiting room that photons are massless and journalists are not.” Not knowing any journalists, I was puzzled at the time. Now I can pass this on to you. (The best gurus are mysterious like that).

    • I was counting on you, Mr Crotchety, to intervene in this one. And how you did. 😉

      Is there a legal way to attempt reaching the speed of light? I’d like to give this a go. Becoming a massless journalist and all that.

    • If you’ve got the money, I suggest you book time at the Large Hadron Collider (the LHC). Check this YouTube out (brilliant):

      If you’re on a tight budget, keep meditating.

    • Cool “rap” video and they managed to make the LHC and all the physics fun. If only Richard Feynman was still alive, I think he would have a fun time in some of these videos playing to the crowd!

    • Well, one of my “thought experiment” is one about the Chinese judicial system.

      Me and my economist friend was talking about the Chinese judicial system and I got us into talking about how would China gets herself into a fair and equitable judicial system. Kind of a “thought experiment” where what if we were ruling China, how would we get out of this mess.

      At the moment, many court cases (those prominent and important enough to be reported by the media) were still show trials. For example, deciding a case in a few short days and sentencing someone to death is a bit too efficient. There was a Canadian model that got murdered in China and the Chinese police were very efficiently (may be too efficiently) in finding “the killer” and sent through the courts. Case close. And then there were many cases of those lawyers that fight for workers’ and human rights in China, and these lawyers being harassed and even arrested by the police/government for doing their legal work.

      Anyway, my friend was more willing to accept the possibility of gradual changes leading to a fair and equitable judicial system. Where I think the Chinese judicial system is not something that gradual changes work.

      To me, if and when the Chinese leaders want to change the judicial system and make it a “fair and equitable” one, it has to be a wholesale change from the lowest to the highest court. A judicial system tends to break at its weakest link.

  2. Apologies for being late and steering a little off topic (though along the lines of kempton’s comments). Can’t help with Einsteins omission…. but….

    I’m a huge fan of thought experiments… especially as they apply in wider contexts. I recently read Alison Gopnik’s (philosopher and developmental psychologist – and sister of Adam) latest book “Philosophical Baby”.
    Her main thesis is that the ability to perform thought-experiments (imagining alternative futures, ‘what if planning’ – she calls these all ‘counterfactual’ thinking) is one of the key things that distinguishes us as humans.

    And my favourite such counter-factual thought experiment is from Rawls in constructing his theory of justice – he thinks the best way to design the rules of a society is by imagining (i.e. thought-experimenting) how those rules work as seen from behind a “veil of ignorance” as to what position you (or each individual) have/has (or will have). Not knowing if you are blessed with strengths encourages you to opt for the set of rules that would be fairest to all. Didn’t explain that well – but as ever there is a good wiki easily a click near by
    Warren Buffet calls this the ‘ovarian lottery’

    If only we applied such thought-experiments more widely (e.g. in health care debate).

  3. Andreas,

    Currently physicists require that massless particles such as photons move at the velocity of light relative to particles with mass. So for a modified version of Einstein’s thought experiment in which:

    1) you recede from the photon’s source,

    2) are caught up with by the photon, and,

    3) absorb it,

    the photon’s measured velocity must be independent of your velocity.

    Moreover, Einstein’s rejection of a light-carrying “ether” forestalls any direct observation of light’s transverse vibrations in that medium. Such waves are now invoked only mathematically: to calculate an emitted photon’s likely point of arrival. So the photon must actually be absorbed somewhere for it’s velocity or wavelength to be measured — one can never observe the wave itself. One might say that light as a vibration in a medium was abandoned but not the mathematical wave associated with that vibration. (The Cheshire cat gone, but not its smile; the physical vibration gone, but not it’s Platonic Form, the sine wave.)

    But there is an interesting wrinkle: your recessional velocity does affect the measurement of the photon’s wavelength. So for the above version of Einstein’s thought experiment the faster you recede, the longer the absorbed photon’s wavelength (this is the famed Doppler Effect). More specifically, the absorbed photon’s wavelength would trend toward infinite the faster you receded from the photon’s source, or the faster the source receded from you, which under relativity is the same thing.

    Of course, Einstein’s version of the above thought experiment required moving at the speed of light and so proved impossible in principle. Perhaps for that reason it was of limited help to him. Special Relativity appears to have been inspired more by his dissatisfaction with Maxwell’s equations and by a sudden insight into the non-absolute nature of time.

    Less clear is the role played by patent applications for synchronized railway clocks, a hot topic at that time, and one that may have helped nurture this sudden insight. The following book explores the role that early 1900s clock synchronization technology may have had on Einstein’s and Poincare’s development of relativity:


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