1.4 Guest Lecture: Space, Time, and Spacetime

MIT 8.20 Introduction to Special Relativity, January IAP 2021
Instructor: David Kaiser

Professor David Kaiser teaches in the physics department at MIT and is also a historian of science. In this guest lecture, he describes how the most accomplished physicists of the mid-to-late 19th century were thinking about motion of bodies through space and time, and how, at the end of that century, a rather young and very little known person named Albert Einstein began asking similar questions but often in very different ways. (01:13:40)

License: Creative Commons BY-NC-SA

[davidtest]

Thank you Dr. Kaiser. Love this lecture. Still struggling to understand special relativity, but your lecture has helped move me forward.

[arman.astrophysics]

This is great. I mean, knowing the history before learning the subject is a great eye-opener and a good motivation to jump into the subject. Thanks for the great content.

[rgudduu]

Super fantastic story. Thanks. On the downside, i expected an explanation or intuition behind how space-time becomes spacetime.

[WayneRiesterer]

I think Minkowski was hinting at displacement that occurs when contriving arbitrary projections; the 'arbitrary axes' representing components of the observation coordinate framework. When he mentions '...space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union of the two will preserve independence...', he his highlighting the mutual dependence of space and time; that either one cannot exist by itself (fades away), but that they are both complementary pairs of 'spacetime'. This is a bit like the binary digits of 0 and 1. Within the context of the binary system, they remain established as distinct components, but if the binary system boundary is removed, neither of them have a context to hold them. Unlike Minkowski's suggestion though, the moment the validation boundary is broken, the validity of the components are simultaneously inverted. The only thing that I'd add is that space and time are not complementary, but rather that time was derived from dynamic spatial observations and has since been mapped to other phenomena.

A requirement for the observation of any kind of change needs a fixed reference so relative comparisons can be made; as everyone knows. It's a bit like the second-hand of a clock. One second is said to have elapsed when the second-hand changes position, but changes in position are spatial concerns and time is really just an abstract wrapper that accounts for this. The same applies in the use of Caesium-133 which is really a count of physical changes that are then 'mapped' to a conceptual time unit.

When considering the Cs-133 standard for a second, 9192631770 transitions representing 1 second in time is only a projection of 0-9192631770 from one axes to 0-1 on another. The important point is that the actual duration is arbitrarily contrived from other more fundamental observations.

I think confusion sets in when people forget that the entire body of knowledge within the scientific community has been built upon a small set of principles that were established a very long time ago. Even though observations, theories and laws are consistent with those primitive principles, it doesn't preclude the possibility that scientific knowledge as an abstract concern is actually floating in a bubble somewhat removed from the actual raw nature of what it sets out to reveal.

Having said this, I absolutely love science or I wouldn't be here watching this amazing presentation and series of lectures on Special Relativity.

[celsiuselements]

Superb lecture. Interesting to hear Einstein's thoughts on Maxwell's equations.

[new-knowledge]

I know...., we have the 4D environment known as space-time. Now of course, photons of light travel across space at roughly 300, 000 km/s. This magnitude of motion is normally labeled as "c". Secondly, if an object was at true spatial rest within space-time, it would now be moving across the time dimension at that very same "c" magnitude of motion. Everything that exists within the 4D space-time environment, is always on the move with this "c" magnitude of motion. Therefore, all that can be changed, is the direction of which the object is traveling within space-time. The more it is directed to be moving across space, the less it will be moving across the dimension of time. If an object is at true spatial rest, the objects entire length will now extend across space only. If its path is altered such that there is now spatial motion, this change in direction also leads to rotation of the object, such that the object no longer extends across space only, but it now partially extends across the dimension of time. This of course has led to the objects spatial length contraction. Anyhow, the rest, is mere details.

[marioabrantes]

Stupendous lecture. Deserved every single second I used watching it.

[tmo]

I find it interesting that this professor encourages us to NOT be like Einstein... I think he had it right by throwing school to the wind and pursuing his own passions. I mean, he's one of the most famous and world-changing people ever.

[Pavan_Gaonkar_abc]

Thank you sir had a great time watching your lecture

[WayneRiesterer]

Nice video :) It seems like Einstein is mixing up the 'event' itself with the 'perception' of it. If two or more people have dissimilar perceptions of an event because each observational frame of reference differs one from another, then the simultaneous flashing of both lights still occur at the same time, but the perception of each light flash can differ temporally since there is relative movement in a velocity-dependent observation. It takes time for the light to convey a visual 'representation' of the event. Any observer will then perceive the event when their perception is coincident with a representation of the event. Concluding that 'the event occurred at different times' is getting confused with 'the perception of the event occurred at different times'. The same can be applied to the length. To myth-bust this, glue a massive tape measure on the side of the train and ask the observers to report the length of it. They can be moving all over the place and they won't report anything that suggests a spatial contraction.

The length of the train represents a scalar magnitude within 1 spatial dimension that is time-independent unless the length of the train is dynamic with respect to time; which it's not when neglecting thermal expansion/contraction and coupling movement. For this reason, it is a snapshot or instantaneous property that is independent of velocity and therefore moving frames of reference are not to be considered when determining the value of length. One experiment that would highlight this, though it's not really needed, is to install a LIDAR unit inside the train and display the length value on digital displays both inside and outside of the train. Regardless of relative movement and position, the value reported by any observer will be the same. It's like a crack of thunder that's heard at different times by people at different distances. If such people were to push a button to record the time they perceived the sound, the time will be different, but the event causing the sound is the same in all cases and happens when it happens. If the event were captured at the source of it and the time of the event transmitted to all observers, it would be the same snapshot in time.

Once upon a time, the atom was considered a particle. With the discovery of sub-atomic particles, the atom can no longer be classified as a particle because it has found to be divisible. The same can be said as one breaks more and more pseudo-particles apart. Now the Higgs is found, is this the last of it? What's interesting is that nature hasn't changed, just people's ideas of it. The atom has always been divisible, so it was never really a particle according to our definition of particles. In fact, we could have called it what we like, it makes no difference to the nature of it. These things often called particles really represent a bounded context and focal point of enquiry that is recognized by someone and given an identity. This is essentially a wrapper around something not yet understood, something that maps to a particular curiosity in the minds of people rather than to nature itself. What is considered a particle in this case is really a well-defined question; which itself is discrete. The characteristics of the question form a closed context boundary and this gives some people the impression it is some kind of individual entity. If anything in nature were truly separated by some kind of definite boundary, it wouldn't have the opportunity to interact with the rest of nature and we would never know about it.

We are immersed and composed of the very nature that we may choose to study. It is easy to get mixed up when taking perspectives on things and trip over ourselves in the study of something which is essentially ourselves.

[new-knowledge]

54:00 Time dilation. Here he discusses light clocks. One is on the train, and another is at the train station. Each observer perceives that it is the other guys clock as being the clock that is ticking slower. Now to determine if a person can truly teach you about SR properly, then challenge them with the following question. We are able to explain how both observers will view the other guys light clock, as being the clock that is ticking slower. However, please explain how this would also apply to the observation of mechanical clocks. If one mechanical clock is positioned right next to each of their light clocks, explain how each observer would see the other persons mechanical clock ticking slower, equally in measure, as to how much the light clock seems to be ticking slower. Explain the mechanics of this.