Frontiers students (and hopefully others of you reading this as well!) learn that we primarily gather information about the Universe by collecting the light that rains down on us from the distant stars (this is at least true of astronomy. Obviously, when it comes to information about what’s around us on Earth, there are other sources…).
Usually when we think of the information contained by light we think of visual images that arrive at our eyes instantaneously. Both of these intuitive notions (visual images and the instantaneous transmission of light) are approximations. Visible light is actually a small fraction of the full spectrum, which includes invisible forms of light such as x-rays, infrared, radio waves. We detect these using instruments built for that purpose (although we can also detect infrared waves and microwaves directly by feeling hotter as a response to the energy they transmit).
The feeling that light emitted from, or reflected off of some object reaches us instantaneously is an illusion due to the relatively short distance-scales we care about as we live our day-to-day lives *and* the enormous, but finite speed of light. A pulse of light, let’s say emitted by a flashlight turned on and subsequently turned off, will propagate through the air at a speed of nearly three-hundred million meters per second (or 186,000 miles per second, or 669,600,000 miles per hour). If the light pulse were confined to traveling around the Earth’s equator, then the pulse would go around the world almost eight times in one second. It is easy to see why people might assume that light is transmitted through space instantaneously.
The fact that light’s nature is not confined to our intuitive, everyday assumptions has some startling consequences. The broad spectrum of light makes the Universe a vastly richer place. Using instruments like x-ray and infrared telescopes, augmented with computers that can generate images from the gathered data by artificially shifting the invisible light into the visible range, we increase our visual vocabulary, and are able to observe remarkable structures that would otherwise be hidden from us.
The finite speed of light means that when I teach a class–my students see me not as I “am” but as I was a five or ten nanoseconds before, since the light bouncing off of me must traverse (an albeit relatively small distance) to reach their eyes. A somewhat less trivial amount of time passes when we consider the light from the sun. The sun is about 93 million miles from the Earth, so it takes about 8 minutes for light to arrive from there. This means that the sun appears to us as it was a full eight minutes ago (by our clocks). More profoundly still, the light from even nearby stars takes years to reach us, which means that we see these stars as they were years ago. The further out into the Cosmos we gaze the deeper into the past we look as well.
This “lookback” time is what allows us to study the history of the observable Universe, since by collecting light from further and further away, we are able to see the status of the things that emitted that light as they were even as far back as ten billion years (even a little further back than that, actually).
So we see that the finite speed of light has implications for how we might measure time. We do not get a picture of the Universe that is simultaneously up to date everywhere–distance intervenes, isolating us in a tiny bubble of “present time” and populating the world around us with objects that a increasingly out of sync with us, the further away from us they are.
Perhaps this is consternating to you, and you ask, “well, why not just redefine what we mean by simultaneous, and say that everything as we see it is the way it is now and that is just by definition. Then we don’t have to get into all this confusing talk of things that we see now looking as they were at some other time.” Embedded in this suggestion is a key aspect of Einstein’s own redefinition of the time’s nature.
To go further toward understanding our more modern view of time (at least in physics) I need to tell you (or remind you) that the speed of light acts as a sort-of universal speed limit and that light is observed to always travel at that speed no matter how you yourself are moving relative to the source of the light. So, no information can be transmitted faster than the speed of light and nobody ever sees light (in a vacuum) that travels slower than 300 million meters per second.
It’s easy to see that these two statements will have profound consequences, but it isn’t obvious what those consequences are. Without going into details, the basic point is that in order to accommodate these postulate, Einstein built a theoretical framework for physics that necessitated that observers in different states of motion will disagree about lengths and time intervals that they measure with their rulers and clocks. In particular, they will disagree about what occurs simultaneously, which is related (at least in flavor) to what I discussed before regarding lookback time (but mind you, these concepts are related but separate from one another).
Our understanding of the nature of time (and light) would be updated as the 20th century wore on, often becoming stranger and stranger. Perhaps I’ll have more to say about these things myself in a future post, but for now, I leave you with a wonderful discussion of what time is from the Leonard Lopate Show: