Credit: Herb Thornby
As you may know, I’m teaching a General Relativity course this term, and so I have spacetime on the brain. So it’s particularly fun when I get relativity-oriented emails. This morning, for instance, I found a question from a reader named Dawn who asks:
Okay, the info out there seems to be all about the effects of speed and mass on time e.g. The Twin Paradox, but I just can’t see the why.
Take this rather interesting question I came across… ‘If you could spin a carousel fast enough to get its rim moving at nearly the speed of light, would time stand still for people on the carousel?’. So, here, in theory, time would go more slowly for the people on the carousel. Fine. Buy why? And I don’t mean mathematically why, I mean physically why.
For time to be affected by speed and mass it must be a ‘thing’ (even if ultimately time may not exist). I have yet to see an animation, model or drawing that shows WHAT is PHYSICALLY happening to this thing called ‘time’.
In the same way that we see how atoms and molecules are affected by heat and then understand why things get hot or cold. What is physically happening to the ‘atoms’ of time when they are being subjected to speed or mass? I would be particularly interested to see this in the spinning carousel example. Being able to affect time without traveling.
I linked the answer to the original question from the HowStuffWorks website for the rest of your edification, but I’m not surprised that Dawn came away from it with more questions than answers.
For those of you who aren’t familiar, let me give you two basic results from relativity:
- Moving observers apparently have slow running clocks. The closer you move to the speed of light, the greater the effect. This shows up, notably, in the twin paradox that Dawn mentions in her question.
- Likewise, clocks run slow in strong gravitational fields. Clocks on earth, for instance, run slower than clocks in deep space by about 1 part in a billion.
Part of Einstein’s genius was that he connected these two effects, and noted that functionally, there’s little difference between an observer who’s moving quickly (at the edge of a Carousel, for instance), and one who’s on the side of a hill. In both cases, you’d be pushed outwards (downwards). I even did a technical blog post wherein I worked out the details for you. Up top of the page, I’ve included a much nicer illustration from my upcoming book to illustrate the basic idea.
But all of that is the “what?” and Dawn, at least, came in already knowing that. Her question more concerns the “why?”.
I’m sorry, Dawn, but I think you’re going to find my answer unsatisfying. The reason is that time (unlike an atom) isn’t a thing. To be blunt, an atom isn’t really a fixed thing either, but I think we should probably leave that level of abstraction for another day.
Let me give you a quick exercise to put your mind in the ride state for what comes next.
I stand up and face North, and having done so, I can now describe the objects around me in various ways. There is a chair about 3 feet to my right, a computer monitor 1 foot in front of me, and so on. These coordinates seem like a perfectly valid way of describing things until I decide to pivot.
Suppose I turn 90 degrees to my right. “Forward”(North) becomes “Left.” “Right” (East) becomes “Forward.” and so on. It would have been ludicrous for me to think of the forward direction as something concrete because I can easily turn and make it some other direction.
Put another way, you’re asking the wrong question.
In a previous post, I tried to tackle this question in a geometric way. The idea is that simply by turning, the coordinates that we might label as “x”,”y”, or “z” get switched around but something — in this case, the distance between any two objects — remains the same.
Time is more complicated because we feel intuitively (and wrongly) that it is somehow wholly different from the 3 coordinates of space. While it has a slightly different behavior, the reality is that space-time is really the thing that is unchanged as we turn around or fly through it at high speeds.
In other words, nothing is happening to time as you travel close to the speed of light. It was, and remains, inextricably combined with space, and you should simply think of the whole operation as looking at spacetime at another angle.