Many others agree. Here’s a short video entitled, ‘Relativity Paradox,’ featuring Michael Merrifield, Professor of Astronomy at Nottingham University. Early on in the video, Merrifield says:You see the idea: whatever processes the astronaut has going on in the rocket, including his own heart-beat … the space-station friend must observe them to be going in ‘slow motion.’ It applies to chemical reactions, to biological phenomena, to radioactive decay, to anything. But when all processes are slowed down like this, we may as well say that time itself has been slowed down: that is the summary, and the phenomenon is called ‘time dilation.’—The Wonderful World of Relativity, Oxford University Press (2011), p. 104.
You see something going past you, something that’s travelling relative to you … the clocks in that reference frame seem to be going slow relative to your reference frame, so that time, you know, the clocks tick more slowly, and everything goes more slowly, biological processes go more slowly, radioactive decay goes more slowly, and so on. So, really, time really is slowed down when something is moving relative to you. (0:21-0:38)
Suppose I have a clock with me that ticks every two seconds. But if you’re moving at a third the speed of light to my left, then from your perspective the time coordinates at which my clock now ticks are slightly further apart. According to you it takes about 2.12 seconds between each tick instead of two seconds. Time literally is running slower for me relative to you … (0:39-0:57)
Having shown these examples, I should emphasize that not everyone freely exchanges the phrases ‘clock slows down’ and ‘time slows down’ in this way. More cautious expositors say simply that a moving clock is observed to run slower without bringing the concept of time into it at all. Here’s an example from the website, Einstein Online:The formula for time dilation [Δt = γΔτ] is often interpreted by saying that time “runs slower” for a moving clock than for a clock at rest, or that a moving clock “runs slower” than a clock at rest by a factor γ.—Special Relativity, Springer (2013), p. 20.
time dilationThe talk here is solely of a clock and other processes slowing down. There is no mention of time itself slowing down. You can also find many other accounts in the literature that are cautious in the same way. So the ready equation of ‘clock slows down’ with ‘time slows down,’ while common, is hardly universal.
In special relativity: From the point of view of an observer (more precisely: an inertial observer), a moving clock goes slower than an identically built clock at rest. All other processes moving alongside the clock (for instance: everything happening aboard a rocket speeding by) are slowed down in an identical fashion.
It should be clear what this passage means. In finding that all processes have slowed down in the astronaut’s rocket, the space-station friend may say that time has slowed down for the astronaut. (He may regard the astronaut as moving through time slower than himself.) That is the intended meaning. But notice that, according to the common practice just examined, this gets things the wrong way around. Imagine that you are the space-station friend, observing the astronaut. The astronaut is moving relative to you and so, as before, you find his or her behaviour and biological processes to be occurring in slow motion. According to the common practice, you should therefore say that time has slowed down for you when compared to the astronaut. (We saw this above: if events in your surroundings slow down, then time slows down for you.) But our relativity expositors would have you say the opposite. According to them, you should say that time has slowed down for the astronaut when compared to you. So their way of talking is at odds with common practice.… whatever processes the astronaut has going on in the rocket, including his own heart-beat … the space-station friend must observe them to be going in ‘slow motion.’ … But when all processes are slowed down like this, we may as well say that time itself has been slowed down …
Is there a mental mnemonic, some sort of intuitive way of thinking about why it is that time slows down when an object is in motion?He then proceeds to sketch the main idea pretty clearly and, if the idea is new to you, it will help to watch the video before we turn to the second source below.And there is … (0:43-0:53)
To get a fuller sense of what Einstein found, imagine that Bart has a skateboard with a maximum speed of 65 miles per hour. If he heads due north at top speed—reading, whistling, yawning, and occasionally glancing at the road—and then merges onto a highway pointing in a northeasterly direction, his speed in the northward direction will be less than 65 miles per hour. The reason is clear. Initially, all his speed was devoted to northward motion, but when he shifted direction some of that speed was diverted into eastward motion, leaving a little less for heading north. …The broad meaning of these passages should be clear and I will also show below that the idea is mathematically sound. Meanwhile, I have one passing observation. The passages are meant to constitute a simple and intuitive explanation of why time slows down when you’re moving. I noted above that what really needs explaining, at bottom, is why your clock slows down when you’re moving. So there is a question here about what Greene takes the relation between ‘time slows down’ and ‘clock slows down’ to be. This question will matter soon but we can leave it unanswered for now.
Special relativity declares a similar law for all motion: The combined speed of any object’s motion through space and its motion through time is always precisely equal to the speed of light. At first, you may instinctively recoil from this statement since we are all used to the idea that nothing but light can travel at light speed. But that familiar idea refers solely to motion through space. We are now talking about something related, yet richer: an object’s combined motion through space and time. …
[So] when you look at something like a parked car, which from your viewpoint is stationary—not moving through space, that is—all of its motion is through time. The car, its driver, the street, you, your clothes are all moving through time in perfect synch: second followed by second, ticking away uniformly.
But if the car speeds away, some of its motion through time is diverted into motion through space. And just as Bart’s speed in the northward direction slowed down when he diverted some of his northward motion into eastward motion, the speed of the car through time slows down when it diverts some of its motion through time into motion through space. This means that the car’s progress through time slows down, and therefore time elapses more slowly for the moving car and its driver than it elapses for you and everything else that remains stationary. …
Moreover, the maximum speed through space is reached when all light-speed motion through time is fully diverted into light-speed motion through space—one way of understanding why it is impossible to go through space at greater than light speed. Light, which always travels at light speed through space, is special in that it always achieves such total diversion.
And just as driving due east leaves no motion for traveling north, moving at light speed through space leaves no motion for traveling through time! Time stops when traveling at the speed of light through space. A watch worn by a particle of light would not tick at all. Light realizes the dreams of Ponce de León and the cosmetics industry: It doesn’t age.
To understand the Special Theory of Relativity at the gut level, a good myth must be invented, and here it is. …This is essentially the explanation given by Greene above and you can also find numerous other advocates of the same idea, especially on the wild wild web. I have not found the idea expressed in any standard text on the subject, however—e.g., you won’t find it in Taylor & Wheeler’s well-known text, Spacetime Physics. Possibly for this reason, some people seem to regard it as a bit of an unorthodox or fringe idea; a useful mnemonic or calculation heuristic perhaps, but one devoid of genuine metaphysical significance. Be that as it may, I have not seen the idea carefully assessed anywhere and have had to evaluate it for myself. What follows are some things that struck me as I turned it over in my mind. As I hope is clear, its key features are these:
Why can’t you travel faster than light? THE REASON YOU CAN’T GO FASTER THAN THE SPEED OF LIGHT IS THAT YOU CAN’T GO SLOWER. THERE IS ONLY ONE SPEED. EVERYTHING, INCLUDING YOU, IS ALWAYS MOVING AT THE SPEED OF LIGHT. How can you be moving if you are at rest in a chair? You are moving through time.
Why are the clocks moving through space perceived to run slower and slower as they travel faster and faster? Because a clock properly runs through time, not space. If you compel it to run through space, it is able to do so only by diverting some of the speed it should use for traveling through time. As it travels through space faster and faster, it diverts more and more speed. How much can it possibly divert? The clock can divert ALL of its speed. Then it is going through space as fast as it possibly can, but there is nothing left for traveling through time. The clock stops ticking. It stops aging.
All this can be depicted in a diagram, which is essentially the cosmic speedometer diagram. Nothing can ever be done to alter the speed of anything. Only its direction of motion through spacetime can be altered. At rest a thing is perceived to speed through time from O to A; but set in motion to the right, R, or to the left, L, its velocity is tilted right or left. In the extreme, the velocity is tilted all the way to C, in which case all the perceived motion is through space and none through time. The diagram is easily calibrated. If the distance from O to A is 1 year, then the distance from O to C must be 1 light year – the distance light travels in 1 year.—Relativity Visualized, Insight Press (1981), pp. 78-81.
Time slows down when you move through space because the faster you move through space, the slower you move through time. This trade-off is necessary to ensure that your overall speed through spacetime remains constant.A centerpiece of this explanation is the idea of moving slower through time as a counterweight to moving faster through space. Indeed, this idea is so central to the explanation that Greene and Epstein cannot reasonably be supposed to be employing the expression ‘move slower through time’ (or the equivalent ‘time slows down’) in a loose or casual fashion, e.g., as just a colloquial way of saying ‘clock slows down.’ Rather, these expressions are obviously meant to be temporal analogues of their spatial counterparts, e.g., ‘move slower through time’ is meant to be analogous to ‘move slower through space,’ and so on. This means that, while the idea of time slowing down figures prominently in the explanation, the idea of your clock slowing down is not, at this stage, coming into it at all. We saw above that some of our relativity expositors may reasonably claim to be using ‘time slows down’ as just short for ‘clock slows down’ whenever they brandish the slogan ‘Time slows down when you’re moving.’ Greene and Epstein, however, cannot do so. And so the “escape route” of the previous paragraph is not available to them, whereupon they seem bound for a head-on collision with the common and established way of talking about time. When Greene and Epstein say, ‘Time slows down when you’re moving,’ what they mean by these words is precisely what the common way of talking denies.
Einstein showed that if you watch me as I’m walking, you will say that time elapses more slowly on my watch than it does on your watch. Time slows for an object in motion. (0:07-0:21)Indeed, all of our previous relativity expositors may have had the intuition too, since they tend to say similar things. But, as we have seen, they have some room to maneouvre and cannot quite be pinned down on this in the way that Greene and Epstein can.
… The clock can divert ALL of its speed. Then it is going through space as fast as it possibly can, but there is nothing left for traveling through time. The clock stops ticking. It stops aging.These intuitions are both natural and tempting. Indeed, when you first study the theory of relativity and learn of how everything slows down in a moving person’s frame of reference, how their clocks slow down, their biology retards, and so on, you never think to question the (oft-made) ensuing assertion that time slows down for someone who is moving. It sounds like such a correct moral to draw; the natural upshot. It is only when you consider the common way of talking about time—the kinds of things we saw in §3—that these intuitions can come to seem suspect. I believe in the end that the common way of talking is correct and that the intuitions are misplaced, but I do understand how compelling they can be, having grappled with them myself. It is important to disarm them, so here goes.
If your clock ticks off just one hour in the time that mine ticks off six, then am I not the speedy one through time and you the laggard?Why would this “intuition” be wrong?