# Speed of Time

Q:
how do we know that every thing is moving at the same "speed" ,for lack of a better word, through time. and why when you travel a positive speed you can slow down your aging but cant travel at a negative speed to speed up your aging.
- Doug (age 12)
IL
A:
Hi Doug,

Interesting question! Time is just a coordinate, like the three spatial coordinates. There are three independent coordinates needed to fully specify a location in space -- call them x, y, and z. To fully specify where and when an event takes place, we need to specify its location in space and also the time at which it takes place, so four numbers are needed to specify an event fully.

We have lots of freedom to define these coordinates, as long as they specify locations and times uniquely. x, y, and z are nice for ordinary space, but they don’t work too well on the surface of a sphere. On the Earth, latitude, longitude, and height are more convenient for many purposes. This is just one of many possible choices. If we choose these coordinates, an object moving at a certain speed may have a very different comparison between its rate of change in latitude as compared to the rate of change of longitude, depending on how close to the North (or South) pole he is. We can construct similarly uncomfortable coordinates for time (who’s to stop us?).

But why? To quote Misner, Thorne, and Wheeler’s fine book "Gravitation", "Time is defined so that motion looks simple." Clocks depend on motion of some sort -- be it a swinging pendulum, a mechanically oscillating crystal, or even the motion of electrons in excited states of atoms. Physicists define time to be the thing that clocks read. If you carry a clock with you on a journey, your perception of time should match that of the clock you carry with you. So you should always "move at the same speed through time" as your clock you carry along.

This clock may not agree with other clocks, however. Einstein’s special theory of relativity did away with the idea that events can be simultaneous if they are in different locations. The difference in time between two events depends on their difference in distance and how fast the observer is moving. So the only way to compare clocks is to bring them together and ask them if they agree.

A clock that travels away and comes back to a stationary clock will read less elapsed time than the stationary clock. See our answers on the "Twin Paradox" for more information about this effect. A clock that is lifted out of a gravitational field, held there for a long time, and returned, will read less time (a consequence of Einstein’s General Theory of Relativity).

Both clocks move with the same "speed" through time because that’s what we mean by "speed" through time -- it’s what the clocks read. But it is possible to travel from one place in space and time to another place in space and time and go a different distance than another traveler might take between the same two places. The same applies to the elapsed time of the journey, because space and time are coupled together in a way explained in Einstein’s Relativity theories.

On speed -- speed is the magnitude, or size, of the velocity vector. Changing direction does not make speed negative -- speed is always a positive number.

Tom

(published on 10/22/2007)

## Follow-Up #1: Time Speed

Q:
Does time have speed? Namely, if you have two points, a and b, which are at significant distances from each other, and each point has a prefect clock, meaning perfectly identical clocks, will one be more delayed from the other? Perhaps time is centered in the center of the Universe, were the big bang occurred? Perhaps the Universe is spreading at a certain speed of time, not at the speed of light?
- Nestor (age 13)
NY
A:

Hello Nestor.

What you seem to be describing is the theory of General Relativity. According to GR, if two clocks are placed in a gravitational field with one being deeper in the field than the other, the time will not pass at the same rate for each clock. The clock that is farther away from whatever is causing the gravitational field will measure time to be passing faster than the clock that is closer to the massive object causing the field. This comes from the effect that gravity has on the passage of time, and not that it affects the mechanics of clocks specifically, of course. This is called time dilation.

Your question about the speed of time though is a bit difficult to understand. If your asking whether time travels at a certain speed through space, then the answer would be no. Time does not travel through space at a finite speed like light or an object that has been thrown. If two clocks were synchronized, at rest with respect to each other and were not in any gravitational field, the two clocks would read the same time no matter how far apart they were.

We also know that time isn't concentric with the universe because the universe has no center, nor did the Big Bang take place at a specific location. The Big Bang was an explosion of space, not an explosion in space. Our observations indicate that the universe is both homogeneous and isotropic, meaning that (on large scales) the structure of the universe is uniform and the same in all directions. There's no indication that any place is more the center than any other place.

Samson

(published on 04/08/2013)

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