# Why do physicists refer to lengths of time?

I am a non-scientist (and most certainly a non-physicist) who has heard pop-science portrayals of General and Special Relativity.

What I want to know is this: if there is no absolute time, why do physicists often seem to make references to absolute time? For example, they’ll say that there is a certain amount of time remaining until the heat death of the universe, and they’ll give an exact quantity of time.

Maybe this is one of those things that you can’t understand without advanced Math?

Cheers,
Joey

If they are giving a time, it is referring to time as experienced in our standard frame of reference. You know, travelling at earthly speeds, etc.

What you have heard as “there is no absolute time” is better stated as “there is no priviledged frame of reference.” If we were travelling at a good percentage of the speed of light, the amount of time left until the projected heat-death of the Universe would be expressed as a different value.

But since we are all in this one frame of reference, it is acceptable to use it as a local constant, especially in a “pop-science portrayal.”

In some contexts, there are actually clear standards for what reference frame to use. When talking about the Universe, for instance, one can define a unique reference frame at any given point such that the current age of the Universe is larger than in any other frame at that point. When we talk about the age of the Universe, that’s the frame that we mean.

Now, we, like most things in the Universe, are very close to being in such a frame, so as an approximation, we often use our own frame. But we don’t have to.

Is that the same reference frame in which the Cosmic Background Radiation has zero Doppler shift? Or can the two frames differ?

There is also no such thing as absolute space, and yet physicists refer to lengths of space. For a given reference frame, one can refer to space and time.

The remaining time until the heat death of the universe isn’t an absolute time, it’s a relative time, between NOW and THEN.

Are you asking how they can mention a relative time without constraining where it’s measured? If one of us traveled to the surface of a white dwarf and the other stayed here, we would observe different time spans until the heat death - do you mean that issue? I think the answer is that practically all locations from which one might time this will be the same, and exotic locations like stellar surfaces won’t be considered.

If there were no such thing as a local time reference, then physicists would rack up tons of overtime.

In practice, they’re the same, but they could in principle differ. Fundamentally, the rest frame of the CMB is just another case of “at rest relative to some object”; it’s just a really big object. You’d need some sort of cosmic conspiracy to make them differ significantly, though, and physicists as a rule don’t believe in cosmic conspiracies (though we do enjoy gedanking about them).