# How do we judge the flow of time from our frame of reference?

Is there a means to determine our frame of reference on past events in space-time? How do I know my current reference (today) is the same as it was two days ago for the rate at which time is perceived? A satellite sits in space and has an atomic clock while another is on Earth there will be a perceivable change over time between the two clocks. If we could, somehow, view past events in the same way would we see a constant change over time or would that change be faster or slower from our current, changing frame of reference?

How do I know the day two days ago was as long as the day today, from my current frame of reference?

• I can't make head or tail of what this question is meant to mean. – tfb Aug 11 '16 at 15:45
• Your words make no sense to me at all. – Omar Nagib Aug 11 '16 at 17:38
• essentially, this question boils down to asking how to determine if we have at all shifted to a different inertial/gravitational frame of reference since some previous time even if we include effects from GR. It's perfectly clear – Jim Aug 11 '16 at 18:26

If you "look in a mirror" (a distant object that is stationary with respect to you), then events reflected by this mirror would reflect "time gone by". So if you have a clock sending pulses at (what you believe to be) 1 MHz, and you look at the pulses as they come back a day later - if our entire universe had changed clocks, the return frequency would appear different.

But that's not what we observe.

Another example: the spectral lines of hydrogen from distant stars (representing the concept of time "a long time ago" still appear in the same place as they do today.

Wouldn't either of these experiments be sufficient to conclude time isn't changing (despite what Bob Dylan might have us believe...)?

• Good enough evidence for me. – Bob Bee Aug 12 '16 at 3:38
• @Floris what about red shifts, such as observed as evidence for dark energy, could that be explained by differences in time? – NationWidePants Aug 12 '16 at 9:36
• BOOM! Take that, Bob Dylan! – Jim Aug 12 '16 at 18:18
• @Floris wouldn't your example only prove observational uniformity and not a consistency in the changes in time itself? – NationWidePants Aug 5 '20 at 10:36
• @NationWidePants I am not sure I understand your question. “Distance” allows us to put two measurements of “time” side by side and we don’t see them shifting relative to each other (other than due to presence of mass/dark matter/ or velocity). That isn’t proof - but it supports the null hypothesis. Until we find an example that it is changing I feel comfortable to go with the reasonable hypothesis it isn’t . But that’s just me. – Floris Aug 5 '20 at 12:41