Our understanding of time gets even more strange but we experience it every day.
Recap of part 1...
In part 1 of this series, we saw that space and time are the same entity called Spacetime and that motion through space and the passage of time are related parts of Spacetime. The more we have of one, the less we have of the other. We also learned that although it may be shocking to us, this was proven experimentally way back in 1971.
But we stopped part 1 pondering why should we care. As its effects although real, are small and not noticeable in our daily lives. Indeed, for many decades, Einstein's theory of Spacetime although proven real, was considered to be a useless theory!
Some more profound implications...
Let's get to the real effects of Time in our lives in a bit as there are still deeper implications of the great mystery of Time. Yet again, I ask you to get ready for the implications coming up as they are significantly mind-bending.
So, let's imagine you and your friend are both stationary. We saw that if you and your friend are both stationary, then you both agree on Time and hence have the same understanding of "Now". But if you start moving through space while your friend is still stationary, then your time ticks slower compared to your friend's, and you both don't agree any more on what time it is now.
Let's represent the green cones below as the universe consisting of all space and all of time. Just like all of space is out there, all of Time is also out there! Let's imagine that you are the yellow dot and your friend is the white dot on these cones and that you both are separated by a great distance i.e. you are located in different galaxies very far away from each other.
Now in the left cone you both are stationary and you both agree on what is "Now" i.e you are both in the same place on the Time axis.
But in the middle cone, you start moving away from your friend (at a leisurely speed of 50 km/hr) while your friend is still stationary. Then you have a difference in relative motion (0 - 50 = - 50) and therefore a different notion of time and your "Now" is angled towards the past and hence to a time many decades back in your friend's life!
Finally, in the cone on the right, you move towards your friend, the relative velocity is ( 50 - 0 = 50 ) and your notion of "Now" is angled towards the future "Now" of your friend many decades in the future in your friend's life.
One final thought. What this means is that the past, the present, and the future already exist and we are simply traveling through the illusion of the passage of time.
Just like all of space is out there, all of time is also out there!
This phenomenon of relative difference in Time between two observers due to relative motion is called Kinematic Time Dilation. There is also another form of Time dilation called Gravitational Time dilation. This is the Time difference between two observers due to the difference in gravity experienced by them Eg: you standing at the bottom of a mountain while your friend is on top of the mountain. ( As we go further away from Earth, the effect of Earth's gravity reduces.)
Back to reality
Okay, so now let's understand the real effects of Time dilation in our daily lives :
Most of us use GPS on our phones. GPS works on the triangulation of satellites orbiting the Earth. Now satellites are moving at great speeds ( ~7000 miles per hour ) and they are also experiencing slightly less gravity than the surface of the earth. Thus clocks on the satellites experience both Kinematic and Gravitational Time Dilation. So the clocks are not synchronised and don't agree with time on earth's surface. This causes a pretty massive positioning error. To fix this, clocks on satellites and the GPS have to be routinely synchronised to get accurate GPS positions.
We all age slightly slower than astronauts in their space shuttles.
Nowadays, we have extremely accurate clocks and several experiments have been conducted with initially synchronised accurate clocks. On top of a mountain compared to the ground, on a town above sea level and sea level, etc. They all prove Time dilation is real and happens every single day in our lives.
So back to the question: It's real, but tiny and not noticeable in our everyday lives. So why should you care? Well, one day we may be able to travel at very high speeds and we may be able to Time Travel with noticeable time-difference effects. Till then we should care as it is reality and we live in that real world and curiosity is the hallmark of human experience.
For an excellent reading on this topic, a highly recommended book is The Fabric of the Cosmos: Space, Time, and the Texture of Reality by Brian Greene
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