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Comment by chubbyrabbit | Hacker News Reader

Yes. However, it's like pointing at the sun and say "That's not the sun, the sun was there 8 minutes ago".

I’m listening to Carlo Rovelli’s book, “Reality is not what it seems”, and I’m still hung up on the concept of the “extended now”. The way he describes it, everything being relativistic, there’s no such thing as an “objective” point of view. So, if I understand his take on your example, we’re not seeing the sun as it was 8 minutes ago. It simply takes 8 minutes for the sun’s “now” to reach us.

That reminds me of a very important aspect of distributed systems (computer science) - there is no global clock!

UTC is a global clock. If I show the time in Sydney and the time here in UK, Sydney will be 150ms behind to me, and 150ms ahead to a viewer in Sydney, but I know the distance therefore I know if the clocks are in sync.

The time dilation between the two places is on the order of femtoseconds/second, a millionth of a clock cycle of a cpu.

UTC is a local clock on which other local clocks are synchronized.

Sounds to me like you could have equally started that post with "UTC is not a global clock".

It's a global clock. UTC is the same time in Miami as it is in Singapore.

The accuracy of the delivery of that timesource is 10ns in theory, and upto 1000ns in practice.

CET and EST are also the same time in Miami as in Singapore. That's just how time zones work. The distinction with UTC is that it isn't tied to any particular physical location, it's not the "time zone" for anywhere.

But UTC is ultimately defined by consensus. We need a reference clock and we need to be able to measure or estimate our skew in order to sync to it.

How do you know you have the correct (to the accepted precision) time?

GPS provides the correct time anywhere on the planet, it's a universal (as far as earth goes) clock, accurate to 10 nanoseconds (3m). The different reference planes that opposite sides of the planet (at the equator) gives you a precision of femtoseconds (micrometers at light speed) so makes no difference there.

The quality of GPS receiver of course is important, it may reduce your precision to to 100ns or even 1000ns, but it will give you the right time. Someone equidistant to two GPS synced clocks will see them both at the same time (within a microsecond)

Why do you have trouble understanding that knowing that light doesn't travel instantaneously?

By that definition there is no meaning to the word "now". If I'm five feet away from you, it takes time for light to travel between us.

Snark aside, using the phrase "now" to describe an in-motion event that is observed over 29000 light years is misleading and should rather say "is now observed as 29000 light years away".

> By that definition there is no meaning to the word "now".

Yes, exactly!

"Now" is an extremely useful concept on Earth, when we're all in approximately the same space going approximately the same speed. Now gets a lot done for us.

Now is not really a concept on the universal scale. "There is no meaning to now" is a great take-away from general relativity!

Since the sun isn't turning around the earth, I believe the sun is still where you point at.

Technically the Sun is moving as part of the arms of the Milky Way galaxy, which itself is moving through space on a collision course with Andromeda, so where you point at will be off.

This would be unnecessarily pedantic but not wrong* if not that the heart rotates on itself, so you are pointing at a 8 minutes old direction.

*All these discussion make no sense at all, as neither synchronous remote events nor the concept of what rotates around what are meaningful. This is not to say that we should not have this conversation, but to say that you are being pedantic without bringing anything to the table.

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