The universe appears "old", but it is still less than 10,000 years old

[Alex K]

I think I disagree. How do you measure the time it takes light to travel from A to B, and how do you define the time interval delta T which it took? You will implicitly use the "isotropic" synchronization convention to do it. The point is not that this is a deep physical difference, it's just that light travel times for one-way trips are a convention dependent quantity.

But that's not my point.

Regardless of what convention you might wish to use.
Independent of any measurement, it still takes light time to traverse distance.
It is never instantaneous.

No, I really hate to be the devil's advocate here when it means that I am on the side of Statler, but you are simply repeating the assertion. How do you know that it takes light time to traverse a distance?

[Zen Badger]

So numerous experiments proving the calculated speed of light don't count then?

[Alex K]

So numerous experiments proving the calculated speed of light don't count then?

They either are round trip times, in which case they are convention independent, or they depend on the synchronization convention and implicitly use the isotropic convention (which is the one that says that everyone at rest with respect to you has the same time, which you get if you transport a clock infinitely slowly to that person).

p.s. what do you mean by calculated speed of light?

[Zen Badger]

So numerous experiments proving the calculated speed of light don't count then?

They either are round trip times, in which case they are convention independent, or they depend on the synchronization convention and implicitly use the isotropic convention (which is the one that says that everyone at rest with respect to you has the same time, which you get if you transport a clock infinitely slowly to that person).

p.s. what do you mean by calculated speed of light?

Regardless of whether they are round trip times, it still took time for the light to traverse that distance.

And Ole Romers discovery in 1676 that light had a finite velocity was based on the measurement of light travelling in one direction, not a round trip.

Or am I missing something?

[Alex K]

I think in the "geocentric" synchronization convention, Romer's measurement would simply be interpreted differently: the moon appears to be covered later by Jupiter, because they have increased their distance and have therefore gone to a distance where time is lagging behind somewhat. Remember that in this slanted "geometry", moving around in space radically changes your time frame. Romers interpretation of the delay in terms of travel time simply assumes that time on Jupiter is the same as his. This is also roughly what you get (up to time dilation due to movement of jupiter) when you use standard SRT synchronization convention. The anisotropic picture is an artificial mess, don't get me wrong.

[Zen Badger]

But we are agreed that c is a constant though?

[Alex K]

But we are agreed that c is a constant though?

I'd say c is the unique constant speed of light which you measure for a round trip, or for a one way trip using the homogeneous isotropic convention for what time means.

[Zen Badger]

But we are agreed that c is a constant though?

I'd say c is the unique constant speed of light which you measure for a round trip, or for a one way trip using the homogeneous isotropic convention for what time means.

I'll take that as a yes then.

[Alex K]

I'd say c is the unique constant speed of light which you measure for a round trip, or for a one way trip using the homogeneous isotropic convention for what time means.

I'll take that as a yes then.

Well, all I can say is what I say above

The trouble with special relativity is that for two events in spacetime which are too far apart to be connected causally, there is no unique notion of which happened before or after the other. In standard SRT synchronization, it depends only on the relative speed of the observer (not on relative position) which of the two events happens first. This ambiguity in temporal order of causally disconnected events can be exploited to further drop the shared timeframe which is usually assumed at least for observers at rest to each other.

To summarize the whole thing, if you strip away the mathematical shenanigans, what Waldorf wants us to do is the following: you observe light from Andromeda tonight - let's call the moment in time on Andromeda when it left there "now" rather than "3 million years ago", and act as if no further time had passed on Andromeda after the departure of this light. This is in principle a completely skewed, but consistent way to view the world because nothing that could have happened after that on Andromeda is causally connected to us yet.

This renaming of points in time at different places in space allows you to put the origin of all the astronomical light which we observe now in your newly defined "present".

[Zen Badger]

I'll take that as a yes then.

Well, all I can say is what I say above

The trouble with special relativity is that for two events in spacetime which are too far apart to be connected causally, there is no unique notion of which happened before or after the other. In standard SRT synchronization, it depends only on the relative speed of the observer (not on relative position) which of the two events happens first. This ambiguity in temporal order of causally disconnected events can be exploited to further drop the shared timeframe which is usually assumed at least for observers at rest to each other.

To summarize the whole thing: if you strip away the mathematical shenanigans, what Waldorf wants us to do is the following: you observe light from Andromeda tonight - let's call the moment in time on Andromeda when it left there "now" rather than "3 million years ago", and act as if no further time had passed on Andromeda after the departure of this light - this is in principle a completely skewed, but consistent way to view the world because nothing that happened after that on Andromeda is causally connected to us yet.

I understand all of that.
But that is simply a case of perception. It's doesn't mean that it is what is actually happening.
And it doesn't mean that the light from Andromeda got here instantly.
It still took it 2.2 million years to complete the journey, no matter how we wish to view it.