I’m struggling to wrap my mind around these things too.
So your answer seems to indicate that scaling up (to the level of the entire universe) gets one closer to being able to identify some absolute time. But scaling smaller brings out the relativistic difficulties. I would have guessed exactly opposite of that --that over large scales (e.g. between our galaxy and some other quite distant galaxy) there would be the greater relativistic differences due to radically different motion reference frames; and that, say, within our galaxy there would be smaller discrepancies between proposed “universal” clocks. But if I understand your answer correctly, we have more difficulties than ever with our closest neighbors. Logically extending that further in, shouldn’t globally agreed-upon “time stamps” be near impossible in the even smaller context of our planet? I think I’m misunderstanding your answer. But thanks in advance for your patience …
While we’re plying Instructor Hesp with the impossible demands to turn us all into relativity experts, I might as well throw this on the heap too.
How does the spatial geometry work for the CMB. I can visualize a big bang exploding (spherically) into space. [okay --I know there was no ‘space’ before that, and that the big bang is space itself expanding…] But still, I can visualize this blob getting bigger, no problems. Here’s my mental exit point, though: So the original ‘background’ microwave radiation is tied to this event and is busy going at the speed of light … where? Wouldn’t it be outward? And if we are all going roughly outward from the big bang but not at the speed of light, obviously, then wouldn’t all this radiation be long gone away from us traveling to (or along with) the expanding outer edges of space? How come we’re seeing it from all directions as if the stuff is (…lingering!) (…bouncing or swirling around!)? The geometry just doesn’t fit, and I suspect my problem is with time. Time too is tied to space and somehow that makes this cosmic “kablooie” into a timeless kind of thing. Kudoes to you if you can expand my mental horizons.
I also find background radiation to be a tricky concept. The only thing I can think of is that the Big Bang happened when the universe was infinitesimally small, so that the radiation was everywhere that was at the time. During the expansion phase, I have heard that space expanded faster than the speed of light, so that the radiation that was heading towards where the earth is now spread out along with the expanding universe, and has continued heading in our direction for the billions of years since the Big Bang. The Big Bang theory was considered unlikely until the cosmic background radiation was discovered in the early 60s, when the temperature represented by the radiation exactly matched predictions of the Big Bang theory.
That does make some sense to me. So as the big bang continues to happen, with us somewhere in it, we are still in the midst of the CMB that is also in it. But shouldn’t it not be isotropic? You would think it would be glowing brighter in the spot of the sky where the big bang originated. I guess if we are still in it, we can’t point to somewhere and say it started there. Because that would presume we could be in ‘pre-existing’ space watching real space expand from a definable location. And we can’t have space outside of space!
Maybe I should just dig up an old Keith Green album and see if the cosmic kablooie left any snacks in the cupboard.
My limited understanding is that right here is as much the origin of the big bang as anywhere else. It is sort of like blowing up a balloon and asking which point on the surface was the origin of the expansion of the balloon. All the points are where it began. And if there is someone in one what was one of those primordial galaxies 12 billion light years away seen by the Humble telescope looking our way, they will also see primordial galaxies.
That is not what I intended to say, so I’m happy that you ask this question. Einstein’s theory of General Relativity is actually defined in such a way that for very close neighbors the spacetime looks flat to first order approximation. So your initial intuition of close neighbors having less disagreements was correct.
The largest problems come in when we try to compare small regions of space that are far apart from each other. That could be a small region within our Milky Way compared to a small region in another galaxy. So there is some truth to your intuition that the problems exist at larger distances.
Now, if we look at sufficiently large scales, we essentially “lose” the resolution to distinguish galaxies from each other. Then the curvature of spacetime looks approximately the same everywhere. That makes it easier to compare clocks, as long as we don’t try to compare “zoomed in” regions with each other.
It might sound weird, but changing the synchrony convention actually does not change the physical properties of the system. Therefore it also leaves unchanged phenomena such as redshift.
The interesting thing about synchrony conventions is that they cannot be empirically verified or falsified.
For example, if you measure the speed of a ray of light travelling from a satellite (clock 1) to Earth (clock 2), you first need to decide what is an appropriate way to synchronize those two clocks.
Something similar holds for other possible effects. There is one very technical article that predicts a global shift in the gravitational potential as a result of anisotropic synchrony. However, global shifts are immeasurable.
Because of this, we will first be looking at theoretical considerations in the coming two posts. Most importantly, Maxwell’s theory of Electromagnetism describes light as electromagnetic waves always travelling at constant speed in vacuum.
Don’t worry, there will be enough opportunity to talk about the empirical issues when we start zooming in on the “ASC model” which Lisle proposed. His assumption that God used the ASC to describe a one-week creation event taking place 6,000 years ago does produce a number of serious conflicts with observations. Stay tuned for more .
Taking our current observations, it is impossible to say whether the Big Bang had a spatial “origin”, also because the CMB is homogenous and isotropic. Either we just can’t see the “edge” yet, or the Universe is infinite. This reminds me of a famous quote, (probably falsely) attributed to Einstein:
“There are two things infinite: the Universe and the stupidity of mankind. However, I’m still not sure about the first one.”
It might be strange to consider the expansion of something that is already infinite. The expansion of an infinite Universe might be visualized by imagining a metal fence with horizontal and vertical bars separated from each other by a certain distance. Consider the fence to be infinitely high and infinitely wide. Now imagine that the distance of separation between the bars starts to grow… The infinite fence is “expanding”, but of course remains infinite… There is no spatial origin of the expansion of this infinite fence. In the same way, the Universe does not need an origin in space.
What implications would gravitational lensing of distant supernovae have for the anisotropic synchrony convention? (Article of interest here) Since the light from the same supernova arrives at different times by different paths, wouldn’t that falsify the idea that the one-way speed of light is infinite?
Hi James,
Using the ASC only changes the way in which events are time-stamped, not the actual physical phenomena like gravitational lensing or redshift. This point is also emphasized in the animation: both plots look exactly the same.
A hand-waving way to look at it would be that the rays of light that undergo gravitational lensing are moving in different directions (they wouldn’t have arrived at earth if their paths wouldn’t be bent along the way). Therefore, even though they left at the same time, their paths differ in how much they deviate from the direction that points directly towards the observer. This results in a time difference since the speed of light has a directional dependency within ASC.
Strictly speaking, we’re walking on thin ice with this way of arguing because Lisle’s proposal centers on the formalism of Special Relativity (SR) which only describes flat spacetime. Gravitational lensing is a consequence of the curvature of spacetime caused by massive objects. That takes us into the realm of General Relativity (GR). It’s not straightforward to generalize statements on chronology from SR to GR.
Hi Mike, thanks for your question. In short, the answer is no. Roemer’s measurement does not prove that the one-way speed of light is constant because it already assumes the standard synchronization (Roemer used Newtonian absolute time).
In the ASC, the synchronization of clocks depends on the distance to the observer. So when the distance from Earth to Jupiter is large, the clocks near Jupiter are set differently than when the distance to Earth is relatively small. The different settings of these clocks correct exactly for the difference in distance. So applying the ASC would give you zero minutes traveling time for both distances, i.e., infinite speed would be measured in both cases (towards the observer on Earth).
Thanks for the suggestion, Paul. The link took me to a Scientific American site, but with apologies that the page was missing beyond that. So it appears to be a partially broken link.
"The interesting thing about synchrony conventions is that they cannot be empirically verified or falsified.
For example, if you measure the speed of a ray of light travelling from a satellite (clock 1) to Earth (clock 2), you first need to decide what is an appropriate way to synchronize those two clocks."
Just recently I came up with a new idea to measure one way speed of light and/or synchronize distant clocks:
Let’s have two light sources at points A and B separated by distance d and sending constantly (perpendicular to AB) signals to clocks at A’ and B’ (see attached diagram)
Let’s have an opaque rigid rod of the length d (it can be measured against AB while at rest with AB, so accuracy can be high) traveling with constant speed v (non-relativistic) parallel (and very close to) the line AB from B towards A. Initially the light from B to B’ will be blocked and the light from A to A’ will be allowed to be transmitted. When front end of the rod will start cutting off the light from A to A’ the light from B will start to be transmitted to B’ . At this moment, we will have both clock at A’ and B’ synchronized.
Alternatively when front end of the rod will start cutting off the light from A to A’ we can start the clock at A and when the light from B will start to be transmitted to B’ we can also send the light from B to A .When the light from B arrives at A the clock at A will measure one way speed of light. This method would need only one clock.
We can improve the accuracy of the measurement sending the rod from A to B with the same speed v and measure one way speed of light from A to B. The average 2 way speed of light from A to B and from B to A has to be c.
.