## Approaching light speed.

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• 141

I'm well aware of comoving and proper distance metrics. Using a commoving metric to argue that the speed of light is not constant, again, is misleading. It's like having an ant walk down a rubber band, and as you stretch the rubber band you argue that the velocity of the ant is slowing, and even reversing, because its distance to the end of the rubber band is increasing.

The fact is that the speed of light, measured from ANY inertial frame of reference (any velocity), is constant. That is the context of this thread. Your presentation is adding levels of complexity and concepts which are beyond the reach of most non-cosmologists, and are completely unnecessary to answer to the question posed by the OP.
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It's like having an ant walk down a rubber band, and as you stretch the rubber band you argue that the velocity of the ant is slowing, and even reversing, because its distance to the end of the rubber band is increasing.
We understand each other then.

The fact is that the speed of light, measured from ANY inertial frame of reference (any velocity), is constant.
I agree. I only interjected because you didn't say 'inertial' the first time (below), and light moves at different speeds as measured by most (all?) non-inertial frames. You also didn't say 'in a vacuum', but most people know that restriction.
From the perspective of any observer in any frame of reference, all photons travel at the speed of light

The distinction is important because the web is full of pages that 'prove' Einstein wrong by making the same assertion and then showing that the rule is violated by any of my examples or others.
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I botched up my question. Let me clarify by a simple thought experiment:

I start out driving along a straight road to a destination 1,000 miles away. To an external observer I drive at a steady 100 mph, with the distance between my front bumper and my destination shrinking at 100 mph. However, I was unaware that clocks tick slower in my car than outside stationary clocks, so at the end of one hour I pass the 200 mile road post, and think, wow, I am going really fast, twice the speed limit! But that’s not possible.

So, my destination is approaching my car at that speed. But that’s not possible either. So I surmise that I am driving at the speed limit and the distance between my car and my destination is shrinking that fast as well. Hence, the rate of change of the distance between car and destination is changing more than I originally assumed.

To an external observer the actual space between vehicle and destination is not contracting, but the distance between them is. It's a playground for metrics.
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Thanks. Pretty heady stuff for one unversed in physics . . . :cool:
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The selected quotes from the above physicists concern infinity, which I did not mention in my comment. I did reference infinite time.
My comment concerned chatGPT's response to a reference to an 'edge', which implies a spatial boundary with matter, light, and stars on one side and nothing on the other. He does say that such a thing isn't well defined. Again, I don't disagree with any of his responses.
There are models of finite size universes, but most/all(??) of them curve in on themselves, much like Earth having finite surface area without anywhere having an edge. It was the reference to that edge at which I balked.

:up:

I think ChatGPT meant "edge" in the higher dimensional sense, where the finite/infinite distinction is relevant. We can say that the Earth has no edge on the 2D surface. Whereas the edge, or boundary, in 3D is defined by the surface (i.e., ~6,000km from the center of the Earth).
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Let me clarify by a simple thought experiment:

I start out driving along a straight road to a destination 1,000 miles away. To an external observer I drive at a steady 100 mph, with the distance between my front bumper and my destination shrinking at 100 mph.

At that speed, there is no appreciable difference between the distance for you and the stationary observer. Your velocity would need to be six million times faster (about 87% of the speed of light) to contract the distance for you to 500 miles.

However, I was unaware that clocks tick slower in my car than outside stationary clocks,

Your clock ticks at the same rate for you as it always has. But you measure the stationary clock as slower. The stationary observer also measures your clock as slower.

so at the end of one hour I pass the 200 mile road post, and think, wow, I am going really fast, twice the speed limit! But that’s not possible.

You are going six million times the speed limit. But the road posts were calibrated for essentially stationary observers. So the post says 200 miles when you have only travelled 100 miles.

So, my destination is approaching my car at that speed. But that’s not possible either. So I surmise that I am driving at the speed limit and the distance between my car and my destination is shrinking that fast as well. Hence, the rate of change of the distance between car and destination is changing more than I originally assumed.

In your reference frame, you are at rest. So the destination is approaching you at 0.87c.

To an external observer the actual space between vehicle and destination is not contracting, but the distance between them is. It's a playground for metrics.

Spacetime stays the same (the spacetime interval is invariant). In that sense it's like looking at a round table from an angle - it appears oval, or contracted along one dimension. But it's more than perceptual, since each observer measures the other as contracted and, indeed, you travel only 500 miles to reach your destination.
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Thanks. Pretty heady stuff for one unversed in physics . . . :cool:

:rofl:
I knew what I was supposed to do ( :zip: ) but I didn't, I couldn't. I was compelled to stay write.:cool:

Pardon me if I've insulted math in anyway.
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You take my simple thought experiment (satire) far too literally as an exercise in relativity. :roll:

Pardon me if I've insulted math in anyway

:wink:
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:smile:
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You take my simple thought experiment (satire) far too literally as an exercise in relativity.

Just grounding the thought experiment in some real numbers - there needs to be Lorentz symmetry between the observers. Weren't you also asking a question?
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Just grounding the thought experiment in some real numbers

Thanks for your input. It's a subject I haven't studied beyond the basics of time dilation.
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It's a subject I haven't studied beyond the basics of time dilation.

OK. Time dilation and length contraction are related in the expected way. If you travel half the distance (i.e., at 87% of the speed of light), then it takes half the time.
• 13

Are you folks in astro? I'm graduating with an MS in astro in a matter of weeks (though starting PhD bridge... probably in the fall, I'm taking spring off). Right now I'm researching galaxy quiescence at 0.6 < z < 2.5 and hopefully publishing morphological metrics from CANDELS soon. Happy to see others at least with a cosmo background here
• 141
Are you folks in astro? I'm graduating with an MS in astro in a matter of weeks (though starting PhD bridge... probably in the fall, I'm taking spring off). Right now I'm researching galaxy quiescence at 0.6 < z < 2.5 and hopefully publishing morphological metrics from CANDELS soon. Happy to see others at least with a cosmo background here

I'm an old electrical engineer, but modern Physics foundations and astrophysics and cosmology have been a hobby since I was a kid, just scratching the surface. Very cool work you're doing there, I would have thought galaxies were anything but quiescent in the early universe (the z range on your research)... I imagine you've been all over those new deep field images.
• 13

We call it "high noon" because of the high star formation rates, but there's still plenty of quenching especially at the "nearer" z (but that's the point, I end up making a lot of figures splitting into four redshift bins of equal comoving space against different metrics so you can kind of "watch" galaxies march towards quiescence under different definitions and with different metrics: sersic n vs. z, axial ratio vs. z, sersic n vs. axial ratio, UVJ in different redshift bins, SFR vs. compactness). I only wish it were possible to more easily cognize a higher dimensional plot so put more of them together in a single plot lol.

I actually haven't had the pleasure to use ANY JWT data even for fun, I've been so busy with CANDELS. :( (They are lovely to look at, though!)

Pleasure to meet you
• 141
We call it "high noon" because of the high star formation rates, but there's still plenty of quenching especially at the "nearer" z (but that's the point, I end up making a lot of figures splitting into four redshift bins of equal comoving space against different metrics so you can kind of "watch" galaxies march towards quiescence under different definitions and with different metrics: sersic n vs. z, axial ratio vs. z, sersic n vs. axial ratio, UVJ in different redshift bins, SFR vs. compactness). I only wish it were possible to more easily cognize a higher dimensional plot so put more of them together in a single plot lol.

I actually haven't had the pleasure to use ANY JWT data even for fun, I've been so busy with CANDELS. :( (They are lovely to look at, though!)

Pleasure to meet you

I think I remember CANDELS survey going from 1 to 8 z, right? Man, must be daunting trying to consolidate data from the different metrics. So many galaxies in that survey to sift through as well, like hundreds of thousands?

Always great to hear from a physicist that's actively interested in Philosophy, pleasure ins mine!
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Are you folks in astro?
Similar to staticphoton, I do something else (low level database implementations) but I read what I need to in order to support or criticize various philosophical views. It is something new to me to interact with somebody actually working in the field.

I only wish it were possible to more easily cognize a higher dimensional plot so put more of them together in a single plot lol.
I've seen 3D plots done with a tool that allows manual rotation/PoV/zoom controls. It conveys a lot more info than the 2D plots, and it works real time. Not sure of the tool used to build it, but I have played with the controls. The one I saw plotted all the nearby large galaxies' peculiar movements for the last 6 BY or so, including Virgo SC but I think not going so far as the great attractor. The plot negated expansion, so it looks like we're headed for Virgo, but of course we'll never get there. It really helped me see our own movement and Andromeda chasing us from behind. Our peculiar motion is actually away from it.

Not sure how I'd implement an interface to convey 4 or more dimensions of data.
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