How to Measure The Universe One Step At A Time

A short history explaining how philosophers and astronomers have managed to measure all the size of the universe, starting with the size of the Earth and progressing through to galaxies billions of light years away.

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[MikeKobb]

IMO, there’s not nearly enough of this sort of “how we know what we know” educational content available to most people. Carl Sagan did a good job of presenting pieces of it, like the story of Eratosthenes and the initial measurement of the Earth’s diameter, but I don’t think I’ve seen anyone build out the whole distance ladder in a single clear explanation like this. Bravo.

[QuantumHistorian]

There's one more thing to say about the distance ladder: it's why astronomers have a bunch of weird different units (AU, Parsecs, red-shift, etc...).

Each one corresponds to a different method (AU to orbital mechanics, parsecs to parallax from the earth's orbit, light shift from the expanding universe). To convert it back into something conventional like metres you have to convert it step by step because that's how the calibration works (eg, to convert parsecs into metres, you first have to know how many metres are in an AU). But the calibration is often inaccurate and is improved in time. So it's best to just leave results in the "natural" unit it was measured in, and if someone wants to translate it into metres at some later point, they can use the most accurate calibration values available then. This is much easier than republishing every table of astronomical distances every time there's a tiny improvement in any of the data at lower rungs of the distance ladder. Giving published work a longer shelf life is just common sense, even if it means using particularly weird and unconventional units.

For example, we can measure red-shift of distant galaxies pretty accurately now, a couple of significant figures or so. However, converting red-shift to metres is HUGELY difficult. First you need a good value for the Hubble Constant; we don't have that, there are even two different values computed different ways that are incompatible with each other. Secondly, you have to either assume the Hubble Constant constant (which it isn't, because the universe is accelerating), or you have to estimate what it was at every instant in time between when the light was emitted from the observed galaxy and when we observed it. But that is very hard and estimates vary considerably when we get to very high red-shifts. So we know distances to far away things in terms of red-shift pretty accurately, but have only a very loose idea for what that distance is in metres or equivalent units.

[DrummertheCody]

The distance ladder has always fascinated me. Especially the standard candle of type 1a supernovae. Such a specific set of circumstances yet it is common enough to be used as a standard. Mind blowing how big our universe is.

[llydrsn]

6:27 "the calculations for these would take a few years" and here I was pouting and racking my brain studying engineering with a scientific calculator on hand. These ancient astronomers are really, really amazing!

[armienn]

The accelerating expansion of the universe was being discovered just as I started school, so to me it's what I've always read about. I was so surprised to hear your experience of it, since I usually forget how recent it is.

[MoonWeasel]

Me: "This galaxy is a few million light years away"
Other dude: "How do you know that?"
Me: *inhales deeply*

[EnglishMike]

It's mind boggling to realize that there are people still alive today who were born when the vast majority of astronomers thought the Universe was little bigger that our own galaxy, the Milky Way.

[xfdrtgfd]

Interesting. I always thought that it was a little strange to choose the earth sun distance as a unit, but it makes more sense now that I know that there was a time when it was useful unit but it was not known exactly how big it was.

[astromelow]

Fun fact, us astronomers still don't actually know exactly what causes type Ia supernova!

As mentioned in the video, they could originate from accreting white dwarfs exceeding the Chandrasekhar limit. Alternatively they might be caused by the mergers of double white dwarfs that slowly spiraled into one another due to gravitational radiation. Either way, they can still be used as standard candles as the underlying mechanism doesn't really matter when it comes to the luminosity of a type Ia supernova.

[OzoneTheLynx]

7:52 Gaia is about to have its 3. Data release next month. It isn't as glamorous as something like Hubble is with its beautiful pictures, but the huge amounts of basic measurements is incredibly important as a foundation for a lot of astronomy.

[Ficon]

Elite Dangerous is such a great visual demo of just how immense the distances are.

[tomhill]

Great summary of a lot of important concepts in astrophysics. No wonder your channel is where I prefer to go than television!

[GiladTeller]

"Space is big. You just won't believe how vastly, hugely, mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space." (Douglas Adams)
Happy Towel Day!

[Clewis]

Thank you for the detailed description. I've always been curious of how we could possibly measure such great distances

[donjones]

4:08 One key to understanding why heliocentrism took so long to be accepted is that naked-eye observations did not provide accurate enough observations. The figures obtained could be used in calculations supporting geocentrism, heliocentrism, and the even odder Tychonic system that had the Sun orbiting the Earth but the rest of the solar system orbiting the Sun. Scott touches on this at 6:26.

Of courser the religious objections were a big impediment, but that issue is too well known to get into fully. The aspect of the planets being held in perfect crystalline spheres was certainly appealing philosophically and was supported by naked-eye observations that placed the planets in perfect circles. This was taken to support the idea of the perfection of God's creation. Then along came telescopes and the unpalatable observations that the planets moved in eccentric orbits.

[alexeynezhdanov]

Scott, given that the far galaxies are red-shifted - does that mean that we also observe them in slow-motion? I guess - yes, but I was never able to find a number of how much slower time runs in these galaxies compared to ours. And even more mind-boggling thing is that they also observe us red-shifted and therefore for them the time in our galaxy is slower...

[damianmcgrath]

Brilliantly done. Fantastic summary of the history of our understanding of the scale of the Universe! Thank you.

[TolgaPala]

Many thanks for this amazing video. This brings back a lot of memories from my first semester as an astronomy student in the introduction to astronomy lectures.

[crashovride]

Fantastic video!! Most of these things I had no idea about. Science is amazing and thank you for bringing it to us!

[philbigdog]

Really great video Scott - this is like a synopsis of all the astronomy books I've read over the years.