# How is speed in space measured? **URL:** \ **Category:** Factual Questions\ **Created:** [November 14, 2018, 2:38pm UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503 "2018-11-14T14:38:39Z")\ **Posts on this page:** 7\ **Page:** 2
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [November 15, 2018, 2:15am UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503/21 "2018-11-15T02:15:09Z")
> [@Chickenwrangler](#): > > Another point with expanding Universe. Presumably the Big Bang worked like any explosion on Earth and flung bits of future Planets in all directions. So all the bits moving in one direction have a speed relative to the point of the Big Bang another speed relative to each other and yet another speed to bits of future planets going in the opposite direction? > > Slight deviation, do we know exactly where the Big Bang happened and what is there now? No, that’s precisely the wrong presumption. The Big Bang worked nothing like any explosion. Less than a second after whatever happened initially all of space was expanding in the same way. There is no place where Big Bang happened, or there is, but it is everywhere. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [November 15, 2018, 2:24am UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503/22 "2018-11-15T02:24:31Z")
> [@Chickenwrangler](#): > > So while Voyager is hurtling away through space, the Earth is orbiting the Sun at 107000Kph or thereabouts and will sometimes be travelling in the same direction as Voyager and sometimes away from Voyager, therefore giving two different readings of Voyager speed relative to the Earth? Yes. I believe they adjust its speed based on measured doppler shift to allow for that. That is, when they report it in the news or whatever. So what we hear is its speed relative to the Sun. > [@](#): > > Another point with expanding Universe. Presumably the Big Bang worked like any explosion on Earth and flung bits of future Planets in all directions. No it did not. This is one of the biggest misunderstandings about the Big Bang. It’s mostly the result of the name – it really needs a better one, say the Big Expansion. The Universe itself expanded and carried all that matter along with it. > [@](#): > > Slight deviation, do we know exactly where the Big Bang happened and what is there now? There’s no such point. Or you could say it happened everywhere. The point that it happened at is now stretched out to the entire size of the Universe. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 15, 2018, 4:35am UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503/23 "2018-11-15T04:35:32Z")
Heh. Last week I was thinking of posting the question: Can an object be motionless in space? ---
**Author:** ![rat\_avatar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/rat_avatar/32/255_2.png) [@rat\_avatar](https://boards.straightdope.com/u/rat_avatar)\ **Post date:** [November 15, 2018, 5:25am UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503/24 "2018-11-15T05:25:20Z")
> [@Johnny\_L.A](#): > > Heh. Last week I was thinking of posting the question: Can an object be motionless in space? In reference to what? or can you provide any constraints? An object is “motionless” if it is not rotating in space, is not under acceleration, or you don’t compare it to something else with a frame you consider inertial. but I should add something in as a caution, _straight_ lines won’t look _straight_ to your eye, they are [geodesics](https://en.wikipedia.org/wiki/Geodesic) While still a simplification, which ignores the tidal force etc…the Earths orbit around the sun is following the Geodesic or the shortest path on a curved surface. There is no centrifugal or centripetal force with this orbit and it is merely an artifact of the geometry of spacetime. Pretend that we lived in a universe a x billion years in the future when we can’t see other galaxies, and pretend you can’t see the sun or the other planets. Without the tidal force or the smaller terms of frame dragging (also related to geometry in GR) you wouldn’t even notice we were “orbiting” an invisible sun. This flawed analogy is just another rephrasing the idea that nature doesn’t care how we label points in space-time and coordinates do not automatically have some real “physical” meaning. The geometry of spacetime matters and the causal structure does have some features that are not relative. The spacetime interval between two events can be space like, time like, or null and that interval is invariant. This lets us know that event _foo_ caused event _bar_ caused event _baz_; Not all observers will be able to see this but the ones that can with particular constraints will always agree. The point being that outside of the effects in observing a causal chain of events movement is 100% relative until you consider the time dimension. Time is kind-of like a dimension except that it is not a degree of freedom, so it really doesn’t work for some definitions so lets just call it the temporal dimension to avoid side tracks. Typically paths through space-time are visualized in “world line” which just exist, and yours would start at birth and end at death in this example. It just exists and isn’t moving but the points along it are “events”. Your preferred shortest path through this is the geodesic, and things get a bit weird here. Your speed along that world line is always _constant speed_ in for a lack of a better term. Any movement you make in the three degrees of freedom moves you away from this shortest path and that longer trip slows your clock. The implications of this will break our common intuitions around the world especially when you consider gravity. This post is already too long but as an example in this model of thinking when an apple falls and hits the earth, it is actually more correct of the earth rising to meet the apple. As running into the earth pushes the apple away from it’s shortest path through space-time. For the same reason the arced path of a basketball being thrown is actually following a straighter line through space-time than you are traveling sitting on your chair as you are constantly being accelerated away from your preferred shortest path. This 1960’s video is very much worth your time to help with the concepts of a frame of reference. While it is almost impossible to share how GR works without teaching someone the math the SR topics are much more accessible and the very well tested implications of SR/GR are really fun to challenge yourself with. But the important part to remember is that the spacetime interval is THE invariant quantity in relativity. Distance is not invariant under relativity which is why the question you ask is problematic in that domain. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [November 15, 2018, 5:44am UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503/25 "2018-11-15T05:44:57Z")
I’m glad cannabis is legal in Canada now. Pfffff… ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [November 15, 2018, 12:40pm UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503/26 "2018-11-15T12:40:36Z")
> [@Nava](#): > > And Voyager’s speed is calculated in relationship to Earth. For that calculation we’re geocentric; since we’re observing from Earth we provide the speed “as seen from Earth”. We could also calculate its speed in relationship to the Sun, or say that it’s stationary and that we’re moving at 17km/sec, but neither of these would provide more-useful information and they’d be more complex (one mathematically, one mentally) so we don’t bother. Nitpick: Voyager’s speed is usually quoted relative to the Sun these days. We’re at the point in the mission where [the distance between Earth and Voyager I actually decreases for part of the year,](https://what-if.xkcd.com/38/) since the Earth’s speed relative to the Sun is greater than Voyager’s speed relative to the Sun. (That article was written in March 2013, so you might try checking [this page](https://voyager.jpl.nasa.gov/mission/status/) in March if you want to see the numbers clicking down instead of up.) ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [November 16, 2018, 3:20am UTC](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503/27 "2018-11-16T03:20:22Z")
> [@rat\_avatar](#): > > In reference to what? Exactly. I was thinking about ‘intergalactic space’; somewhere between galaxies. By motionless, I’m thinking along the lines of ‘zero kinetic energy’. But as you say, speed is meaningless without a frame of reference. Is kinetic energy the same? Or, consider galaxies that are approaching each other. With reference to one galaxy, you’re approaching it at _x_ meters per second. But you’re _also_ approaching the _other_ galaxy of _x_ meters per second. [Previous page](https://boards.straightdope.com/t/how-is-speed-in-space-measured/824503.md?page=1)