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Please ignore relativistic effects and the effects of the expansion of space-time due to the expanding universe theory for the purposes of this question.

Whenever someone asks what is the speed of X, we always ask "with respect to what"? We can calculate the speed of something only with respect to another object. A train's speed is calculated relative to the ground. A spacecraft's relative to the earth/sun and so on. It seems fair enough.

I understand there is no universal frame of reference. Because, well, there is no sense of direction in space, except relative to some other heavenly body.

Agreed there is no universal frame of reference. But I think that actually implies that there is no universal origin in the universe. Certainly not that there isn't a universally acceptable notion of distance.

1 meter is 1 meter, no matter where you are. If I am covering 10 meters every second, my speed is that much.

It is only that we dont feel the speed with which the earth rotates, since we are on it. When we say a car moves at 60 kmph, that is with respect to the earth, but in reality, if we were to measure it with respect to space, our actual speed would be earth's speed in space (+ sun's speed in milky way etc ) + 60.

Other bodies only give us a reference point to feel speed. They shouldn't dictate how we measure it.

The gist - Isn't it incorrect to measure our speed with respect to another body (say earth), just because we can't feel/visualise our speed in empty space, which should be the actual speed of any body.

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closed as unclear what you're asking by ACuriousMind, Floris, Danu, yuggib, Ryan Unger Jul 30 '15 at 20:54

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    $\begingroup$ I don't understand this question. In the preamble, you seem to understand the issues. $\endgroup$ – innisfree Jul 27 '15 at 8:20
  • $\begingroup$ What is "speed in empty space"? You concluded yourself that there is no universal origin. How would you measure your speed in otherwise completely void space? What does speed/velocity even mean in that context? $\endgroup$ – Nephente Jul 27 '15 at 10:16
  • $\begingroup$ Isn't it incorrect... no, it's quite correct because I'm going 0 m/s and the rest of the world moves around me. $\endgroup$ – Kyle Kanos Jul 27 '15 at 15:09
  • $\begingroup$ A lack of universal origin doesnt imply the lack of a universal grid that could indicate distances, even if no body was present anywhere. $\endgroup$ – Yuganka Sharan Jul 28 '15 at 6:42
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When physicists use the word velocity it has a precise definition that is meaningful and unambiguous. If I measure the displacement from me to you then the result is a vector i.e. it tells me how far apart we are and in what direction you are. The velocity tells me how this vector is changing in time.

The point is that I can do this for any pair of objects: me and you; me and the Earth; you and the Milky Way; and so on. In every case the displacement between the objects is a vector and the relative velocity tells me how that vector is changing with time.

So there is no actual speed because velocity is necessarily linked to the separation between a pair of points, and is relative because choosing different pairs of points gives us a different velocity. All the measurements of velocity are equally valid, but you need to specify the pair of points you are using to understand what the velocity is telling you.

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  • $\begingroup$ That is why my question isn't about velocity, it's about speed... $\endgroup$ – Yuganka Sharan Jul 27 '15 at 7:33
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    $\begingroup$ @YugankaSharan: Speed is the magnitude of the velocity, so everything I say about velocity applies to speed as well $\endgroup$ – John Rennie Jul 27 '15 at 8:29
  • $\begingroup$ John, there's a typo in your answer. Distance is a scalar. $\endgroup$ – John Duffield Jul 27 '15 at 10:18
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    $\begingroup$ Whether using the word distance in this context does any harm I'm not sure, but for completeness I'll change it to displacement. $\endgroup$ – John Rennie Jul 27 '15 at 10:55
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Velocity and speed of a body are measured with respect to another body (see other answer). This is the only possible definition. The "actual" speed respect to the "empty space" is not well defined instead. And this is because we are not moving inside an "empty space" or an "aether" which fills the universe. To grasp the concept think about how you will define what you call the "actual" speed with respect to the empty space. Do you measure the speed of the car with respect to the Earth, or to the sun, or to the galaxy, or to the stars visible from the Earth? Or respect to the visible Universe, or from the whole Universe (which comprises what lies beyond the visible one)? It is possible to see that this reasoning is kind of circular and actually reminds me the story about the universe built on turtles all the way down...

In short: there is no privileged reference frame, and therefore any (inertial) reference frame is in principle just good enough. That said, in most practical situations it makes much more sense to consider the speed of a car with respect to the ground that its speed with respect to the galaxy.

Further reads:

Reference frame

Aether

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  • $\begingroup$ I don't care about the practical aspects for this question. I want to understand why the concept is built this way. $\endgroup$ – Yuganka Sharan Jul 27 '15 at 8:20
  • $\begingroup$ Why, imagine a grid in empty space. That should be enough to measure speed. $\endgroup$ – Yuganka Sharan Jul 27 '15 at 8:20
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    $\begingroup$ Well, the fact that all (inertial) reference frames are equivalent is not a practical issue. How you draw a grid in empty space? Where and how? $\endgroup$ – sintetico Jul 27 '15 at 8:57
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Isn't a physical frame of reference useless for calculating speed?

There aren't really any physical frames of reference. You can't step outside and point up to the clear night sky and say "Look, there's a reference frame". You can point to the Moon and the stars, but they are what they are. You can use them in your reference frame, but that reference frame is an abstract thing.

Whenever someone asks what is the speed of X, we always ask "with respect to what"? We can calculate the speed of something only with respect to another object. A train's speed is calculated relative to the ground. A spacecraft's relative to the earth/sun and so on. It seems fair enough.

Yes, it's simple enough.

I understand there is no universal frame of reference. Because, well, there is no sense of direction in space, except relative to some other heavenly body.

Actually, there is. See the CMB reference frame: "From the CMB data it is seen that our local group of galaxies... appears to be moving at 627±22 km/s relative to the reference frame of the CMB". Also check out this answer where ghoppe quotes Professor Douglas Scott saying "There clearly is a frame where the CMB is at rest, and so this is, in some sense, the rest frame of the Universe".

1 meter is 1 meter, no matter where you are. If I am covering 10 meters every second, my speed is that much.

Relativity is all about how your measurements are altered by your motion, and by gravity.

if we were to measure it with respect to space, our actual speed would be earth's speed in space (+ sun's speed in milky way etc ) + 60.

Correct.

Isn't it incorrect to measure our speed with respect to another body (say earth)

No, because motion is relative. Whether it's relative to the Earth or relative to the Universe. It isn't called relativity for nothing.

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  • $\begingroup$ I have mentioned in the first line to ignore relativistic effects for the purposes of the question. $\endgroup$ – Yuganka Sharan Jul 27 '15 at 10:41
  • $\begingroup$ Sorry Yuganka. It's quite hard to avoid relativity when we're talking about motion. $\endgroup$ – John Duffield Jul 27 '15 at 10:58
  • $\begingroup$ I understand that John... I just meant - consider my question for speeds when relativity can be ignored. Say where v < 0.001 c. $\endgroup$ – Yuganka Sharan Jul 27 '15 at 12:12
  • $\begingroup$ I looked at it again, and I still say it isn't incorrect to measure your speed with respect to another body. Your speed with respect to the body called space or the universe is circa 627km/s. Your speed with respect to the body called the Earth is maybe 60kph. $\endgroup$ – John Duffield Jul 27 '15 at 15:55

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