We measure space objects speed in relation to earth. But since earth is also moving through space, how do we know the true speed of objects without a universal “non moving” point?


The speed of an object is measure relative to the observer. But since all objects are moving in relation to all objects, how can we know the true speeds of things? Wouldnt two observers on different planets measuring an object in space come up with different answers?

In: Physics

Why does there need to be a “true speed”? And yes, two observers in different planets would measure the same event differently. The key though is that they can both go through something called a Lorentz Transformation, and get the answer that the other would have. There is no true 0 speed, the same way there is no true 0 longitude. In both cases all coordinate systems are valid.

All speeds are relative.

On a universal scale, scientists will try to measure the speed of things in relation to the center of the known universe, and use that as a hypothetical non moving point.

But I’m most practices, that isn’t really necessary or useful, what is more useful is having a speed relative to soemthing more useful, like say relative to earth, our home.

Or relative to the sun, which is effectively the center of our solar system.

Or relative to the center of the Milky Way galaxy, or one galaxy relative to another galaxy. That stuff is typically more helpful/applicable

There is no true speed, and there is no universal fixed point. We can only ever measure movement in relation to something. This is sometimes called Newtonian relativity.

A teacher I had in college brought up this, basically poking fun at Star Trek. In the show they would say full stop, but full stop compared to what? If Earth, then what ever your trying to look at would zoom past you. He liked pointing out that speed is just like time, a concept made up by humans that ultimately means nothing to the universe.

Note, that there is also “proper speed”. When you visit a star light years away (like alpha centuri), you yourself can be there in 3 days (considered you have enough energy). So talking very simply, from your point of view, you travel FTL in proper distances.

Observers on Earth, will see that you will take 3 years. And this is where time dilation and length contraction come into play. As you are near FTL, your way towards the star contracts, as such you only need some days.
However, for the observers on the earth, your way does not conract, instead they see in difference in time perceived.

Everyone here has told you that all speed is relative, and there’s no “true speed”. Which is true!

*But* it is possible to measure an object’s speed relative to the cosmic background radiation. The cosmic background radiation is the afterglow from the big bang, it is pretty much the same (with small variations) in all directions. If there is a universal rest frame in the universe, that’s it.

Our velocity relative to the cosmic background radiation is about 370 km/s, in the direction of the constellation Leo. That’s about 0.1% of the speed of light.

it might give you vertigo to think *we dont!*.

I once got caught in a blizzard on a snowboard. I couldn’t see so I slowed down and stopped to regroup.
Then I sat down.
Turns out i hadn’t stopped, it was just so white and noisy and powdery that I had no cues left about my speed relative to earth.

I ate shit in the most unexpected way.

I am a physicist and there is a lot of bad information in this thread.

~~There absolutely is~~ We can easily define a universal reference frame. Yes relativity is a thing, but it’s pretty easy to define a fixed point in the universe. You can find this in several ways:

1. You can look at the average velocity of every object in the universe and define a universal reference frame based on the reference frame where that average velocity is zero.
2. You can also look at the Doppler shift in the cosmic microwave background (CMB) which is the remaining heat glow from the big bang. Doppler shift for the CMB is analogous to when you hear the pitch of a train horn change from higher to lower as it passes you. We have good reason to believe the universe is isotropic, meaning the same in every direction. So we should expect that the CMB is the same in every direction, too. However, in one particular direction the frequency of the light is slightly higher, and the opposite direction is slightly lower due to Doppler shifting.

These above methods can give us a good absolute reference frame to measure any velocity relative to. For our galaxy, we are moving though space at about 600 km per second.

To anyone doubting the validity of defining a fixed reference frame, I also wanted to mention something known to physicists as Mach’s principle. This principle tells us how it’s possible for a rotating object to “know” when it’s rotating. If you spin around, you can feel your arms being “pulled” outwards, but how could this ever happen without there being some kind of universal reference frame? Mach explained this effect as the distant stars acting as a fixed reference point for the rotation and centrifugal effects. Einstein further explained this effect in general relativity via “frame-dragging”. But honestly this effect is still stumping physicists to this day, and we don’t fully understand how something could ever “know” it’s rotating. Our best explanation is that all the matter in the universe somehow provide the necessary reference frame for the rotation.

Edit: Changed wording to clarify my point. Thanks for the silver.

Edit2: Spelling