# Measuring speed..

Discussion in 'General Science & Technology' started by ZMacZ, Mar 27, 2018.

1. ### ZMacZRegistered Senior Member

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(When I say absolute zero speed I mean absolute speed zero, as in opposition to the speed of light,
absolutely 300.000 km/s)

My question is this:

Is this true or not ?

(and why do I only notice stuff missing till after posting..)

Last edited: Mar 27, 2018

3. ### gmilamValued Senior Member

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Absolute speed of zero... Aren't we always at zero relative to light?

5. ### ZMacZRegistered Senior Member

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Umm.I don't actually have to answer that right ?
Even a speed of a brisk walking pace, 5 kmh, is a fraction of the speed of light, albeit a small one..
I'm talking standing still absolutely, in the middle of space...
(No speed from the planet revolving around a sun, no speed from the solar system moving/revolving
away from the center of the galaxy, and no speed from our galaxy moving/revolving away
from the site of the Big Bang....no speed whatever.THAT absolute zero speed..)

Hmm..maybe the point you stay in when moving at the speed of light minus the speed of light ?

Last edited: Mar 27, 2018

7. ### gmilamValued Senior Member

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You don't have to answer it. I don't really know the answer - but I am pretty sure that absolute zero speed is meaningless. The Earth is always moving, the solar system is always moving, the galaxy is always moving. As far as I know, we can only measure our speed relative to something else. And we always measure light at the same speed.

8. ### ZMacZRegistered Senior Member

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I think ur not without wisdom..

The thing is, in order to go from relative to absolute, we need a way to find out the exact speed we have, as an absolute..
Not a relative to anything else..
Once we know that, we can know exactly where we came from..
Since the Earth revolves around the sun etc etc etc, we can tell we are actually moving all the time..
But the LS is an absolute (as far as we can tell at least), so measuring the differences in timing in light,
from the moment of emission to receiving, we can tell our speed.)

If I could only make a 10 Thz ticker.then i could make the estimation at home..
(it would decrease the distance required to 30 m..)

9. ### billvonValued Senior Member

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Well, "relative" is all that matters when it comes to measuring speeds. There is no absolute speed, only speeds compared to something else (the Earth, the solar system, the galaxy, the center of mass etc.)
Nope; you can't, not if you are right there observing it. Google "Michelson Morley" for the experiment that proved this.

Edited to add - unless I misunderstood you. You can compare the rates of time passing on both devices, and use that to determine the _relative_ difference in speed between them both.

10. ### DaveC426913Valued Senior Member

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No. Speed is always relative to something. And it is up to you what you pick to use as a reference.

No matter how fast you are moving, relative to the ground, the sun the galaxy, the Virgo supercluster, you will always measure the speed of light in a vacuum as c.

If you got in a spaceship and zoomed away from the Earth at 0.999999999999c, you would still measure the speed of light as c.

There is no such thing as absolute speed.

11. ### ZMacZRegistered Senior Member

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LS is constant right ?
So basically LS - LS = zero speed correct ?

(the only way for this to NOT be correct is saying that LS isn't a constant ?)

12. ### DaveC426913Valued Senior Member

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Sure. Zero speed is you in your own frame of reference. You are always stationary in your own FoR.

The speed of light is always measured as a constant. You are always measuring in your own FoR. What you cannot do is try to measure your speed relative to some fictitious absolute FoR of the universe.

13. ### James RJust this guy, you know?Staff Member

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ZMacZ:

Standing still relative to what? There is no absolute standard of rest.

You can be stationary relative to the centre of our galaxy if you like. That idea about negating motion "aawy from the site of the Big Bang" won't work, though, because the Big Bang happened (is still happening, technically) everywhere at once. There is no centre to the universe.

The speed of light is measured relative to something. You can be at rest relative to that thing if you like.

Sorry, but it can't be done. There is no absolute standard of rest.

Only relative to the sun, in that case. Not absolutely moving.

No, because the speed of light is the same in all (inertial) reference frames.

You could perform your experiment in the opening post in two different frames travelling at different (constant) velocities and you'd get exactly the same result in both cases, in the frame where you made the measurements.

14. ### sweetpeaValued Senior Member

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I sometimes read on the web that the CMB can act as a kind of absolute frame, the thing I don't understand about that idea is, the CMB is just photons and being such are moving.

15. ### exchemistValued Senior Member

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Yes I don't see how that can work, seeing that radiation moves at c relative to everything!

16. ### James RJust this guy, you know?Staff Member

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The CMB comes equally from all directions, more or less (because the big bang happened everywhere at once).

If you start moving around, then the CMB coming from in front of you is Doppler shifted towards the blue, while the CMB from behind is red-shifted. You can use that to judge how fast you're moving relative to where stuff was when the CMB was emitted, I guess, but it's just another reference frame. There's nothing "absolute" about it.

17. ### exchemistValued Senior Member

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That's an interesting point.

So, then, do we see the CMBR at the same frequency from all directions, or do we see it representing different temperatures, depending on which direction we look? My understanding was that it was the same in all directions. Which would seem to put the Earth at rest, relative to the source of the CMBR. A bit of an odd coincidence, surely? Or have I misunderstood?

18. ### NotEinsteinValued Senior Member

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Here's a picture of the CMB measurements done by COBE: https://ned.ipac.caltech.edu/level5/Sept02/Kinney/Figures/figure6.jpg
The top one is what was measured. It was then compensated for the movement of the Earth (that's the dipole contribution), and the contributions of the Milky Way were subtracted (that the "noise" in the vertical middle). The anisotropies are then exaggerated (the color-scale is re-adjusted), resulting in the bottom picture.

I remember hearing somewhere that the inertial frame in which the CMB is at rest differs per position, and thus it can't be used as an absolute frame.

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19. ### exchemistValued Senior Member

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Aha, thanks. So the Earth is moving relative to the rest frame of the CMBR.

Is the CMBR rest frame an expanding one, and thus not strictly inertial, or something?

20. ### NotEinsteinValued Senior Member

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Exactly. It's mostly a combination of our orbit around the sun, and its orbit around the center of the galaxy, if I remember correctly.

My guess would be that it is inertial, but consider two objects stationary in this rest frame at two different locations. Due to the expansion of the universe, they are drifting apart. I don't think that can possibly be the same inertial frame?

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21. ### exchemistValued Senior Member

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Nice point. It's getting late now but I might research that a bit tomorrow or Monday.

22. ### Q-reeusValued Senior Member

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Not to seem too critical, but....the case of motion wrt CMBR has been mentioned in numerous earlier threads that most members would have read. Somehow such things become ever fresh. Ground hog day.

23. ### exchemistValued Senior Member

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Thanks Q-reeus, I had not realised that.

I confess I'm a bit selective about threads on cosmology, as the subject seems so often to bring the cranks and nutters (nebel, Trivedi/God and so forth) out of the woodwork, which can maek the threads confusing and irritating to read. I'll take a look and see what I can find - unless you happen to have to hand a particular one you can recommend me.