Can you determine absolute motion?

DaleSpam

There is nothing wrong with using a reference frame where the whole universe is moving.

Yes, proper acceleration is absolute, even in a non-inertial reference frame.

No, it is not true in any frame-independent sense.

1MileCrash

No.

No. That changes nothing. You're confusing yourself here.

Who's to say the moon and earth weren't drifting in a constant speed in any direction I want? No one, because it's relative. And who's to say that when you moved away from the earth wrt the earth, you ceased motion while it continued (along with the moon, stars, whathaveyou) in a constant speed and direction?

The universe is a big, big place. The stars and the moon that you can see are literally just about nothing, they are relatively moving just as everything else is and they are not absolute reference frames.

See above, so?

You are moving relative to the moon, along with the earth, and they are moving relative to you. Neither is a reality because you could never define any of them as being "still" to begin with!

Pretend there is an absolute reference frame that we've located, just to put this into your noodle. Relative to it, our galaxy is moving in a given direction (X) at .12 C.

You blast off in your ship going .12 C in direction Q (opposite of direction X). So, who's moving? According to our pretend absolute reference frame, when you turned on your thrusters, you stopped moving while the galaxy kept moving at .12 C in direction X.

No. The statement you're looking for is "I moved away from the earth, AND the observable bodies not the whole universe moved away from me. Our net change in speed with regards to each other is absolute, but whether I slowed down, sped up, or changed direction with regards to the earth or the earth with regards to me is relative. Which is perfectly possible."

See above. You absolutely underwent acceleration and the earth did not, therefore you are younger. That means nothing regarding who was "moving."

According to any reference frame, you accelerated, that's why you're the younger twin. But did you stop, speed up, or change direction? That's relative! Only the net change in speed of your ship is absolute. Therefore no, the twin paradox in no way suggests that the space ship twin is "absolutely moving" only that it absolutely changed it's speed.

If I could paint a mental picture, ignoring the "speeds not exactly adding" fact for simplicity:

From the earth's reference, you blast off at .1 C in a -> direction, then after some time you turn around and travel back towards earth at a <- direction at the same speed.

Reference Frame A, which is whatever the hell I want, initially observed earth as traveling .1 C in -> direction. Therefore when you got in your ship and left, you traveled .2 C at direction ->, then when you "turned around," according to A, you ceased motion while the earth "caught up with you" by traveling it's constant .1 C. You absolutely accelerated.

You absolutely changed speed by .2 C, and A observed that change of speed as you stopping motion.

Reference Frame B, which is also whatever the hell I want, initially observed earth as traveling .1 C in <- direction. WRT B when you launched your space ship, you stopped! You completely ceased motion. You are traveling .0 C, while the earth continues <- @ .1 C. Now, it's time for you to "turn around", which B observes as you starting to move again, this time in a <- direction, at .2 C.

So according to reference frame B, the earth was in constant motion, you stopped motion when you left the earth, and then you accelerated back towards the earth at a speed of .2 C.

You absolutely changed speed by .2 C, and when you turned around, B observed you starting to move, relatively.

Now, reference frame C, which is also whatever the hell I want (this is physics, I can do that.) initially observes earth traveling .7 C in direction ->. It sees you speed away from earth at .8 C, and percieves your "turning around" as a mere "slowing down" to .6 C while the earth catches up with you.

You still absolutely changed your speed by .2 C when "turning around." But here, you just slowed down, relatively.

None of these reference frames are preferred, because none of them can be concluded as being of "absolute rest" wrt space itself, therefore, all observations are equally correct.


Nope. You can only say with absolutely certainty that you experienced a net change in speed.

Yes it does.

rede96

Firstly, thanks for the replies.

I think the penny has dropped, but I am still struggling with one thing.

If I am at rest wrt to the earth and then I accelerate for a short time then stop accelerating, I would notice that the earth and I are moving apart. So two things could have happened.

a) I slowed down and the earth carried on its merry way

or

b) I speed up and moved away from the earth.


In both those cases it was I that accelerated. So I can say that I broke the symmetry not the earth (As it did not accelerate.)

But I can't say that I moved away or the earth moved away.

However, if someone on earth sent a beam of light in my direction, couldn’t I tell using the Doppler effect if it was the earth receding or if I was moving away?

DaveC426913

Yes, if your craft accelerates you can conclude that it is you moving away from the Earth rather than the Earth moving away from you.

Both scenarios would result in the same Doppler shift, so no.

rede96

Thanks.

DaveC426913

HOWEVER, once you stop accelerating, you revert to an inertial frame of reference, from which you will determine that you are stationary, like any other FoR.

(Note that, even though you know you accelerated away from Earth, you have no way of claiming that the Earth was stationary while you were on it, so it is not stationary either. All you can say is that you and the Earth are equally valid inertial FoR).

DaleSpam

Yes, exactly.

No, the Doppler effect is fully relativistic.

harrylin

Hi Physicist1231,
Was your first post of this thread sufficiently answered?

Assuming that relativity is correct, "The Omnipotent Point of view" cannot be disproved by experiments - as JesseM also mentioned. Perhaps because it has no practical use (how could we use a view that we can't determine!), it's less well known from the peer reviewed literature.

Relativity jargon is positivistic: only operational definitions are used, based on phenomena (appearances). Now, "Light approaches any reference point at C" is poorly stated, and therefore true or false depending on your references. The approaching speed of light is "relative" to the used reference system: it is set by definition equal to the receding speed in special relativity. You can read the definition here, in section 1:
http://www.fourmilab.ch/etexts/einstein/specrel/www/

As a result, the (apparent) "closing" or "approaching" speed has been made c wrt a reference point that is at rest in the reference system that you use, by appropriately regulating the clocks (see also Einstein 1907, Jahrbuch Radioelectr. Electr.4, 414).
It seems that you correctly understood that it is not c relative to a point that is "moving" in your reference system - indeed, that would be paradoxical, as the vector subtraction (c-v) with c=constant and v=/=0 cannot equal c.

The impossibility to determine absolute motion (in the original, Newtonian sense) is directly related to the impossibility to determine the absolute one-way closing speed of light, which in turn is directly related to relativity of simultaneity.

Harald

xxxx0xxxx

The force required to change direction is applied in your rest frame, and is independent of relative motion. For you, your mass is always your rest mass. For others not at rest with you, you have additional energy/mass in the form of momentum.

Your thrusters do not change your net momentum, since they only produce angular acceleration (you do lose rest mass however).

chingel

But can you determine absolute rotation? The further away you go on a merry-go-around the harder it tries to throw you out. If the universe is spinning around a center, can we absolutely determine by how much?

DaleSpam

Yes. Rotation is a non-inertial motion.

xxxx0xxxx

Well, from the point of view of the moon, earth rotates around it, and I would say the universe rotates around us every 24 hours :)

If you are within some sort of rotating structure, you will experience centripetal force if you appear to be stationary with the structure, and you will observe coriolis motion of objects moving relative to the structure; otherwise the interior structure will be moving with respect to you in a circle ... and yes these would be measurable absolute effects.