3
$\begingroup$

Earth is traveling at a certain velocity. Earth orbits the Sun at velocity X. The sun is orbiting the center of our galaxy at velocity Y. The galaxy is orbiting (something?) at velocity Z. On and on...

In relation to zero, Earth has a velocity of X + Y + Z + (?).

Is it theoretically possible that there would be a point in space where nothing is moving and velocity is essentially 0?

If so, what would time look like at this point in space?

$\endgroup$
0
9
$\begingroup$

Yes.

The velocities you list (X, Y, …) are all velocities with respect to some reference frame. But all reference frames are arbitrary, and you can always define a reference frame where the velocity of some object is exactly zero, as long as it's not accelerating.

For instance, Earth's velocity in the Sun's reference frame is X, but in its own reference frame, it's zero.

This is not just semantics; it is probably one of the most profound concepts in physics that there is no such thing as an "absolute frame of reference". Before Einstein, people thought that time and space are absolute, but Einstein taught us that everything is relative.

In cosmology, there is a special reference frame in which all matter on average is zero. The coordinates of this frame are called comoving coordinates, and in many cases it makes sense to regard this as the most natural reference frame. But it is important to remember that it is still just a choice, and that any other reference frame is equally legit, although a bad choice of reference may make certain calculations harder or impossible to solve. In comoving coordinates, the cosmic microwave background (CMB) is isotropic, i.e. it looks the same no matter which direction you look. But since Earth is traveling with a non-zero velocity in comoving coordinates (at an average of 369 km s$^{-1}$), the CMB is slightly blueshifted in the direction in which we're traveling, and slightly redshifted in the other direction.

What would time look like? Time runs slower, the faster you move. In any non-accelerating reference frame (an inertial system), time runs "as it should", but an observer in this frame who measures how time runs in a frame that has a non-zero velocity with respect to his/her frame, will see it run slower.

$\endgroup$
15
  • 4
    $\begingroup$ To be fair to the pre-Einstein folk, the principle of relativity goes back to Galileo, and plenty of people didn't agree with Newton that space was absolute. (And for Newton himself, Galilean invariance of physics is actually a theorem in Principia, but he thought that space must be absolutely anyway for conceptual reasons.) Note that having space relative is enough for the answer to this question to be morally the same, even if time was thought to be absolute. $\endgroup$ – Stan Liou Mar 16 '15 at 20:16
  • 1
    $\begingroup$ 600 km/sec is the usual figure for our velocity with respect to CMB: apod.nasa.gov/apod/ap140615.html $\endgroup$ – Wayfaring Stranger Mar 19 '15 at 11:50
  • 1
    $\begingroup$ Okay, settled: 369 km/s! But thanks for prompting me to actually look up the number rather than just guessing :) $\endgroup$ – pela Mar 19 '15 at 14:10
  • 1
    $\begingroup$ Oh, cool, @pela. I've been wrong all these years, but now have it straightened out. Thanks. $\endgroup$ – Wayfaring Stranger Mar 19 '15 at 14:28
  • 1
    $\begingroup$ I… I thought that was what I did point out. I don't agree that the question is invalid and should not be answered. I think it's a great question, which gives you and me the opportunity to explain that velocities always should be expressed in relation to something, but that sometimes this "something" is so obvious that we don't need to. Which we did. $\endgroup$ – pela Mar 20 '15 at 11:21
3
$\begingroup$

The question itself is wrong, actually.

There is no such thing as absolute velocity, which is what you're assuming in your question. Velocity is always relative to a frame of reference.

Your speed relative to your chair is zero, but it's not zero relative to the airplanes flying over your house.

When you say "a point in space where nothing is moving and velocity is essentially 0", you must add "relative to such-and-such frame of reference", otherwise your statement makes no sense.

By that token, any object in any point of space has zero speed relative to any frame of reference rigidly attached to it, and non-zero speed relative to other frames. You can't talk about speed unless you specify the frame against which it is measured.

Speed doesn't even exist by itself. It's always relative to a frame. It's not a property of an object or (even worse) a property of "a point in space". It's a relation between an object (the moving thing) and another object (the frame). "Points in space" don't have properties themselves, space is featureless.

Cars may seem like they have a speed as an intrinsic property, but that's just their speed relative to the ground. The car's speed relative to its driver is very different (and hopefully equal to zero). And the car's speed relative to a comet in the Solar System is yet again different.

Any further considerations, such as the flow of time, etc., are invalid as long as you're not asking the right question. Again, speed can only exist relative to something and, as such, it depends on the choice of the frame. And time depends on that whole causal chain.

$\endgroup$
1
  • $\begingroup$ I am strongly disagree with the current science about the idea Florin and I tell you why.... In my point of view there is no fix physical object in space unless if it would be made artificially. I believe light would give a reference point in this matter to calculate the absolut zero speed in speed as light speed Is constant. Also because speed is relative to time it would also tell us that would be the point where time is running In the absolute fastest in space. The calculation: We need two laser beam shooted opposite direction the same time away from each other. Let’s call them X and Y. The $\endgroup$ – Attila Varga Nov 20 '18 at 11:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.