# Do Distant Planets Move Faster Than Light Seen From Earth?

• I
• Machu
In summary: Thanks. So basically all spinning reference frames don't count, or are there any exceptions?There are no exceptions that I know of.

#### Machu

From the observation point of Earth, you can say that the sky "moves" while the Earth is still. From this point, an observer on Earth would see a distant galaxy travel a circumference millions of light years lomg around the Earth in only a day - resulting in a velocity faster than light. Special relativity says superluminal speeds are impossible so what technicality keeps this from breaking the law of physics?

We recognize that our frame here on Earth is a non-inertial, rotating frame of reference and thus those specific rules of SR do not apply.

Note that SR is based off of two postulates. From wikipedia's article on SR:
• The Principle of Relativity – The laws by which the states of physical systems undergo change are not affected, whether these changes of state be referred to the one or the other of two systems in uniform translatory motion relative to each other.[1]
• The Principle of Invariant Light Speed – "... light is always propagated in empty space with a definite velocity [speed] c which is independent of the state of motion of the emitting body" (from the preface).[1] That is, light in vacuum propagates with the speed c (a fixed constant, independent of direction) in at least one system of inertial coordinates (the "stationary system"), regardless of the state of motion of the light source.
Here "inertial coordinates" means a non-accelerating system of coordinates, which excludes rotating reference frames.

Dale
Machu said:
Special relativity says superluminal speeds are impossible so what technicality keeps this from breaking the law of physics?

Special relativity tells us that information cannot be transmitted at a speed faster than the speed of light in a vacuum. If, for example, you could get a particle to travel faster than this speed you could use it transmit information at this speed. Such a thing is not possible. As far as we know, according to the laws we understand.

Thanks. So basically all spinning reference frames don't count, or are there any exceptions?

Machu said:
Thanks. So basically all spinning reference frames don't count, or are there any exceptions?

There are no exceptions that I know of.

Machu said:
So basically all spinning reference frames don't count, or are there any exceptions?
Rotating coordinate systems are non-inertial, and the laws of physics need to be described differently in such systems. This is true in Newtonian physics as well. There the change consists of adding a list of "fictitious forces" (The D'Alembert, centrifugal, Coriolis, & Euler forces) that apply to all matter in the non-inertial system, depending on position and velocity. For instance, if we want to describe the sun as moving around Earth, the "force" accelerating it toward Earth is the Coriolis force, and it has to "work twice as hard" because of the centrifugal "force" pushing outward.
There is a corresponding set of rules for SR physics, but I don't know it. I assume it is very complicated and ugly. Anyway, the "no FTL" rule will not apply in its simple form.

I find it helpful to think about things in terms of "chasing a light beam". No matter what reference frame you are in, whether accelerating or not, nothing can ever beat a beam of light in a race. You can set up a coordinate system in a rotating frame of reference, and you'd have to account for the increasing tangential velocity that objects have the further away from you they are, but "FTL" then becomes not a single speed, but a variable speed. This makes the math extremely complicated, but in the end the result is the same. If you release an object and a beam of light from any location and send them towards any other location, nothing will every beat that beam of light. This even holds true in the expanding and curved space of General Relativity.

## 1. How do we know that distant planets move faster than light seen from Earth?

We can observe the movement of distant planets through a variety of methods, such as measuring their position in the sky over time or studying the light they emit. These observations have shown that distant planets do indeed move faster than light seen from Earth.

## 2. Can we see the actual movement of distant planets from Earth?

While we cannot see the actual movement of distant planets from Earth, we can observe the changes in their position and motion over time. This is due to the limitations of the speed of light and the vast distances between Earth and these planets.

## 3. How does the speed of light affect our perception of distant planets?

The speed of light is the fastest speed at which any object or information can travel through space. This means that the light we see from distant planets has taken a significant amount of time to reach Earth, therefore our perception of their movement is delayed and may not accurately reflect their current position.

## 4. Do all distant planets move faster than light seen from Earth?

Yes, all distant planets move faster than light seen from Earth. This is due to the vast distances between these planets and Earth, which means their motion appears much slower to us due to the limitations of the speed of light.

## 5. How does the theory of relativity play a role in the movement of distant planets?

The theory of relativity, specifically the theory of special relativity, helps to explain the observed motion of distant planets. This theory states that the laws of physics are the same for all observers, regardless of their relative motion. Therefore, the apparent faster movement of distant planets seen from Earth is a result of our relative motion and the limitations of the speed of light.