# Relative Speed of Light Between Objects

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• Skaperen
In summary, the first assumption is that each observer would see the other object approaching at 150% of the speed of light, but that can't be because true speed of anything is relative to the observer.
Skaperen
TL;DR Summary
Given that motion in space is relative to the observer and that motion is limited to the speed of light, I am wondering how that relationship determines relative speed of objects that would seem to exceed the speed of light between them.
This is one of my thought experiments where I am drawing a big blank, If you have 2 objects approaching a 3rd object from opposite directions (just enough off to avoid collision) at 75% of the speed of light, the first assumption is that each observing the other would see the other object approaching at 150% of the speed of light. That can't be. What speed would each appear to be approaching as viewed by the other? I'm certain this must be less than 100% since true speed of anything is relative to the observer.

Just because an observer has accelerated to a speed that would pass the 3rd object at 0.75c does not mean their speed relation to any other object (doing the same at 180 degrees) can exceed 100% the speed of light. I would like to know what formulas apply to a case like this including at different speeds (0.8c and 0.9c) and different approach angles, (such as 120 degrees or 91 degrees).

There is a distinction between the separation rate that I measure between two objects that I see as in motion and the relative velocity either of those objects ascribes to the other. According to me the distance between two objects may increase at up to 2c, but each one will measure the other doing less than c. From my perspective this is because their rulers are length contracted and their clocks are time dilated and incorrectly synchronised, so I can understand their measures even if I don't agree. From their perspective it's my rulers and clocks that are acting oddly.

One should not confuse this issue. By definition the relative velocity between too (pointlike) objects is the velocity of one object in the rest frame of the other (at least the latter must have positive invariant mass of course). See Sect. 1.5 and 1.6 in

https://itp.uni-frankfurt.de/~hees/pf-faq/srt.pdf

## 1. What is the relative speed of light between two objects?

The relative speed of light between two objects is the speed at which light travels between them. This speed is always constant and is approximately 299,792,458 meters per second.

## 2. Does the relative speed of light change depending on the objects' distance?

No, the relative speed of light remains constant regardless of the distance between the two objects. This is one of the fundamental principles of the theory of relativity.

## 3. How does the relative speed of light affect time dilation?

The relative speed of light is a key factor in time dilation, which is the phenomenon where time appears to pass slower for objects that are moving at high speeds. The faster an object moves, the slower time appears to pass for it.

## 4. Can the relative speed of light be exceeded?

According to the theory of relativity, the relative speed of light is the maximum speed at which anything in the universe can travel. This means that it cannot be exceeded by any object, regardless of its speed or energy.

## 5. How is the relative speed of light used in space exploration?

The relative speed of light is crucial in space exploration as it determines the maximum speed at which spacecraft can travel. It is also used in communication systems between Earth and spacecraft, as signals must travel at the speed of light to reach their destination in a timely manner.

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