Science a GoGo's Home Page Forums General Discussion Physics Forum Dear ORAC I use C to explain Earth's speed

Okay we are going to change the two photons moving away from each other with rockets that do 0.6 the speed of light and moving away from each other.


<---- 0.6c ---- ROCKET 1| ROCKET 2 ---- 0.6c ----->


Now this is where things get interesting.

The two rockets are moving away from each other at 1.2c that is a fact however you will never see that and that's why it gets interesting.

Even from side on like I have drawn the two rockets as they pull away will be red shifted via Doppler movement and you will see them moving much slower than they really are.

So you will observer the relative movements of the two rockets as below the speed of light.

The important bit that usually confuses people with relativity is they think that a combined speed of two objects can't exceed the speed of light.

That is incorrect Relativity says you can not directly observe movements faster than the speed of light and also that no ONE object can move faster than the speed of light.


So you need to be careful when doing these problems are we talking about observation or calculation.

In the above example even though you observe the rockets doing slower than the speed of light you can calculate the correct speeds correctly by measuring the red shift.

So Relativity says you can never directly observe a speed greater than c. However you can calculate speeds greater than the speed of light and valid range of two objects is -2c to 2c.

The difference between calculated speed and observed speed is the thing that most confuses people with relativity.

I am checking you are not falling into this trap.

In your examples vo+c if you are talking about a calculated speed is perfectly valid within relativity you haven't broken any rules. If you are talking that you can directly observe vo+c then we need to find what is wrong.