# How do I calculate the event horizon?

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1. Sep 12, 2015

### Spring

I am clearly talking about black holes. The event horizon is the limit where even a photon won't escape it.

I tried to calculate it in the easy way using enegry calculation

m * MG/R = mc^2 / 2

but I do not know if I am using the right equation or even if I can devide by the m because it equels to zero, deviding by which is mocking the foundations of math and physics.

If the way to calculate it is tricky and scientific I will be disappointed because I wanna understand it well but I will still try and listen.

2. Sep 13, 2015

### stevebd1

The first equation is correct but the second one (to my knowledge) should be the Newton equation for kinetic energy-

$$E_k=\frac{1}{2}mv^2$$

If you replace the second equation with this one, then you should be able to rearrange to get the equation for escape velocity and from that, you should be able to establish an equation for the event horizon (or the Schwarzschild radius). This is a basic way of establishing the EH, for a more accurate and GR related solution, you should look at the Schwarzschild metric. You might also find the following thread of interest-

Last edited: Sep 13, 2015
3. Sep 14, 2015

### Spring

The second equation is ½mv2 but I used c (speed of light) to calculate it for light.

Once again, I am unsure because I devided by m of photon which equals to zero, deviding by nothing.
Also because there is a different way to calculate the energy of a photon. Ep = hf .
That way it means that every where there is a photon who can escape with high enough frequency.

4. Sep 14, 2015

### AgentSmith

Zero is the rest mass of a photon. Of course, photons are not normally at rest. High frequency implies high energy.