[Physics] Earth still exists – does this fact tell us anything about LHC safety

anthropic-principleearthlarge-hadron-colliderprobability

When LHC was about to be launched there were many fears that it would destroy the world. To counter them scientists tried to carefully examine all possibilities and concluded that there is nothing dangerous. While most of their arguments sounds fine and reasonable to me there is one which I never understood. It is mentioned in many places, and here are some examples of it:

we can conclude that the existence of our solar system proves that mini black holes can not be dangerous.

http://arxiv.org/abs/0807.3349

If we ignore these strong theoretical arguments, we could pursue another path. Huge numbers of high-energy cosmic rays have hit the earth over its lifetime. Thus, we can argue, nature has already carried out the LHC experiments many times. If we are still here, the LHC must be safe. This is a standard argument that was worked out carefully by Jaffe et al.

http://physics.aps.org/articles/v1/14

It has always been reassuring that higher-energy cosmic rays have been bombarding the Earth since its creation with no disastrous side-effects.

http://arxiv.org/abs/0806.3414

This report explains why there is nothing to fear from particles created at the LHC. In fact, collisions just like those the LHC will make have been produced by cosmic rays bombarding the earth throughout its existence. It would take about 100,000 LHC experiments to match the number of cosmic ray events that have already occurred. We can rest assured that our planet will not be affected by the four experiments about to be conducted in Geneva.

Statement by the Executive Committee of the DPF on the Safety of Collisions at the Large Hadron Collider.

So, basically, from those two facts:

  1. For billion of years events similar to those at LHC were happening on Earth.
  2. Earth still exists.

We are concluding that:

  1. If we will recreate these events at LHC Earth will still exist.

In my opinion (3) does not follow from (1) and (2).

Imagine the following scenario: out of all planets with life, 99% are destroyed by some events caused by cosmic rays before the appearance of scientists on these planets who will conduct experiments at particle accelerators. However, at 1% of lucky planets evolution will have enough time to create curious scientists who will build particle accelerators. Scientists on these planets will then argue that it is safe because their planet still exists. But on 99% of planets there is no one to argue. Sounds like survival bias.

If you are asking why conditions on our planet are comfortable for life, the answer is obvious – because there is no one to ask such questions on planets which are not comfortable for life, so if you are asking one you must be on a proper planet.

If you are asking why our planet was not destroyed by some hypothetical cosmic ray event, the answer should be – because on planets destroyed by such events there is no one to ask, it shouldn't be – because such events are impossible.

So, using billions of years of Earth existence as an argument is wrong. Instead you can use existence of other bodies (which do not affect existence of life on Earth) as an argument.

What do you think, is this correct?

Best Answer

Your reasoning demonstrates precisely why formal logic alone is insufficient to study nature. In particular, it lacks the ingredient of inductive inference that is a cornerstone of empirical science.

A cosmic ray striking the Earth is not some random act of the gods that can have any imaginable consequence whatsoever. It is a cosmic ray striking the Earth. Sure, by formal logic we cannot see 100 trillion black ravens safe cosmic rays and conclude anything about the next raven cosmic ray. But everything we know in physics indicates the collisions in the LHC should be similar to the ones in the atmosphere -- otherwise, you'd probably be forced to believe that subatomic particles care about where in the universe they are with respect to human beings and their constructs.

Note that your reasoning could be used to disprove the safety of anything whatsoever. I assure you that never before in the history of the observable universe has user number 40492 ever asked a question about particle collider safety on Stackexchange. Sure, millions of other users have asked questions, and sure billions of things have been done on the internet, and sure no electronic device has ever been known to initiate a supernova and destroy its solar system, but perhaps you asking this very question could be the trigger for such an event. And by the anthropic principle we would never know until we detonated the planet.

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