How long can one survive without a heartbeat?

Difficult one this! The common perception is 3-4 minutes but we had a patient a while ago who went into cardiac arrest shortly after arriving. After several cycles of CPR, a decision was taken to discontinue the resuscitation ( and the heart was scanned to confirm that there was no cardiac activity)
To our amazement ( horror!) the patient continued to take the occasional gasping breath for the next 20 minutes.

There is also the notion within Emergency Medicine, that you are not dead until you are 'warm and dead' so there are relatively frequent cases of resuscitation continuing for many hours in patients who are extremely cold. There are many documented cases of children particularly being revived after very long periods of time ( obviously CPR must continue)
Why do people stop believing in God?

The answer to this question might vary, depending on what Atheist you ask. Some will tell you they grew out of religion the more educated they became. I have one friend who said working at a children's hospital is what made him walk away from religion, as she refused to believe in a god that would allow

Does light travel fast because it doesn't have mass?

It travels at the speed it does because it is just a field disturbance, not an actual thing that moves. Sometimes we need to think of light as something we call a

What is a fear of aliens known as?

One possible definition is  xenophobia from https://en.m.wikipedia.org/wiki/... , but may not cover explicitly fear of extraterrestrials of extraterrestrial abductions. Xenophobia may apply more to fear of earthbound people from other countries or cultures.