Someone told me that because of the relative air pressures inside and out, it would be impossible for any human to open the cabin during flight.Is this true?

probably not, but you'de be an idiot if you did!

Why do not you try it? Make sure l am not on the plane first though

No, only way is thru hydraulics or explosion, the compressed air inside against the air pressure from the outside would make it impossible

You can open it because the pressure inside the door is more (approx 4psi) than outside so it pops the door open
And as soon as u loosen the seal u & everything inside will be sucked out, not just due to the pressure difference inside & outside, but the velocity of the air that goes over the door causes a low pressure area immediately in front of it causing anything in close vicinity of the door to be pulled out.

In flight though it is normally locked by the pilot/staff & controlled hydraulically

In normal i.e. pressurized cabin, it would be difficult to open the door. Aircraft doors r designed in such mechanism so when closing the doors, it will go through the door frame in, & the doors locks in such a way that it is bigger than the door frame. Think of a putting a cork into a bottle, but from the inside. That system reduces the needs of stronger door-lock mechanism that can withstand differential pressure of more than 8psi.

It is possible to open it at 38000ft, but u need to depressurize the cabin first.

first of all it is not allowed, but if u do manage to open it , then every one on board will be sucket out in space and the plane will crash. u want to try it, go ahead and see what happens.

the cabin in pressurized and all the cabin doors open into the aircraft.
So the doors are pretty much locked in until the cabin is depressurized.
You could always try

If you depressurized the aircraft you could. It would be a little windy though in the cabin not to mention a nippy -20 to -40 degrees so you may want to put on a jacket. WIth the air moving around the aircraft it may make it a bit hard to pust the door open, but if you nosed the plane up and got it close to a stall the air moving around the plane should be light enough to open the door.

It depends on the aircraft. Modern airliners have plug-type doors that r pressed into their frames more tightly as the airplane ciimbs higher; to open them in flight, you'd have to overcome the pressure inside the plane that is holding the door against the frame, which amounts to nearly 20 tons in the case of a typical door—more than even a gorilla could manage.

Some smaller pressurized planes (not airliners) do not have plug-type doors, & these can be opened in flight. And of course, if the plane is not pressurized, the doors can easily be opened.

So the answer is no, for all modern airliners, but yes for some other, smaller aircraft (most smaller aircraft can not get anywhere near 38,000 feet, however).

In normal commercial aircraft, your friend is wrong. The pressure inside the cabin is forcing the door shut. The pilots/flight attendants do not have any sort of magical locking device preventing someone from opening it, it is just air pressure. If the cabin was depressurized, then theoretically, anyone could walk up to the door and turn the handle.

The pressure differential is around 8 PSI. Even if it was 0.1-0.2 PSI, it would be nearly impossible to open.

yes you could but you you would be sucked out right when you open it so it would be stupid