In short, thanks to the answers and comments posted so far. No actual money is magically disappeared when the stock price goes down but the value is lost. The value changes of a stock is similar to the value changes of a house.
The following is the long answer I came up with based on the previous answers and comments alone with my own understandings. Any experts who find any of the following is 200% out of place and wrong, feel free to edit it or make comments.
Everything below only applies if the following are true: The stock price is only decreasing since the IPO because the company has been spending the money but not making profits after the IPO. The devaluation of the stock is not the result of any bad news related to the company but a direct translation of the money the company has lost by spending on whatever the company is doing.
The actual money don’t just disappear into the thin air when the stock price goes down. All the money involved in trading this stock has already distributed to the sellers of this stock before the price went down. There is no actual money that is literally disappeared, it was shifted from one hand to another, but again this already happened before the price went down.
For example, I bought some stocks for $100, then the price went down to $80. The $100 has already shifted from my hand to the seller before the price went down. I got the stock with less value, but the actual money $100 did not just go down to $80, it’s in the hand of the seller who sold the stock to me.
Now if I sell the stock to the same seller who sold the stock to me, then I lost $20, where did the $20 go? it went to the seller who sold the stock to me and then bought it back at a lower price. The seller ended up with the same amount of the stocks and the $20 from me. Did the seller made $20? Yes, but did the seller’s total assets increased? No, it’s still $100, $80 from the stocks, and $20 in cash. Did anyone made an extra $20? No. Although I did lost $20, but the total cash involved is still there, I have the $80 , the seller who sold the stock to me and then bought it back has the $20. The total cash value is still $100. Directly, I did lost $20 to the guy who sold me the stock when the stock has higher value and then bought it back at a lower price. But that guy did not increased his total assets by $20. The value of the stock is decreased, the total money $100 did not disappear, it ended up from one person holding it to 2 people holding it.
I lost $20 and nobody gained $20, how is that possible? Assume the company of the stock never made any profit since it’s IPO, the company just keeps spending the money, to really track down where the $20 I lost is going, it is the company has indirectly spent that money. So who got that $20 I lost? It could be the company spent $20 for a birthday cake, the $20 went to the cake maker. The company never did anything to make that $20 back, so that $20 is lost.
Again, assume the stock price only goes down after its IPO, then buying this stock is similar to the buying a sport car example from JoeTaxpayer (in one of the answers), and buying an apple example from BrenBarn(in one of the comments from JoeTaxpayer’s answer).
Go back to the question, does the money disappears into the thin air when the value of the stock goes down? No, the money did not disappear, it switched hands. It went from the buyer of the stock to the company, and the company has spent that money.
Then what happens when the stock price goes down because bad news about the company? I believe the actual money still did not just disappear. If the bad news turn out to be true that the company had indeed lost this much money, the money did not disappear, it’s been spent/lost by the company. If the bad news turn out to be false, the stock price will eventually go up again, the money is still in the hand of the company.
As a summary, the money itself did not disappear no matter what happens, it just went from one wallet to another wallet in many different ways through the things people created that has a value.