Blocking of Google.cn: This would result in people having to go straight to the Hong Kong domain. If it would be a block across the subdomains as well, then it would mean the loss of e.g. Google Music Search China (a service accessible from China, which still doesn’t forward to any Hong Kong domain). Such a block might result in the loss of a lot of casual users, as they might not know of the Hong Kong domain or not bother to go to google.com. For anyone seriously interested in getting Google results though, nothing would change – the Google.com results in English and in Chinese are still available, as is the Hong Kong domain.
Blocking of Google.com.hk: If Google.cn is available but the Hong Kong domain is blocked, then it would effectively block people going to google.cn, because Google’s forward would fail. Sites like Google Music China would survive, though. If such a block would be created, then casual users may be lost to Google (similar to above, with a block of google.cn), though serious Google users could simply go to Google.com’s English or Chinese search sites.
Blocking of Google.com.hk and Google.com: Currently and in the past, users from China can access Google.com fine, with some exceptions (certain subdomains or search featurs services, like Google’s cache, may not be available, and web search is sometimes temporarily blocked after certain uses). If authorities would decide to block access to not only Google.com.hk but also Google.com itself, then people would not be able to easily see Google results anymore. Casual users might simply stop using Google but now, even people who do care about who provides their search results and who would wish to see Google results may feel the need to go to a competing engine like Baidu. Only those willing to use a program that routes around the block – such programs are available for use in China – would now be able to access Google’s search engine. However, such workarounds may slow down the connection quite a bit, and not be a very good alternative. Another option would be to go to another country domain – say, google.de – and switch the interface language to English or Chinese... unless authorities block access to all Google country domains, that is.
How will the authorities react then? Nobody on the outside will be able to tell you for sure. Some reports are coming in that the Google Hong Kong domain is already blocked, which may start the loss of casual users who don’t want to go alternative routes to access Google. [Update: Google’s Alan Davidson said that Google had noticed “intermittent” censorship of the Hong Kong site.] However, I can’t reproduce this block at my location in China at the moment.
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