Given the new 12" Retina MacBook doesn't have a Thunderbolt and only has a USB-C port, how do I connect it to an existing Thunderbolt display?
As @Kevin McKenzie already said, and just to expand on it.
At present there is no adapter for the USB-C to Thunderbolt.
Thunderbolt Gen 1 and 2 don’t support USB 3.1, Type C or even 3.0 for that matter. An adapter wouldn’t have fixed this problem. However, Apple is releasing Thunderbolt 3 later in 2015. It’s not ready yet, because it’s developed with Intel and now Gen 3’s functionality is tied to Skylake chips – which will replace Broadwell shortly. I know. You’re thinking “But Broadwell just came out!” That’s true, but only because of delays. Now, you may wonder how a new MacBook without a Thunderbolt port or a Skylake chip will support a Thunderbolt Display.
Well, in short it won’t. That is unless Apple releases a firmware update for their Thunderbolt cables. Which they won’t. So it probably won’t support it. Considering the last time they updated it, I think we’ll see an update at WWDC, with a USB C port on the display itself.
Right now, you don't. The Thunderbolt display requires a Thunderbolt connection as input, and while Apple did announce some USB-C dongles yesterday, none of them were USB-C to Thunderbolt. They may well come out with something in the future, or a third party may, or they may be releasing a replacement for the Thunderbolt display soon, one that also supports input via USB-C.
If you go to the technicality the Thunderbolt Display, according to iFixit, is technically a DisplayPort monitor internally coupled to a Thunderbolt dock station with DisplayPort output, and the internal DisplayPort connection is actually detachable from the dock station assembly as a Mini DisplayPort cable (!)
So if you are brave enough to give the monitor a hack, with some quick drilling action on the chassis and a Mini DisplayPort extension cable you can actually detach the display from the internal docking station and use it as a standalone display and use the docking station portion of the machine independently. Ugly hack and probably not worth it since Apple already had a Mini DisplayPort+USB Cinema Display in the past.
Using one of that and a third party USB Type C to USB+Mini DisplayPort+Power adapter you can use that last generation Cinema Display on your MacBook actually.
Apple have released a Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter.
I have one of these. It works well, still providing access to the extra ports on the back of the Thunderbolt Display.
2 issues I have with the adapter: (1) it is quite long and slightly unpretty and (2) the Thunderbolt Display intermittently does wake from sleep, requiring that I unplug and replug the adapter.
If your display is Thunderbolt, it won't run even if you connect it properly to a mini-DisplayPort signal.
If you have a DisplayPort display, you can get things to work as follows:
You need three items to make this work.
- Apple's USB C to HDMI video adapter
- an active HDMI to DisplayPort adapter
- an inexpensive Mini DisplayPort to DisplayPort cable
This only lets the display itself work and you would need to connect USB to gain speakers and camera functions.
Apple will surely have a USB-C Display coming soon. The display would provide USB2.0, ethernet, firewire, thunderbolt, etc. The kicker is that the display could also charge the laptop through the USB-C cable. So you basically get a docking station in your display with a single connection to USB-C. Very slick, even if the Monitor is 2x the cost of what it should be. =(