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Time Machine, like many backup utilities, creates incremental backups of files which can be restored at a later date.^[1] It allows the user to then restore the whole system, multiple files, or even a single file. It works within iWork and iLife and other programs, making it possible to restore files without leaving the application.

Time Machine allows the user to back up Apple Macintosh computers through Apple's AirPort networking (as long as the computer wishing to be backed up is not connected wirelessly), but does not support backing up to network attached storage drives. On January 15, 2008, during his keynote at Macworld, Steve Jobs introduced Time Capsule, which adds network storage capability to Time Machine and adds the ability to back up wirelessly.

User interface

Time Machine's user interface uses Apple's Core Animation API. Upon its launch, Time Machine "floats" the active Finder or application window from the user's desktop to a backdrop depicting a galaxy and star field. Behind the current active window are stacked windows, with each window representing a snapshot of how that folder or application looked on the given date and time in the past. When toggling through the previous snapshots, the stacked windows extend backwards, giving the impression of flying through a 'time tunnel'. While paging through these "windows from the past", a previous version of the data (or presently deleted data) may be retrieved.

Requirements

Time Machine requires a non-booting hard-drive or partition to be connected to the computer.^[2] It can back up to internal hard-drives or partitions, but it is recommended that you back up to an external hard drive connected by USB or Firewire. According to Apple, it can only be backed up to network drives if they are being hosted by another computer running Leopard (including Leopard Server). Further, the volume needs to be formatted with the Mac OS extended file system, with journaling enabled. External hard drives typically are pre-formatted with the FAT32 filesystem. To use Time Machine, the hard drive must first be formatted, which erases all data. The hard drive will not require formatting if already using the HFS filesystem.

Similar functionality in other operating systems

Microsoft's Windows Server 2003 and some versions of its Windows Vista operating system feature Shadow Copy (or a variation of it called previous versions in Vista), which creates snapshots of files and folders on a specific volume at a specific point in time that can later be retrieved. Both Time Machine and Shadow Copy use snapshots to achieve a similar useful purpose, however there is a major difference in the way in which Time Machine has been implemented. The main purpose of Windows Vista's previous versions feature and its Backup and Restore Center is to back up major operating system files and files from the user's home directory (Documents, Pictures, Downloads, etc). Further, Windows Previous Versions allows for retrieval of old data from snapshots taken with System Restore. And while Time Machine serves these functions, its interface allows for users to restore individual deleted photos from within iPhoto, and deleted items from within Mail, and the Address Book.

Unlike competing products, Time Machine only allows for limited network backup functionality. Backing up to other machines is limited to Apple Macintosh computers running Leopard; Time Machine will not backup to systems running Microsoft Windows or Linux.

TimeVault is a GNOME-based application which will allow similar functionality. A port to KDE is underway. FlyBack is another alternative for GNOME users.

There are various ways in which a person could "travel into the future" in a limited sense: the person could set things up so that in a small amount of their own subjective time, a large amount of subjective time has passed for other people on Earth. For example, an observer might take a trip away from the Earth and back at relativistic velocities, with the trip only lasting a few years according to the observer's own clocks, and return to find that thousands of years had passed on Earth. It should be noted, though, that according to relativity there is no objective answer to the question of how much time "really" passed during the trip; it would be equally valid to say that the trip had lasted only a few years or that the trip had lasted thousands of years, depending on your choice of reference frame.

This form of "travel into the future" is theoretically allowed using the following methods:^[12]

• Using time dilation under the Theory of Special Relativity, for instance:
  • Traveling at almost the speed of light to a distant star, then slowing down, turning around, and traveling at almost the speed of light back to Earth^[30] (see the Twin paradox)
  • Orbiting Earth for long periods of time (practical, but insignificant);
• Using time dilation under the Theory of General Relativity, for instance:
  • Residing inside of a hollow, high-mass object;
  • Residing just outside of the event horizon of a black hole

Additionally, it might be possible to see the distant future of the Earth using methods which do not involve relativity at all, although it is even more debatable whether these should be deemed a form of "time travel":

Time travel themes in science fiction and the media can generally be grouped into two main types and a third, less common type (based on effect—methods are extremely varied and numerous), each of which is further subdivided. These classifications do not address the issue of time travel itself, i.e. how to travel through time, but instead call to attention differing rules of the time line.

1. The time line is consistent and can never be changed.

1.1 One does not have full control of the time travel. One example of this is The Morphail Effect. This concept of time can be referred to as circular causation. For examples of circular causation, see Robert A. Heinlein's story By His Bootstraps.

1.2 The Novikov self-consistency principle applies (named after Dr. Igor Dmitrievich Novikov, Professor of Astrophysics at Copenhagen University). The principle states that if you travel in time, you cannot act in such a way so as to create a paradox.

1.3 Any event that appears to have changed a time line has instead created a new one. It has been suggested that travel to the past would create an entire new parallel universe where the traveler would be free from paradoxes since he/she is not from that universe^[35].

1.3.1 Such an event can be the life line existence of a human (or other intelligence) such that manipulation of history ends up with there being more than one of the same individual, sometimes called time clones.

1.3.2 The new time line might be a copy of the old one with changes caused by the time traveler. For example there is the Accumulative Audience Paradox where multitudes of time traveler tourists wish to attend some event in the life of Jesus or some other historical figure, where history tells us there were no such multitudes. Each tourist arrives in a reality that is a copy of the original with the added people, and no way for the tourist to travel back to the original time line.

2. The time line is flexible and is subject to change.

2.1 The time line is extremely change resistant and requires great effort to change it. Small changes will only alter the immediate future and events will conspire to maintain constant events in the far future; only large changes will alter events in the distant future. (Example: The Saga of Darren Shan, where Major events in the past cannot be changed, but minor events can be affected. This is explained as if you went back in time and killed Hitler, another Nazi would take his place and do all his evil deeds.)

2.2 The time line is easily changed (example: Doctor Who, where the time line is fluid and changes often naturally; even changes to the traveler's own timeline are possible, though it is suggested such an act would destroy most of the Universe).

3. The time line is consistent, but only insofar as its consistency can be verified.

3.1 The Novikov self-consistency principle applies, but if and only if it is verified to apply. Attempts to travel into the past to change events are possible, but provided that:

-They do not interfere with the occurrence of such an attempt in the present (as would be the case in the Grandfather Paradox), and

-The change is never ultimately verified to occur by the traveler (e.g. there is no possibility of returning to the present to witness the change).