# High speeds necessary for time travel are relative to what ?

We know *everything* is moving including earth, sun and entire galaxies. They say if you travel at high speeds , time slows down. But since earth it self is moving at high speeds, it is difficult to say what our actual speed if we attempt to fly in space at high speeds.

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It’s relative to the speed of light.
= 299,792,458 m/s

I think in this case the high speeds you refer to, are speeds approaching the speed of light. The speed of light (or more specifically the speed of a massless particles) is a universal standard, often given the notation ‘c’.

Great point. yeah, i think you would measure relative to the speed at which the galaxy, thus earth, is moving through space.

In order to experience time dilation, you need to reach speeds approaching the speed of light relative to whatever frame of reference you are using for comparison. There is no “master” frame of reference, but if you want to experience a time shift relative to Earth’s time, then you must travel at near the speed of light relative to Earth. If you want to experience a time shift relative to time as measured in the center of the Virgo Galaxy Cluster, then you must travel at near the speed of light relative to that cluster, etc. Time is not proceeding identically in those two locations, since they are moving relative to each other (albeit fairly slowly in a galactic sense). So you must pick your reference frame before you can determine relative velocity and a resulting time dilation. Your experience of time is based entirely on your location and your velocity relative to whatever you choose as a frame of reference.
ADDED: Since the speed of light is constant in all inertial frames of reference, using it as a starting reference is not helpful. Your time change is only meaningful as compared to the time measured at a certain location. If you are comparing to the time here and now, then your motion must be relative to “here and now.” Motion relative to the speed of light still requires a frame of reference to base the time dilation upon.

It is relative to the speed of light. Einstein explained how to relate speed to other objects, stating that it is impossible to know whether you’re moving or not if you cant relate your speed to another object, i.e. Earth around the Sun. You can read more about it in Theory of General and Special Relativity.

It’s not meaningful to claim that “everything is moving including Earth”, etc., without specifying a reference frame. The whole point of Relativity is that there is no absolute rest frame against which all speeds are measured. The reference frame of the Earth is just as good of a rest frame as that of the Sun, or the galaxy, or whatever.
The high speeds you are talking about are relative to the observer. When any observer observes a body traveling at high speeds relative to himself, he will detect that its clocks tick too slowly. For instance, if a spaceship is traveling to Vega at nearly c relative to Earth, then observers on Earth will see its clocks running slow. Of course, to observers on the spacecraft, the spacecraft is the rest frame and it is Earth that is traveling backwards at nearly c, so their clocks run normally and the Earth’s run slow.

Two things would be necessary for the comparison, usually it would be in reference to where you started from, like Earth for instance. Earth’s speed compared to other things, like the Sun or the center of our galaxy is not that fast, not compared to light speed, it is not a relativistic speed.

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