September 27 2013, 10:14 PM   #8
Edit_XYZ
Fleet Captain

Location: At star's end.
Re: Neutrinos

PurpleBuddha wrote:
 Collingwood Nick wrote: If we can create these particles, and detect them, and they are tachyons, theoretically we should be able to construct a Dirac radio and start receiving messages from the future.
Here is something that has always bothered me. I am also a lay person, so I am hoping someone can show me what I am misunderstanding.

I do not see how traveling faster than light speed (or sending information faster than light speed) would mean you could receive messages from the future. Even if information were transmitted at 10 times the speed of light, nobody receives that information until after the event creating it happens. In this hypothetical they would simply receive it 10 times faster than it would take the light from the event to get there, but the event still happened in the past. I see no bridge between the past and present being created in the relativistic sense just based on sending information faster than light speed. Where am I off on this?
Special relativity includes the relativity of simultaneity:
You have POV S, for which two events happen simultaneously in places 1 and 2. In a coordinate system, the line connecting these two events is the axis of simultaneity for POV S.
You have POV G, for which two events happen simultaneously in places 1 and 2. In a coordinate system, the line connecting these two events is the axis of simultaneity for POV G.

The axis of simultaneity of POV S and the axis of simultaneity of POV G are NOT parallel. They intersect at an angle, and then, if you are in POV S, the axis of simultaneity of POV G goes into your past (if you are in POV G, it's the other way around).
I recommend you google for a coordinate system depicting this.

The problem is - if you're limited to the speed of light, no matter how fast you send a signal from POV G, along the axis of simultaneity of POV G, it can't reach the past of POV S; the signal is not fast enough, it takes too long to travel along the axis of simultaneity of POV G.

If you can travel FTL, though, the signal can travel fast enough along the axis of simultaneity of POV G to reach into POV S's past. If you have FTL, you can always find POVs where the FTL signal travels into the past.

Most designs for time machines you might have heard of use frame dragging to make an object travel slightly faster than light. As per general relativity, they should work, too - but you must have a rotating singularity/etc at your disposal.
Some are using wormholes - for them, you need negative pressure AKA antigravity, etc. But all are based on the relativity of simultaneity.
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