# Is the speed limit as light speed just based on what we see? [duplicate]

I know, the title makes no sense, generally. Let me explain. I am just asking a question for which I found an answer nowhere (not even here). i think similar questions has been asked before and, no perfect answer is got(at least to what I ask).

The question is simple, why is light speed the limit for speed of particle? From many sources, I heard the answer, if we travel faster than light, we will reach somewhere before light reaches there and we will see and be in the past.

My question is, if we travel faster than light, surely we will be able to see the past. It is just because photons reach there later. We will see photons from a time before we started the journey, and probably the past. Of course it make sense. We are just seeing the past. Now we are seeing the past of sun since it take time for photons to reach here. In the same manner, we will see (and just see) the past of earth when we travel faster than light. That doesn't mean we are at the past. If we return back, we will reach the very normal time we would have felt if we neither went or returned.

I don't know if it makes any sense or not. I am not a graduate in physics. I am just 16.

## marked as duplicate by John Rennie, user12029, Bill N, CuriousOne, John DuffieldMar 15 '16 at 20:54

• Possible duplicate of How does faster than light travel violate causality? – John Rennie Mar 15 '16 at 18:47
• You say: "I heard the answer, if we travel faster than light, we will reach somewhere before light reaches there and we will see and be in the past." No reliable source will tell you that! The causality violations you get in special relativity from arbitrary faster-than-light travel are very real, "I traveled back in time and killed myself before I could travel back in time" causality violations. – user12029 Mar 15 '16 at 20:03
• Possible duplicate of How does faster than light travel violate causality? – user12029 Mar 15 '16 at 20:05

First, you should understand that the speed of light is constant. If you try to make a running competition against light, the speed of light relatively to yours will always be the same, no matter the fast you try to go. No matter what the reference is, or if it's moving or not: it's always the same and this is simply because it's what we observe. Accept this.

Now, if you accept this you should google for "Special Relativity" and try to understand what are the implications of such observation. You will see that for the speed of light to be constant, no matter if the source or observer is moving or not, something very weird must happen: the spacetime can be deformed. The "faster you go" the more compressed the spacetime will be, and that's why when you approach the speed of light the time will pass slower to you.

You misunderstood the "seeing the past". The example you gave it's not what would happen. After reading about special relativity you may understand that while you approach the speed of light, the time that elapses will approach zero. "If" you could go faster than light, the time would start to go "negative", i.e. you would be travelling back in time.

Since you are 16 with not much background in Physics, I believe that this is the best I can do to keep it simple and short.

Time is kind of more abstract and complicated. Specially if you think about moving backward in time.

For example - It is said that if someone travels at FTL, time would start running backward - meaning it would start going into the past.

The immediate past is - that you just crossed speed of light. Meaning you are sent back to the place and time where you crossed the speed of light? That would put you into a loop. I am not sure if going back in time really a thing, or just a mathematical outcome without corresponding reality.

Other possibility is - at FTL, the traveler is randomly thrown "somewhere" into the past without traversing backward from that point, and then time switched to forward flow. Because, any bacward flow of time will put you into a loop.

Speed limit and causality -

There may be other ways for causality to be violated at FTL.

A very simple way I can think of is - The cause (the forces) itself travel at c. And the forces themselves travelling at c, can not make anything move faster then themselves. Therefore causality just can not be violated. The effect can not be made to move faster than the cause, by the cause.