I was recently listening to Star Talk radio and NDT mentions how if one were to beat a light beam (using a wormhole) that one could travel to the past. How would this work theoretically?
In: Physics
It’s not clear to me exactly what the Star Talk radio show was discussing, so there could be 2 different concepts going on here.
First, theoretically speaking, traveling through a wormhole would allow you to get from one part of the universe to another nearly instantaneously regardless of distance. Because light takes time to travel over that same distance, you could arrive ahead of the light that had been sent out earlier, and it would appear that you had traveled into the past relative to where you came from.
For example, let’s say you’re on Earth today and go through a wormhole that takes you 60 light years away from Earth. Because it takes light from Earth 60 years to reach your new location, when you look at Earth (say, through a very powerful telescope), the light you see (and the radio transmissions) would be from 60 years ago. It would seem like you’ve traveled through time (although in reality, you haven’t traveled through time, you’ve just caught up to the light waves we sent out in the 1960’s).
But there’s a larger concept about time travel and the speed of light that is covered by special relativity. That idea is, if it were possible to travel faster than the speed of light (IE. you could get a spaceship to move at a speed faster than the *c* – the speed of light in a vacuum), then you would be traveling backward in time.
The reason this is true is due to the fact that the passage of time isn’t constant for everyone – it’s relative. Imagine for a moment that you’re a police officer with a radar gun sitting in your car parked on the side of the road. You measure the speed of a car driving by. The car is traveling 60 miles per hour, so your radar gun reads 60 miles per hour.
Now imagine you’re the same police officer, but now you’re driving in the same direction as the car. You’re going 60 miles per hour, they’re going 60 miles per hour, and when you look at the car with just your eyes, it doesn’t look like the other car is moving (that is, it’s not getting further away or closer to you). If you measure it’s speed with a radar gun now, it will say 0 miles per hour.
>!Actually, a police radar can adjust for the motion of the police car, so it would still know the other car is moving 60 miles per hour, but it’s doing math to determine that – it’s not determining that by measurement alone. From the radar guns perspective, it doesn’t know if you’re parked measuring the speed of another parked car, or if you’re driving measuring the speed of a driving car. The software has to read the speed of your car to determine the speed of the other car.!<
So what we’ve determined is, the speed measured by a speed detector (the radar gun) changes depending on the motion of the detector – if your police car is moving, then the same 60mph car you’re measuring will come back as a different value then if your police car is parked.
If we repeat this experiment, but we measure the speed of light in a vacuum instead, this behavior doesn’t follow. If you measure the speed of light using a non-moving detector, you’ll measure *c*. But, if you move the detector, you *still measure c* as the speed of light. How can this be?
Well, let’s discuss what speed is actually a measure of. It’s a measure of a distance (in this case, miles) traveled over a given period of time (in this case, 1 hour). Hence the units “miles per hour.”
If we write speed as an equation, it would be:
s = d / t
where s is the speed, d is the distance, and t is the time.
In the case of our experiment with light, the speed of light didn’t change, so s is constant. But, when we moved the detector, we changed d (the detector moved to a new distance). So, the only way for d to change but s to stay constant is if t also changes.
This is special relativity – the time you experience is dependent on the speed you are traveling.
If you blast off in a rocket from Earth and you get to very very high velocities, 1 second for you will take much longer than it does for someone on Earth. After 1 second for you, many seconds may have passed on Earth (you’d have to be at very high speeds – much higher than any rocket we’ve ever built).
Because the speed of light has to stay *c* for all observers, regardless of the speed they travel, this means that if you were able to reach the same speed as light, time would have to stop for you. If it didn’t, then you’d be like the police car traveling the same speed as the car next to you – you saw that other car traveling at 0 speed.
If you travel the same speed as light and time doesn’t stop for you, light will appear to be traveling at 0 speed, but since we said it has to travel at *c* for all observers, this isn’t possible.
If you were to exceed the speed of light, the only way that light could continue traveling at speed *c* relative to you is for you to be traveling backwards through time. At that point, physics pretty much breaks down.
According to our current understanding of physics the difference between traveling faster than light and arriving before you started is just a matter of perspective.
Literally!
What looks like moving faster than light from one reference frame looks like moving backwards in time from another.
There are no ways to go faster than light that we know of, but if there were you would be hard pressed to explain why that would not also functionally constitute a time machine.
Most Sci-Fi wants to travel between stars in reasonably short amount of time, but doesn’t want the hassle of having their protagonist having easy access to time travel, so they conveniently ignore that part of physics for easier storytelling. Some franchises like star trek are just horribly inconsistent in how they apply their rules.
Really though the speed of light is the speed of causality, even without explicit timetravel, any story involving ftl breaks how effect should always follow cause and not preceded it. If you have ftl you are involved wibbly-wobbly, timey-wimey stuff whether you acknowledge it or not. Your best cause of action is to throw in some techno babble, wave your hands and hope your audience will be willing to suspend their disbelief if they notice.
It’s a result of Einsteins relativity.
An object that moves at the speed of light, doesn’t move through time in a resting observers frame of reference. Time dilation at lightspeed causes no time to pass anymore.
For this to work you need to drop some intuitive thinking like “Events can be simultanous”, because if they are or not depends on the observer.
So a resting observer would view a FTL object as travelling backwards through time. Also mass of the object would become negative.
Tachyons (particles faster than light) interacting with regular particles would allow a telephone into the past too, wich would cause the same paradoxes as timetravel, and would conflict with causality.
All this are reasons why we are pretty sure that Faster Than Light can’t exist.
Wormholes are theorethically possible, but we can’t know what kind of properties they would have to “fix” any breaking of causality. They connect two points in spacetime, but as we have never seen one we don’t know if they even have a mechanism that creates one, and If so how it decides wich spacetimes they connect.
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