Does time go backwards if object is going faster than light?

[adimantium]

Lets pretend I am in my spaceship going in one direction at 0.9c. My brother is in his spaceship going the opposite direction also at 0.9c. Oh and there is a big clock on the side of our ships. When we cross paths, to me it looks as if I'm not moving and my brother is moving at 1.8c. I am aware that the formula for time dilation is t0 = t√1-$\frac{v^{2}}{c^{2}}$ and that velocities don't simply add together, the formula for that is V$_{3}$ = $\frac{v_{1}+v_{2}}{1+\frac{v_{1}v_{2}}{c^{2}}}$. Using that formula for velocity the speed would be about 0.9945c. this only makes sense to me if the two velocities are in the same direction. So let's just forget the formula for velocity. I see my brother moving faster than light. Let's also pretend I have some kind of telescope that can perceive vision instantaneously. Using the time dilation formula, I would be left with t√-0.8 I know you can't really find the square root of a negative number, so what would I see? Would I see his clock run slow but forward like normal, not move at all, or backwards?

 

[Bill_K]

I am aware that the formula for time dilation is t0 = t√1-$\frac{v^{2}}{c^{2}}$ and that velocities don't simply add together, the formula for that is V$_{3}$ = $\frac{v_{1}+v_{2}}{1+\frac{v_{1}v_{2}}{c^{2}}}$. Using that formula for velocity the speed would be about 0.9945c. this only makes sense to me if the two velocities are in the same direction. So let's just forget the formula for velocity.

No, please don't forget it! It's correct. The formula the way you've written it assumes that the velocities are in the opposite direction. Moreover there's a recent thread which discusses how to add velocities that are in any direction.

 

[Ibix]

You can't just "forget" the formula for velocity addition. That's the way velocities add. So you don't see your brother traveling faster than light.

On a related note, nothing goes faster than light so you cannot perceive faster than light. It's common practice to imply that all players in an experiment are smart enough to subtract out lightspeed delay, which can confuse some students. But if something happens a light year away, we cannot know about it for a year.

So: you would see his clock running slow because of time dilation, but sped up (as he approaches) or slowed even more (as he moves away) because of the Doppler effect. You would never see the clock running backwards, or showing an imaginary time.

 

[D H]

So let's just forget the formula for velocity. I see my brother moving faster than light.

You do not see your brother moving faster than light. That formula that you want to forget is precisely what you need to use to explain why you see your brother moving at a speed less than the speed of light.

 

[sardar]

I can confidently say that time does not go backwards, regardless of an object's speed. This is a fundamental concept in physics known as causality, which states that the cause must always precede the effect. Therefore, even if your brother appears to be moving faster than the speed of light from your perspective, time will still be moving forward for him.

In this scenario, there are a few important concepts to consider. First, the formula for time dilation only applies to objects moving at sub-light speeds. Once an object reaches the speed of light, the formula becomes undefined. Therefore, it is not accurate to say that your brother is moving at 1.8c.

Secondly, the concept of relative velocity is important to consider. In this scenario, both you and your brother are moving at 0.9c relative to each other. This means that from your perspective, your brother's clock will appear to run slower, while from his perspective, your clock will also appear to run slower. This is known as time dilation and is a well-established concept in physics.

Finally, the idea of a telescope that can perceive vision instantaneously is not possible according to our current understanding of physics. The speed of light is the fastest possible speed in the universe, and it is impossible to have instantaneous communication or observation over large distances.

In conclusion, while it may seem like your brother is moving faster than light from your perspective, time will still be moving forward for him. The concepts of time dilation and relative velocity explain the apparent differences in time between two observers moving at different speeds.