Traveling at a speed faster than light

[Saurabh Kumar Singh]

Imagine yourself to be bat. You can't see anything. You don't have eyes. All you can do is echolocate, using ultrasound.
Now imagine something is moving away from you faster than the speed of sound. Can you locate it ? Can you perceive its existence ?
If the answer is yes, how ?
If no , then from the bats perspective it should be highest speed that can be achieved .
Then how can we, as humans say that speed of light is maximum in the universe because we won't be able to detect anything moving faster than speed of light. So is special relativity wrong ?

 

[berkeman]

Welcome to the PF.

So is special relativity wrong ?

No. You can start by reading the FAQ that is pinned at the top of the Relativity forum where you have started your thread:

https://www.physicsforums.com/threads/faq-experimental-basis-of-special-relativity.229034/

 

[Orodruin]

Special relativity is based on the observation that the speed of light is invariant, not that it is the largest possible speed - that is a consequence of the theory.

As a bat, you might notice that the speed of sound changes depending on wind and how you move relative to it. The point is that this does not happen for light. Regardless of how you move, light always has the same speed relative to you. That is the basis of SR.

 

[russ_watters]

Imagine yourself to be bat. You can't see anything. You don't have eyes. All you can do is echolocate, using ultrasound.
Now imagine something is moving away from you faster than the speed of sound. Can you locate it ? Can you perceive its existence ?

If it doesn't emit its own sound (unlikely), then no. But since supersonic planes make noise, in real life the answer is yes.

If no , then from the bats perspective it should be highest speed that can be achieved.
Then how can we, as humans say that speed of light is maximum in the universe because we won't be able to detect anything moving faster than speed of light.

Because we can detect other things besides visible light And if something moving faster than light gave off its own light, we'd see that.

 

[Dale]

Imagine yourself to be bat. You can't see anything. You don't have eyes. All you can do is echolocate, using ultrasound.
Now imagine something is moving away from you faster than the speed of sound. Can you locate it ? Can you perceive its existence ?

Sure. If it emits sound then you could perceive it directly. You could also set up objects that would emit sounds upon collision with the object. You could set up a network of echolocators throughout the region of interest.

Supersonic jets are very audible.

Then how can we, as humans say that speed of light is maximum in the universe because we won't be able to detect anything moving faster than speed of light

This is not correct. He speed of light is the maximum speed because it is the invariant speed, not because we could not detect faster objects. Using the methods described above we could easily detect FTL objects using light.

Even a bat, using echolocation only, with sufficiently precise measurements could determine that the speed of sound is not invariant and that there is a finite speed which is invariant. They would simply call that speed "the invariant speed" rather than "the speed of light". Nothing about SR would change

 

[SiennaTheGr8]

@Saurabh Kumar Singh:

Imagine that you're sitting on the ground and a beam of light shoots past you, from east to west. How fast would you say it's moving? Obviously you'd say it's moving at the speed of light ($c$), yes?

Now, imagine that while this is happening, your friend Alice flies past you on a rocket traveling at three-fourths the speed of light ($3c/4$), also from east to west. How fast does Alice say the beam of light is moving? Perhaps you'd think that she says the light is moving at $c - 3c/4 = c/4$, since she's moving in the same direction as the light. But that's not correct! For Alice, the light is moving at $c$, just as it is for you.

That's what we mean when we say that the speed of light is invariant. Even if Alice were traveling at $.99c$ relative to you, you'd both agree that the light beam is traveling at speed $c$.

Here is the takeaway: Alice can't "catch up" to the light. From her own perspective, she can't even begin to catch up to it! No matter how much she accelerates westward, she always measures its speed to be $c$.

Of course, as she accelerates, you (on the ground) will say that her speed gets closer and closer to $c$, but will you ever see her reach that speed? Well, no, because "catching up to something" isn't a matter of perspective. If you say she and the light beam are moving westward at the same speed, then from her perspective the light must be traveling at speed $0$ (not $c$). But that violates the condition that the speed of light is invariant.

So you see, if the speed of light is invariant, then it's a speed we can never accelerate to.