# Time Halt and Speed of Light

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I know when something is accelerated it will be moving slower.

and I know when something is going faster than the speed of light - in theory - time would go backwards.

so when something IS going the speed of light, does time stop, or just move very slowly?

thanks.

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Using the formula for time dilation at relativistic velocities, I think that time would stop.

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It gives you a number divided by zero, which doesn't work.

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I know when something is accelerated it will be moving slower.

and I know when something is going faster than the speed of light - in theory - time would go backwards.

Common misconception. Time would not go backwards at FTL speeds' date=' it would become imaginary (a product of the square root of -1)

so when something IS going the speed of light, does time stop, or just move very slowly?

thanks.

It would be undefined. Look at the time dilation equation:

$T = \frac{T'}{\sqrt{1- \frac{v^2}{c^2}}}$

Note that as v equals c, the equation becomes

$T = \frac{T'}{0}$

And division by zero is undefined.

But since nothing with mass can travel at c, it'll never come up.

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ah, ok, thanks guys.

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Common misconception. Time would not go backwards at FTL speeds' date=' it would become imaginary (a product of the square root of -1)

[/quote']

Wow, talk about confusing math with reality. Time would NOT become imaginary. Time would be time. The EQUATION for time dilation for speeds faster than light would YIELD an imaginary number. That basically says nothing other than the fact that equation was not developed to deal with FTL speeds.

Remember, there is actually a real world out there. Things don't become imaginary because you model the system mathematically.

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What is it with you, Saint? Everyone knows that mathematical theories model the universe. But everyone also knows that overwhelming experimental confirmation makes for a strong case for the truth of a model. It is not unreasonable to answer a question about time with the predictions of SR, because there is no reason to think that SR should not apply. And besides, Mag qualified his question with the expression "in theory", so really what's the problem?

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What is it with you, Saint? Everyone knows that mathematical theories model the universe. But everyone also knows that overwhelming experimental confirmation makes for a strong case for the truth of a model.

I'm just trying to make sure that when someone asks a valid question about reality, they are not told that space and time become imaginary numbers based on an equation. Or, if that represents a reasonable model of reality, could you please tell me what imaginary time is.

It is not unreasonable to answer a question about time with the predictions of SR' date=' because there is no reason to think that SR should not apply. And besides, Mag qualified his question with the expression "in theory", so really what's the problem?[/quote']

You are correct that it is not unreasonable to answer a question about time with a prediction of SR. What Janus presented was not a prediction of SR. Unless you too believe that time being imaginary makes any sense.

It would be more useful to say that we don't know what happens to the rate of vibration once one passes the speed of light wrt something else. My vote is nothing, but that's me. But you'll really have a few choices:

1) stop

2) faster

3) reverse

To say that it becomes an imaginary number shows a scary dependence on math. You need to think through the reality of the scenario before you. Not just slap down an equation, and plug in the numbers.

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You are correct that it is not unreasonable to answer a question about time with a prediction of SR. What Janus presented was not a prediction of SR.

Yes' date=' it is a prediction of SR. He even cited the relevant equation!

Unless you too believe that time being imaginary makes any sense.

It would be difficult to make sense of it, even mathematically. But even so that in itself does not preclude its occurance.

But that's not the point. The point is that the "imaginary time" prediction is regarded as a reason that faster-than-light travel is impossible.

It would be more useful to say that we don't know what happens to the rate of vibration once one passes the speed of light wrt something else. My vote is nothing, but that's me. But you'll really have a few choices:

1) stop

2) faster

3) reverse

Why did you stop at those?

To say that it becomes an imaginary number shows a scary dependence on math.

Why?

You need to think through the reality of the scenario before you.

Yes, and you also need to have the fullest possible picture of reality before you. This includes what we know about electrodynamics and how it necessitates relativity. Once you've got the full picture, then you're in a position to start thinking about what must be true in reality. And even then, you cannot rule theories out on a priori grounds, unless the theory is self-contradictory.

Not just slap down an equation, and plug in the numbers.

I, for one, have never met a physicist who does that.

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While I agree with you that time will not be imaginary and time will be time, I think you are oversimplifying the concept of "imaginary". A number or concept is imaginary because it does not exist in a certain "real" domain. In the case of time, I think we haven't even defined the relationship between this imaginary domain and how it affects our real world domain. For example in electric circuits, imaginary current does NOT simply mean 1. no current, 2. backward current, 3. forward current. It is a more complex "tendency" of current behavior that cannot be explained if we limit ourselves to only 1 domain. Mathematics allows us to study behavior beyond simple physical observation, so I would personally not attribute those simple options to imaginary time without deeper analysis.

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To say that it becomes an imaginary number shows a scary dependence on math. You need to think through the reality of the scenario before you. Not just slap down an equation, and plug in the numbers.

The equation leaves you with an imaginary answer. Since we've defined things in these equations in terms of real numbers, the answer is unphysical and we conclude that it cannot happen.

The only scary part is when one does not acknowledge the assumptions that went into the formulation of the equation.

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I should definitely take back my statement about the options one will have. I agree with all of you that there are more possible answers than the three I posted. Sorry for that one.

Swansont, Tom, et al - Time does not become imaginary. The equation YIELDS an imaginary result. As I said, it would be more useful to simply say that we don't know. Janus did not say that. He said that time would become imaginary. I think that's a dangerous way to look at the world. And, for Swansont, I think it shows that Jauns did not acknowledge the assumptions that went into the formulation of the equation.

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Swansont, Tom, et al - Time does not become imaginary. The equation YIELDS an imaginary result. As I said, it would be more useful to simply say that we don't know. Janus did not say that. He said that time would become imaginary. I think that's a dangerous way to look at the world. And, for Swansont, I think it shows that Jauns did not acknowledge the assumptions that went into the formulation of the equation.

this is an entirely semantic argument (as usual). It simply depends on your definition of 'time'. The way Janus defined time, it does become imaginary, so his comment was completely appropriate.

The obvious question to you is, why would you define time differently from Janus' definition? What insight of FTL travel do you have that we do not? Do you have personal experience? I suspect you don't, so any alternate definition you can come up with will be just as arbitrary.

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hm, so from this i gather that the equation gives us an imaginary number, which would be impossible in the physical world, and therefor we say its immpossible.

i dont understand the "assumptions going into the equation part" though.

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this is an entirely semantic argument (as usual). It simply depends on your definition of 'time'. The way Janus defined time' date=' it [b']does[/b] become imaginary, so his comment was completely appropriate.

The obvious question to you is, why would you define time differently from Janus' definition? What insight of FTL travel do you have that we do not? Do you have personal experience? I suspect you don't, so any alternate definition you can come up with will be just as arbitrary.

I would disagree that it's entirely semantic. It's an important point. That equation was not developed to provide answers (or predictions) for FTL speeds. Period. To better illustrate this point you need to look no further than the equation itself. It uses c as it's upper limit for providing real answers. If it had been developed for FST speeds, it would not use c as it's upper limit. The backbone of SR is not simply the constancy of c, but also that it is the universal "speed limit". Everything was developed based on that idea. Since it was clearly developed with the assumption that FTL speeds are impossible to achieve, it cannot provide useable answers to inputs beyond c.

Janus even alluded to this in his response.

But since nothing with mass can travel at c' date=' it'll never come up.

[/quote']

Which brings me back to my original point. Since the equation clearly doesn't provide modelling of reality for FTL speeds (because it was developed under the assumption that nothing can go FTL), why would anyone agree that time being imaginary is a prediction of SR? It's not a prediction of SR. SR predicts that nothing can go FTL. It says nothing about FTL time, space, color, etc... because it was not formulated to that. That's where understanding the intent of a theory vs the letter of the equations comes into play. If you simply accept the SR equations as Gospel, and apply them where they are not applicable, you'll get nonsense. You have to understand why you get nonsense, and not simply accept the fact that reality becomes nonsense because the equations say so.

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I would disagree that it's entirely semantic. It's an important point. That equation was not developed to provide answers (or predictions) for FTL speeds. Period. To better illustrate this point you need to look no further than the equation itself. It uses c as it's upper limit for providing real answers. If it had been developed for FST speeds' date=' it would not use c as it's upper limit. The backbone of SR is not simply the constancy of c, but also that it is the universal "speed limit". Everything was developed based on that idea. Since it was clearly developed with the assumption that FTL speeds are impossible to achieve, it cannot provide useable answers to inputs beyond c.

Janus even alluded to this in his response.

Which brings me back to my original point. Since the equation clearly doesn't provide modelling of reality for FTL speeds (because it was developed under the assumption that nothing can go FTL), why would anyone agree that time being imaginary is a prediction of SR? It's not a prediction of SR. SR predicts that nothing can go FTL. It says nothing about FTL time, space, color, etc... because it was not formulated to that. That's where understanding the intent of a theory vs the letter of the equations comes into play. If you simply accept the SR equations as Gospel, and apply them where they are not applicable, you'll get nonsense. You have to understand why you get nonsense, and not simply accept the fact that reality becomes nonsense because the equations say so.[/quote']

Which of the two postulates of special relativity denote c as the upper bound for speeds?

I think you have a causality issue here.

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Swansont, Tom, et al - Time does not become imaginary. The equation YIELDS an imaginary result. As I said, it would be more useful to simply say that we don't know. Janus did not say that. He said that time would become imaginary. I think that's a dangerous way to look at the world. And, for Swansont, I think it shows that Jauns did not acknowledge the assumptions that went into the formulation of the equation.

It is clear that the question that was asked regarded the theory (I know when something is going faster than the speed of light - in theory - time would go backwards.). So saying "time becomes imaginary" is the same, to me, as saying "the time variable becomes imaginary" and the resulting conclusion that the result is not physically meaningful.

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Much greater speed than c is required to freeze the time

V=s/t , so smaller time, bigger distance. When t is near 0, s is close to infinity

It takes some 8 minutes to get from sun to earth. I time stopped for photons, there would be no delay. simple

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I would disagree that it's entirely semantic. It's an important point. That equation was not developed to provide answers (or predictions) for FTL speeds. Period.

It was developed to provide answers for inertial frames when gravitational effects are negligible. It was not developed with the restriction that speeds must be less than c.

To better illustrate this point you need to look no further than the equation itself. It uses c as it's upper limit for providing real answers. If it had been developed for FST speeds' date=' it would not use c as it's upper limit. The backbone of SR is not simply the constancy of c, but also that it is the universal "speed limit". Everything was developed based on that idea.

Since it was clearly developed with the assumption that FTL speeds are impossible to achieve, it cannot provide useable answers to inputs beyond c.

[/quote']

You are simply mistaken. Neither postulate of SR puts c as the "speed limit". That result is deduced from the postulates.

It's obvious that you haven't studied relativity. So now the big question is, why do you insist on pushing your opinions on it? Why are you trying to convince us of something that we know is false?

Which brings me back to my original point. Since the equation clearly doesn't provide modelling of reality for FTL speeds (because it was developed under the assumption that nothing can go FTL), why would anyone agree that time being imaginary is a prediction of SR? It's not a prediction of SR.

It is a prediction of SR.

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Much greater speed than c is required to freeze the time

V=s/t ' date=' so smaller time, bigger distance. When t is near 0, s is close to infinity

It takes some 8 minutes to get from sun to earth. I time stopped for photons, there would be no delay. simple[/quote']

I´m afraid I must tell you that above is not really the topic here. They are talking about relativity and the effect of time dilatation.

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Swansont/Tom - I never said that the speed limit it was a postulate. Why do you keep telling me I did?

The equations in SR use c as the thing against which some relative velocity is compared (or divided). And the equations cease to provide usable information for relative velocities greater than c.

You're acting as though I'm bashing relativity here. While I have done that before, I'm not doing it here. I'm just saying that you can't take the equations past c and get anything useful.

Those equations are designed to provide imaginary answers for FTL speeds.

Tom - Do you truly believe that SR predicts imaginary time? Is any misuse of an equation to be called a prediction of the base theory?

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Swansont/Tom - I never said that the speed limit it was a postulate. Why do you keep telling me I did?

The equations in SR use c as the thing against which some relative velocity is compared (or divided). And the equations cease to provide usable information for relative velocities greater than c.

You're acting as though I'm bashing relativity here. While I have done that before' date=' I'm not doing it here. I'm just saying that you can't take the equations past c and get anything useful.

Those equations are designed to provide imaginary answers for FTL speeds.

Tom - Do you truly believe that SR predicts imaginary time? Is any misuse of an equation to be called a prediction of the base theory?[/quote']

It was when you said That equation was not developed to provide answers (or predictions) for FTL speeds. Period. To better illustrate this point you need to look no further than the equation itself. It uses c as it's upper limit for providing real answers. If it had been developed for FST speeds, it would not use c as it's upper limit.

This implies that you think c as a speed limit was part of the development, and it wasn't.

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It was when you said That equation was not developed to provide answers (or predictions) for FTL speeds. Period. To better illustrate this point you need to look no further than the equation itself. It uses c as it's upper limit for providing real answers. If it had been developed for FST speeds' date=' it would not use c as it's upper limit.[/i']

This implies that you think c as a speed limit was part of the development, and it wasn't.

Saint also goes on to say

"The backbone of SR is not simply the constancy of c, but also that it is the universal "speed limit". Everything was developed based on that idea. Since it was clearly developed with the assumption that FTL speeds are impossible to achieve, it cannot provide useable answers to inputs beyond c."

Where he quite literally says that SR was based on the Speed of light being a universal speed limit.

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Nevermind. I'm wrong, and you are all correct. When something travels FTL wrt something else, it's time and length both become imaginary. Of course it makes no physical sense, but that's just the way it is becuase the SR equations say so. And, since SR was apparently constructed to deal with every possible speed, all numbers (useful or otherwise) produced by an equation with a defined upper limit for real answers should also simply be accepted. Beyond that, we should accept that reality will follow that equation into the imagination of the mathematician. Solid reasoning, folks.

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Swansont/Tom - I never said that the speed limit it was a postulate. Why do you keep telling me I did?

Because you did. The others have already explained why.

The equations in SR use c as the thing against which some relative velocity is compared (or divided). And the equations cease to provide usable information for relative velocities greater than c.

You're acting as though I'm bashing relativity here. While I have done that before' date=' I'm not doing it here. I'm just saying that you can't take the equations past c and get anything useful.

[/quote']

And you think this because...?

Those equations are designed to provide imaginary answers for FTL speeds.

Great, now you've got "imaginary answers for FTL speeds" as yet another postulate!

No, Saint, SR was designed to answer the question, "What type of universe is it that has the same speed of light for all inertial observers, and the same laws of physics for all inertial observers." As long as those two things hold true, then we are in the domain of applicability of SR.

Tom - Do you truly believe that SR predicts imaginary time?

I wouldn't have said it if I didn't think it.

Is any misuse of an equation to be called a prediction of the base theory?

It is only a misuse in your ill-informed opinion.

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