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# Why nothing can go faster than speed of light.

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When you describe a problem in relativity it's a needed step to be clear on the observer. Just an fyi

And what a speed is being referenced against, in case it's not the observer's frame.

Semantics just or can you say rather that the speed of light is relative to everything equally ?

That may be a better way to say it.

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Semantics just or can you say rather that the speed of light is relative to everything equally ?

Semantics now I would say. But by not being relative, we simply mean that all inertial observers will agree. You do not need to take into account your relative speed with respect to the source of the light.

It seems that the speed of light is just one example of the speed limit of everything,isn't it?

It seems a fundamental speed limit, which in practice many physical processes usually come nowhere close to. For example, the speed of electron flow through a wire cannot exceed c, for fundamental reasons. However, the flow of charges in a physical wire is a lot slower than that; really snail's pace in the classical Drude model.

Do we need a speed limit of some kind just so as not to break causality and does light (and massless particles just fill that slot?)

I sometimes like to think in this way. The causal structure of space-time demands that there is a natural speed limit. And equivalently we need a physical constant with units of velocity in special relativity so that we can mix space and time (i.e., ct has units of length). It just 'happens' that this speed is the speed of light.

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I sometimes like to think in this way. The causal structure of space-time demands that there is a natural speed limit. And equivalently we need a physical constant with units of velocity in special relativity so that we can mix space and time (i.e., ct has units of length). It just 'happens' that this speed is the speed of light.

From that view, it should not be too surprising that something goes the speed limit. The most common (to us) thing happens to be light.

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Try to explain why nothing can go faster than speed of light without using the term relativistic mass.

Can it be done?

it's not hard. You do need to start off with the axiom that the speed of light in a vacuum is the same for all observers though. Some people struggle with that bit.

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I have held a long time (50 year!)inclination to believe that there should be no universal speed limit.I feel that I have seen the error of this (unconscious) disposition.

Let's hope this sticks☺

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The idea of Strange, but a bit simplified:

The speed of light is the same for all observers. A rocket flies by, and sends a lightbeam in the direction of its flight. So the astronaut measures a speed of c in the direction of his flight. I see the lightbeam also with a speed of c. If the rocket would be faster than c, I would see the astronaut measuring the light beam behind him, but for the astronaut it would be in front of him. That is logically impossible, so I will see the rocket always with a velocity lower than c.

on thing worth mentioning here is that you see a red beam of light moving away from your rocket, whereas someone on the ground sees a blue beam of light (or something blue shifted anyway, depending on how fast you're going and what you emit)

I have held a long time (50 year!)inclination to believe that there should be no universal speed limit.I feel that I have seen the error of this (unconscious) disposition.

Let's hope this sticks☺

essentially the speed of light is 1/sqrt(mu*epsilon) which are invariant of the speed of the observer. everything else just falls out of that.

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it's not hard. You do need to start off with the axiom that the speed of light in a vacuum is the same for all observers though. Some people struggle with that bit.

I thought that was just an experimentally observed fact rather than an axiom

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I have held a long time (50 year!)inclination to believe that there should be no universal speed limit.I feel that I have seen the error of this (unconscious) disposition.

Let's hope this sticks☺

Good luck reformulating Maxwell's equations and then showing how any new ideas are mathematically consistent with all of the measurements that agree with special relativity.

This thread is not the place to discuss though.

If you want to read the speculation forum rules and start a thread in there.

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Good luck reformulating Maxwell's equations and then showing how any new ideas are mathematically consistent with all of the measurements that agree with special relativity.

This thread is not the place to discuss though.

If you want to read the speculation forum rules and start a thread in there.

Perhaps you have misinterpreted my post . The last thing I wanted to be was speculative. I thought I was rejoining scientific orthodoxy after 50 years in the wilderness (I now accept there must be a universal speed limit -before I had an unconscious acceptance of a no speed limit universe).

I am not trying to reformulate anything -I am just a slow learner trying to understand SR.

The error I was referring to was my own (is that where you misinterpreted me?)

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I thought that was just an experimentally observed fact rather than an axiom

well a mathematician wouldn't use the word axiom

to put it another way, Special Relativity is simply Galilean relativity (or invariance) reformulated to include Maxwell's Equations, and General Relativity includes the equivalence principle.

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I thought that was just an experimentally observed fact rather than an axiom

In essence it's both. Einstein was taking what was known for E&M and applying to to kinematics. In hindsight it may seem strange to have the postulate, but at the time nobody knew that it applied (or how it applied) to mechanical systems.

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Perhaps you have misinterpreted my post . The last thing I wanted to be was speculative. I thought I was rejoining scientific orthodoxy after 50 years in the wilderness (I now accept there must be a universal speed limit -before I had an unconscious acceptance of a no speed limit universe).

I am not trying to reformulate anything -I am just a slow learner trying to understand SR.

The error I was referring to was my own (is that where you misinterpreted me?)

Very sorry, definitely misread it!

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Very sorry, definitely misread it!

whew, apology accepted

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That is a start I suppose. Something is moving and you add some more energy it moves faster why can't this go on forever?

If the SOL is the universal speed limit, it might be better to ask: "Why is the speed of light the speed it is?" because it doesn't have the constraint of mass to hold it back? What I've learnt, so far, is that it is limited by the properties of the vacuum of space.

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Why do you need to communicate?

The question doesn't really make sense. The speed of light isn't relative to anything. But I suppose you can consider the difference in speed between the light (i.e. c) and the craft (0.5 c) which is (rather painfully obviously) 0.5c.

If one of your inputs in your calculations is the speed of a missile from the perspective of the inertial observer, then I suppose that information has to be communicated.

It might seem obvious but I wanted to check the relationship of the craft's speed compared to the speed of light at the full range of speeds.

It would seem intuitive that the speed expressed in fraction of c relates to how fast the light front moves compared to the craft by simple subtraction.

For instance if the inertial frame is not moving from my perspective (external observer) the laser light wave is moving at speed c away from it.

So at all speeds, the speed of light minus the inertial frame velocity from the perspective (external observer) equals how fast the light front moves ahead of the inertial frame from the perspective (external observer).

Do you agree with that bolded statement?

Edit: I removed the word "stationary" ahead of the term "external observer" for all motion is relative.

Edited by Robittybob1

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If one of your inputs in your calculations is the speed of a missile from the perspective of the inertial observer, then I suppose that information has to be communicated.

It is a thought experiment. No communication is necessary.

So at all speeds, the speed of light minus the inertial frame velocity from the perspective (external observer) equals how fast the light front moves ahead of the inertial frame from the perspective (external observer).

Do you agree with that bolded statement?

I'm not sure I understand it. You seem to be saying that if the speed of is a fraction of c, then the speed is that much less than c. But that is (a) tautological and (b) not very useful.

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It is a thought experiment. No communication is necessary.

I'm not sure I understand it. You seem to be saying that if the speed of is a fraction of c, then the speed is that much less than c. But that is (a) tautological and (b) not very useful.

Well it could be a thought experiment or it could be an actual situation. If you on the ground use parameters that were recorded on the inertial frame you would need to think how you get to know these facts.

It might seem basic but does it hold true when the inertial frame is going forward but fires the laser to the rear? Does the light waveform separate from the craft at over 1c?

Is the maximum value of that separation 2c? I.e craft going forward at near c and fires a laser to the rear at c

Edited by Robittybob1

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Well it could be a thought experiment or it could be an actual situation.

No, it was definitely a thought experiment.

It might seem basic but does it hold true when the inertial frame is going forward but fires the laser to the rear? Does the light waveform separate from the craft at over 1c?

Is the maximum value of that separation 2c? I.e craft going forward at near c and fires a laser to the rear at c

The craft will see the light moving away at c.

Someone on Earth will also see the light moving at c.

But, yes the maximum separation speed is 2c - but it is important to note that no one, in any frame of reference, sees anything moving at more than c.

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No, it was definitely a thought experiment.

The craft will see the light moving away at c.

Someone on Earth will also see the light moving at c.

But, yes the maximum separation speed is 2c - but it is important to note that no one, in any frame of reference, sees anything moving at more than c.

OK but how does the inertial frame measure the speed of light (in the situation where it is moving forward yet firing the laser to the rear? As I see it the inertial frame (IFoR) is not aware it is moving so it doesn't have the concept of forward and backward, but they are noted by the external observer (EO) .

Is the experiment to measure the speed of light done within the craft or exterior to it? I'm thinking it would have to be done in the craft, inside the IFoR.

Edited by Robittybob1

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OK but how does the inertial frame measure the speed of light (in the situation where it is moving forward yet firing the laser to the rear? As I see it the inertial frame (IFoR) is not aware it is moving so it doesn't have the concept of forward and backward, but they are noted by the external observer (EO) .

Is the experiment to measure the speed of light done within the craft or exterior to it? I'm thinking it would have to be done in the craft, inside the IFoR.

Wherever the observer is, it's always c. Note that strange stressed that the combined apparent SOL is 2c, but the speed of each, the light and craft, is still c, or nearly, since massive objects can't quite get there.

Edited by StringJunky

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OK but how does the inertial frame measure the speed of light (in the situation where it is moving forward yet firing the laser to the rear? As I see it the inertial frame (IFoR) is not aware it is moving so it doesn't have the concept of forward and backward, but they are noted by the external observer (EO) .

Is the experiment to measure the speed of light done within the craft or exterior to it? I'm thinking it would have to be done in the craft, inside the IFoR.

It doesn't matter how they measure it, or even if they don't (it is a thought experiment, remember). I doubt there is any way of measuring the speed of light from a laser. Although, I suppose you could reflect it off something at a known distance and time it.

Edited by Strange

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It doesn't matter how they measure it, or even if they don't (it is a thought experiment, remember). I doubt there is any way of measuring the speed of light from a laser. Although, I suppose you could reflect it off something at a known distance and time it.

I think that's how they do it by bouncing it off well-spaced apart mirrors.

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It doesn't matter how they measure it, or even if they don't (it is a thought experiment, remember). I doubt there is any way of measuring the speed of light from a laser. Although, I suppose you could reflect it off something at a known distance and time it.

Would thoughts of time dilation and length contraction confound the experiment? I am wondering how they would explain the light from the forward direction going less distance per sec than to the rear directions as determined and defined by the EO?

I think that's how they do it by bouncing it off well-spaced apart mirrors.

That's how they could try that, you have one source of light and split the beam of light into forward traveling and rearward light beams. Bounce them off the front and back walls and time their return.

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Would thoughts of time dilation and length contraction confound the experiment?

Time dilation and length contraction are a consequence of the speed of light being invariant, so your question doesn't make much sense.

I am wondering how they would explain the light from the forward direction going less distance per sec than to the rear directions as determined and defined by the EO?

Again, I don't really understand what you are asking. "distance per sec" is known as speed and so all observers will see light moving at the same "distance per sec".

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I think that's how they do it by bouncing it off well-spaced apart mirrors.

One method has a spinning mirror, which rotates a certain amount between the first bounce and the return. The deflection and rotation speed tell you the elapsed time.

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