Why stop at speed of light?

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  • #1
Gaijin
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Why can nothing(in theory) go faster that light.

what is stoping it and what governs the speed of light itself?

?
 

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  • #2
James R
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It is not true that nothing can go faster than light. For example, if you point a flashlight at the moon, then you can sweep the beam across the moon's surface in a fraction of a second. Work out the speed of the spot from the flashlight as it crosses the moon's surface, and you'll get an answer greater than the speed of light.

What is true is that that no material object (with mass) can ever be accelerated to the speed of light. (The spot on the moon is not a material object.)

The reason you can't accelerate an object to the speed of light is that you'd need to provide an infinite amount of energy to do that, and an infinite energy source isn't available.
 
  • #3
pervect
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Gaijin said:
Why can nothing(in theory) go faster that light.

what is stoping it and what governs the speed of light itself?

?

The speed of light appears to be a universal speed limit. Hypothetical particles called "tachyons" have been proposed which always go faster than light, and could never go slower than light, but have never been observed.

As far as "what is stopping it" goes, if you add together a velocity of .99*c to itself 100 times, you still get a velocity that's below the speed of light. This happens because of the relativistic velocity addition formula.

v1+v2 = v1+v2/(1+v1*v2/c^2)

you can add together ANY number of velocities lower than 'c', and the result will still be lower than 'c'.
 
  • #4
Pengwuino
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James R said:
It is not true that nothing can go faster than light. For example, if you point a flashlight at the moon, then you can sweep the beam across the moon's surface in a fraction of a second. Work out the speed of the spot from the flashlight as it crosses the moon's surface, and you'll get an answer greater than the speed of light.

That is incorrect. Your transmitting a beam of photons at the moon. When you turn the flashlight, you are not moving anything at the speed of light, your simply re-directing the photons. A photon that just hit hte moon does not leave the moon simply because you change the direction of the flashlight after that.
 
  • #5
James R
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Pengwuino,

You misunderstand.

If you were standing on the moon's surface, and I was standing on the opposite side of the moon, then you would see a spot of light appear to move from you to me across the moon's surface in a certain amount of time. Calculate the distance divided by the time it takes the spot to move, and you'll get an answer greater than the speed of light.

I know that the same photon doesn't travel from you to me, but that's not what I'm talking about. I have demonstrated that something can move faster than the speed of light.
 
  • #6
Dr.Brain
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Gaijin said:
Why can nothing(in theory) go faster that light.

what is stoping it and what governs the speed of light itself?

?

Logically , for cause and effect to be seen in perfect order , the speed of light needs to be less than c.

I would like to ask you 'What would be the the effect if the speed limit was 2c?"

BJ
 
  • #7
quantumdude
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Dr.Brain said:
Logically , for cause and effect to be seen in perfect order , the speed of light needs to be less than c.

That I can't agree with. It isn't simply a matter of logic, because the Galileo vs Einstein question cannot be decided by logic alone. The matter must be (and has been) decided by experiment.

I would like to ask you 'What would be the the effect if the speed limit was 2c?"

In that case the laws of electrodynamics could not be the same for all inertial observers.
 
  • #8
Pengwuino
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James R said:
I know that the same photon doesn't travel from you to me, but that's not what I'm talking about. I have demonstrated that something can move faster than the speed of light.

This argument has been brought up before and no THING has moved faster then the speed of light. A perception has moved... but no THING has moved. No THING has been displaced at >c.
 
  • #9
Pengwuino
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Tom Mattson said:
That I can't agree with. It isn't simply a matter of logic, because the Galileo vs Einstein question cannot be decided by logic alone. The matter must be (and has been) decided by experiment.

Actually it is logical... but its the NEW logic... or "improved" logic. Bring in the idea that time is a dimension, and all of a sudden its logical that nothing can go beyond c. Train a child that there are 4 usable dimensions and logic now dictates that nothing can go beyond C. Now... this is probably symantecs but i sure am in a typing mood today.

Dr. Brain said:
I would like to ask you 'What would be the the effect if the speed limit was 2c?"

Well since c is the speed limit of light, its impossible to say "what if its 2c" since in this new world, our 2c woudl be the new c at 600,000km/sec
 
  • #10
Integral
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Gaijin said:
Why can nothing(in theory) go faster that light.

what is stoping it and what governs the speed of light itself?

?
The universe has a nasty habit of doing things, then failing to tell us why. We can only observe and make predictions based on those observations.
 
  • #11
Pengwuino
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We should blow up one planet for every problem we can't understand. The universe deserves it for not being nice to us.
 
  • #12
Gokul43201
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Pengwuino said:
This argument has been brought up before and no THING has moved faster then the speed of light. A perception has moved... but no THING has moved. No THING has been displaced at >c.
Well, no massive "thing" at least. And James never claimed that a "massive thing" was made to move faster than c; in fact he specifically disclaimed this.

Another simple example is the intersection of a pair of lines. This too, is a thing, but not a massive thing (even though the "lines" may be).

To the OP : While SR explains (as James did) why it is not possible to accelerate something up to c, it does not insist that there not be things traveling at speeds greater than c. That, however, violates causality.
 
  • #13
Dr.Brain
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Tom Mattson said:
That I can't agree with. It isn't simply a matter of logic, because the Galileo vs Einstein question cannot be decided by logic alone. The matter must be (and has been) decided by experiment.



In that case the laws of electrodynamics could not be the same for all inertial observers.

If things could travel at a speed greater than light , we could 'see' the effect before the 'cause' . This is one of the basic things that I would like to introduce to 'Gaijin' . 'Perceiving' the images of an event is a surety that it has happened , we get these images in an ordered manner , that is from cause to effect . But objects exceeding the speed of light would reverse the order of images certainly.In that case we would have the subjects 'Future' instead of 'History' and History would have been unsure of. :uhh:

Any spacetime point can be taken to be surrounded by spacetime region divided into three regions , two timelike and one space-like, this spacetime point can be affected in several ways . The same point in past-time coordinates would effect the point in present , and this point (at origin ) can affect the future, example it can shoot a bullet , which will hit the wall in the future . Future is always affected by past and present , Have you ever wondered why future cannot affect your present?? Gaijin?? :rolleyes: ... This is where the limits end.This is where the limit 'c' comes into play.
 
  • #14
cliowa
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James R said:
The reason you can't accelerate an object to the speed of light is that you'd need to provide an infinite amount of energy to do that, and an infinite energy source isn't available.

I think there you're turning things upside down. As far as I know, Einstein started from the idea, that the speed of light is the same everywhere (in every inertial system) and thereby got to the conclusions that are now so popular: e=mc^2, length contraction, time dilatation...

Your "explanation", that there would be needed an infinite amount of energy to accelerate an object to the speed of light, is no reason, it simply follows from the theory Einstein developed (and that was found to be correct by experiments).

So, I would say that the best argument for the speed limitation by c is what Dr.Brain mentioned writing about cause and effect. This isn't so obvious, considering quantum theory, for example, where deterministic reasoning simply doesn't work anymore.

@Pengwuino: Could you explain, what exactly you mean, by saying "logical"? If you feed a child the necessary information (that is: you keep on telling that c is constant), it will surely find it the most natural thing, just like religious people, that are grown up with their religion, find it to be obvious. But there's no logic in it, as far as I get your idea.

greetings...Cliowa
 
  • #15
Pengwuino
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cliowa said:
@Pengwuino: Could you explain, what exactly you mean, by saying "logical"? If you feed a child the necessary information (that is: you keep on telling that c is constant), it will surely find it the most natural thing, just like religious people, that are grown up with their religion, find it to be obvious. But there's no logic in it, as far as I get your idea.

No no i mean like... ok, bring yourself into before you were taught SR. You thought what was logical in your mind is that you can go as fast as you want and that time isn't a "dimension". That was your logic because all you were taught before learning SR is that you just add speeds and such and that time can't really be screwed around with.

If you were being taught abotu velocities for the first time and were also taught about special relativity at that time, your "logic" would dictate that time is something that can be messed with like it is in SR and that you can't just constantly add up velocities.

Its symantics I think because to me logic = obeys laws of physics. Since all the universe is is a bunch of laws of physics, nothing can be illogical to me. Well... ok, humans can be illogical for whatever reason they are but the rest of the world isn't :) hehehe. But then again on the more detailed level, whatever makes people do "illogical" things has a logical sequence of neurons firing and brain things doing this and that and I am just still typing for no reason and i don't even know what I am talking about anymore... i think I am going to eat this taco now.
 
  • #16
Pengwuino
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Gokul43201 said:
Well, no massive "thing" at least. And James never claimed that a "massive thing" was made to move faster than c; in fact he specifically disclaimed this.

No, he said a light beam can move faster then c. But what is a light beam? A lot of photons. The photons don't move though lateraly (towards the other side of the moon) but the perceived "beam" does. The "beam" isn't a solid object however. Its like firing a stream of water at a house and then moving to fire it at a house 400 light years away. No molecule of water actually moved faster then allowed but the perceived "stream" moved.
 
  • #17
cliowa
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Pengwuino said:
No no i mean like... ok, bring yourself into before you were taught SR. You thought what was logical in your mind is that you can go as fast as you want and that time isn't a "dimension". That was your logic because all you were taught before learning SR is that you just add speeds and such and that time can't really be screwed around with.

Well, the thing is, that the "old view" perfectly fits our experience! It is absolutely counter-intuitive that the Doppler effect at some point just won't "happen" anymore! Probably you're (at least partly) correct in calling the humans illogical and the universe logical, but i wouldn't call things this way. Because the physics theories are man-made, describing what is happening, but not why it is happening that way, I would take as the base the human being. Logic means getting from an idea to the conclusion in reasonable, rational steps. So it is pretty much our definition (what is rational). I would therefore take the human arguing and reasoning as being logic and the rest as not. But, as you see, I'm not really getting anywhere: it's a pure matter of definition (this is not to say it's trivial).

What a pity I don't have a taco around.
 
  • #18
krab
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Pengwuino said:
... this is probably symantecs but i sure am in a typing mood today.
...but apparently, not in a spelling mood. "symantec" is a computer software company.
 
  • #19
Pengwuino
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krab said:
...but apparently, not in a spelling mood. "symantec" is a computer software company.

Well my other favorite forum is a computer hardware forum so I'm just thankful there were no famous physicists named Microcroft.
 
  • #20
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cliowa said:
I think there you're turning things upside down. As far as I know, Einstein started from the idea, that the speed of light is the same everywhere (in every inertial system)
Saying the speed of light is frame-independent is not equivalent to saying that c is an upper limit (without the framework of SR). In other words, the "upperlimitness of c" is not a postulate of SR, it is a result.

Your "explanation", that there would be needed an infinite amount of energy to accelerate an object to the speed of light, is no reason, it simply follows from the theory Einstein developed (and that was found to be correct by experiments).
All explanations of physical phenomena must follow from some theory. Please explain anything to me without using the results of some theory.

Causality alone (without SR) is not sufficient to make speeds greater than c impossible. It is the framework of SR that tells you that v > c violates causality. In Newtonian mechanics, for instance (with Galilean transforms), traveling faster than c would not violate causality.

But one must note the important distinction between traveling faster than c and accelerating past c. The latter is impossible, according to SR, because it violates energy conservation (as James explained). The former is impossible because, according to SR, it would violate causality.
 
  • #21
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Pengwuino said:
No, he said a light beam can move faster then c. But what is a light beam? A lot of photons.
It sounds like you are arguing for the sake of argument. Please quote to me where he said that the light beam (or the photons) move faster than c. In both posts, he says that it is the "spot on the moon" that moves faster than c - the "spot" is not a physical body.
 
  • #22
Gokul43201
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cliowa said:
It is absolutely counter-intuitive that the Doppler effect at some point just won't "happen" anymore!
Could you explain what you mean by this last bit? I'm not sure I follow.
 
  • #23
DaveC426913
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Gaijin said:
Why can nothing(in theory) go faster that light.

what is stoping it and what governs the speed of light itself?

?
Do not think of the speed of light as a “speed limit”. Time and space are one and the same, and light is an integral part of this. It is non-sensical to think of them separately.

I am going to draw an analogy where “direction” is substituted for “speed”. And then ask why there is a “direction limit”, and why we can’t exceed it.

Consider:
You and I are in a vehicle with two driver’s seats. Mine faces North, yours faces West. My controls govern how fast the car goes North and South. Your controls govern how fast the car goes East and West. We can operate independently. With coordination and a slide rule, we can pick any direction of the compass and go that way. (i.e. I go 40mphN and you go 40mphE and we will go NW at ~56mph.)

But today, I want to go shopping in Northtown, and start the car going North at 40mph. But you don’t want to shop in Northtown, you want to shop in Westville, which is due West from our current location. You accelerate the car to go due West as fast as possible.

If you go at 100mph, you’ll still only be heading West NorthWest. If you double your speed, we’ll be heading more West, but will still be heading slightly North. Increasing effort on your part only causes an incremental and ever-decreasing gain in Westwardness.

Simply put, no matter how fast you go, we will not achieve a due West heading. You’d have to travel West at an infinite speed. How fast would you have to go to “exceed due Westness”, so that you are actually traveling “a little bit South”? It is nonsensical to ask the question.
 
  • #24
quantumdude
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Pengwuino said:
Actually it is logical... but its the NEW logic... or "improved" logic.

When I say "logic" I mean exactly that: no more, no less. Logic is content-free, with regards to the physical world. You aren't going to derive a speed limit of c from truth-functional propositional schema or first order quantification. Once you start filling in those p's and q's with statements about the non-analytic world, you are infusing meaning to the logical variables with statements based in experience. And that cannot be said to be 'only' logic.
 
  • #25
Pengwuino
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Gokul43201 said:
It sounds like you are arguing for the sake of argument. Please quote to me where he said that the light beam (or the photons) move faster than c. In both posts, he says that it is the "spot on the moon" that moves faster than c - the "spot" is not a physical body.

No, the "spot" is a collection of photons. When you move the flashlight, a new spot is created on the opposite side of the moon. Its not the same spot. It'll look pretty much exactly the same, but nothing about that first spot exists at the location of the new spot. Its like saying if i introduce a new flashlight into the situation, that the "spot" is actually existing twice at the same time.
 
  • #26
Gokul43201
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Pengwuino said:
No, the "spot" is a collection of photons.
No, the spot is a set of co-ordinates.
 
  • #27
Jelfish
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When you take a flashlight and shine a spot on a wall and then move the flashlight, the spot "travels" along the wall. There's no assertion that the photons that make up the initial spot are the same photons that make up the spots after it has moved. The spot is a conceptualization of an object. Objects of mass cannot be accelerated to c, but the situation that James mentioned does not involve massive objects. It deals with the illusion of an object which is completely reliant on human perception. Such illusions are not necessarily bound by the laws of physics.
 
  • #28
Ubern0va
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If those arent enough for you here is my answer, it humours the determined...

One cannot go faster than the speed of light simply due to logical reasoning. That is, if one were to take into account the theory of relativity, which states that, the closer you get to the speed of light, the faster the world moves around you and the slower you move relative to the world (i.e. the less you progress through time). That having been said, the logical inference to make is that when you hit a point past the speed of light, you will begin to go back in time relative to the world around you, though the world around you should actually begin to go slower and slower the faster you go after surpassing the speed of light.

That having been said, let's take the following example into account, and as well take the obvious problems of energy restrictions out of the equation for the sake of argument (though, truly, they also prove the same as this argument does). Let us say that a rocket managed to accelerate to the speed of light and beyond. There are three important instances to take into account: the first is the instance before reaching the speed of light, the second is the instance when you reach that particular velocity, and the third is the, entirely hypothetical, instance after you reach it.

If one is to travel past the first instance, then, relative to the world, that person should appear to be stationary. Relative to him or her self, however, the person would be moving at a great speed and the world around him would be aging infinitely fast (even though he or she would not be able to see it). This person would now have experienced all of time in an instant and would be very likely to have been obliterated in some sort of universal collapse (unless that is you conduct this outlandish experiment in the vast reaches of space, where gravity has little to no effect on you and your rocket).

Again for the sake of argument, (i.e. some people are ignorant enough to need more proof) let’s say that we found a way to get passed that bump in the road. The next instance of importance is your ‘reaching a speed beyond the speed of light’, a Sci-fi fanatic’s dream. Should you, however, reach that speed, you would not go back in time very far. You would actually just go back to a moment before the third instance (the reaching of a velocity greater than C). You would then again be traveling at a velocity = the speed of light, not beyond it. Since the instances I speak of are unimaginably small periods of time, before you could react, you would again be propelled to a speed beyond that of light. After which you would go back in time an instant and the event would repeat itself over again. Thus, you would be stuck in a loop, destined to surpass the speed of light and go back in time to do it again for all of eternity.

Other than that, everything that the other guys said.
 
  • #29
James R
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Pengwino:

This argument has been brought up before and no THING has moved faster then the speed of light. A perception has moved... but no THING has moved. No THING has been displaced at >c.

A spot of light is a thing, if you ask me. If you want to argue about whether it is really the "same" spot as it moves across the moon's surface (since it is made up of different photons at different times), then we can have that philosophical discussion. We can also argue about whether you are the same person today as you were when you were 3 years old, because all the cells in your body have probably been replaced since then.

But it's all beside the point. My initial post was unambiguous.


Cliowa:

Your "explanation", that there would be needed an infinite amount of energy to accelerate an object to the speed of light, is no reason, it simply follows from the theory Einstein developed (and that was found to be correct by experiments)

The statement that it requires an infinite amount of energy to accelerate something to the speed of light is either a true or false fact about the universe, regardless of whether Einstein said it, or I said it, or Mathatma Gandhi said it.

So, my explanation is an explanation. I just use Einstein to justify my explanation, but it stands on its own. Even you admit it is consistent with available experimental evidence.
 
  • #30
Vern
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I predect that nothing can exceed the speed of light because the final irreducible constituent of all physical reality is the electromagnetic field.

But I think someone said that about 150 years ago.
 
  • #31
Pengwuino
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I hope we're not confusing anyone with our argument over symantics/symantecs (whatever the word is that isn't a virus protection company).
 
  • #32
DaveC426913
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Pengwuino said:
I hope we're not confusing anyone with our argument over symantics/symantecs (whatever the word is that isn't a virus protection company).
My concern as well. I tried to address the OP's question via analogy in post #23.
 
  • #33
russ_watters
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Tom Mattson said:
When I say "logic" I mean exactly that: no more, no less. Logic is content-free, with regards to the physical world.
Expansion: most people think SR is illogical at first glance, but the reason is that most people don't see that the logic is only as good as the starting premise. If the starting premise is flawed (as it is with Newtonian physics), then good logic will lead to wrong conclusions. So the converse is that SR is illogical only if you try to figure it out using the wrong starting premise.
 
  • #34
Pengwuino
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russ_watters said:
Expansion: most people think SR is illogical at first glance, but the reason is that most people don't see that the logic is only as good as the starting premise. If the starting premise is flawed (as it is with Newtonian physics), then good logic will lead to wrong conclusions. So the converse is that SR is illogical only if you try to figure it out using the wrong starting premise.

Yes, exactly what i meant. Thank you for condensing my 2 pages of confusion into a coherant statement.
 
  • #35
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James R said:
A spot of light is a thing, if you ask me. If you want to argue about whether it is really the "same" spot as it moves across the moon's surface (since it is made up of different photons at different times), then we can have that philosophical discussion. We can also argue about whether you are the same person today as you were when you were 3 years old, because all the cells in your body have probably been replaced since then.

But it's all beside the point. My initial post was unambiguous.


No your initial post is just plain wrong. Apply the word "thing" to a spot of light if you must, but understand that the only movement taking place is photons moving from the light source to the moon and back to the observer (and traveling at C not above). No photons move sideways (that the observer can see) you are simply creating a set of events that happens faster than the speed of light. No philosophical discussion required.
 

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