A question about the speed of light and electromagnetism

[johnfrancismurray]

TL;DR Summary

an observation that light has both a 'true' and 'apparent' speed.

hello, I am interested in the speed of light as well as electromagnetism.
I understand that time stops at the speed of light, which is approx 300000km/s.

but because time stops at the speed of light it appears to me to have two speeds- those being the speed it experiences which is instantaneous (because time has stopped so all destinations at any distance will be arrived at instantaneously) and the speed a third party experiences it at which is approx 300000km/s (traveling through a vacuum).

this would help explain the infinite and finite characteristics of the speed of light ie. it takes an infinite amount of energy to accelerate an object with mass up to the speed of light, with the amount needed rising exponentially as time slows down to zero exponentially.

It reminds me of wind speeds that a sailing boat experiences, those being 'true' the wind speed for a stationary object in the same place as the boat, and 'apparent' the wind speed experienced across the deck of the moving boat- like light has a 'true' and 'apparent' speed.
please feel free to correct me if I am mistaken in any of my facts.

many thanks
johnmurray

 

[Dale]

Light doesn’t have an experience. I don’t think anything meaningful can come of pretending that it does.

It does have an affine parameter. I am skeptical about the usefulness of any notion of the speed with respect to an affine parameter.

 

[Ibix]

Time does not stop at light speed - that makes no sense, despite its appearance in popularisations of relativity. The correct statement is that proper time cannot be defined along the paths that light follows, so there is no meaning to your "speed it experiences".

 

[johnfrancismurray]

yes Im not trying to anthropomorphize light, I know that wouldn't make sense. Im just talking about time ceasing to exist for something traveling at the speed of light (theoretically).

 

[johnfrancismurray]

yes Im not trying to anthropomorphize light, I know that wouldn't make sense. Im just talking about time ceasing to exist for something traveling at the speed of light (theoretically).

I forgot to add thanks Dale

 

[Dale]

yes Im not trying to anthropomorphize light, I know that wouldn't make sense. Im just talking about time ceasing to exist for something traveling at the speed of light (theoretically).

What does time ceasing to exist mean? When something ceases then it was happening earlier and it stopped happening later. But what does it mean to happen later if it is time itself that stops happening? How does the concept of having previously stopped even make sense without time existing?

I am afraid that this is just a logical dead end. You get tied up in mental knots. It is better just to stick with the standard formulation. It picks a narrow path through some very thorny issues.

 

[FactChecker]

It reminds me of wind speeds that a sailing boat experiences, those being 'true' the wind speed for a stationary object in the same place as the boat, and 'apparent' the wind speed experienced across the deck of the moving boat- like light has a 'true' and 'apparent' speed.

I don't think that the behavior of time and speed in Special Relativity can be likened to anything that is a common experience at low speeds. The geometry of spacetime is not nearly apparent in common experience at low speeds.
That being said, even at low speeds, there are extremely sensitive measurements that agree with SR, as opposed to Galilean relativity. Also, electromagnetic effects are easy to see at slow speeds, but their ties to SR are not obvious.

 

[phinds]

Im just talking about time ceasing to exist for something traveling at the speed of light (theoretically).

But is DOESN'T "cease to exist". It never existed in the first place.

 

[johnfrancismurray]

I don't think that the behavior of time and speed in Special Relativity can be likened to anything that is a common experience at low speeds. The geometry of spacetime is not nearly apparent in common experience at low speeds.
That being said, even at low speeds, there are extremely sensitive measurements that agree with SR, as opposed to Galilean relativity. Also, electromagnetic effects are easy to see at slow speeds, but their ties to SR are not obvious.

thanks FactChecker, I only used the true and apparent analogy to describe that something can be at two speeds simultaneously, it is just a metaphor and not the message. The message that I want to put forward here is that light has an infinite and a finite speed and qualities associated with these two. I think this is all established science but not really acknowledged, it means that things traveling at light speed exist both in time and outside of time.

 

[johnfrancismurray]

What does time ceasing to exist mean? When something ceases then it was happening earlier and it stopped happening later. But what does it mean to happen later if it is time itself that stops happening? How does the concept of having previously stopped even make sense without time existing?

I am afraid that this is just a logical dead end. You get tied up in mental knots. It is better just to stick with the standard formulation. It picks a narrow path through some very thorny issues.

Hey Dale thanks again, what I meant was if something accelerated up to the speed of light (I know that is physically impossible (but isn't that because light speed is also an infinite speed as I was saying) then at that moment time would cease to exist for that object. I see your point Dale if time stopped it could not have a moment in which it started, although is that also problem with the theory of the 'big bang' for you? I can see the mental loop you're referring too

 

[johnfrancismurray]

But is DOESN'T "cease to exist". It never existed in the first place.

isn't that so interesting phinds, it never existed in time and yet it also at a measurable finite speed (speed of course = distance over time), it's another duality of light

 

[phinds]

isn't that so interesting phinds, it never existed in time and yet it also at a measurable finite speed (speed of course = distance over time), it's another duality of light

You are mixing apples and oranges, so it's not surprising that your conclusion is bananas.

 

[DaveC426913]

Hey Dale thanks again, what I meant was if something accelerated up to the speed of light (I know that is physically impossible (but isn't that because light speed is also an infinite speed as I was saying) then at that moment time would cease to exist for that object. I see your point Dale if time stopped it could not have a moment in which it started, although is that also problem with the theory of the 'big bang' for you? I can see the mental loop you're referring too

The speed of light is not an infinite speed.

Since you can't accelerate any object to the speed of light, it is meaningless to pretend you can.

Time doesn't cease to exist for objects.
[Mentors' note: this post has been edited in an effort to keep the discussion in this thread focused on the physics]

 

[PeroK]

isn't that so interesting phinds, it never existed in time and yet it also at a measurable finite speed (speed of course = distance over time), it's another duality of light

The concept of time has two related but distinct meanings in relativistic physics. The first is coordinate time. This is the time as measured in a given frame of reference and used to measure the time that an event took place and, along with spatial coordinates and measurements of distance, to calculate the speed of an object.

In Special Relativity, it is simplest to use inertial reference frames, which are the frames of reference in which Newton's first law of motion holds. Two inertial reference frames may be related by a relative velocity. This relative velocity, however, must be less than $c$, the speed of light in vacuum.

Therefore, there is no inertial refrence frame associated with the rest frame of light. In fact, in every inertial reference frame, the speed of light is measured to be $c$. Which is one of the original postulates of relativity.

The second concept of time, which applies in the theory of relativity, is the proper time of a massive particle. This is the time measured by the particle. You can imagine this as the time recorded by a perfect clock moving along with the particle. Light moves along paths where proper time cannot be defined. The physical consequences of this are well understood.

Popular science sources, however, are often not satisfied with things that cannot be defined (like $\frac 1 0$) and so they like to make a big deal of it. Instead of saying simply: proper time cannot be defined along a light-like trajectory, they prefer to say that "time does not exist" or "the photon experiences no time". This sounds mysterious and of great philosophical interest. Whereas, if you study relativity systematically, then there is no issue at all.

 

[Ibix]

isn't that so interesting phinds, it never existed in time and yet it also at a measurable finite speed (speed of course = distance over time), it's another duality of light

No.

There are two different ways to measure time. One is to establish a network of synchronised clocks at rest with respect to each other and read the timeoff the clock you are currently passing. The other is to carry a clock with you. The former is called coordinate time and the latter proper time. The former is always available and can be used with light - that's how we can measure light speed. The latter cannot be used to measure speed, since the appropriate distance would be the distance you travel in your rest frame which is zero by definition.

All "time is not defined for light" means is that you cannot even in principle build a clock that travels at the speed of light. That is obvious - some parts of a clock travelling at $c$ would have to exceed $c$, which is impossible.

 

[johnfrancismurray]

Time does not stop at light speed - that makes no sense, despite its appearance in popularisations of relativity. The correct statement is that proper time cannot be defined along the paths that light follows, so there is no meaning to your "speed it experiences".

The concept of time has two related but distinct meanings in relativistic physics. The first is coordinate time. This is the time as measured in a given frame of reference and used to measure the time that an event took place and, along with spatial coordinates and measurements of distance, to calculate the speed of an object.

In Special Relativity, it is simplest to use inertial reference frames, which are the frames of reference in which Newton's first law of motion holds. Two inertial reference frames may be related by a relative velocity. This relative velocity, however, must be less than $c$, the speed of light in vacuum.

Therefore, there is no inertial refrence frame associated with the rest frame of light. In fact, in every inertial reference frame, the speed of light is measured to be $c$. Which is one of the original postulates of relativity.

The second concept of time, which applies in the theory of relativity, is the proper time of a massive particle. This is the time measured by the particle. You can imagine this as the time recorded by a perfect clock moving along with the particle. Light moves along paths where proper time cannot be defined. The physical consequences of this are well understood.

Popular science sources, however, are often not satisfied with things that cannot be defined (like $\frac 1 0$) and so they like to make a big deal of it. Instead of saying simply: proper time cannot be defined along a light-like trajectory, they prefer to say that "time does not exist" or "the photon experiences no time". This sounds mysterious and of great philosophical interest. Whereas, if you study relativity systematically, then there is no issue at all.

Thanks Perok,

The concept of time has two related but distinct meanings in relativistic physics. The first is coordinate time. This is the time as measured in a given frame of reference and used to measure the time that an event took place and, along with spatial coordinates and measurements of distance, to calculate the speed of an object.

In Special Relativity, it is simplest to use inertial reference frames, which are the frames of reference in which Newton's first law of motion holds. Two inertial reference frames may be related by a relative velocity. This relative velocity, however, must be less than $c$, the speed of light in vacuum.

Therefore, there is no inertial refrence frame associated with the rest frame of light. In fact, in every inertial reference frame, the speed of light is measured to be $c$. Which is one of the original postulates of relativity.

The second concept of time, which applies in the theory of relativity, is the proper time of a massive particle. This is the time measured by the particle. You can imagine this as the time recorded by a perfect clock moving along with the particle. Light moves along paths where proper time cannot be defined. The physical consequences of this are well understood.

Popular science sources, however, are often not satisfied with things that cannot be defined (like $\frac 1 0$) and so they like to make a big deal of it. Instead of saying simply: proper time cannot be defined along a light-like trajectory, they prefer to say that "time does not exist" or "the photon experiences no time". This sounds mysterious and of great philosophical interest. Whereas, if you study relativity systematically, then there is no issue at all.

Thanks Perok, I appreciate what you've done here. Co-ordinate time and Proper time are the terms I should have been using - thankyou Ibix also.

 

[johnfrancismurray]

Thanks Perok,

Thanks Perok, I appreciate what you've done here. Co-ordinate time and Proper time are the terms I should have been using - thankyou Ibix also.

Perok

Thanks Perok,

Thanks Perok, I appreciate what you've done here. Co-ordinate time and Proper time are the terms I should have been using - thankyou Ibix also.

Perok when you say 'the physical consequences are well understood' you leave your point there, i would like you to go on. Also you say popular science isn't satisfied with things that cannot be defined but why should they be, this is after all an attempt at understanding the physical world.
thanks again
john

 

[johnfrancismurray]

The speed of light is not an infinite speed.

Since you can't accelerate any object to the speed of light, it is meaningless to pretend you can.

Time doesn't cease to exist for objects.

This is word salad.

why can you not accelerate anything to the speed of light Dave? please elaborate.

[Mentors' note: this post has been edited in an effort to keep the discussion in this thread focused on the physics]

 

[renormalize]

why can you not accelerate anything to the speed of light Dave? please elaborate.

Because accelerating any object with nonzero mass (no matter how tiny) to the speed of light requires an infinite amount of energy.

 

[johnfrancismurray]

Because accelerating any object with nonzero mass (no matter how tiny) to the speed of light requires an infinite amount of energy.

yes, they gain mass as they approach the speed of light, time also slows down for that object. my original point was that light speed is an infinite speed, if that were so it would take an infinite amount of energy to accelerate to that speed and any distance would be arrived at instantaneously, both of these can be said to be true and of course it is not at all surprising that it is also impossible to travel infinitely fast or at light speed.

 

[phinds]

yes, they gain mass as they approach the speed of light,

No, they do not. You have been reading pop-sci stuff about "relativistic mass" which is now a deprecated concept in physics and has been for a long time.

time also slows down for that object.

No, it does not. It APPEARS to, to other observers but not to the object itself.

my original point was that light speed is an infinite speed,

Which, as has been pointed out, it is not.

You would be well served to forget everything you've seen/heard/read in the pop-sci realm and read some actual physics.

[Mentors' note: this post has been edited in an effort to keep the discussion in this thread focused on the physics]

 

[johnfrancismurray]

then tell me, why does it take infinite energy to travel at light speed - approx 300000km/s?

 

[phinds]

then tell me, why does it take infinite energy to travel at light speed - approx 300000km/s?

Actually, I think saying that you could travel at c if you had infinite energy at your disposal is just another way of saying that you CAN'T travel at c.

Think about it this way. If you are going at .9c and you shine a flashlight out in front of you, the light leaves the flashlight at c, so you are clearly not traveling at c because if you were the beam wouldn't move away from you.

Now increase your speed to .999c. This changes nothing. The light STILL moves away from you at c, so you clearly aren't traveling at c.

You can do this as long as you like, adding 9's. It doesn't change anything, it just takes more and more energy to not move at c, with the flashlight beam always leaving you at c.

I think the "infinite energy" thing is a pretense that if you could just get your .99999 ... to become 9-bar, then you would be there, and that that is a realistic limit, but 9-bar (=1) is mathematical and could only be physical if you had an infinite amount of something.

Of course, my statements all require that SR be a valid description of reality. It IS, of course, but if you choose not to believe that then the rest is immaterial to you.

 

[PeroK]

then tell me, why does it take infinite energy to travel at light speed - approx 300000km/s?

If you are serious about understanding this, then you could try Morin's book on SR. The first chapter is free here:

https://davidmorin.physics.fas.harvard.edu/books/special-relativity/

In general your questions reveal that you have acquired a mish-mash of ideas from popular science sources, which are very difficult to consolidate into a coherent understanding of the theory of relativity. You may think we are a bit down on pop-science here (and perhaps that's true), but you should realise that pop-science is a dead-end and can't be extended into the "real" understanding of theory - as you would get as a physics student.

I'd even say that you ought to forget everything you think you know about relativity and start fresh with Morin's book.

The short answer to your question is that the energy of a massive particle is given by:
$$E = \frac{mc^2}{\sqrt{1 - v^2/c^2}}$$Where $v$ is the speed of the particle and $c$ is the speed of light. You can see that $E \to \infty$ as $v \to c$.

This equation reduces to the classical equation for kinetic energy: $KE = \frac 1 2 mv^2$ in the case where $v$ is much smaller than $c$. If your maths is up to it and you know the binomial theorem, I can leave that as an exercise for you to prove for yourself.

There's a fascinating video of experiments at MIT in 1962 to confirm this (this is how popular science should be, IMHO!).

 

[Ibix]

then tell me, why does it take infinite energy to travel at light speed - approx 300000km/s?

Because the required kinetic energy is $(\gamma -1)mc^2$, where $\gamma=1/\sqrt{1-v^2/c^2}$, and the $\gamma$ approaches infinity as your velocity approaches $c$. Note that when $v$ is small compared to $c$, $\gamma$ is approximately $1+\frac 12\frac{v^2}{c^2}$, which is where Newton's formula comes from.

Of course, you can ask where the $\gamma mc^2$ formula came from. The answer, if you keep back-tracking, eventually leads you to the postulates of relativity. Fundamentally, the reason is that the speed of light is $c$ for all observers. That means that you can see that you need to increase your speed by $3\times 10^8\mathrm{ms^{-1}}$ to catch up with light. So you accelerate for a bit and take stock, and you will find that you still need to increase your speed by $3\times 10^8\mathrm{ms^{-1}}$. And you always will. Accelerating to $c$ is forbidden directly by the postulates of relativity. The only question is if we live in a universe governed by relativity - and PeroK has just posted one experimental demonstration of that in the post above mine.

 

[Ibix]

It's worth noting that dividing the distance I measure you to travel by the time I measure you to take is a quantity called velocity. Dividing the distance I measure you to travel by the time you measure you to take is a quantity called celerity. The two values are always different, and celerity is indeed infinite for light (if one is careful in interpreting "time you measure" in terms of "interval along worldline").

So the OP is talking about something that exists in physics, but it's just a choice between two different quantities and not any kind of duality. Nobody uses celerity for anything that I'm aware of because everything it does velocity also does, and with less potential for mathematical problems.

 

[Nugatory]

This thread is temporarily closed for moderation.

 

[Nugatory]

After a fair amount of surgery on this thread:

Several digressions have been edited out of this thread because they are best covered in threads of their own, or by reference to existing threads on the subject:
- The mistaken idea that mass increases with speed.
- The mistaken idea of wave/particle duality
- What to make of popularizations written by serious physicists

More than one poster is fortunate that @Greg Bernhardt has so far resisted my pleading that "making unnecessary work for the mentors" should be an infractable offense under the forum rules. Please, please, everyone, if you dislike the tone of a post... Don't escalate by replying in kind, just report the post and let us mentors take care of it. Yes, our response in this thread was not as timely as it might have been - but we're unpaid volunteers and it was Christmas Day, for crissakes.

The thread is still closed for moderation, but will reopen soon.

 

[Nugatory]

Because accelerating any object with nonzero mass (no matter how tiny) to the speed of light requires an infinite amount of energy.

We should be cautious about this line of argument. The standard formulas of SR are derived using assumptions that imply that $v\lt c$ so cannot be expected to apply when it is not. Thus the infinities that appear in these standard formulas when we plug in $v=c$ (and likewise the negative numbers that appear under a square root sign when we plug in $v\gt c$) aren't telling us anything about how the world works, they're the math telling us that we're misusing it.

I'm happier with the argument that no matter how much energy we add to an object initially at rest relative to us, the resulting speed of the object relative to us will be less than $c$. This result follows from the velocity addition rule without introducing any infinities or trying to make sense out of a division by zero.

 

[Nugatory]

OK, thread open again.

 

[Dale]

It's worth noting that dividing the distance I measure you to travel by the time I measure you to take is a quantity called velocity. Dividing the distance I measure you to travel by the time you measure you to take is a quantity called celerity. The two values are always different, and celerity is indeed infinite for light (if one is careful in interpreting "time you measure" in terms of "interval along worldline").

So the OP is talking about something that exists in physics, but it's just a choice between two different quantities and not any kind of duality. Nobody uses celerity for anything that I'm aware of because everything it does velocity also does, and with less potential for mathematical problems.

@johnfrancismurray you should really pay attention to this post above by @Ibix

Physicists use technical words with precise meanings. Speed is one of those words. The speed of light is a very specific quantity and it is finite, not infinite. Saying that it is infinite is simply wrong no matter how many ways you try to say it. In fact, the fact that the invariant speed is finite is the single key difference between relativistic and Newtonian mechanics.

There is a different quantity called celerity which has units of speed, and which goes to infinity as an object's speed goes to $c$ in an inertial frame. There are also related quantities called rapidity which is the hyperbolic angle between two four-velocities and which is infinite if one of those four-velocities is light like.

 

[Mister T]

why can you not accelerate anything to the speed of light Dave? please elaborate.

It's assumed that light travels at speed $c$ for all observers, regardless of their speed relative to the source of the light beam. Therefore, if you were to chase after a light beam in an attempt to catch it, it would recede from you at speed $c$ regardless of your speed. Thus you can never catch up with it, or in other words, you can never travel at speed $c$.

The only question is whether or not that assumption is valid. Many if not most physicists wouldn't accept it when Einstein proposed it almost 120 years ago. But since that time it has been shown to be valid and is an accepted fact of life for thousands of scientists, engineers, and technicians working in hundreds of places all across the globe.

but because time stops at the speed of light

The passage of time is not defined at light speed. That is very different from saying the passage of time is zero at light speed. In the latter case it is being given a definite value, which can't be done for something that's undefined.

 

[A.T.]

Popular science sources, however, are often not satisfied with things that cannot be defined (like $\frac 1 0$) and so they like to make a big deal of it. Instead of saying simply: proper time cannot be defined along a light-like trajectory, ...

Unlike $\frac 1 0$, the proper time interval approaches the finite value of $0$ in the limit of $v \rightarrow c$.

 

[ohwilleke]

The passage of time is not defined at light speed. That is very different from saying the passage of time is zero at light speed. In the latter case it is being given a definite value, which can't be done for something that's undefined.

I'm not convinced that the oversimplification that time is not passing in the reference frame of a particle with no rest mass has no use.

This oversimplification could be useful in understanding why individual photons, gluons, and gravitons do not decay, no matter how much energy they carry.

It could also be useful in a heuristic understanding of why forces carried by bosons with zero rest mass don't experience CP violation, which is equivalent to time symmetry violation. If there is no experience of the passage of time in the reference frame of the massless particle, then it can't have time symmetry violation, ergo the forces it carries can experience CP violation. And, indeed, none of the forces carried by zero rest mass bosons do experience CP violation.

 

[phinds]

Unlike $\frac 1 0$, the proper time interval approaches the finite value of $0$ in the limit of $v \rightarrow c$.

But I assume that you are NOT implying that a photon does experience (zero) time.