Special relativity/speed of light

[geordief]

I am trying to understand this.Would I be correct in saying that the reason that the maximum speed possible is the speed of light solely down to the fact that matter and energy are composed of light? (Does time only come into existence when two particles or events interact with each other?)

If we were to find something more fundamental than light would the maximum allowable speed become a function of that new particle/element ?

As a second point suppose we were living in a medium without direct perception of light (say a blind dolphin without a physics degree) would the speed of sound (it's only sense perhaps) take over the role of the speed of life in this new universe?
Would it be possible to visualise the effects of phenomena at the speed of light be creating a robot with just the one sense and which was able to travel through water at speeds up to the speed of sound?
(I appreciate that sound waves just derive from light but ,if one 's world view is shackled to just that phenomenon would a kind of cousin of special relativity exist in that apparently bounded universe?)

 

[my_wan]

Your description is a bit contorted but subtract the bogus sensory associations and I suspect there is a seed of an idea here.

(Does time only come into existence when two particles or events interact with each other?)

Is is not so simple as you imply below, but I would say that yes an interaction or process rate is needed to define time, and talking about time outside of a process rate is physically absurd. I would not say that time has an existence of its own outside process sequences. From here out you went off the ledge.

If we were to find something more fundamental than light would the maximum allowable speed become a function of that new particle/element ?

My opinion would be no. To change the maximum allowable speed for a mass particle requires changing the processes that defines time. So if you change the processes that define a mass in such a way then it changes the effective clock rates which then entails measurements that erase the effective increase in speed.

This is what Special Relativity does more or less. If you are traveling toward a star 10 light years away at 86% the speed of light then that star is not 10 light years away but 5 light years. From your perspective you did not get their faster than the speed of light because your clocks were slow, you got there faster because the distance was shorter.

As a second point suppose we were living in a medium without direct perception of light (say a blind dolphin without a physics degree) would the speed of sound (it's only sense perhaps) take over the role of the speed of life in this new universe?
Would it be possible to visualise the effects of phenomena at the speed of light be creating a robot with just the one sense and which was able to travel through water at speeds up to the speed of sound?
(I appreciate that sound waves just derive from light but ,if one 's world view is shackled to just that phenomenon would a kind of cousin of special relativity exist in that apparently bounded universe?)

No. What you can and cannot sense has nothing to do with the effective rate of the processes that define time. Changing your sense of time does not change the process rate but changing the local process rate (as in GR) can change your sense of relative time. Changing process rates only changes your measurements to insure that any apparent change in the speed of light is erased by that change in effective clock rates resulting in apparent changes in distance.

This is more fundamental than merely what you can sense. Becoming aware of a faster process does not change the processes that define what you are, or any mass particle is for that matter. Hence it cannot provide a mechanism to escape the limits imposed by the processes that define you. Specially Relativity allows you to get anywhere in as close to the same time you left as you want. It simply says that you cannot get there before your left, and to anybody who stayed back where you left from you took more time than you experienced. Physically it boils down to saying you cannot race yourself and win with or without relativity. No sense of some other process is going to change this.

 

[WannabeNewton]

The speed you are talking about is the currently known cosmic speed limit. It is not special to light; there is a common misconception about this because when Einstein postulated it the only known field that propagated waves was electromagnetism. All massless particles travel at the speed of light and photons happen to be a class of these. So you should take into account that photons are not special in any case when talking about the cosmic speed limit.

 

[Superstring]

If I'm not mistaken, the reason Einstein originally postulated that c is constant for all observers is because this is what is implied if the laws of physics are the same in all inertial reference frames AND Maxwell's equations are correct.

 

[my_wan]

If I'm not mistaken, the reason Einstein originally postulated that c is constant for all observers is because this is what is implied if the laws of physics are the same in all inertial reference frames AND Maxwell's equations are correct.

Yes, in my opinion this is a much better interpretation than a cosmic speed limit. What does speed even mean in a coordinate independent space. The notion of a cosmic speed limit seems to imply some sort of absolute cosmic meaning to distance and time which cannot be exceeded. Which is counter to anything relativity describes.

 

[geordief]

thanks (I am trying to make sense of your replies)
Could I ask , as a separate question, whether this theory (the special relativity one) is forever counter-intuitive ?
Does there come a point when you feel at home with its concepts or are we just hard wired to get a surreal feel from it?
I know that quantum theory is not considered to be something you can ever get your mind around like that but does the same apply to SR?
Would it help me to learn a bit about Maxwell's equations ?

 

[Teinor]

thanks (I am trying to make sense of your replies)
Could I ask , as a separate question, whether this theory (the special relativity one) is forever counter-intuitive ?
Does there come a point when you feel at home with its concepts or are we just hard wired to get a surreal feel from it?
I know that quantum theory is not considered to be something you can ever get your mind around like that but does the same apply to SR?
Would it help me to learn a bit about Maxwell's equations ?

Special relativity is counter intuitive to start with, but I find you can feel pretty comfortable with it. Quantum physics is always surreal because it contains concepts that we can't even start to visualise, like spin, which has no complete analogy in the part of the world we experience. Special relativity is easier to visualise, eg. you can think about a super fast rocket shooting past you looking much smaller, and accept that although you've never gone that fast, those effects do happen and you can imagine them. There are some things that are initially confusing until a couple of years into a physics degree, like the twin paradox, but you do find it the mechanics aspect makes sense and feels fairly physics by the end.

Unless you're a physicist at around undergrad level, I wouldn't recommend looking at Maxwell's equations directly, as covariant electromagnetism is difficult without knowing a lot of maths (4 Vectors etc).
The gist of it is, is that in special relativity we postulate that the laws of physics take the same form in all inertial frames, so if you have eg a fridge magnet on your fridge, if you took it in a rocket, while you're moving at the same speed as it, so it looks stationary to you, the field lines you see if you sprinkle iron filings about are exactly the same. It turns out that if you just add speeds together the way we think of doing in a low velocity case (eg throw a ball in a moving car, speed of ball is speed you threw it plus speed of car) Maxwell's equations come out different and wrong. But if you set the speed of light as a 'speed limit', and start using Lorentz transformations, you get the right answers.

Sorry if this is all a bit garbled, I found Michio Kaku's book Einstein's Cosmos good for starting to get an idea of how it all works, it's popular science so no maths, but give a good description of the ideas, and a nice biography of Einstein. Otherwise if you're reallllly interested, you'll need to get a textbook and learn a lot of maths!

 

[geordief]

thanks a lot.I don't suppose I will ever learn the maths now.Maybe in the next 20 years or so there will be brain plugins for that on the National Health!

 

[my_wan]

If you can accept time dilation as something fairly intuitive then the issue that is intuitively most problematic is often simultaneity.

Suppose we have a space that can travel so close to the speed of light the difference in negligible. Now we have a colony on a planet 10 light years from Earth. On 1/1/2030 you are standing on Earth looking at this planet you know that what you are looking at is 10 years old so what you are looking at is from 1/1/2020. So if you wanted to get there right now you would need to get there on 1/1/2030. Any faster and you would get there before you even left. You jump into the spaceship and take off very near the speed of light. You get there, in a few minutes due to time dilation, and look and sure enough it is 1/1/2030 on the colony. You really did get there in just a few minutes. You also look back at Earth and notice that is still 1/1/2030 on Earth, the same day you left. But if you see the Earth from 10 light years as 1/1/2030 then you know that is from 10 years ago. So Earth is really at 1/1/2040.

So by the definition of simultaneity used neither you or the colony aged any during your trip to the colony, but the Earth aged 10 years. So can you say the effect was "just" your time dilation if the colonies clocks agree with yours even as measured from Earth before you even left on the trip? The colony did not accelerate or take any trip trip that would involve time dilation, yet it still agrees with your clocks both before and after your trip.

So saying that getting there at almost the same time you left was "just" because your clocks slowed down is not really completely valid, since the destination clocks agree with yours both before and after the trip as measured by you on both ends. Only Earth clocks now disagree. The colonies clocks will only disagree to if you go back home the same way. This is why the Star Trek notion that you need to go faster than light to get there at the same time you left, even for the clocks at the destination, is bogus.

It gets even more complicated. The people on the colony watch will see you get on the spaceship in 1/1/2030 and get there a few minutes later. Only when they seen you get on the spaceship they figure that, since it is 10 light years away, you must have gotten on the spaceship in 1/1/2020. Only you did not from your perspective, you got on at 1/1/2030.

So both the Earth and the colony disagrees on when right now is by 10 years, and it is possible to prove that the disagree is both 10 year before and 10 years after now. Depending on which colony sends a ship to prove which one is really before and which one is after now. They are both right.

By getting the physical situation straight you can work out the basics without dealing directly with the math. Then the math will start making more sense to many people once they get the physical picture. One very important thing to note. The sequence of events at a given location, regardless of inertial will never be out of causal order. Any apparent out of order events will only be the result of not seeing it for some time after it occurs. Defining simultaneity is easy for a given inertial observer and is never violated. It is only when you try to define simultaneity for separate points in space that it becomes impossible except relativistically.

This can be understood if you try to ask how many nows exist between now and then since time dilation is real. If time is quantized that means one now for you may have to represent say 1.36 nows for somebody else. How can you have only a part of a now?

I still suggest you learn the math, as it will clean up all the messiness in all the wild scenarios you can imagine. But there is nothing wrong with getting a leg up with some qualitative reasoning if you do not start making claims about how things really are based on some limited qualitative cases you have chosen.

 

[geordief]

thanks
well I have digested (more or less) the first 2 paragraphs of your last post and my aim is too get to the end in one piece.I will report back if I satisfy myself that I have understood what you said.As Shackleton said...

 

[x007bit]

Excuse me, my wan, but at the beginning of your thought experiment we are looking at the other colony (lets call it Hoth ) from 10 light years away which means although we are seeing the planet from 2020. It is already 2030 at Hoth as light took 10 years to reach us. So, I think, if we travel near the speed of light we would be there 10 years later at 2040, which would make seeing a 2030 Earth not paradoxical.

Now, I'm not an expert at physics, so perhaps my opinion is fallacious in some way and that's why there are divergences between our statements. I hope that you can clear up anything weird I've proposed.

 

[Antiphon]

Let's mark the spaceships with the date they take off using a can of paint.

Calenders on Earth and Hoth are synchronized by a light beam sent from the midpoint.

On Earth, on 1/1/2030 you mark your ship and take off. On that date on Hoth, they see Earth's activities at 1/1/2020, ten years before you take off. On Hoth, they will see 1/1/2030 happening on Earth when on Hoth it becomes 1/1/2040. A few minutes later, on 1/1/2040 you will arrive on Hoth with your ship painted with 1/1/2030, and your ship's time reading 1/1/2030.

 

[x007bit]

Ok. Now my brains f****d up. I undestand that from Hoth's frame of reference our trip only took a few minutes, because we started the trip before they saw us. But why would our clock be affected by this. Why wouldn't it say 2040 instead of 2030? I understand that some experiments have shown that clocks moving at different velocities seem to record time differently, but why does it happen?

 

[my_wan]

Excuse me, my wan, but at the beginning of your thought experiment we are looking at the other colony (lets call it Hoth ) from 10 light years away which means although we are seeing the planet from 2020. It is already 2030 at Hoth as light took 10 years to reach us. So, I think, if we travel near the speed of light we would be there 10 years later at 2040, which would make seeing a 2030 Earth not paradoxical.

Now, I'm not an expert at physics, so perhaps my opinion is fallacious in some way and that's why there are divergences between our statements. I hope that you can clear up anything weird I've proposed.

Your right. You see 20 years tick by in your few minutes travel time on the planet you are headed toward and just a few minutes time tick by on the planet you are leaving.

 

[danR]

thanks
well I have digested (more or less) the first 2 paragraphs of your last post and my aim is too get to the end in one piece.I will report back if I satisfy myself that I have understood what you said.As Shackleton said...

I'll try to simplify the whole matter for you. Einstein real reason for saying the speed of light is the limit and it's fixed is because of empirical evidence: experiments showed no matter which way light's speed is measured it's always the same. And people have asked 'why', and Feynman's famous reply was, approximately 'Because that's the way the universe works, and if you don't like it, go find another one.'

The matter has been stated more formally, and as an 'axiom': there is a velocity that everyone agrees on.

What is surreal is that we are used to thinking of single parameters: length, time, 'weight'/mass, etc. as someone the metric centre of the universe, and we can have a standard length, etc. somewhere and everyone agrees on that fixed unit (metre, gram, whatever).

No, the universe doesn't seem bound by that idea. The most fixed thing is a ratio: the distance light travels in a unit of time. This all seems rather slippery.

It would seem scientists have gotten used to it, however, since some standards are based on composite elements, such as light frequency, for example.
________

Would a new particle change everything? I can't answer that. We are so accustomed to relativity, that conjectured particles such as faster-than-light tachyons, are required to have special properties bound by the speed of light anyway. For example, the faster a 'tachyon' goes than the speed of light, the more energy it loses.

_______

Is light-speed analogous to sound-speed? Perhaps. I can't do the math, but if someone calculated, or actually tested, acoustic photography, I wonder if something like the Penrose-Terrell rotation would appear in the 'image': side-on, non-spherical images might appear rotated for objects going at relatively different velocities from the observer/imager, in air or water.

Penrose-Terrell effect of relativity:

http://faraday.physics.utoronto.ca/PVB/Harrison/SpecRel/Flash/ContractInvisible.html

Note: requires Flash.