is relativity logically necessary?

[aa]

hi everyone

 

I don't have an aptitude for physics so I can't answer this for myself but I was curious about something--

 

Is relativity logically necessary? I mean, would it be possible for a God to create a universe with just Newtonian, and not relativistic, physics? If not, why? And if so, in what way would it be different from this universe?

 

thanks

[big314mp]

hi everyone

1) Is relativity logically necessary?

 

2) I mean, would it be possible for a God to create a universe with just Newtonian, and not relativistic, physics? If not, why?

 

3) And if so, in what way would it be different from this universe?

thanks

 

I broke this down into numbers to make it easier for me to answer.

 

1) This question doesn't mean much, as relativity is observed in experiments. It is far more than just a logical convenience. For example, GPS satellites must take into account that their clocks run slightly slower than ours, in order to produce accurate locations.

 

2) Assuming your god was omnipotent, then anything is possible.

 

3) See above.

[Severian]

Is relativity logically necessary? I mean, would it be possible for a God to create a universe with just Newtonian, and not relativistic, physics?

 

That is actually a very difficult question to answer. I think you could make a Newtonian universe, but whether or not this is complex enough to maintain stars, planets and ultimately life, is not clear.

 

The vast majority of fundamental phenomena have relativity built into them, and I am fairly sure that many of the mechanisms that are essential for particular necessary phenomena wouldn't work without relativity. However, perhaps one could think up analogous Newtonian mechanisms to do the same jobs?

[Pete]

Is relativity logically necessary? I mean, would it be possible for a God to create a universe with just Newtonian, and not relativistic, physics? If not, why? And if so, in what way would it be different from this universe?

I find myself in agreement with big314mp to the extent that the question could have been stated a bit better. Something is said to be necessary when it is of an inevitable nature or when it is logically unavoidable, that cannot be denied without a contradiction or determined or compulsory, i.e. required. One must first refine more of what property of releaitivity one is speakling about before you can determine if it meets the requirement that is being sought after. For example: all scientific principles must be infallible, i.e. all reasonable compliance to a theory must be born out in experiments. One might as of a theory "Has it been proven to be valid under any and all possible circumstances. Since it would take an infinite amount of time, money and reseources to be completely tested then I believe there is a practical limit where the wise physicist would not demand that all possible experiments be carried out. Muff said.

[gcol]

If relativity, like God (I was not the first in this thread to mention it) did not exist, man would surely find it convenient to invent it, and indeed he has done both. Where would maths be with nothing finite, no limits, merely an infinity of infinities? why, we would even find it convenient to invent extra dimensions to tidy things up a bit. .....oh dear done that too. It seems to work so far, in the universe we know, as far as we know.

 

I would love to live long enough to hear someone say "Well, it was ok while it lasted, but now we know it is old hat. Say thank you Mr. Einstein but it is time to move on"

[swansont]

If relativity, like God (I was not the first in this thread to mention it) did not exist, man would surely find it convenient to invent it, and indeed he has done both. Where would maths be with nothing finite, no limits, merely an infinity of infinities? why, we would even find it convenient to invent extra dimensions to tidy things up a bit. .....oh dear done that too. It seems to work so far, in the universe we know, as far as we know.

 

I would love to live long enough to hear someone say "Well, it was ok while it lasted, but now we know it is old hat. Say thank you Mr. Einstein but it is time to move on"

 

Relativity is an observed effect, rather than being invented. Some theory may supersede it someday, but the effects we have documented will be included in the new theory; consider that we still find Newtonian gravity useful, even though it is "old hat."

 

I think that Severian is right in that the implications of not having relativity are way too involved to know the answer of designing a universe. It's all interconnected.

[Mr Skeptic]

Without general relativity we wouldn't have any idea how to make a universe. I think it would be better to rephrase your question so as to not involve God, as that will just result in trouble. I don't think that relativity is a logical necessity, but other aspects of physics would also have to be changed if there were no relativity. It could result in things becoming more complicated, which may be the opposite of what you wanted.

[ajb]

One thing that springs to my mind is spin. Spin is really a relativistic "thing" rather than a quantum mechanical thing. In non-relativistic quantum mechanics spin is just "bolted on" to the Hilbert space.

[aa]

Thanks for your replies.

 

Wow! This gets way beyond me..."spin" is just "bolted on" to the "Hilbert space"...! Don't have a clue what that's all about.

 

Anyway, the consensus seems to be--

No, relativity is not logically necessary, at least as far as anyone can tell.

 

I guess I was thinking that if you can think through the implications of relativity in this universe, then you could also do the same for the opposite case--the case of a universe that is exactly the same as this one--except--without the principle of relativity.

 

But I guess you can't do that, it would be too difficult.

 

I gather that what you can say is just that without relativity, the universe would be very different from how it is.

[Pete]

Thanks for your replies.

 

Wow! This gets way beyond me..."spin" is just "bolted on" to the "Hilbert space"...! Don't have a clue what that's all about.

Same here. I guess he means that in non-rel qm spin is added ad hoc where perhaps in rel qm it can be derived?

[iNow]

...

Edited November 7, 2008 by iNow
Answered my own question

[Severian]

Same here. I guess he means that in non-rel qm spin is added ad hoc where perhaps in rel qm it can be derived?

 

When you consider a quantum mechanical wave equation, usually people think of Schroedinger's Equation. However, that is non-relativistic - it doesn't take relativity into account.

 

You can try an construct an equivalent equation which includes relativity in two ways. The first way gives you something called the Klein-Gordon equation, which we now know describes particles like the photon. The second way gives you the Dirac Equation, which we now know describes particles like the electron.

 

The neat thing is that if you just work through the maths, you discover that any particle described by the Dirac Equation must have an intrinsic angular momentum we now call "spin". This is a prediction of the equation.

 

So the spin of an electron is a prediction of making its equation work with relativity.

 

Of course, you can just give a particle an intrinsic angular momentum in a non-relativistic framework, but that is 'ad hoc', and not a prediction.

[ajb]

Same here. I guess he means that in non-rel qm spin is added ad hoc where perhaps in rel qm it can be derived?

 

Yes, exactly as Severian states.

[pioneer]

Is relativity logically necessary? I mean, would it be possible for a God to create a universe with just Newtonian, and not relativistic, physics? If not, why? And if so, in what way would it be different from this universe?

 

The answer is, based on our current understanding, it is easier to explain the bible version with relativity. I am only playing the devil's advocate and am not claiming one way or the other. Follow the logic to see why relativity is important.

 

With relativity we can get time dilation. If I was on space-ship moving near C my clock would slow. To use numbers, one day in my life on the ship might take 2 millions years on earth. If I was looking out my window on the ship, at a new planet that was about to form humans from apes, the entire 2 millions years of evolving from apes to their modern version of humans, would appear to happen in 1 day in my reference. If I filmed this to show others, it would look like time lapse photography.

 

If we then stopped the space ship, and showed the advanced humanoids on that planet, my little time lapse movie, and they knew nothing of relativity, they would all freak out. They would say, this is a trick. We could not have evolved in 1 day, like the movie shows. I would be called crazy or a trickster, even if I used great care to film under the highest standards of science. But once you explain relativity then it would all sort of make sense.

 

Using math and relativity, once could simulate genesis time scales by where we place our reference. If we start our reference at C and slow down just slightly below C we can watch 10 billions of years go by in one of our spaceship days. At C--, we may only see 4-5 billions years as the solar system snaps into form. At C---, now we may only see 1 billions years, as the earth goes from barren to full of life, etc. If we assume relativity and use the right reference one can simulate the referenced needed at each step.

Edited November 8, 2008 by pioneer

[mooeypoo]

Excuse me, but the bible version states that the world went "POOF" into existence. You can't explain that with either theory.

 

If you are refering to explaining the universe (however you believe it was created) by Newtonian physics alone, the answer is absolutely not.

 

Relativity is more than necessary, it is inevitable. We have holes in our understanding of the universe without it.

 

As I wrote in another thread, if you want a relatively simple example for this, you can read up on Mercury and "The missing planet" - Vulcan.

 

The story in a nutshell:

Before Einstein (way before), we began calculating the orbits of the planets using Newtonian principles. We managed quite well - impressively well, even, but there was one major problem: Mercury just didn't fit.

 

In other words, the orbit we observed was about 43 seconds longer than the calculations predicted. The calculations that DID work for other planets. They didn't work for Mercury. So scientists began thinking - perhaps there's another planet in a closer orbit to the sun. Its gravitation would 'shift' Mercury off its 'predicted' orbit and it would explain the missing 43 seconds.

 

They called this planet "Vulcan".

 

And they looked for it. A while. A long while, even. But it was nowhere to be found. Today we know there's no such planet. But we don't need it anymore. Newtonian physics explains many things, but it came short on this one.

 

Until Einstein came along. General Relativity doesn't just explain the behaviour of Mercury better than Newtonian physics, it accurately predicts its motion, and best of all - calculations based on General Relativity yield *precisely* the missing 43 seconds.

 

Brilliant.

 

In one awesome theory we managed to explain the missing time and "weird" behaviour of Mercury.

 

If you are not comfortable with all the other subjects that Relativity is crucial to, I think this is one story that shows why it's so critically important, and one of the simpler ways of seeing that we can't really manage without it.

 

~moo

[Pete]

The neat thing is that if you just work through the maths, you discover that any particle described by the Dirac Equation must have an intrinsic angular momentum we now call "spin". This is a prediction of the equation.

Are you saying that all particles must have a non-zero intrinsic spin?

[insane_alien]

it can have an intrinsic angular momentum of zero. you were the one who added in that limit from who knows where.

[Pete]

it can have an intrinsic angular momentum of zero.

Not according to Severian.

you were the one who added in that limit from who knows where.

I did what? Where?

[insane_alien]

severian said they have intrinisic angular momentum, he did NOT say that the angular momentum had to be non-zero. the higgs boson is a spin zero particle(if it exists which is probable)

[ajb]

The Dirac equation describes spin-1/2 fermions. No way by itself can you say anything about other "types" of states/particles/fields.

 

Anyway, what I was thinking of really was the fact that fields are classified by the (irreducible) representation of the Poincare group they form, which is labelled by mass, spin and helicity. The observed fact that we have bosons and fermions in nature is really inherently a relativistic thing.

 

Spin-1/2 representations are important as you can build other representations via tensor products.

 

(Of course you have spin reps of O(N) also, so spinors are not "special" to the Lorentz group.)

[Pete]

severian said they have intrinisic angular momentum, he did NOT say that the angular momentum had to be non-zero. the higgs boson is a spin zero particle(if it exists which is probable)

So who was talking about angular momentum? Not I. I was talking about intrinsic angular momentum, i.e. spin.

[insane_alien]

thats what i was talking about. hence my use of the word intrinsic.

[Severian]

Are you saying that all particles must have a non-zero intrinsic spin?

 

No. Spinless particles are bosons, so described by the Klein-Gordon Equation, not the Dirac equation.

 

Having said that, we have never observed a fundamental (ie. non-composite) spinless object.

[Pete]

thats what i was talking about. hence my use of the word intrinsic.

I'm merely trying to figure out what you were referring to when you said you were the one who added in that limit from who knows where. since I never brought up anything other than intrinsic spin. But you claimed here that I added something. Please show me what I said which led you to this conclusion. Thanks.

No. Spinless particles are bosons' date=' so described by the Klein-Gordon Equation, not the Dirac equation.

[/quote']

Ah! Interesting. I was not aware of that. Thanks! So you have to know whether the particle is a boson or a fermion before you even decide what equation to use, huh?

[Severian]

So you have to know whether the particle is a boson or a fermion before you even decide what equation to use, huh?

 

No - not really. They are just different representations of the symmetry groups which you use to define your theory.

 

In fact, in supersymmetric theories, the fermions and bosons are just different projections onto orthogonal directions.