My Problems With Relativity

[OK I have to go now as guests are arriving so this may need some editing still. Hopefully it kinda makes sense]

A few of my problems with relativity, both of which might expose my extreme leymanish understanding of relativistic issues! Both problems are about the relatativistic effects of velocity.

I am sure we’ve all heard the example of time dilation due to velocity where a space ship heads away from the earth to alpha centuri at close to the speed of light and, due to its velocity, we’d see time slowing down due to relativity …

My problem with that whole example is this:

To the people on the spaceship, they would be the stationary observers and the earth would be moving away relative to them at close to the speed of light and hence (surely) they’d see time slowing down for us.

a. Is this the case?

b. If so then how can the two simultaneously be true in any traditional layman’s view of what time is?

c. When physicists say ‘time slows down’ for an object are they talking about the observed rate at which time passes for the object rather than whats actually happening to it? i.e. we see things happening more slowly on the object rather than them actually slowing down?

I assume all of the above would be true of the theoretical claim that an object ‘travelling faster than the speed of light’ would travel backwards through time.

Realtive to what? The earth I assume. Surely to any observer on the object would ‘see’ (for want of a better word) the earth relative to them moving faster and so perceive that moving backwards through time.

Is what the physicists are actually describing the fact that we would see what happened to the object in reverse order - i.e. we would see it moving backwards through time? This makes sense to me, for an object approaching the observer at least.

This would be my understanding of the situation, but I’d thought I’d ask for an expert opinion.

As far as I know although we have some physical evidence of relativity due to gravitation (flying clocks around the world etc) there’s none for relativity due to velocity, other than that it correctly predicts what we as ‘stationary’ observers can expect to see (i.e. it correctly predicts what will be observe in certain situations NOT what is actually happening).

Essentially relativity seems to map the universe on to a space which describes what we observe, and thus the models are dictated by or choice of viewing media. We have the conceit of believing all there is to the universe is what we perceive … and I have a real problem with this.

Imagine, if you will, a race that evolved to an enlightened state without knowledge of EM radiation, lets say in the dark seas of Europa.

Without light they no eyes, no perception of EM radiation at all all sensory input comes from sonar. They develop a perfectly good working model of their perceived universe, with relativity based around the speed of sound, perhaps a little earlier in their history than us, given that any perceived distortions due to velocity would exaggerated and more easily measured than those of light.

As a result, europan science correctly predicts that as the velocity of an object approaches the speed of sound the mass of the objects approach enourmous proportions (supersonic travel is well beyond the current capabilities of Europan science).

We, on the other hand, with our extra weapon of observation through light, would see no such thing. Europans science merely adds extra mass to the object so that, at the velocity they perceive it moving at (due to their choice of viewing medium) the object’s mass etc fits their other perceived experimental data.

What their science predicts is what they will observe, not what is actually happening, and the same, I believe is the case with Relativity.

(Not a perfect analogy I know … but you understand my my point I hope).

Surely a better approach for either science (human/europan) to take would be to understand the behaviour of their viewing media, take this in to account when making observations, but then, when they have their observations, do a little more work and create a model of the universe as it actually is.

At the very least physicists should be aware of what they’re doing (i.e. predicting what might be observed) and stop disappearing up their navels, making idiotic predictions about the universe about such things as time travel!

Such an approach would certainly save a lot of red faces on both europa and planet earth… (Note: unable to perceive ‘redness’ as such, traditional Europan scientists would describe such a phenomena as ‘time dilation due to embarrassment’, the speed of sound accelerated by the warmed fluid around the cheeks joke).

Anyhoo comments/answers please. I may be completely wrong in my understanding of this whole field. As I have said I don’t have an in depth knowledge of these issues!

Let’s concentrate on this for a moment rather than the rest of your post.

a. This is the case. Relativity theory says that time is meaningful to you only inside of the one particular reference frame you choose to measure from. But see below because this is only part of the answer.

b. Throw out the layman’s view. That’s the revolution of relativity.

c. Inside any particular reference frame time moves at exactly one second per second for you. This is still true if you’re moving near the speed of light. You can only tell that something unusual has happened if you move outside of your reference frame and attempt to sync up your clock with a clock in a different frame.

The physics folk will be here in a second to tell you how important that is. Basically, it’s the difference between the special theory of relativity and the general theory of relativity. We really want to apply the general theory here because it cuts out a lot of problems.

Say you’re taking a trip to the star Lalande 21185, which has planets orbiting it and is only 8.23 light years away. You accelerate at 1 gravity, to make the calculations smoother, for half the distance and then decelerate at 1 gravity for the second half of the journey so you come to a dead halt at the star.

One gravity acceleration doesn’t sound like much but it brings your speed up to 98% of the speed of light by the halfway point, so all the relativistic effects come into play.

Here’s what you see onboard the ship. It takes you 2.2685 years to get you to the halfway point and another 2.2685 years to get you to Lalande 21185 for a total of 4.537 years. If you turned around and went home you would arrive there after another 4.537 years. Call it 9 years total.

On earth, however, they would have calculated your voyage this way. It takes you 4.9986 years to get to the halfway point and another 4.9986 to get to Leland 21185, and then the same time to return. Call it 20 years.

Now you get out of your ship and compare chronometers. Yours reads 9 years and you are 9 years older in every possible way. The people on earth say that 20 years have gone past and they are 20 years older in every way.

How do you reconcile these two viewpoints? You can’t. There are no layman’s terms to express this difference. It’s purely a function of relativity. Time and space are part of the same whole. They are not constant. Speed is the only constant. The faster you speed, the more time and space must change to keep the equations straight.

The rest of your analogy doesn’t work. Sound is not a universal constant. It has no effect on anything. The speed limit, capital C, the theoretical speed of light in a perfect vacuum, is a constant. That was Einstein’s tremendous insight. Everything stems from that one fact. Everything is relative - except the theoretical speed limit. Time really is affected because of this.

Flying clocks around the world is exactly an example of this. So are the effects of speeding particles in an accelerator up to 99.99+% of the speed of light. The equations predict the outcomes perfectly. There can be no doubt about this.

And nothing can go faster than light. Forget trying to make any sense of faster than light travel. Although the equations permit it in theory there are so many obstacles to describing the effects that’s it would hopelessly confuse any conversation.

  1. You can be aware of time passing, and you can keep clocks of whatever accuracy you like, and rulers, and so forth. You will feel a force, a heaviness, a resting-against-something, when you are being accelerated, like in an elevator. You can feel the heaviness because you are close to a big mass, or because you are changing velocity; in fact at a fundamental level these two are actually identically the same thing. A key element of any relativity, the Einsteinian one included, is that all your experiences work out correctly and nothing ever appears wrong to you. Things only look weird when you compare notes with somebody else who has been moving differently than you have.

  2. The velocity c is a deeply important thing. It is the speed of causes. It is the limiting speed at which any influence can spread. Light and other electromagnetic radiation and the radiation of gravitational field changes happen to propagate at this speed when nothing interferes with them, but c isn’t something about light, it’s something about cause and effect.

** DaveTheJackal**–Welcome to the SDMB! :slight_smile:

DaveTheJackal wrote:

Without light they no eyes, no perception of EM radiation at all all sensory input comes from sonar. They develop a perfectly good working model of their perceived universe, with relativity based around the speed of sound, perhaps a little earlier in their history than us, given that any perceived distortions due to velocity would exaggerated and more easily measured than those of light.
Having no eyes may not be the drawback you think. We can’t directly perceive things like X-rays, cosmic rays, etc. We developed knowledge about them through indrect methods. For instance, infared was discovered (by Newton I think) while working with a prisim. In an attempt to determine the temperature associated with each color (or something like that) he discovered that the highest temp came in a zone next to red where there appeared to be nothing but, “normal” light.
As a result, europan science correctly predicts that as the velocity of an object approaches the speed of sound the mass of the objects approach enourmous proportions (supersonic travel is well beyond the current capabilities of Europan science).
You’re also mistaken about the Europen science. Thier models would have fallen apart when actual measurements didn’t show the expected change in mass. Regardless of what they thought or expected there would have to be some serious questions when the real world didn’t fall into place.

Perhaps i wasn’t explicit enough. My basic premise was the Europans have no concept of EM radiation at all. The fastest (if not only) thing they know is sound.
Jeeze i deliberately didn’t go in to the far reaches of the whole Europan confusion over heat radiation (they’re essentially very very cold blooded so more heat to them means time passes faster. To them temperature and the rate as which time passes are the same thing © ™. Ho hum so much for trying to keep things simple.

I didn’t mention anything about europan science falling apart.

As far as I am concerned it would hold together.

For example… the first time a Europan was shot with an earthen (supersonic) rifle they’d see the bollet going backwards through time. Moving faster than sound the first thing the europan on the receiving end of the bullet would perceive is the impact of the bullet itself. The sound waves from its procession through the ocean would then hit, giving the europan the impression of something moving progressively further away from the point of impact (since the bullet would move ahead of the sound it created the bullet would hit befoe the sound in created say 1 second before impact) … effectively moving backwards in time.

Put another way … what does the europan percieve?
The bullet moves faster than the speed of sound.
First perception: the impact of the bullet.
1 sec later it perceives where the bullet was 1 second before impact (as the sound from that event reaches the Europan - ok maybe slightly more than 1 sec as the sound has to travel through the ocean).
2 secs later it perceives where the bullet was 2 seconds before impact (as the sound from that event reaches the Europan)… further away than where the bullet was 1 second before impact (less sound wave travel time).
3 seconds later it perceives where the bullet was 3 second before impact (as the sound from that event reaches the Europan)… further away than where the bullet was 2 seconds before impact (less sound wave travel time).

Effectively the Europan perceives the bullet moving backwards through time, impact preceded by its position 1 second before impact … then it’s position 2 second before impact etc etc … proving rather than disproving europan relativistic science the bullet is, using Europan relativism moving backwards through time.

That don’t say that they’re right about the physical nature of the universe though. They are describing what they would perceive rather than what’s happening. Sadly, like most modern human scientists, the Europan physicists community have forgotten this is what their equations do.

Europan physics predicts that objects moving faster than the speed of sound are observed to move backwards through time … and that’s just what is observed. However, just like human physicists they’ve forgotten that what they observe isn’t necessarily what’s actually happened.

Europan science would not be right just because they could explain the whole shooting incident with their voodoo relativistic witch doctor theories, just like human relativistic physicists, by any common sense understanding they’d be talking nonsense - even if they’d predict what they’d observe correctly.

More examples and explanations later … atm fighting off the effects of 2 bottles of wine wot ho! l8rs :wink:

Why does this discrepancy occur? I’ve never done much reading on the topic, but you seem to have an excellent grasp on it and write in an especially lucid style. I’ve always wondered about this basic part of the theory.

Yadda yadda yadda.

a. No they haven;t been able to prove anything by flying clocks around the world, how could they? After all, regarding velocity, each clock would be stationary and every other clock moving relative to it would be expected to be slower through time, so (logically) each clock involved in the would be expected to be behind the others… since it and only it would be stationary relative to the others.

Meh they’ve given some evidence for relativity due to gravitation (eg planes flying at different altitudes) but that’s about it.

It’s a scientific oxymoron. There is no (and i doubt there will be) any verifiable scientific proof of relativity due to velocity.

b. I’d rather keep the lay man’s view and remind the physicist that their mathematical short hand of describing what they see on their instruments ain’t necessarily what’s happening.

The revolution your referring to runs under the slogan ‘forget what’s really happening let’s concentrate on what we can observe and what’s easy to formulate’. Einstein’s ‘revolution’. Easy equations do not make reality.

c. again you’re not making sense - to me at least.

Whatever star system astronauts headed out to they’d perceive time slowed down on earth at the same rate as people on earth perceived time slowed for them… since at any one point each would be traveling (relative to the other) just as close to the speed of light as the object of their attentions would observe them to be traveling at.

So if, at any point a return to earth as made, all clocks (barring natural phenomena) would synchronize.

Count me in as an anti revolutionary.

d. you’ve missed the point of the Europans completely, to them sound is a universal limit, i was trying to point out how conceited we are to think we’ve discovered the fastest thing in the universe by giving an example of a science as badly limited (though perhaps more obviously … to us humans at least) as ours.

Anyways - you haven’t dissuaded me from my convictions - much booze - need bed.

Define “simultaneously”. Since time depends on your reference frame, two events which occur at the same time in one reference frame may well occur at different times in some other reference frame. The only situation where you can say that two events unambiguously occur at the same time is when they also occur at the same place. So if two space travellers meet up again and shake hands, they’re going to have to be able to agree, then, on which one is older, but if they don’t ever meet again, each one will say the other one is older.

We have a middling amount of experimental evidence of General Relativity (the relativistic theory of gravity) though not nearly as much as we’d like. But the evidence for Special Relativity (which deals with speeds) is absolutely astounding. Special relativity is the most thoroughly tested theory in all of physics, and possibly in all of science, and it has passed all of its tests with straight A+.

There’s no reason that your hypothetical Europans would develop a theory of relativity based on sound, because the Universe doesn’t work that way. Measurements of space and time are not affected by relative motions comparable to the speed of sound the way they are by the speed of light. It’s not just that light happens to be the fastest thing we know of, it’s that there’s an inherent speed built into the fabric of the Universe, and light just happens to be one of a few things which travels at that speed. Relativity deals with the inherent speed built into the Universe, and works just as well even if there doesn’t happen to be anything which moves at that speed.

Ugh. That’s a much harder bit to explain since it means envisioning space-time.

But the simplest way I can explain it, hopefully not too oversimplified, is to think of distances in space-time as including a spatial component and a time component. A trip at relativistic speeds covers a greater amount of space-time than does the corresponding trip at non-relativistic speeds. (Even if the other clock stays still, it moves through time so there is always a space-time distance.)

The equation for it spells it out:
ds[sup]2[/sup]=c[sup]2[/sup]dt[sup]2[/sup] - dx[sup]2[/sup] - dy[sup]2[/sup] - dz[sup]2[/sup].

ds is the space-time distance between two events, dt, dx, dy, and dx are the four dimensions of space and time that are traveled.

But because the traveling clock and the earth-bound clock wind up in the same spatial location, i.e, the ds is equal is both cases, something has to give. We know that the spatial distances are longer, so the time component must be shorter. You can think of it as time speeding up or as less time elapsing. As far as we can tell, time appears absolutely normal inside any reference frame, so the less time elapsing explanation works better.

This all comes from the notions that there are no absolute reference frames, that the one constant must be a limit on speed, and that both space and time are equivalent dimensions in a four-dimensional space-time. Time cannot be considered something separate outside of space. All stem from Einstein’s totally counterintuitive inspiration.

Dave, if you’re determined to reject the last one hundred years of science there’s nothing anybody can do for you. You don’t grasp even the basic notions of the physics involved, let alone how general relativity works. Your explanations are at the level of creationism, in that you throw out all theory and experimentation and decide to go with anything that feels right to you. If you want to do that and remain ignorant, fine, but please don’t try to argue it in front of actual physicists like Chronos. It’s just insulting to him, even though he will patiently try to educate you. I won’t. Everything you say is just plain wrong. If you want to learn what is right, then people will spend all the time needed to help you. If not, don’t blame anybody but yourself.

No offense, but a layman’s understanding is not an understanding at all.

Here’s the thing. The speed of light is constant in all inertial frames.
If I’m standing still, and you’re running at 6 mph, and I throw a ball at 10 mph in the same direction you’re running, it looks to you like the ball is moving only 4 mph.

But if you’re running at half the speed of light, and I shine a flashlight, the beam of light is moving away from you at the speed of light. Not half the speed of light as you would intuitively expect. All of relativity is derived from that.

Oh, and the “clocks on airplanes” experiment does give evidence for time dilation by velocity. The planes have to start at rest, take off and accelerate while doing so. I don’t know enough about how, but that makes a difference.

I’m not at all rejecting the theory, it correctly predicts what you will see from any particular frame of reference and has been proven to do so, my argument is that it doesn’t tell you much about what’s actually going on.

Going back to your earlier example, with the starship that accelerates to 0.98c.

From the point of view of the astronaut at mid journey, the point of 0 acceleration, he is stationary and it is the earth that is moving away at 0.98c.

He can shine a torch in any direction and the light beams will shoot off relative to him at c. In fact if he shines his torch back at earth he’d observe the earth receding at 98% of the speed of the photons emitted by the torch.

Viewing the earth he’d correctly observe things going slow there, since, relative to him the earth is moving at 0.98c.

As a result, using relativistic predictions the astronaut would expect to see his clock ahead of those on earth upon his return (since the earth is moving a relativistic velocities).

Please note I am not saying that observations from either earth or the spaceship would disprove relativity. They’d back it up 100%.

The ‘problem’ (for me) comes when the spaceship returns to earth, the spacemen and earth observers once again share a common frame of reference - each expecting their clocks to be ahead of the others.

Am I missing the point … does relativity have an explanation for this situation that i’m not aware of …?

I think you’re dismissing Relativity too fast, before you truly grasp the idea here. And that’s the hard part. It’s counter-intuitive with our everyday speeds and experiences. Just like your hypothetical Europans, us humans have limitations. We are not arrogant and we realize this. We may be able to directly detect a VERY small part of the much broader EM spectrum, but we have no experience, and therefore have not evolved the necessary sensations to pick up on the outcomes of relativity. Why? Because we never even come close to relativistic speeds.

You’re also making an obtuse assumption. Just because these Europans can’t directly detect light (only sound), this means absolutely nothing. The speed of sound is not a universal constant. Please be clear on this: Light itself has nothing to do with C (the universal speed limit). It just happens that light is one of the very few things that can travel at that speed, because it is massless. We call it “The Speed of Light” only for convenience. We could just as well call it “The Speed of Gravity”. The Europans surely have noticed gravity, haven’t they?

Other than the fact that in their neutral buoyancy environment makes gravity pretty much irrelevant for them, forget the Europans for now, they were meant to be an abstract (if imperfect) example of why modeling the universe based on your own personal frame of reference is not always a good idea… and the sort of nonsensical results/observations this can lead to.

This confused me as well for a long time. The answer is that for there can be a difference between the two frames of reference if one is accelerating. For example, a oerson in an elevator can feel the effects of acceleration at the begioning and end of their trip. That’s how they can tell that they, and not the building are the one moving.

I hope I’m understanding you right, but the whole key to special relativity is the idea that no matter how fast you are (or are not) accelerating, of you turned on a flashlight, it will always appear to you to be moving away from you at 100% the speed of light. Everything is relative to that. If you were traveling at 99% C (relative to the earth), and shone a flashlight out of your ship in ANY direction, it would STILL recede from you at 100% C. You might expect that if you shone the light in the direction of your movement (since you’re going 99% C) that the light would appear to be moving away from you at 1% C. Nope. So there’s the crux of the whole thing right there. It’s still moving at C relative to you.

The point of science is to create a hypothesis. You make some observations and come up with a hypothesis that might explain what it is you’re seeing. Now you need some evidence to test your hypothesis so that it may become a theory. So you create a way, an experiment, to test your ideas (which are likewise based on your keen observations). After you perform some experiments, you obtain data and hopefully are able to extract some evidence from this data. If it supports you hypothesis, GREAT! You now have a theory. Now you need other people to test your theory in their own way. The results you consider evidence for your theory HAVE to be reproducible by everyone else. There are no ifs, ands or buts about this. The more evidence that is accrued in this fashion supporting your theory, the harder it will be to refute. If you doubt relativity, you have to come up with observations that fit with the mountain of reproducible evidence that has been observed over the last 100+ years, develop your own hypothesis, and start experimenting. Otherwise… you got nothing.

It might be time better spent to learn the theory (and physics in general) you’re trying to refute (even if only in part), before you start hypothesizing. There are real-live physicists here that have extensive experience, training and understanding. Keep an open mind and learn from them.

That doesn’t hold water surely and why in my previous example I deliberately used the example of the astronaut at the point of 0 acceleration.

Remember the earth is moving at one helluva rate to start with - relative to the ‘centre’ of the universe at least.

If the spaceship moves against the earths movement it’s infact ‘decelerating’ for much of its journey.

Anyhoo such considerations are irrelevant as relativity (wisely) does away such fixed points of reference and correctly predicts what, from your own frame of reference, will be observed (as I have said before, this I do not dispute at all).

Whether it explains what is actually happening in the universe is another question!

There is no center of the universe. This is why this is so confusing to you. You need a better understanding of some of the basics of physics and space-time before you can really grasp this subject, me thinks.

Please re-read my original post. You’re missing the point completely.

I stated that relative to the spaceman light shines at the same velocity in all directions. I do not at all dispute that any of relativity predicts about what you can observe and measure using relativity what I am saying is that all it does is predict what, under certain circumstances, you will observe and measure.

You still have not even attempted to answer the central question i posed - who would have made the correct prediction of which clock would be faster - the astronaut from his frme of reference or the people on earth from theirs - relative to each’s frame of reference the other has been moving at relativistic velocities.

Isn’t that what I just said?