Question regarding relativity and "infinite" speed - WTF?!

[MattusMaximus]

Howdy all,

I am engaged in a discussion on another message board over at the Christian and Atheist forums (you have to sign up to see the thread, sorry), and I'm wrangling with a chap who seems to be pushing some kind of nonsense to me about relativity. This fellow, Mitch McKain, does seem to understand some relativity, but he seems to be explaining things in a really weird way that doesn't make sense to me.

So, I wanted to come check with some of you to see if anyone here can translate just what the hell he's talking about, because from what I know of relativity (though I'm no expert) this just seems like word salad to me...

In the OP he states the following:

The fact is that the speed of light in our universe is an infinite speed. It takes an infinite amount of energy to get to that speed, but at that speed you could go anywere (all the way accross the universe) in no time at all. The problem is that it is space-time you are traveling and you cannot travel space alone, so although you could go the 100,000 light years accross the galaxy in no time at all (with enough energy), you MUST travel 100,000 years into the future at the same time. If you didn't then there would be reversals of causality itself. This is what you get when you try to talk about going faster than an infinite speed -- it means getting to your destination before you even left which just doesn't make any sense at all -- except when you want to make fun SF books and movies.

That first line is what is troubling me. So I challenged him on it, and he responded thusly...

The fact is that the speed of light in our universe is like an infinite speed. No it is not an infinite velocity. It is 299,792,458 m/s. That is indeed a finite number. But nevertheless the physics does mean that this does have a lot of the characteristics that we would expect of a infinite speed.

Can anyone here who has more knowledge of the subject please shed some light on just what the hell he's talking about, or is my initial suspicion correct and he's completely full of crap?

Thanks in advance!

Cheers - MM

 

[Vorpal]

At least some of his comments make almost-sense if you assume that he's just confusing proper speed aka celerity, dx/dτ, with, uh, actual speed, dx/dt.

 

[Pulvinar]

Howdy all,

I am engaged in a discussion on another message board over at the Christian and Atheist forums (you have to sign up to see the thread, sorry), and I'm wrangling with a chap who seems to be pushing some kind of nonsense to me about relativity. This fellow, Mitch McKain, seems to push himself as some kind of physics expert there (he says he "teaches online", whatever that means) and he does seem to understand some relativity, but he seems to be explaining things in a really weird way that doesn't make sense.

So, I wanted to come check with some of you to see if anyone here can translate just what the hell he's talking about, because from what I know of relativity (though I'm no expert) this just seems like word salad to me...

In the OP he states the following:



That first line is what is troubling me. So I challenged him on it, and he responded thusly...



Can anyone here who has more knowledge of the subject please shed some light on just what the hell he's talking about, or is my initial suspicion correct and he's completely full of crap?

Thanks in advance!

Cheers - MM

He's correct in that from the point of view of light, light travels at an infinite speed. A way to see this is to put yourself on the "crest" of a lightwave. You leave on the crest and arrive at your destination still on the same crest. In order for that to happen, in your frame of reference no time at all must have passed-- the light wave didn't have a chance to "wave". Covering a distance in no time requires infinite speed. Not useful of course since traveling along with light is impossible.

 

[MattusMaximus]

At least some of his comments make almost-sense if you assume that he's just confusing proper speed aka celerity, dx/dτ, with, uh, actual speed, dx/dt.

And by proper speed you mean the speed as measured from the frame of reference at rest wrt the observer, yes? And celerity is the speed as measured from another reference frame? Just trying to really pin things down here. Thanks, btw!

 

[Twiler]

Wouldn't an infinite speed imply movement in no time, i.e. instant movement?

So, the speed of light can't be infinite in that sense, or 'light-year' would be meaningless.

 

[MattusMaximus]

He's correct in that from the point of view of light, light travels at an infinite speed. A way to see this is to put yourself on the "crest" of a lightwave. You leave on the crest and arrive at your destination still on the same crest. In order for that to happen, in your frame of reference no time at all must have passed-- the light wave didn't have a chance to "wave". Covering a distance in no time requires infinite speed. Not useful of course since traveling along with light is impossible.

Yes, this is what I was wondering. I suspected that he was getting at this, but he never bothered to mention it in the discussion (or, at least, he didn't clarify it very well).

He also said something else which confused me. He has stated on numerous occasions that within a particular frame of reference that the speed of light doesn't act like a barrier and that there is no limit on how fast one can travel in a particular frame (at least, he made it sound that way). Again, I'm wondering if he really knows what he's talking about and he's explaining it poorly, or if he's just making stuff up.

 

[MattusMaximus]

Wouldn't an infinite speed imply movement in no time, i.e. instant movement?

So, the speed of light can't be infinite in that sense, or 'light-year' would be meaningless.

Yes, but as was pointed out above, this only makes sense in the context of things viewed from a photon's frame of reference.

 

[MattusMaximus]

Incidentally, one other thing he said which tweaked me was that he felt no need to introduce people to the concept of a frame of reference when talking about relativity because they wouldn't necessarily have the mathematics background to understand it all. It seems to me that is a very stupid way to approach the subject, since frames of reference underlie the entire subject, and I know how to teach the concept in a very non-mathematical manner.

This would also make sense (that is, his avoidance of frames of reference) given his apparent refusal to clarify that he seems to be talking about things from the reference frame of photons. Really damn sloppy, if you ask me, especially because he claims to be teaching relativity online.

 

[sol invictus]

This part "the speed of light in our universe is like an infinite speed. No it is not an infinite velocity" is particularly alarming. Speed is defined to be the magnitude of velocity. So how can the speed be infinite, but not the velocity?

Anyway, the speed of light is obviously not infinite, it's c - it takes finite time for light to go from point A to point B.

 

[sol invictus]

He's correct in that from the point of view of light, light travels at an infinite speed. A way to see this is to put yourself on the "crest" of a lightwave. You leave on the crest and arrive at your destination still on the same crest. In order for that to happen, in your frame of reference no time at all must have passed-- the light wave didn't have a chance to "wave". Covering a distance in no time requires infinite speed. Not useful of course since traveling along with light is impossible.

It's true that no time passes - but in the reference frame of the light that's because the distance is Lorentz contracted to zero, not because the speed is infinite.

 

[Vorpal]

And by proper speed you mean the speed as measured from the frame of reference at rest wrt the observer, yes? And celerity is the speed as measured from another reference frame?

It's analogous to proper acceleration, d²x/dτ², which is the acceleration a particle would measure with an accelerometer, but as a speed, it's kind of a bastard offspring, being observer-distance per traveler-time. Physically, it would be momentum per unit mass, dx/dτ = (dt/dτ)(dx/dt) = γv, which does go to infinity as v→c.

Mind, part of his argument is rather incomprehensible to me, so I'm not sure if that's what he's doing or not. Parts of it are sort of right: you can cover any distance in arbitrarily small amount of your time by accelerating sufficiently near c because you'll be time-dilated ("at that speed you could go anywhere in no time at all"), which is suggestive of celerity. Considering it "infinite" is as well. Going above lightspeed/infinite celerity does make you go backwards in time in some inertial frame, too.

But despite rattling off a bunch of statements that make some sense individually, there doesn't seem to be a coherent argument. "The problem is that it is space-time" doesn't actually imply anything (why not Galilean spacetime?). So at best it's a very strange repackaging of relativity rather than a derivation: you can't go faster than lightspeed because it is "'infinite'" but it is "'infinite'" because of relativity.

 

[Pulvinar]

Yes, this is what I was wondering. I suspected that he was getting at this, but he never bothered to mention it in the discussion (or, at least, he didn't clarify it very well).

He also said something else which confused me. He has stated on numerous occasions that within a particular frame of reference that the speed of light doesn't act like a barrier and that there is no limit on how fast one can travel in a particular frame (at least, he made it sound that way). Again, I'm wondering if he really knows what he's talking about and he's explaining it poorly, or if he's just making stuff up.

Well it is true if you're always measuring your own speed. No matter how much you accelerate you never reach a barrier (other than running out of means to accelerate more). Nothing like crossing the sound barrier in air. From other frames you appear to make less and less progress as you approach the speed of light.

 

[mike3]

This part "the speed of light in our universe is like an infinite speed. No it is not an infinite velocity" is particularly alarming. Speed is defined to be the magnitude of velocity. So how can the speed be infinite, but not the velocity?

Anyway, the speed of light is obviously not infinite, it's c - it takes finite time for light to go from point A to point B.

But he said it is "LIKE" an infinite speed, not that it "IS" an infinite speed.

 

[sol invictus]

But he said it is "LIKE" an infinite speed, not that it "IS" an infinite speed.

It's "like" an infinite speed only in that you arrive at a seemingly distant destination in zero time. But as I said, that's not because the speed is large - it's because the distance is small (it gets Lorentz contracted in your rest frame).

The two explanations aren't the same. For instance, objects will get distorted because they are Lorentz contracted in one direction but not in the other two - and that wouldn't happen if it were just a matter of going very fast.

So it's really not at all like infinite speed.

 

[MattusMaximus]

But he said it is "LIKE" an infinite speed, not that it "IS" an infinite speed.

Actually, in the beginning he said it WAS an infinite speed, and he only changed it to "like" an infinite speed after I pinned him down on it. Then he dismissed the difference as being irrelevant.

 

[MattusMaximus]

At least some of his comments make almost-sense if you assume that he's just confusing proper speed aka celerity, dx/dτ, with, uh, actual speed, dx/dt.

Vorpal, here is Mitch's response to what you said...

I am not confused about the difference here in the slightest. I was simply speaking to a different audience who would not know the difference between these things. The question I wonder about is whether maximus knows the difference between these two things since he didn't respond to my questions about proper time which is what that funny looking t or greek letter tau stands for.

When I am talking about the speed of light being like an infinite speed I am certainly talking a speed with respect to proper time and it is interesting to hear that there is a relevant term, "celerity" (no matter how rarely used) defined as gamma times velocity, which rapidly approaches gamma in light units (such as light years per year). This is a quantity that does indeed go to infinity as the velocity approaches the speed of light, and it is in same units as a velocity. In these terms my point would be that celerity would be much more like what many people would think of as a speed because that is the quantity that is relevant to how quickly you can get to a destination as YOU measure time.

Btw folks, Mitch has agreed to come over to this Forum to join in. I'm looking forward to the ensuing discussion

 

[Roboramma]

Actually, in the beginning he said it WAS an infinite speed, and he only changed it to "like" an infinite speed after I pinned him down on it. Then he dismissed the difference as being irrelevant.

It's interesting that he points out the importance of mathematics in understanding relativity. Given that, if the speed were infinite he should be able to derive that mathematically.

He seems to be saying something like this:

I am on the earth and I look off at a distant galaxy, millions of light years away. Given that it's millions of light years away, it should take at least millions of years to get there. But I get in my spaceship, accelerate to extremely close to the speed of light, and somehow, in an arbitrarily short period of time, I've arrived at my destination. Maybe it took me a year or a month or a day, but that's only dependent upon how close to the speed of light I got. Thus, since I can cover any distance in an arbitrarily short period of time by approaching the speed of light, that speed must be infinite.

This completely ignores what actually accounts for that fact: that it's not the speed that is potentially infinite, but the distance that is potentially infinitesimal. And if he were willing to look at the lorentz contraction he would see that.

In other words, when doing his analysis of what's going on he take the distance in one frame (the rest frame before I set out) and the time in another (the frame after I've accelerated) and uses them to compute the velocity (infinite! Or at least, arbitrarily large!). This is completely contrary to relativity: you can use any reference frame, but you can't just switch between them. If you could, you could literally get any value for anything. I could show that the speed is zero.*

*Start out in a reference frame near the speed of light toward the destination and then decelerate to a frame at rest with respect to the journey's origin. The distance begins at zero (or arbitrarily close to it). Use that distance. Now, how long does the journey take? Let's look in my rest frame with respect to the origin: millions of years! Thus, the time it takes to go 0 meters is millions of years. That's 0m/millions of years = 0 m/s.
The moral of the story is that if you switch reference frames midway through your examination, you can end up with any result you choose.

 

[MattusMaximus]

To be fair to Mitch, I think he is taking into account Lorentz contraction, but he seems to keep wanting to cling to this notion of infinite speed. My primary criticism of him all along hinges on his use of terminology, which seems quite confusing.

 

[Vorpal]

But you can formalize it mathematically quite easily. All he seems to be doing is taking the spatial part of the four-velocity and mistakenly calling it "velocity" and its magnitude "speed" (which is really specific momentum). Other than a conflation of vocabulary, the entirety of special relativity for massive particles can obviously be put in those terms, since normalization of four-velocity means this "velocity" uniquely determines the four-velocity.

The point of this and the rest of the argument is still impenetrable to me, though.

 

[MattusMaximus]

Well it is true if you're always measuring your own speed. No matter how much you accelerate you never reach a barrier (other than running out of means to accelerate more). Nothing like crossing the sound barrier in air. From other frames you appear to make less and less progress as you approach the speed of light.

But this seems to imply that within a given frame you can measure your own speed as greater than c, since there is no limit on your speed. I thought that went completely against special relativity. Am I missing something?

 

[dasmiller]

deleted for clarity

 

[MattusMaximus]

You get into dangerous territory any time you say "speed" without adding "with respect to." When you "measure your own speed," what would you be measuring it with respect to?

No matter how much you accelerated, you wouldn't find anything that you could measure as moving faster than c with respect to you (or vice versa).

Got it. I thought as much, but Mitch seems to be implying otherwise in his statements. Again, this has led to much confusion between us, which is one reason why I was hoping to get him to post here, so that others more knowledgeable than I can see what he has to say. Hopefully he'll show up soon.

 

[Modified]

But this seems to imply that within a given frame you can measure your own speed as greater than c, since there is no limit on your speed. I thought that went completely against special relativity. Am I missing something?

Just as before, as long as you use distance measurements from the non-accelerated frame (we'll assume, I guess, that the origin and destination are in the same inertial frame), like, for example, the distance between mile markers on a highway. It's a point that most non-physics-educated people certainly don't understand: that from your point of view you can just keep accelerating at a constant rate forever. I expect most people have the idea as you get "closer to light speed" it becomes more difficult or takes more energy to maintain acceleration, and don't know that "closer to light speed" is a meaningless statement on its own.

 

[WhatRoughBeast]

But this seems to imply that within a given frame you can measure your own speed as greater than c, since there is no limit on your speed. I thought that went completely against special relativity. Am I missing something?

It takes a special blend of mixing things up, but in a sense he's correct.

If you want to travel to Alpha Proxima (4.3 ly away), and you have the ability to get very close to c (that is, you can give yourself an indefinitely large energy), then you can do it in an arbitrarily short time - as measured in the moving frame. Since you know the distance beforehand, and you have a measured (short) travel time, you can then infer an arbitrarily high speed.

The trick is to use the distance from a rest (yeah, yeah. You know what I mean) frame and the time from a moving frame. And that's cheating.

 

[mitchellmckain]

Greetings, my name is Mitchell Mckain. I have a masters in physics from the University of Utah where I worked on a couple of PHD project in theoretical physics but I lost interest in these and decided to pursue a project of my own which was a relativistic spaceflight simulator called relspace that can be found on my homepage which you can find by simply serching for "relspace" since it will not let me post the url here at this time.


In anycase I am now here so you can abuse me person. LOL

The post which maximus is asking about was one addressed to someone with very little mathematical ability who had questions about the possibilities of faster than light travel. My response which is quoted below was not only to make it clear that was no such thing, but that it really doesn't make any sense and that any expectation for such a thing is really misguided anyway. Maximus complains that the terminology is "sloppy". I think that is evitable when you leave the mathematical precision of proper physics to talk with people with no head for mathematics about what its conclusions are. The suggestion that I should use the more precise term celerity is a helpful one if I wanted to broach this subject with a class in physics. But I don't think that even that kind of minimal accuracy was really going to be all that helpful explaining things to the person I was talking to.

Maximus was outraged that I was not sticking to the textbook physics dogma on the subject. My complaint to him is that it is my experience that public at large have some serious misunderstanding with regards to relativity and it was my endeavor in my unorthodox treatment of the subject to dispell some of those misunderstandings.

Perhaps this should be in a different thread.

Traveling faster than the speed of light is not mathematically possible -- not without changing the structure of the entire universe. It is the locally minkowsky structure of space time that makes this not only impossible but nonsensical. Non-physicists think of this as some kind speed limit but it is nothing of the kind. You can go anywhere you like as fast as you like -- it has nothing to do with a speed limitations at all. Traveling near the speed of light warps space to shorten the distance to your destination. That is why you can go anywhere as fast as you like. There is even a measure of speed in special relativity that could be called "warp speed" and it represent the real speed with which you can go places --the lorentz contraction factor gamma.

gamma = 1/square root of ( 1 - v^2/c^2) where v is the usual velocity and c is the speed of light

Travelling at gamma = 10 would very much like traveling 10 times the speed of light as far as getting to your destination is concerned. Your velocity (according to the usual definition) is only about 99.5% of the speed of light and that is how fast you would see your destination approach you. But traveling at that speed warps space-time itself so that your distance is only 1/10 of what it was when you were not traveling at that speed. So at that speed you would travel 10 light years in only slightly more than one year.

The fact is that the speed of light in our universe is like an infinite speed. It takes an infinite amount of energy to get to that speed, but at that speed you could go anywere (all the way accross the universe) in no time at all. The problem is that it is space-time you are traveling and you cannot travel space alone, so although you could go the 100,000 light years accross the galaxy in no time at all (with enough energy), you MUST travel 100,000 years into the future at the same time. If you didn't then there would be reversals of causality itself. This is what you get when you try to talk about going faster than an infinite speed -- it means getting to your destination before you even left which just doesn't make any sense at all -- except when you want to make fun SF books and movies.

The hypothetical mechanism that I was referring to was, of course, the Alcubierre drive. Something that I didn't know was that there was some more recent work on the idea since I was an undergraduate, many relevant citations of which are in the (url reference removed) I wasn't aware of Coule's paper, in particular, which seems pretty conclusive: The physics is meaningful, but the engineering won't fly, since you need an Alcubierre drive to build an Alcubierre drive. It isn't going to happen in this universe.

There ARE logical contradictions involved. Some are even mentioned in the article, such as requiring the existence of something that already violates SR (tachyons).

Unless the beginning and ending (which must be already occupied according to the article) are required to be at rest with respect to each other (refer to these as Alcubierre "rails") then it is easy to show that violations of causality are implied, but I am pretty sure that I can to do this even in that case with a bunch of Alcubierre "rails" in different inertial frames, by transmitting messages between the endpoints of different "rails" that happen to be in proximity of each other at the time. That would demonstrate that this drive is not logically consistent with the over all Minkowsky structure of space-time.

I think that this shows that the claim that time dilation does not apply must actually wrong. There is something fundamentally wrong with that whole idea. Time dilation involved in travel near the speed of light is NOT a local effect and so the idea that some local alteration of the space-time metric can alter this doesn't make any sense.

According to special relativity, points in space-time are seperated by either time-like or space-like distances, and that is only real distinctions between past present and future. Being able to travel faster than the speed of light in a vacuum would mean you can traverse space-like distances and that would imply reversals of causality because there is no order to time over such distances, and thus there would be no logical reason why you could not traverse another space-like distance to get back to where you started from at a time before you even left.

 

[randman]

This part "the speed of light in our universe is like an infinite speed. No it is not an infinite velocity" is particularly alarming. Speed is defined to be the magnitude of velocity. So how can the speed be infinite, but not the velocity?

Anyway, the speed of light is obviously not infinite, it's c - it takes finite time for light to go from point A to point B.

I think it makes sense. What is velocity if you have no time elapsing. How is there then a speed from the perspective of the photon? It is everywhere it will ever be all at the same time from that perspective.

Interesting observation.

 

[Gazpacho]

But this seems to imply that within a given frame you can measure your own speed as greater than c, since there is no limit on your speed.

No. There is no limit on acceleration. You can drop buoy A in space, accelerate to 0.9c relative to it, drop buoy B, and accelerate to 0.9c relative to buoy B. Your speed as seen from to buoy A is then 0.994c. Because speeds do not add linearly in special relativity, you can do this an arbitrary number of times. In this sense (the sense of supposing that speeds add linearly when they really don't) c is an infinite speed.

 

[Aepervius]

Wouldn't an infinite speed imply movement in no time, i.e. instant movement?

So, the speed of light can't be infinite in that sense, or 'light-year' would be meaningless.

From the frame of reference of the photon no time passes.

See it that way : if you go toward light speed as human, time shorten. You can never go up to light speed as it would require infinite energy, but the limit of this is not dilating time to a constant, it asymptotically go to infinitely small. That was the basis of the novel delta tau.

Now transposed to the photon which is always at c speed is the conclusion that the time "does not pass" for it, and no matter where it travel, from the POV of the photon it is always at "infinite" velocity and no time passes.

I am not sure this is really "physical" but that is the conclusion one could come to.

ETA: Getting confused with the chap language myself corrected some stuff.

 

[JJM 777]

Greetings, my name is Mitchell Mckain.

Hello Mitch,

A penny for your thoughts about time dilation and/or time travel. Is time travel possible:
- at all?
- to your own biological PAST (your body becoming younger as you travel)?
- to the PAST of others (without you yourself becoming younger)?
- to your own biological FUTURE (your body becoming older as you travel)?
- to the FUTURE of others (without you yourself becoming older)?

 

[MRC_Hans]

Howdy all,

I am engaged in a discussion on another message board over at the Christian and Atheist forums (you have to sign up to see the thread, sorry), and I'm wrangling with a chap who seems to be pushing some kind of nonsense to me about relativity. This fellow, Mitch McKain, does seem to understand some relativity, but he seems to be explaining things in a really weird way that doesn't make sense to me.

So, I wanted to come check with some of you to see if anyone here can translate just what the hell he's talking about, because from what I know of relativity (though I'm no expert) this just seems like word salad to me...

In the OP he states the following:



That first line is what is troubling me. So I challenged him on it, and he responded thusly...



Can anyone here who has more knowledge of the subject please shed some light on just what the hell he's talking about, or is my initial suspicion correct and he's completely full of crap?

Thanks in advance!

Cheers - MM

He is not right, but he is also not wrong. It is indeed a good point that from the position of the traveller, you can accelerate infinitely, and keep shortening your (subjective) travel time.

Other than that, I suspect he is just trying to seem smarter than he is, by expressing things in a round-about way. I don't notice any sensation or downright absurd claims. It is usually not worth the trouble to engage in word-play with people like that. I suggest simply replying along the lines of "Well I suppose there is nothing wrong with expressing it that way", and then then cut to the chase by asking him what implications he sees from his viewpoint. In other words, you can express things in many ways, but the interesting thing is: How does it impact our perception of reality?

Hans

 

[MRC_Hans]

Greetings, my name is Mitchell Mckain.

Welcome to the lion's den! .... Really our bark is worse than our bite, it's just that we're so many .....

I have a masters in physics from the University of Utah where I worked on a couple of PHD project in theoretical physics but I lost interest in these and decided to pursue a project of my own which was a relativistic spaceflight simulator called relspace that can be found on my homepage which you can find by simply serching for "relspace" since it will not let me post the url here at this time.

That sounds fun, but .... if I may ask, what do you do for a living?

In anycase I am now here so you can abuse me person. LOL

Actually, people rarely get abused here, unless they really ask for it.

The post which maximus is asking about was one addressed to someone with very little mathematical ability who had questions about the possibilities of faster than light travel. My response which is quoted below was not only to make it clear that was no such thing, but that it really doesn't make any sense and that any expectation for such a thing is really misguided anyway. Maximus complains that the terminology is "sloppy". I think that is evitable when you leave the mathematical precision of proper physics to talk with people with no head for mathematics about what its conclusions are. The suggestion that I should use the more precise term celerity is a helpful one if I wanted to broach this subject with a class in physics. But I don't think that even that kind of minimal accuracy was really going to be all that helpful explaining things to the person I was talking to.

Maximus was outraged that I was not sticking to the textbook physics dogma on the subject. My complaint to him is that it is my experience that public at large have some serious misunderstanding with regards to relativity and it was my endeavor in my unorthodox treatment of the subject to dispell some of those misunderstandings.

In short: You simplified it somewhat for the sake of the peasants.

That is not a bad thing, but IMHO, your wordiness betrays your purpose, and lures more knowledgeable persons (relative to the peasants) to hold your explanations to standards they can't honour.

Hans

 

[mitchellmckain]

Hello Mitch,

A penny for your thoughts about time dilation and/or time travel. Is time travel possible:
- to the FUTURE of others (without you yourself becoming older)?

First of all time dilation and time travel (into the past) don't belong in the same sentence here. They have nothing to do with each other. Time dilation may sound to the non-initiate like time travel but it really isn't like that at all. Time dilation is an established fact, orthodox special relativity and like the other issue discussed here completely a consequence of the Minkowsky space-time geometry of our universe.

Time dilation means that the last of the option that you listed does occur. Again this is a measured and established fact both in particle physics and orbital measurements.

Now on to the topic of time travel which I would identify with this option only:

- to the PAST of others (without you yourself becoming younger)?

I know that it is not absolutely excluded by GR (EPR bridge and all that) but the fact is that GR is more of a theoretical framework than can be used to describe all sorts of things that not really physical. Furthermore there are puzzles that are not really nailed down all that well by the mathematics of physics such as times arrow -- though I don't think the problem is insoluable -- in fact I think there are some really substantial clues in quantum physics.

The upshot I guess is that I think that there really is an arrow of time and so my answer is no. I don't believe that this is possible and in much the way that tachyons are used in quantum field theory to rule out theories as non-physical, I think that anything that implies time travel should be considered a logical impossibility.

As for the other two...

- to your own biological PAST (your body becoming younger as you travel)?
- to your own biological FUTURE (your body becoming older as you travel)?

I certainly don't see anything in physics that suggest any possibility of such things.

 

[MattusMaximus]

Hi Mitch. Thanks for coming over here to further the discussion. And welcome to the Forum!

In short: You simplified it somewhat for the sake of the peasants.

That is not a bad thing, but IMHO, your wordiness betrays your purpose, and lures more knowledgeable persons (relative to the peasants) to hold your explanations to standards they can't honour.

Hans

Well said, Hans. Thank you for putting it in a much more succinct and elegant manner than I have put it before now. This has been, essentially, the crux of my criticism regarding what Mitch has been saying all along. I am also concerned that by him taking the shortcuts and being loose with the terminology (as I've complained at length) that he is actually reinforcing various misconceptions about relativity as opposed to overcoming them. But perhaps this is just pendantic nitpicking on my part because I am in the business of overcoming physics misconceptions.

Now I'm off to teach class all morning, but I'll try to drop back in later today to dig through the rest of the thread posts in more detail.

Cheers - MM

 

[Vorpal]

Maximus complains that the terminology is "sloppy". I think that is evitable when you leave the mathematical precision of proper physics to talk with people with no head for mathematics about what its conclusions are. The suggestion that I should use the more precise term celerity is a helpful one if I wanted to broach this subject with a class in physics. But I don't think that even that kind of minimal accuracy was really going to be all that helpful explaining things to the person I was talking to.

In any case, there's some utility in avoiding language that liable to me misunderstood by many already familiar with relativity that happen to pass by, both in regards to them and your intended audience, should they choose to continue study relativity from more orthodox sources. Perhaps if celerity is too highfalutin, how about speediness?

Since you mention Lorentz gamma and length contraction anyway, it doesn't seem all that difficult to define what you mean more precisely. At some point you're going to have to acknowledge that what you're talking about is not the same thing as the speed someone else measures you going by, so using the same terms just invites confusion for everyone involved.

... but I am pretty sure that I can to do this even in that case with a bunch of Alcubierre "rails" in different inertial frames, by transmitting messages between the endpoints of different "rails" that happen to be in proximity of each other at the time. That would demonstrate that this drive is not logically consistent with the over all Minkowsky structure of space-time.

Or rather that arbitrary construction of such drives on a Minkowski background violates causality. Which is worrying, but not quite as strong a conclusion.

I think that this shows that the claim that time dilation does not apply must actually wrong. There is something fundamentally wrong with that whole idea. Time dilation involved in travel near the speed of light is NOT a local effect and so the idea that some local alteration of the space-time metric can alter this doesn't make any sense.

Time dilation of the type present in STR, i.e., between inertial frames, is definitely local, because inertial frames themselves are local. I don't think I understand your objection.

 

[Cuddles]

Well said, Hans. Thank you for putting it in a much more succinct and elegant manner than I have put it before now. This has been, essentially, the crux of my criticism regarding what Mitch has been saying all along. I am also concerned that by him taking the shortcuts and being loose with the terminology (as I've complained at length) that he is actually reinforcing various misconceptions about relativity as opposed to overcoming them. But perhaps this is just pendantic nitpicking on my part because I am in the business of overcoming physics misconceptions.

Have to say, I'm very much with Mitch here. As already noted, no simplification is going to be perfect, but I don't see anything particularly bad about what he's saying. All he's said is that it's impossible to travel faster than light, but if you travel at a relativistic speed then it can seem as though you have since you'll have experienced much less time during your journey than you would expect in a non-relativistic universe. That is, you can travel 10 light years while ageing less than 10 years.

Nothing he's said (at least, nothing that's been quoted here) is in any wrong, and it doesn't even look particularly sloppy to me. It's just an attempt to explain some relativistic quirks to someone who doesn't understand it. I suppose it could be tidied up a little bit. For example:

Traveling near the speed of light warps space to shorten the distance to your destination. That is why you can go anywhere as fast as you like.

would be more accurate if it read:

That is why you can go anywhere in as short a time as you like.

But other than that really quite minor correction, I don't see what the fuss is about.

 

[sol invictus]

Well, there are obviously no rules for what's going to help someone understand something. But to me, talking about infinite speed for light is very misleading (especially when you add that it's not an infinite velocity, which simply doesn't make sense). If you watch a light pulse go by, it definitely moves at finite speed: c. If you watch two light pulses approach head-on, their relative speed is 2c - a bit difficult to understand if you'd been told that the speed of light is "infinite", no?

If you were yourself moving very close to c, you'd see objects approaching you at nearly speed c (again, finite), but the distance to them would be shortened due to length contraction and therefore they'd arrive in a very short time. So in no case are there any infinite speeds involved.

 

[Cuddles]

Well, there are obviously no rules for what's going to help someone understand something. But to me, talking about infinite speed for light is very misleading (especially when you add that it's not an infinite velocity, which simply doesn't make sense). If you watch a light pulse go by, it definitely moves at finite speed: c. If you watch two light pulses approach head-on, their relative speed is 2c - a bit difficult to understand if you'd been told that the speed of light is "infinite", no?

And if you travel 100,000 light years while only experiencing one second, that's a bit difficult to understand if you've been told that nothing can ever travel faster than light, no? And that is exactly the point that mitchellmckain was addressing. Try reading his actual post:

Travelling at gamma = 10 would very much like traveling 10 times the speed of light as far as getting to your destination is concerned. Your velocity (according to the usual definition) is only about 99.5% of the speed of light and that is how fast you would see your destination approach you. But traveling at that speed warps space-time itself so that your distance is only 1/10 of what it was when you were not traveling at that speed. So at that speed you would travel 10 light years in only slightly more than one year.

There's nothing vaguely wrong or misleading about this at the level being discussed. He's simply addressing the point that you can appear to travel faster than light if you don't know the underlying relativity, but that it isn't actually the case because relativity explains it all. He even specifically says it only appears that way because of length contraction. To be honest, I'm astonished none of you seem to have seen things explained this way before, since it's really quite a common and simple way of explaining it.

 

[Roboramma]

To be honest, I'm astonished none of you seem to have seen things explained this way before, since it's really quite a common and simple way of explaining it.

I actually recall a science fiction story (it was in a collection of very old science fiction, from the '30s or '40s maybe) in which the hero sets out to disprove relativity. He gets in his spaceship and starts accelerating, and keeps accelerating at the same rate. His accelerometer shows that his acceleration isn't slowing and as he clocks up his speed (by simple addition) he passes the speed of light. Having measured the distance to his destination star before setting out, he finds that he completes the journey in much less time than would have been required for a ray of light. He then concludes that relativity is wrong.
I don't recall the conclusion of the story (maybe he returns to earth and finds everyone aged?), but anyway he somehow finds out that everything that happened was just as is predicted.

Sorry for a bad memory, it was 7 or 8 years ago that I read it. But anyway, my point is that, at least in as much as this story resembles mitch's post, it's not a particularly new way of looking at things.

 

[Roboramma]

First a note: my first post in this thread was in response to MM's post saying that Mitch was suggesting that it "really was" an infinite speed. Reading his full words in context it's clear that I have at least much less to disagree with than I thought.

That said, the bit that does bother me is simply that looking at it this way makes it seem as though there is a "real" distance between, say, here and the Andromeda galaxy, and that when you accelerate you affect space , but that initial distance is the real one, so when you look at how fast you're going you compare the time it took to that real distance.

That really does seem to be the implication to me, and I think it's very misleading in that it actually distracts from the central idea of relativity.

 

[Vorpal]

To be honest, I'm astonished none of you seem to have seen things explained this way before, since it's really quite a common and simple way of explaining it.

I've never seen it called a speed. Not even once, anywhere, although I have seen relativity presented in much the same terms, "map-distance per traveler-time" (and it's really just part of the standard four-vector formalism anyway). There's a good reason for that: the concepts of inertial frame and observer is absolutely central to special relativity and what he's referring to simply isn't a speed measured by any such observer, so calling it such really is "asking for it."

That said, it's still just a vocabulary issue. That's been clear since even before the context was made known in post #25, which does illuminate the point quite a lot.