# How do you define speed in space?

Discussion in 'Hangar Talk' started by cowman, Jan 22, 2020.

1. ### Brad WPre-takeoff checklist

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yes, unless the travel speed is very slow...in which case I'd think meters per second, kilometers per hour, etc...
and I have no idea why metric seems to make more sense to me in this context...but it just seems better than miles per hour

2. ### FastEddieBFinal Approach

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Because we have 10 fingers?

3. ### RavioliFinal Approach

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In the late 80's we called our cell phones HHPC's. (Hand held personal communicators)

It didn't catch on. ;(

4. ### Half FastEn-Route

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But if the story's aliens only have 8 digits, their number system might be base eight instead of base ten. A clever writer can create everything from humorous confusion to world-destroying catastrophes around a few simple base mistakes or unit errors.

5. ### FastEddieBFinal Approach

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I’ve always thought that a system in base 12 would be a lot more elegant.

6. ### PalmpilotTouchdown! Greaser!

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It seems to me that the concept of speed has no meaning unless it's relative to some other object.

7. ### PalmpilotTouchdown! Greaser!

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Here's a question: Suppose you have three objects, labeled A, B, and C, all traveling along the same straight line. B is moving away from A at 90% of the speed of light, and C is moving away from B at 90% of the speed of light. So does that mean that C is moving relative to A at 90% + 90% = 180% of the speed of light, which is impossible?

I expect that the equations of the theory of special relativity have a solution to this conundrum, but I haven't gotten around to doing the math.

8. ### FastEddieBFinal Approach

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Thanks, Einstein!

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9. ### Juliet HotelPattern Altitude

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Are atoms not objects?

10. ### FastEddieBFinal Approach

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The math is WAY beyond me, but the implication of that math is if an object were to be accelerated to the speed of light, it’s mass would become infinite and time would stop. Which, of course, would present its own problems.

11. ### MooneyDriver78En-Route

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Mass doesn’t change, that’s a common misconception, it’s momentum that approaches infinity.
It’s thought that worm holes or invisible (to us) dimensions can be used to travel faster by taking a short cut.
Star Trek star blurring gives an indication of speed, they also use increase engine sounds.

Tom

12. ### FastEddieBFinal Approach

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13. ### PalmpilotTouchdown! Greaser!

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Just call me Captain Obvious!

14. ### PalmpilotTouchdown! Greaser!

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Define "stop"!

15. ### SoonerAviatorFinal ApproachPoA Supporter

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No need for being creative, just pretend we sent up a Mars rover . . .

16. ### PalmpilotTouchdown! Greaser!

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17. ### kathCleared for Takeoff

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They do, indeed. (And this is exactly the kind of question that Einstein daydreamed about at the patent office.)
First, throw out the idea that "Vac = Vab + Vbc" for a situation like this. It works down here at pokey slow speeds and for Physics I, but not when things get relativistic.

When you take into account how both space and time get all mixed together in the theory, the new formula is:
Vac = (Vab + Vbc)/(1 + Vab*Vac/c^2)
...so in your example, where Vab = 0.9c and Vbc = 0.9c, the answer is C is moving relative to A at 0.9945c, or 99.45% of the speed of light.

You can play with the formula and see that there is NO WAY you can get something to break the rules (go past "c"), if you start with V's that obey the rules.

Edit: you can also notice that as the two V's approach c, the answer does too. And that if the two V's are small compared to c, you get the old Physics I formula back.

http://www.math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html

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18. ### MooneyDriver78En-Route

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Newtons equation for gravity F=GmM/r^2 doesn’t change because of relativistic speeds. So, no such thing as relativistic mass.

Tom

19. ### MasonPattern Altitude

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If you're driving at the speed of light and turn on your headlights, how far ahead can you see?

20. ### kathCleared for Takeoff

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The "onboard clock" of an object traveling at the speed of light, would appear to be stopped or "frozen", with time not passing at all.
From the point of view of the object itself, its clock would seem to be running fine, but the rest of the universe would look like it was stopped.

Happens all the time. ...To photons, which are massless particles so they're the only ones who can get away with it, and not get into "infinite relativistic mass" troubles.
If a photon were carrying a clock, it would appear to be stopped.
If you were a photon, you'd feel normal but you could cross the whole universe in zero time because of length contraction.

To the edge of the universe, because it will be contracted to zero size and the light (traveling at c) will cross it in no time.
But only if you look straight ahead.

This is, absolutely, my favorite lecture of the whole year of Physics I...

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21. ### MooneyDriver78En-Route

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22. ### smvPre-takeoff checklist

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Your light buckets would fill with light until they fill up and flow over. The light flowing over the edges would set fire to the plastic surrounding the light assemby and you would crash in a blaze of glory.

23. ### Sac ArrowTouchdown! Greaser!

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Could you bottle the light and reuse it?

24. ### DaleBEn-Route

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Well... you'd FEEL like you'd gotten there in no tme, but then you'd get where you were going and find that billions of years had passed, right? Like "hypersleep" in the movies, but without the yawn and the hangover.

Not that you'd care, since you're a photon.

25. ### smvPre-takeoff checklist

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If your bottling equipment is fast enough, yes... That is how they make light bulbs.

26. ### MatthewTouchdown! Greaser!

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It needs to be a dark bottle.

27. ### Sac ArrowTouchdown! Greaser!

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But, if it is holding light, that would, by definition, make it a light bottle.

28. ### MatthewTouchdown! Greaser!

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You'd have to weigh it before and after to know how much is in it. Once you open it, you let it all out. Unless it's clear, then it's already gone. Because science.

29. ### write-stuffEn-Route

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It's all relative, right? If there is nothing to compare your position to, you are motionless. This from someone who doesn't know his mass from his energy.

30. ### MauleSkinnerFinal Approach

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Doesn’t matter what your speed measurement reference is...if it can’t measure the acceleration or deceleration due to farting, it’s not going to be believable.

31. ### Sac ArrowTouchdown! Greaser!

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Is there such a thing as absolute speed? We all know what relative speed is. If the entire universe was travelling all together as a unit at the speed of light, would that make the whole mass-time thing different than if it was standing still?

32. ### PalmpilotTouchdown! Greaser!

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In the aftermath of the experimental confirmations of the theory of relativity, I don't think anyone has come up with a definition of absolute speed that would make it observable or measurable.

33. ### Larry in TNCleared for Takeoff

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This video demonstrates the principal in as easy to understand way as I've ever seen.

That doesn't mean that I actually understand it, but it makes sense as I'm watching the video.

34. ### deonbPattern AltitudePoA Supporter

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Base 8 or 16 is much simpler.

Humans can instinctively do multiplication but not division. The reason for that is base 10 is horrible for division - if I ask you to slice a pizza into 10 equal pieces it will lead to amusing results without pre-measurement. Slice it into 8 (or 16) though and it’s easy - just divide it in 2 three times.

Humans can naturally judge balance and can divide most things in 2 equal parts. So any numeric system that is a power of 2 would mean even a 4 year old would be able to understand division. Having more intuitive access to math available at the time your brain is developing comprehension skill would be a good thing.

It also makes logarithms easier - you can rewrite base 16 very easily into base 8, 4 or 2. Base 10 can only easily be rewritten into base 5, which is not an interesting base mathematically.

35. ### SaltyFinal ApproachPoA Supporter

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I call bs.
Calculate 0x24 / 0x0C without a calculator and without converting to decimal first. I can easily do it in decimal. Even simple multiplication in hex is hard, only because we didn’t grow up memorizing the tables. 0x0A * 0x0C = ?, Childs play in decimal.

Ask someone to divide a pizza into 0x0A pieces and it’s the exact same difficulty as your example.

36. ### DaleBEn-Route

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Only because, I think, we haven't thought in hex since early childhood. If we'd grown up learning to count 0-10 in hex instead of 0-10 in decimal, I think we'd be just as comfortable with that system -- except that we don't have fingers for A-F.

37. ### SaltyFinal ApproachPoA Supporter

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Doesn't change my point. Dividing a pizza by 0x0A is no easier than dividing it by decimal 10. children wouldn't inherently understand division better. Sure, division of numbers evenly divisible by the base would be easier, but that's universally true, including base 10.

38. ### MauleSkinnerFinal Approach

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Using the pizza example, then, 1 divided by 2 would be 2.

I see it working for fractions but not division.

39. ### DaleBEn-Route

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But that is my point. You can calculate 30/10 in your head instantly, simply because of the decimal notation. You can do the same thing in hex... 0x30/0x10 yields the same result and is just as easy to do. 12/5 in decimal is no easier than, say, 0x0c/0x05. Just as dividing a pizza into 10 slices requires the exact same amount of work whether you call it decimal 10 or hex 0x0A.

I'm not disagreeing with your argument that doing division in hex is no easier than doing division in decimal. Well... actually it might be, since there are more cases where /2 works out to a whole number. But my point was that this statement:
is only true because, as you correctly noted, we didn't grow up learning a hexadecimal numbering system. If we had, it would be just as easy for us to do as decimal is for us now.

Last edited: Jan 24, 2020
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40. ### MatthewTouchdown! Greaser!

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But the average person has less than 10 fingers...