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Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Mariana on 12/02/2019 10:37:32

Title: Is it possible to achieve relativistic speed?
Post by: Mariana on 12/02/2019 10:37:32
Shafin wants to know:

Is it possible to achieve relativistic speed? Nearly a speed of light in order to travel the deep space?

What do you think?
Title: Re: Is it possible to achieve relativistic speed?
Post by: syhprum on 12/02/2019 11:07:55
Once you get up to really high speeds CMR photons begin to look like gamma rays and the tiniest dust particle carries megatons of energy.
Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 12/02/2019 11:47:58
I don't really know. My point of view is that 'c', still being a 'constant', is observer dependent. There is no 'golden standard' for it. Why is pretty simple. As long as all uniform motions are equivalent then a acceleration can only be defined from your local frame of reference. Doesn't matter what you define as your 'speed' relative something else if there is no 'golden standard' for the speed you presume yourself to have.

So, it's a balance. And it involves a lot more than just a 'speed'.
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What I mean is that you're free to choose any 'anchor' to define that acceleration from.
doesn't matter, 'c' will be 'c,' and in any uniform motion you will have the same (equivalent) access to 'proper time' and 'proper length'.
Title: Re: Is it possible to achieve relativistic speed?
Post by: Halc on 12/02/2019 13:37:48
Is it possible to achieve relativistic speed? Nearly a speed of light in order to travel the deep space?
I am already moving at relativistic speed, no acceleration needed.  Take a muon born in the upper atmosphere.  It is going to decay in only a few μsec and only due to Lorentz contraction is my distance from it so short that I can get there (moving at .995c) to measure it before it decays.

Quote from: yor_on
My point of view is that 'c', still being a 'constant', is observer dependent. There is no 'golden standard' for it.
The observed value is the golden standard.  It is the same for all observers, hence a constant.
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Doesn't matter what you define as your 'speed' relative something else if there is no 'golden standard' for the speed you presume yourself to have.
That speed is also a golden standard.  For every observer, that observer will presume himself to have speed zero, also a constant.
Said observer could presume himself from a different reference frame, but then light speed and everything else changes from the constant values it would otherwise have.
Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 12/02/2019 13:40:59
No :)
Look it up for yourself.
Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 12/02/2019 14:22:59
Sorry Halc, I'm tired. And you're pretty smart too :)

I'll give you a example.

You have a sun. You have three ships at 'relative speeds'. You need three to prove that although relative speeds are equivalent there still is a difference in displacements. Or maybe you just need two using the sun, but I'll stay with three. All three shoot a laser towards that sun, it reflects it by a mirror being at rest with it outside its 'gravitational potential'

What will the three ships measure?
'c' or not?

They do have different speeds.
Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 12/02/2019 14:33:20
What relativity builds on, when it comes to defining a 'commonly shared universe' isn't 'c', although there is where it starts. It's 'proper time' and 'proper length'.
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And the proofs of that is LorentzFitzGerald transformations, and SpaceTime Intervalls. They work, and they will give you the viewpoint of that other frame of reference, translated into its 'proper time' and 'proper length'.
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I am tired, saw that I wrote 'contractions' instead of 'transformations',
Anyway :)
Title: Re: Is it possible to achieve relativistic speed?
Post by: Halc on 12/02/2019 17:54:40
Said observer could presume himself from a different reference frame, but then light speed and everything else changes from the constant values it would otherwise have.
Poorly worded, yes.  In a reference frame in which I am moving, I can have greater than light speed delta-V between myself and something coming at me.  Delta-V is not velocity or speed, so c is still a constant speed.
Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 12/02/2019 18:42:08
Actually, stating that 'c' is observer dependent isn't anything new. Every constant comes from locally made experiments. There exist no 'objectively made experiment' uncoupled from the experimenter that I can think of. What I think could be a possible interpretation is to take 'proper time and length' seriously, as a sort of 'golden standard' for relativity. But that craves one to turn everything around.
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If one do one will have to redefine what uniform motion 'is', and with that one will need to take a new look at accelerations.That's unavoidable from a definition in where proper time and length are equivalent for all uniform motions becoming a 'golden standard'. But I tend to do so at times. Uniform motion seems extremely weird to me.
Title: Re: Is it possible to achieve relativistic speed?
Post by: Halc on 12/02/2019 21:38:36
Proper time and length is about as frame dependent as things get, by their very definitions.
But what else do we have?  The 'radius' of a black hole has no direct meaning (it is not the distance to the center of it), but one can find a meaningful value for it from the proper length of its circumference.
Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 13/02/2019 10:48:41
Yes Halc, from what I call a 'global point of view' they are frame dependent. But you can turn it around and use a local definition. If you do then proper length and time becomes a equivalence for all uniform motions locally defined. That fits 'black box experiments' and how uniform motion behave generally. If 'time' was a 'illusion' you have a third alternative, but that one I don't agree with.
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The problem with using that 'global definition' is that it presumes a outside, from where you can observe those 'frame dependencies' and I doubt that one and so do relativity as far as I've seen. The 'local definition paints' your universe for you on the other tentacle, and that painting is always your reality.
Title: Re: Is it possible to achieve relativistic speed?
Post by: Halc on 13/02/2019 13:01:58
Just now saw this post, directed at me.
I'll give you a example.

You have a sun. You have three ships at 'relative speeds'. You need three to prove that although relative speeds are equivalent there still is a difference in displacements.
You've not told me where these ships are, so while they have relative displacements from each other by definition, I cannot 'prove' that those displacements are any significant number larger than the physical dimensions of the ships.  I assume the ships are not inside each other.

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Or maybe you just need two using the sun, but I'll stay with three. All three shoot a laser towards that sun, it reflects it by a mirror being at rest with it outside its 'gravitational potential'
I don't think a mirror near the sun is outside its gravitational potential.

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What will the three ships measure?
There is a mirror at the surface of a significant gravitational source like a star.  There are multiple observers at the same distance from it, but moving towards, away, and in orbit about it.  They all shoot lasers at the sun.  The one moving away will measure the longest time to get his reflected light because he's moving away so the light has further to go.  It will also be redshifted.
The one moving in will see the reflected light blueshifted, and in the shortest time for the same reasons.
The stationary one will see no shift in the beam.  All of them will measure c when timing their signal, meaning that if they drop a tape measure to the mirror to get the distance to it, the time of the light will be twice that distance over c.
Title: Re: Is it possible to achieve relativistic speed?
Post by: Halc on 13/02/2019 13:09:53
The problem with using that 'global definition' is that it presumes a outside, from where you can observe those 'frame dependencies'
If one is outside, there is no view of the universe at all.  No light escapes to 'outside', so there is no measuring of it at all.  To assume there even is a state to measure presumes some principles that I personally find unlikely to be true since I think cause and effect are confined to light speed.
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The 'local definition paints' your universe for you on the other tentacle, and that painting is always your reality.
That wording corresponds very close to my personal view.
Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 13/02/2019 14:07:28
Halc, it is a simple explanation. What it say is that those ships have different speeds relative each other as they each one do a two mirror experiment being in uniform motion. All three reach a same result, namely 'c'. Doesn't matter that one can prove that they all have different uniform motions.
Title: Re: Is it possible to achieve relativistic speed?
Post by: MikeFontenot on 13/02/2019 18:11:44

Is it possible to achieve relativistic speed? Nearly a speed of light in order to travel the deep space?


Here's some hard data:  Suppose a space traveler accelerates at 1g for 1 year.  1g is a perfectly reasonable acceleration ... powerful cars can do it for 5  or 10 seconds.  At the end of 1 year of 1g acceleration, the space traveler will be going about 0.774c (relative to an inertial observer co-located and initially stationary with the traveler at the starting point) ... i.e., a little more than 77% of the speed of light.  That's fast enough to produce the well-known relativistic effects.

But 1 year is a long time to maintain 1g of acceleration of a spacecraft big and heavy enough to carry (and support) a few human beings.  It is in principle possible, but we don't have the capability of doing that yet.  It would require an enormous amount of energy.  Perhaps it might be possible to carry along that much stored nuclear energy.  Or perhaps enough anti-matter could be taken along.  Or perhaps ways might be found to continuously transmit enough radiant energy from the earth to supply the space craft.  Practical or not, though, it isn't impossible in principle.

Title: Re: Is it possible to achieve relativistic speed?
Post by: yor_on on 15/02/2019 08:55:41
That's true Mike, but for each percent of 'lightspeed' gained from that first relativistic speed you described you will need more power than before, and for the last few millionths of percents you should reach a demand of needing more 'energy' than existing in the universe.
Title: Re: Is it possible to achieve relativistic speed?
Post by: MikeFontenot on 16/02/2019 20:01:23
That's true Mike, but for each percent of 'lightspeed' gained from that first relativistic speed you described you will need more power than before, and for the last few millionths of percents you should reach a demand of needing more 'energy' than existing in the universe.

For each year of 1g acceleration, the same amount of energy is required, no matter what your starting speed is. If you first accelerate for 1 yr at 1g, your speed will be 0.774c (as I already said).  If you then accelerate for an additional year, it will require the same amount of energy as was required for the first year of acceleration, and your speed will then be 0.968c.  It's unlikely that any trip would require a speed faster than 0.97c, and 0.77c would be plenty for most trips.
Title: Re: Is it possible to achieve relativistic speed?
Post by: Janus on 16/02/2019 21:37:34
That's true Mike, but for each percent of 'lightspeed' gained from that first relativistic speed you described you will need more power than before, and for the last few millionths of percents you should reach a demand of needing more 'energy' than existing in the universe.

For each year of 1g acceleration, the same amount of energy is required, no matter what your starting speed is. If you first accelerate for 1 yr at 1g, your speed will be 0.774c (as I already said).  If you then accelerate for an additional year, it will require the same amount of energy as was required for the first year of acceleration, and your speed will then be 0.968c.  It's unlikely that any trip would require a speed faster than 0.97c, and 0.77c would be plenty for most trips.

The big problem is that you aren't just accelerating the payload. you have to accelerate whatever fuel/reaction mass your ship uses.  Even if you had a 100% efficient hydrogen fusion rocket, you would need ~ 5750 kg of hydrogen per kg of dry ship mass to achieve that 0.774c.     And if you want to stop at your target rather than fly by it at 0.774c, it will take ~32,830,839 kg of hydrogen per kg of dry mass.   As the amount of fuel goes up, The amount of mass devoted to just superstructure for the ship goes up and the relative fraction of payload goes down.
Title: Re: Is it possible to achieve relativistic speed?
Post by: MikeFontenot on 16/02/2019 23:35:11
[...]
  Even if you had a 100% efficient hydrogen fusion rocket, you would need ~ 5750 kg of hydrogen per kg of dry ship mass to achieve that 0.774c.     And if you want to stop at your target rather than fly by it at 0.774c, it will take ~32,830,839 kg of hydrogen per kg of dry mass.   As the amount of fuel goes up, The amount of mass devoted to just superstructure for the ship goes up and the relative fraction of payload goes down.

I'm not understanding why it takes more energy to go from 0.774 to zero than it does to go from zero to 0.774.  I can see that for only one acceleration, you aren't having to accelerate the entire fuel mass during the whole acceleration, because the fuel is mostly used up near the end of the acceleration.  Is the effect simply caused by the fact that for two accelerations, you are having to accelerate all the extra fuel which will be needed for the second acceleration during the entire first acceleration?
Title: Re: Is it possible to achieve relativistic speed?
Post by: Janus on 17/02/2019 00:03:18
  Is the effect simply caused by the fact that for two accelerations, you are having to accelerate all the extra fuel which will be needed for the second acceleration during the entire first acceleration?
That's the basic gist of it.  You have to accelerate the fuel/reaction mass that you are going to burn later, and that takes more fuel/reaction mass to start with...   It compounds very quickly.  It doesn't matter whether the later acceleration was to increase your velocity or decrease it.
Title: Re: Is it possible to achieve relativistic speed?
Post by: Halc on 17/02/2019 13:17:14
It's unlikely that any trip would require a speed faster than 0.97c, and 0.77c would be plenty for most trips.
You are trying to get to star "Goomba" which is 1000 light years away.  You have a ship that can accelerate at 1g for 2 years and decelerate for 2 years, and coast in the middle.  You will die when you are 20% of the way there.  You have need to go much faster.  So I keep the juice up for 3 years at each end and get the ship up to .996c.  It still takes 90 years to get there, and I'm not likely to live that long, so I need more.  I keep it up for 4 years each way, achieving .9995c and now it takes ~38 years to get there.  I can still be alive when I arrive, at least if I was young enough when I departed.

If I keep the acceleration on the whole way, I can make the trip in under 13.5 years, getting up to .999998c at the midpoint.

So there very much is reason to go faster than .97c.
Title: Re: Is it possible to achieve relativistic speed?
Post by: MikeFontenot on 17/02/2019 14:12:30

That's the basic gist of it.  You have to accelerate the fuel/reaction mass that you are going to burn later, and that takes more fuel/reaction mass to start with...   It compounds very quickly.  It doesn't matter whether the later acceleration was to increase your velocity or decrease it.

Thanks.  It IS pretty clear that we won't be sending humans to explore our galaxy, or even using unmanned missions to send us back reports.  The value of the twin "paradox" scenarios is really only in what they tell us about the way special relativity works.
Title: Re: Is it possible to achieve relativistic speed?
Post by: PmbPhy on 17/02/2019 18:32:57
Is it possible to achieve relativistic speed? Nearly a speed of light in order to travel the deep space?
Sure. Relativistic speeds are accomplished daily at particle accelerator labs. Also, cosmic rays contain relativistic particles. So you have relativistic particles hitting your body all day long. As far as deep space goes, we'll need shielding to block such harmful rays.