Spacecraft to travel at speed of light

[Snark]

55 minutes ago, Green Baron said:

But the ship observer cannot see (but knows because he follows our forum) that time on earth passes 10 times faster than time on board (it lies behind). He cannot see because a message sent to him from earth is heftily red shifted.

Well, that aspect of things is symmetric.  Messages he receives from Earth are red-shifted... but messages that Earth receives from the ship are red-shifted, too.  While he's in flight, neither one has time "passing faster" than the other, as far as either of them are able to observe:  each one of them feels normal, and each one sees the other as having time slowed down.

Also, let's be careful not to muddy the waters with red-shift issues-- yes, they're a big deal, but they're also not the same thing as the Lorentz dilation.  He could be coming from a place one light-year away from us in one direction, and headed to a place one light year from us in the other direction.  We'd see him blue-shifted as he approaches us, no Doppler shift at all as he passes us, and red-shifted as he recedes from us.  He'd observe the same thing, too.

[mikegarrison]

20 hours ago, DDE said:

It’s an oft-repeated, annoying myth that sci-fi has any more predictive power greater than astrology does.

It's not the "predictive power" of technology that is interesting. The true predictive power of science fiction is predicting how people might react to various technologies.

For instance, in June 1970, James H. Schmitz wrote a story where one of his characters overhears something and wants to know more about it, so she sits down at a comm-net terminal and does a keyword search. Bingo, stuff starts appearing on the screen. Sound familiar? He wasn't exactly predicting Google, but he was predicting that if people have an interconnected data network, they will use it to search for things. He also predicted that people would use that same network to send each other messages, to play online games with each other, and to act as their alarm clock and general appointment calendar. She also routinely does comm-net searches on herself to find out what information other people might be able to learn about her from the net.

He didn't predict the internet, really, but he asked himself, "If there was this comm-net that everyone had access to, how would people use it?"

Edited January 1, 2019 by mikegarrison

[Green Baron]

Thousands of predictions were and are made. And some of them really resemble things from today's reality. But otoh there is no guarantee that a prediction may have a manifestation in the future. It may be best to enjoy them on a fictional basis, until they eventually come into being.

To the ship flying of at ludicrous speed: there is no way to tell (if there is no angular movement involved) if it comes or goes but a red or blueshift in its transmissions, provided the recipient knows the "original" wavelength or frequency, right ?. I didn't mean to "muddify" anything, just point that out. Doppler shifts are not an issue as such, just a method to tell the own frame from the other.

[DDE]

8 hours ago, Green Baron said:

Thousands of predictions were and are made. And some of them really resemble things from today's reality. But otoh there is no guarantee that a prediction may have a manifestation in the future. It may be best to enjoy them on a fictional basis, until they eventually come into being.

"They laughed at Galileo. They laughed at Kopernicus. But they also laughed at Bozo the Clown."

[snip]

[sevenperforce]

On 12/30/2018 at 9:44 AM, JohnDuke said:

I don’t understand why it’s so hard, I have a learning disability where I can do and see mechanical things easier than most so my thoughts were, everything we have seen as kids like 20,000 Leagues under the Sea, Star Trek & lots of other things are a reality now so why is this so hard?

Lots of good answers so far. Here's another way of looking at it.

How much is 2+2?

Well, it's 4, obviously. 

And for thousands of years, we could all readily pick up two rocks or marbles or blades of grass, combine them with another two, and count find out that 2 + 2 does indeed equal 4.

But suppose someone decided to really measure it. Like, super precisely. Far more precisely than you'd ever need for ordinary applications. They had this whole massive system of lasers and whatnot and they sat down and they measured and they realized, suddenly, that 2 + 2 is not 4. Instead, they found, 2 + 2 = 3.9999999999999437.

It would be a shocking result, and everyone would initially expect it was some sort of error with the measurement apparatus. So someone else designed a different system, and they measured, and they found that 2 + 2 = 3.9999999999999435.

So they tried again, and again, and with many different ways of measuring over many years, they ended up with the consistent answer that 2 + 2 is actually ever so slightly less than 4. 

That's what happened with relativity. We had been measuring things in very straightforward ways ever since Galileo and Newton, and then Einstein came along and said "Hold up, we haven't been measuring things closely enough; it's actually just a little different, and once you have a lot of mass or are going very fast then it gets more and more noticeable." As counter-intuitive as it is, measurements and speed and energy all start to warp when you look really really closely, because the things we are measuring (space and time) have a fabric all their own, and that fabric ends up getting stuck to what we are using to measure them.

One consequence of this: When you push things to nearly a million times the speed of sound, space and time get so twisted up together that adding more thrust slows your perception of time but doesn't actually make you move any faster. 

That upper speed limit also happens to be the speed we perceive light as moving.

[wumpus]

18 hours ago, Snark said:

Also, let's be careful not to muddy the waters with red-shift issues-- yes, they're a big deal, but they're also not the same thing as the Lorentz dilation.

Really?  I'd assume that it is a direct result of that, and that the change in wavelength was exactly the same as the Lorentz contraction.  But that's only because of the special nature of light, other doppler shifts vary directly with velocity with respect to the medium (light deals with Lorentz dilation because there really isn't a "medium").

31 minutes ago, sevenperforce said:

That's what happened with relativity. We had been measuring things in very straightforward ways ever since Galileo and Newton, and then Einstein came along and said "Hold up, we haven't been measuring things closely enough; it's actually just a little different, and once you have a lot of mass or are going very fast then it gets more and more noticeable." As counter-intuitive as it is, measurements and speed and energy all start to warp when you look really really closely, because the things we are measuring (space and time) have a fabric all their own, and that fabric ends up getting stuck to what we are using to measure them.

The guys who did the measurement were Michelson and Morley.  From the Maxwell equations (and before), it was assumed that since light was a wave it had to have an "aether" to move through.  Michelson and Morley set up an apparatus to measure the speed the Earth traveled through this medium.  They got absolutely zero.  No matter how carefully they checked, it was always zero (hint: zero is easy to check with lasers, but they were invented long after this happened).  Einstein claimed to be only peripherally aware of this issue (he must have known that was why Lorentz created his transforms), but I can't see anyone els accepting relativity without the Michealson-Morley results.

There's one more footnote: the LIGO experiment is an even more sensitive experiment that measures something very similar to the Michealson-Morley experiment (it looks for a variation in the speed of light between two points over time instead of in different directions) and finally found valid data a year or two ago.  So it isn't *quite* accurate to say it is "always zero".

PS: Michelson and Morley are also famous for getting the Nobel Prize out of null data (their famous experiment).  They were expecting a useful number.  What they got shocked the world.

[Vanamonde]

We've trimmed the thread a bit. There's no need to mock each other, especially when the person in question is asking to learn more about a subject. 

[Snark]

3 hours ago, wumpus said:

Really?  I'd assume that it is a direct result of that, and that the change in wavelength was exactly the same as the Lorentz contraction.  But that's only because of the special nature of light, other doppler shifts vary directly with velocity with respect to the medium (light deals with Lorentz dilation because there really isn't a "medium").

Not quite.  Geometry matters.  Let's say you're on Earth, looking up at three glowing objects out in space.  They're all glowing in the same monochromatic light, with some wavelength λ (as seen from Earth, when the objects are at rest relative to Earth).

Now suppose the three objects-- call them A, B, and C-- suddenly start moving at half the speed of light, but in different directions.

• A is heading directly towards Earth.
• B is heading directly away from Earth.
• C is heading at right angles to Earth, i.e. not towards or away.

They're all moving at 0.5 c relative to Earth, which means their Lorentz factor will be the same, so they'll all experience the same amount of time dilation.   However... on Earth, you will observe them as having very different wavelengths from before-- and from each other!  In this case, A is heading towards Earth and will be blueshifted.  B, heading away from Earth, will be red-shifted.  C, heading parallel to Earth, won't have any Doppler effect, but it will (I believe) red-shift slightly due to Lorentz dilation.

[Green Baron]

I think, when straight abeam, there will be 0 shift from transmissions of the ship, because light always propagates with c. But a candle, seen thorugh a window on board of the ship, will be slightly redshifted when abeam.

Valid until correction :-)

[magnemoe]

14 hours ago, mikegarrison said:

It's not the "predictive power" of technology that is interesting. The true predictive power of science fiction is predicting how people might react to various technologies.

For instance, in June 1970, James H. Schmitz wrote a story where one of his characters overhears something and wants to know more about it, so she sits down at a comm-net terminal and does a keyword search. Bingo, stuff starts appearing on the screen. Sound familiar? He wasn't exactly predicting Google, but he was predicting that if people have an interconnected data network, they will use it to search for things. He also predicted that people would use that same network to send each other messages, to play online games with each other, and to act as their alarm clock and general appointment calendar. She also routinely does comm-net searches on herself to find out what information other people might be able to learn about her from the net.

He didn't predict the internet, really, but he asked himself, "If there was this comm-net that everyone had access to, how would people use it?"

This, it was loads of predictions about the future. Some sort of global computer network was pretty obvious but most expected it to be more centralized as in the university networks expanded and centralized or AOL than the internet. 
It was obvious for me that the mobile phone would replace the mp3 player, also probably the pda as screen sizes would grow. Did not predict it replacing the camera as I did not think they could add an useful camera in an phone. 
 

[KerikBalm]

Well... it hasn't replaced the camera, just displaced it... While those small cameras are impressive, they still have fundamental physics limitations. So "pure" camera's have had a significant portion of their niche intruded upon, as phone camera's are sufficient for most people's purposes.

But for other purposes, you simple need larger lenses. A cell phone camera simply can't compete with this if the person is actually interested in taking photos beyond selfies to share on instagram:

 

As far as the original question.... we don't even have to bother with relativity. Lets consider what it would take to get to 0.01c with standard rockets... Lets say you want to send a probe to alpha centauri, and to get there in anything close to a reasonable amount of time... 0.01c gets you there in about 450 years. That is a dV of 3,000,000 m/s... or 3,000 km/s To be useful, it will need to be a fairly large payload, if its going to transmit a signal that we can see, or carry something that can do something there... lets assume something similar to what a Saturn V could get to lunar flyby... so lets say 50 tons and a dV of 12.5 km/s.

To get 50 tons to twice that (25 km/s), we need something that could send a Saturn V on a lunar flyby... The Saturn V was about 3,000 tons. So the launcher would need to be 3000/50 = 60 times bigger than the Saturn V.... just to get a 50 ton payload to 25 km/s. To add another 12.5 km/s, we need to make it 60 times bigger again (3600x the size of a Saturn V)... to get to 50 km/ sec, we need the equivalent of 216,000 Saturn Vs. 50 km/sec is just 0.000167 C.

Lets go for 100 km/s... requiring another 50 km/s, that's another 4 iterations... 60^4 * 216,000... that would require something 2,799,360,000,000 x bigger than the Saturn V... and that gets us to a measly 0.00033 c ... we can ignore relativistic effects for that.

That's about 3 trillion Saturn V's to get to one 1/30th of 1% of the speed of light. For the record, only 13 Saturn Vs were made...

Going with Nerva/NTR stages for double the average Isp (assuming the same mass ratios... which would not hold true because NTR dry mass is much higher) would get us to 0.00067c... an ion stage (would need nuclear power) for about quadruple the Isp (again, now assuming unrealistic mass ratios) gets us to 0.00268 c... or 0.27% the speed of light... so we can ignore the difficulty in going from 95% to 99%, getting anywhere close to even 1% is ridiculously hard, because that is ridiculously fast, because light is ludicrously fast.

Edited January 4, 2019 by KerikBalm

[magnemoe]

4 hours ago, KerikBalm said:

Well... it hasn't replaced the camera, just displaced it... While those small cameras are impressive, they still have fundamental physics limitations. So "pure" camera's have had a significant portion of their niche intruded upon, as phone camera's are sufficient for most people's purposes.

But for other purposes, you simple need larger lenses. A cell phone camera simply can't compete with this if the person is actually interested in taking photos beyond selfies to share on instagram:

 

As far as the original question.... we don't even have to bother with relativity. Lets consider what it would take to get to 0.01c with standard rockets... Lets say you want to send a probe to alpha centauri, and to get there in anything close to a reasonable amount of time... 0.01c gets you there in about 450 years. That is a dV of 3,000,000 m/s... or 3,000 km/s To be useful, it will need to be a fairly large payload, if its going to transmit a signal that we can see, or carry something that can do something there... lets assume something similar to what a Saturn V could get to lunar flyby... so lets say 50 tons and a dV of 12.5 km/s.

To get 50 tons to twice that (25 km/s), we need something that could send a Saturn V on a lunar flyby... The Saturn V was about 3,000 tons. So the launcher would need to be 3000/50 = 60 times bigger than the Saturn V.... just to get a 50 ton payload to 25 km/s. To add another 12.5 km/s, we need to make it 60 times bigger again (3600x the size of a Saturn V)... to get to 50 km/ sec, we need the equivalent of 216,000 Saturn Vs. 50 km/sec is just 0.000167 C.

Lets go for 100 km/s... requiring another 50 km/s, that's another 4 iterations... 60^4 * 216,000... that would require something 2,799,360,000,000 x bigger than the Saturn V... and that gets us to a measly 0.00033 c ... we can ignore relativistic effects for that.

That's about 3 trillion Saturn V's to get to one 1/30th of 1% of the speed of light. For the record, only 13 Saturn Vs were made...

Going with Nerva/NTR stages for double the average Isp (assuming the same mass ratios... which would not hold true because NTR dry mass is much higher) would get us to 0.00067c... an ion stage (would need nuclear power) for about quadruple the Isp (again, now assuming unrealistic mass ratios) gets us to 0.00268 c... or 0.27% the speed of light... so we can ignore the difficulty in going from 95% to 99%, getting anywhere close to even 1% is ridiculously hard, because that is ridiculously fast, because light is ludicrously fast.

True, not replaced cameras, outside of the cheapest ones. I simply did not think you could get the optic and system good enough to take pictures who would be usable for anything at all. 
Obviously the optic are very limited. 

As for even getting close to relativistic speeds you will need engines of an entire new class of that we have now, forget ntr or even ion.  
Orion pulse nuclear is supposed to be capable but this require an giant ship just as you need giant bombs for maximum efficiency. Think kilometers in diameter, not the orion battleship or nasa concepts. 
antimatter should work. 
If you have real good fusion it should work. Laser pumped solar sails is probably the most realistic. 

[sevenperforce]

5 hours ago, KerikBalm said:

As far as the original question.... we don't even have to bother with relativity. Lets consider what it would take to get to 0.01c with standard rockets... Lets say you want to send a probe to alpha centauri, and to get there in anything close to a reasonable amount of time... 0.01c gets you there in about 450 years. That is a dV of 3,000,000 m/s... or 3,000 km/s To be useful, it will need to be a fairly large payload, if its going to transmit a signal that we can see, or carry something that can do something there... lets assume something similar to what a Saturn V could get to lunar flyby... so lets say 50 tons and a dV of 12.5 km/s.

When people ask "why can't spaceships go at light speed" they may be asking multiple questions.

Your (excellent) answer illustrates that sometimes you don't have to go into the details of relativity, because the biggest teaching point is that space is HUGE. The distances between everything in the universe are IMMENSE.

Consider: the farthest that human beings have ever traveled from Earth was during the Apollo 13 mission, when Lovell, Swigert, and Haise took an extra-wide swingaround past the far side of the moon. At apogee, they were 407,000 km from Earth...a distance that would take a NASCAR stock vehicle more than seven weeks to traverse, traveling at top speed.

Yet that distance pales in comparison to the size of the solar system. If the solar system (as defined by the orbit of Neptune) was the size of Paris, France, then that 407,000 km would less than two feet. And at that scale, the outer limits of our solar system (defined by the Oort cloud) would be three times the size of our planet.

The speed of light has very little to do with it. The major issue is that space is really, really, ridiculously big.

[wumpus]

11 hours ago, magnemoe said:

True, not replaced cameras, outside of the cheapest ones. I simply did not think you could get the optic and system good enough to take pictures who would be usable for anything at all. 
Obviously the optic are very limited.

The great advantage the cell phone camera has is that it is usually with you.  A bad camera in hand when you need to take the shot is better than the best SLR back on your desk.