# Travelling at the speed of light?

#### Keith Brown

I’ve just started to take an interest in the universe mainly because of the Webb Telescope, but it does bring to mind if it takes about 32 years to reach the end of our solar system how can we venture beyond that ?
To travel to Mars would take two years which is way beyond our capability.
I’m hoping someone can give me an answer.

Moderator
if it takes about 32 years to reach the end of our solar system how can we venture beyond that ?
Easy answer. We can't. We just don't have the technology available to us for an extended mission like that.

To travel to Mars would take two years which is way beyond our capability.
The mission to Mars is about two years (there, stay for a bit, and come home) round trip. It would only take about six months to get there. I wouldn't say it's beyond our capabilities, but the technology is in place. We just need the finances and reasons to go (mostly finances).

-Wolf sends

IG2007 and Helio

#### Helio

I’ve just started to take an interest in the universe mainly because of the Webb Telescope, but it does bring to mind if it takes about 32 years to reach the end of our solar system how can we venture beyond that ?
To travel to Mars would take two years which is way beyond our capability.
I’m hoping someone can give me an answer.
It may be possible [project Starshot] to accelerate very tiny probes to about 20% the speed of light, allowing them to zip by the Centauri stars/exoplanets in about 21 years.

If we could reach speeds near the speed of light we can go great distances due to time dilation. But the energy needed for this is way beyond our abilities.

If we could travel at, say, 78% the speed of light, we would get to Proxima Centauri in 7.8 years, but this ignores a few years for accelaration/decceleration. The extra energy needed for this is about 2x the normal energy since the travelers/ship would have about double the relativistic mass.

#### Helio

The speed of light traveling through a vacuum is exactly 299,792,458 meters (983,571,056 feet) per second. That's about 186,282 miles per second — a universal constant known in equations as "c," or light speed.
Yes. This is true for any frame of reference. The speed of a snowball is seen as traveling faster if you are driving a bus toward the thrower vs. traveling away from the thrower. This is true for every moving thing, except light. Traveling towards light or away from it will still result in a constant, unchanging speed, though its wavelength will shift accordingly.

This was the key to Einstein's SR (Special Relativity). Surprisingly, this constancy for light means that it is time that varies with motion. This, for instance, explains my prior post where travel time is lessened for the travelers, but not the time expired by the travelers as seen from Earth.

Catastrophe

#### Catastrophe

##### "There never was a good war, or a bad peace."
Keith Brown, here is the substance of a previous post, which bears repeating because of the amount of relevant detail:

There is a very good article in Astronomy May 2021, entitled Breakthrough Starshot: A Voyage to the Stars.

Essential reading for anyone who thinks that this is a practical means of space travel.

"A necessary requirement for the Breakthrough Starshot mission is keeping the mass of each sail-equipped Starchip to just a few tenths of an ounce (a few grams). . . . . . . likewise the solar sail itself, which is expected to span up to around 13 feet (4 metres), will need to weigh in at less than 0.035 oz (1 gram). It will also need to be extremely thin, as otherwise the sail would absorb far too much heat and be vaporized by the barrage of laser light . . . . . . "
the barrage of laser light to drive a few grams??

"Mass is the bane of accelerating objects to great speeds. To significantly increase the velocity of a heavy object takes a tremendous amount of energy. So, if the goal is to reach a distant star in a reasonable amount of time, say within a generation, a spacecraft must be extremely tiny and, therefore, robotic. Plus, it still requires an insanely energetic boost to get up to speed."
" . . . propelling a lightsail-equipped nanocraft, or Starchip, [sic] would require hundreds of individual lasers ,spanning roughly 200 acres (1 square kilometer). The array would also need access to enough energy to fire a coherent 100 gigawatt laser beam for several minutes during each . . . launch.
That's roughly the amount of power generated by all the nuclear power plants in the U.S. in a given year."

Another problem occurred to me, which is not mentioned. 200 acres of individual lasers surely would not be very manoeuvrable. Would it be able to follow said Starchip, even for a few minutes (as the Earth turns)? Also, would it not be usable only at a fixed latitude - that at which it was built?

Not to mention stopping when it gets there!

Cat

Helio

#### Atlan0101

Mass acceleration also has to do with the environment the mass is being accelerated in. Mass acceleration may be closed systemically 1-dimensional but if the environment, such as a plane of interplanetary space, or a plane of interstellar space, or a plane of intergalactic space -- in other words potential hyper-planes of distancing when it comes down to continuous powering (continuous acceleration) in them -- is open systemically amenable to differing gains, all the way up to mind boggling differing gains, in distance out of the same amount of gravities of mass acceleration....

#### billslugg

They are proposing a 200 acre array of many small lasers, each of which can be aimed at the craft. After a ten minute acceleration of 10,000 g's, each craft is about 17 million km away, too far for any more laser acceleration. Energy input to the lasers equal to about a Terawatt, or same as current US electrical generating capacity. About 2 million Tesla battery packs would do it also.

#### Pogo

But, not to mention about diffusion of the laser beams by the atmosphere, decreasing the energy delivered to the craft, especially if it’s not near the zenith.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Further to my post #6, where (billslugg) are they siting the (post #8) lasers? To maintain direction on a rotating Earth, especially a 200 acre array, ain't easy! Putting such an array in space would also be a substantial problem (in maintaining direction.

Cat

#### billslugg

They are still in the conceptual phase, no site is mentioned. The array only needs to focus on an individual craft for ten minutes, pretty much anywhere on Earth would do I suppose.

#### Catastrophe

##### "There never was a good war, or a bad peace."
Travelling at the speed of light

"I’ve just started to take an interest in the universe mainly because of the Webb Telescope, but it does bring to mind if it takes about 32 years to reach the end of our solar system how can we venture beyond that?"

Perhaps OP will correct me, if necessary, but the above from #1 appears to indicate human travel.
That being so (?) you can completely rule out light sails for the reason given in post #6. If post #6 is not enough, here is some more information

Probably the most practical and informative approach you can find is in the book "Extraterrestrial" by Avi Loeb, John Murray (Publishers), 2021. Author is Chair of Harvard's Astronomy Department, and all the rest - one of the world's top astrophysicists.

He was approached by a billionaire over a special project. This guy wanted to fund a mission to the Centauri system, to arrive there during his lifetime - unmanned of course. Conventional chemical propulsion would require over 100,000 years. Avi came up with the idea of using a light sail. However, this is totally impracticable for anything over a few grams. They only wanted to take pictures and similar as they passed by. Obviously it would take too long to stop.

The system uses a 100 gigawatt laser beam. It is stated that everything they propose be within existing technological bounds. This is no joke. It is a serious mission. To avoid burning the sails, they had to absorb less than 1/ 100,000 of the (laser) light striking them.

There is an article published in Astrophysical Journal, October 2015, by Avi and James Guillochon on lightsails. It was decided to announce the Starshot Initiative, as they call it, on April 12 2016, On the stage were included Stephen Hawking, Freeman Dyson and Yuri Milner.

I hope this will help, but if you are thinking about manned trips using lightsails - forget it - unless you want to completely ignore the science.

Cat

#### billslugg

"I hope this will help, but if you are thinking about manned trips using lightsails - forget it - unless you want to completely ignore the science." Catastrophe

Actually, you can forget about any method of human interstellar transportation.
No method can work. (Assumptions, humans not in some sort of hibernation, a round trip within their lifetimes. Can't leave Earth until age 18)
Light sail is a one way trip.
Neither chemical, fission nor fusion can provide enough energy for a round trip.
No way to make or store enough antimatter to do the job.

Catastrophe

#### Jzz

Yes. This is true for any frame of reference. The speed of a snowball is seen as traveling faster if you are driving a bus toward the thrower vs. traveling away from the thrower. This is true for every moving thing, except light. Traveling towards light or away from it will still result in a constant, unchanging speed, though its wavelength will shift accordingly.
Not true. Every wave displays similar characteristics. Every wave will travel at a constant speed regardless of the motion of the source or of the observer, the speed of the wave will depend solely on the properties of the medium it is travelling through. You are missing the whole point of relativity, if you don't mind my saying so. It is not the speed of light that remains constant, it is that distances and times change in order to render the speed of light constant in all frames of reference.

Atlan0101

#### Atlan0101

It is that distances and times change in order to render the speed of light constant in all frames of reference." Renders the speed of light as conditionally hard fixed and universal as strong force rock diamond and, at once, as conditionally soft and universal as gravity force bubble elasticity. The reflection of their affinity?!

I've written many times of the universe traveler in the mind's eye. My version of Einstein's traveler in the mind's eye. The universe traveler travels horizon to horizon, the horizon ahead always opening up toward the oncoming traveler from infinity's / origin's point of horizon (one of an infinity ahead). And always recoiling space for space and time for time, into the closing point of infinity's / origin's horizon behind. From out of merger ahead, receding into merger behind . . . The multi-facet, multi-dimension, 'Multiverse' analogue and physic: The 'white hole' ("Exit Only Door") event horizon 'evolutionary set' ever ahead of the traveler || The 'black hole' ("Entry Only Door") event horizon 'devolutionary set' ever behind the traveler. The traveler, ever dead point between the two. Why the recoil is always observed to be constantly precisely exact regarding the "relative universe," the "observable universe," the constancy of the speed of light.

Now someone might see the horizons to be precisely the same as the horizons of the sphere of Earth, always coming round to the same point. Never happen! The traveler is always at the same dead center point of a constant of change, horizon of change always observed oncoming from ahead (always observed oncoming within an oncoming light) to horizon of change always observed receding behind (always observed receding away within an oncoming light), white hole event horizon to black hole event horizon! Einstein's observer in rest frame witnesses both procession and recession of universe in the same horizon.

The traveler will never come back round to exactly the same space and time universe. Fractal self-similar frontier universes always evolving from out of a "Horn of Plenty" horizon ahead, yes, duplication of the gone (though remaining existing infinities apart), no (exclusionary principles will apply also to space and time).

The traveler will trade observed light time (((-) (+)) > (t = 0)) for observed light time reversal ((t = 0) < ((+) (-))), more or less precisely, but the traveler's local time onboard clock will always advance a reality of change, not only for itself, but as a local (quantumly entangled) reflection of distant destination points ahead (in reality, (m-e point-bubble) universe cell divisions, infinitely plural) and distant departure points behind (in reality, (m-e point-bubble) universe cell divisions, infinitely plural).

Universe (U), the united in merger 'set' (t = '0') / universes (u), the countlessly divided, offset, 'constituency' (t ='1') inside / outside, the 'set' (t = '0'). The Cosmological Constant is Base'2' primal base, '0' and/or '1' (infinity (origin) = '1', constant). The CC base is ["and/or"]. You could say the 'Schrodinger's cat' third dimension of two . . . or the second dimension of one . . . or maybe even the first dimension of zero?!

#### Atlan0101

Early morning and wide awake. I have another scenario to describe:

An astronomer on Earth spots a distant Jupiter size object being thrown out of the galaxy by a nearby black hole (rather than being captured by it) at about 99% the speed of light relative to the astronomer sitting at his telescope on Earth. He doesn't see it, but an Enterprise class starship is keeping pace with the object studying it, thus also doing 99% the speed of light relative to the astronomer observing the event through his telescope on Earth.

The astronomer sitting at his scope measures the speed of light to be 'c' relative to himself, as well as his gravity to be 1 Earth 'g'. Those aboard the Enterprise class spaceship measure the speed of light to be 'c' relative to themselves, as well as measuring their gravity aboard ship to be 1 Earth 'g'.

The astronomer on Earth, through his scope, times an event on the Jupiter size object hurling along at 99% percent the speed of light (99% 'c') to take many, many, seconds according to his nearby precision grade clock from beginning to end to occur.

Aboard the Enterprise class starship keeping pace with the object at 99% 'c' as the astronomer observes things to be, the crew time the event to take exactly two seconds according to their on-board precision grade clock from beginning to end to occur.

Of course, the witnessing of all the events by the astronomer on Earth occurs a few thousands of years after the events occur on the distant spot and are witnessed to occur by the crew aboard the Enterprise class starship also on the spot, relative speaking, that is.

The many, many, seconds at the long distance and vast separation in relative velocities (an expanded dimension of space and time), and the two seconds at the short distance and no difference in relative velocities (a contracted dimension of space and time), in fact measure exactly the same length of time of the event (two seconds).

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#### Helio

It is not the speed of light that remains constant, it is that distances and times change in order to render the speed of light constant in all frames of reference.
Speed is simply what we measure and light exhibits only one speed, depending on the medium, of course, which is why it is known as a constant. This fact is important since science is objective-based. So please show where what I said is false. Explaining how it is true is no falsification.

But to your point, Special Relativity is the mainstream model that shows that between two inertial frames this constancy happens as a result of either length contraction or time dilation, but not both, unless proportioned somehow. We have abundant evidence for time dilation, but little for contaction.
On this I think we agree.

#### Catastrophe

##### "There never was a good war, or a bad peace."
billslugg:
QUOTE
"I hope this will help, but if you are thinking about manned trips using lightsails - forget it - unless you want to completely ignore the science." Catastrophe

Actually, you can forget about any method of human interstellar transportation.
No method can work. (Assumptions, humans not in some sort of hibernation, a round trip within their lifetimes. Can't leave Earth until age 18)
Light sail is a one way trip.
Neither chemical, fission nor fusion can provide enough energy for a round trip.
No way to make or store enough antimatter to do the job.
QUOTE

Of course, you are completely correct. I only highlighted lightsails because I assumed (wrongly?) that all other suggestions had already been discarded as impossible.

Cat

billslugg

#### Jzz

But to your point, Special Relativity is the mainstream model that shows that between two inertial frames this constancy happens as a result of either length contraction or time dilation, but not both, unless proportioned somehow. We have abundant evidence for time dilation, but little for contaction.
On this I think we agree.
I think it is best if we don’t confuse SR with quantum mechanics where a sub-atomic particle can be a particle or a wave and vice-versa just never both at the same time. (Why not? Not a statement just a question.). In SR anything goes, lengths can contract and times can dilate for one observer and not for another, each observer sees a different length and a different time. The only reality that exists is for the observer who is seeing that particular frame of reference. BOTH time dilation and length contraction take place simultaneously, for this not to be true would be almost impossible.

Catastrophe

#### Catastrophe

##### "There never was a good war, or a bad peace."
Travelling at the speed of light?

OP asks "how can we venture beyond that? (viz the Solar System).

To summarise, am I correct in stating that any system would need to carry enough fuel to increase speeds approaching anywhere near LS, such that it would be subject to increasing mass, the greater the speed?
In other words, as you approach LS, the mass of fuel on board increases towards infinity the closer you get to LS, thus making the initial theoretical requirement of fuel mass infinite. (Pardon the 'I' word).

An exception is the use of light sails, which possibility has also been shown to be non viable.

Are there any other possibilities of travelling LS?

Cat

#### billslugg

If you were to make an ion engine at the outer limit of what NASA has patented, the exhaust velocity is on the order of 200,000 miles per hour or about 90,000 meters per second. If you planned a really big rocket, with a payload of only 1% of the mass then the rocket equation: final velocity = exhaust velocity x ln (initial mass/final mass) will give a final velocity of 413,000 meters per second or about 14% of the speed of light. That would get to the nearest star to the Sun in 30 years but does not provide for stopping there or returning to Earth. Your target solar system would be in the viewing window for about half an hour.

#### Pogo

Interesting question. If the ship is traveling at a fraction of C such that the mass of the ship is 2x it’s at rest mass, then it follows that the remaining fuel masses 2x it’s rest mass. But the volume decreases as the ship length (and tank length) is shorter in the Z-axis, as well as the shape of the engine nozzles and so forth. I wonder how that affects engine efficiency, the kilograms or liters per delta-V.

#### billslugg

I believe those effects are only as viewed by an observer. Those on the ship are not aware they are being shortened or more massive nor do they see their clock slow down.

#### Helio

I believe those effects are only as viewed by an observer. Those on the ship are not aware they are being shortened or more massive nor do they see their clock slow down.
Yes. Those on the ship will see foreshortening outside the ship, hence their travel distance will seem, or is, shorter.

#### Helio

The other SR effects are quite interesting as well, though this might be too far off topic.

The redshift effect grows with velocity to reach overhead. The light sources ahead will funnel toward the center of travel and become brighter and bluer, etc.

#### Pogo

Oops! Totally forgot about the perception of the observer aboard and the remote observer. The ship itself would not sense any change in that respect. My bad!

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