“The very closest stars would require many years to visit, even traveling at the speed of light, which is impossible according to Einstein’s theory of relativity. Today’s fastest spaceships would require 200,000 years to travel to Alpha Centauri, our closest bright star. The energy required to send a hundred colonists to another star, as Frank Drake has pointed out, would be enough to meet the energy needs of the entire United States over a human lifetime. And these estimates are regarding nearby stars. When we consider the distances across the entire galaxy, and between galaxies, interstellar travel seems absolutely untenable.” –David E. Fisher

So you’ve got dreams of interstellar travel. do you? And you don’t want to wait for multiple generations to come and go before we get there; you want to make it happen in a single human lifetime. The most straightforward strategy of all is to accelerate in one direction at 1g for some time, turn around mid-flight, and decelerate so that you’ll reach your destination at a reasonably low speed.

This plot illustrates how a spaceship capable of 1 g acceleration for 100 years can power a round trip to most anywhere in the visible universe, and back in a lifetime or less. Additional time will have elapsed on earth by the time that you return. Image credit: P. Fraundorf, under creative commons.

This plot illustrates how a spaceship capable of 1 g acceleration for 100 years can power a round trip to most anywhere in the visible universe, and back in a lifetime or less. Additional time will have elapsed on earth by the time that you return. Image credit: P. Fraundorf, under creative commons.

With current technology, this is woefully impossible, as the energies required — as well as the fuel masses you’d need — are simply too great. But with antimatter technology, accelerating close to the speed of light is a real possibility. Reaching it or exceeding it, though? Special relativity simply won’t allow it.

This is an artist's rendition of an antimatter propulsion system. Matter - antimatter arnihilation offers the highest possible physical energy density of any known reaction substance. Image credit: NASA / Marshall Space Flight Center.

This is an artist’s rendition of an antimatter propulsion system. Matter – antimatter arnihilation offers the highest possible physical energy density of any known reaction substance. Image credit: NASA / Marshall Space Flight Center.

Come learn all about our dreams for reaching the speed of light by Christmas, and how close we can actually get!


  1. #1 Sinisa Lazarek
    December 24, 2016

    Christmas came.. at least on this longitude. Things look kinda the same.. so.. guess not. Still at 0.0000001 c 🙂 Merry Christmas!

  2. #2 G
    December 27, 2016

    What’s a reasonable consensus estimate of the highest velocity that could be reached using hydrogen fusion as the means of propulsion?

    And, how does that translate to travel time to a distance of let’s say 50 LY, both a) from the perspective of mission support on Earth and if relativistic effects would be significant, b) from the perspective of the astronauts?

    I don’t think it’s realistic to assume “single lifetime” travel to other star systems. In any case the astronauts will need full life support for an extended duration while they are orbiting a planet and conducting missions to the surface to build a self-sufficient ground station there.

  3. #3 Michael Kelsey
    SLAC National Accelerator Laboratory
    December 27, 2016

    @G #2: There is no “reasonable consensus estimate.” The exact value depends on the details of the spacecraft design, as entered into the (non-relativistic) rocket equation: V-final = V-exhaust ln(M-initial/M-final).

    For something like a Bussard ramjet, the best you can get is a final velocity equal to the exhaust velocity.

    If you have a rocket carrying its own fuel, then M-initial > M-final and you can get higher final velocity by just making your payload (including fuel containers, motor, etc.) as small as possible.

    Translating speed to travel time is left for the reader, as a very simple exercise in special relativity. If you can’t do the math on paper, there are plenty of online calculators to help you.

  4. #4 Sinisa Lazarek
    December 27, 2016

    IMO we have to accept the fact that any interstellar travel will most likely have to involve “generation” ships or a whole fleet of them.. i.e. Battlestar Galactica scenario.

    Unless we invent some super-duper drive. but given that even at c you will need 8 years till the closest star.. the distances involved are just too great for a single human life span

  5. #5 Wow
    December 27, 2016

    It’ll only be 8 years from the POV of the people sitting on Earth. To those in the ship, it could be merely days.

    Any genuine reality of interstellar space travel will have to accomodate that reality: that we will send them out, and we will never know how it went, nor even see them again.

  6. #6 Sinisa Lazarek
    December 27, 2016

    “To those in the ship, it could be merely days.”

    true. my bad. darn lorentz contraction crept up on me.

  7. #7 Wow
    December 27, 2016

    It’s still important to recognise that a round trip would take 8 years, it just won’t be 8 years of the life of the people making the trip.

    And any time spent setting up a permanent or semi-permanent base would be added on for the POV of both parties, unless it’s REALLY close to a black hole…

  8. #8 gallopingcamel
    January 1, 2017

    Happy New Year Michael Kelsey!
    My calculations differ from Ethan’s so I am hoping some of you learned folks can tell me where I went wrong.

    Note that I calculate a trip time of 10.6 years to the galactic center. That is the time for a “Fly By”. The time for a trip that slows the space vehicle down to zero relative velocity is 19.8 years.

    I would be happy to share my calculations with y’all.

  9. #9 gallopingcamel
    January 1, 2017

    “For something like a Bussard ramjet, the best you can get is a final velocity equal to the exhaust velocity.”

    That is why you need the PAR (Proton Annihilation Rocket) which has an exhaust consisting of 938 MeV gamma rays!

  10. #10 Michael Kelsey
    SLAC National Accelerator Laboratory
    January 1, 2017

    @gallopingcamel #8 and #9: Did you do your calculations using the relativistic rocket equation (where you take your fuel with you), or did you make the ramjet/lightsail approximation, where you collect and eject your fuel along the way? The assumptions underlying those two cases will affect your final result.

    Your “proton annihilation” has numerous problems with it. First, you need to either carry with you, or make as you go, a supply of antiprotons equal to the mass of protons you plan to annihilate.

    Second, particle-antiparticle annihilation is the worst system for producing a “rocket.” Since the annihilation occurs at rest, the photons or other products (see below) are emitted isotropically. There’s no net momentum transfer.

    Finally, proton-antiproton annihilation does not produce a pair of super-high-energy gamma rays. It typically produces anywhere from a few to a dozen or so pi mesons, and occasionally some strange (K) mesons, which go on to decay into lower energy gammas (70 MeV each in the pi0 rest frame), electrons or muons, and neutrinos.

  11. #11 gallopingcamel
    January 2, 2017

    I addressed most of your questions in the link in #8. Here it is again:

    I disagree with you about the results of proton-antiproton reactions. If you do it right you get two 938 MeV photons. Then the problem is how do you focus them into a beam. Nobody has a clue how to do this today but can you prove it is not possible?

    Personally I don’t see much hope for matter/antimatter reactors as finding and handling the antimatter seems a stretch!

    It is easy to show (as Bussard did) that fusion rockets are not good enough to power interstellar travel in a human lifetime. However if we could convert matter into energy at close to 100% efficiency it would be possible.

    You are right to be skeptical but remember what Lord Kelvin said a little over 100 years ago:
    ” Heavier-than-air flying machines are impossible.”
    ” There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”

    Lord Kelvin was an intellectual giant but those quotes show that even the smartest people can be wrong.

  12. #12 Michael Kelsey
    SLAC National Accelerator Laboratory
    January 2, 2017

    @gallopingcamel #11: You wrote, “I disagree with you about the results of proton-antiproton reactions. If you do it right you get two 938 MeV photons.”

    Whether you “disagree with me” or not is irrelevant to reality. I do this for a living, as a professional experimental particle physicist. You really have no idea what you’re talking about in this case, probably because you don’t have the training and background to understand the internal details of hadronic interactions.

    Baryons are composite entities. Their constituent quarks have a range of momenta within the bound baryon. The only possible way to get a pair of gammas out of a p-pbar interaction is to just happen to get (a) a perfect zero impact parameter collision, (b) to have BOTH the baryon and antibaryon in a special (essentially zero measure) state with one quark carrying 100% of the momentum, and the other two quarks carrying exactly zero momentum, and (c) to have the specific interaction involve the two 100% quarks annihilate to gammas, and the other quark pairs annihilate to zero-energy photons. This latter is trivially impossible, because quarks have non-zero mass, so even at the zero-momentum-fraction point, they’ll still produce few-MeV photons.

    In reality, the interaction of a mix of six quarks and antiquarks is much, much more complex than that. All of your wishing and hoping won’t make it different.

    Your chase into cherry-picking past quotes (a variation on the tired pseudoscientist trope of “they thought Einstein was wrong, so I must be just as smart as Einstein”) is a better demonstration of your lack of knowledge and training than anything else.

  13. #13 gallopingcamel
    January 3, 2017

    You should get off your high horse and let your imagination run wild. I have my tongue firmly in my cheek as you would know if you took the trouble to read that post I linked.

    Like you I made a decent living building and operating relativistic accelerators so I understand why you get all that junk when you smash protons and antiprotons together in the LHC. Now ask yourself what happens when a thermal proton meets a thermal antiproton.

    An apology would be appreciated but I am not holding my breath.

    Here is a paper that explains what we built at Duke 20 years ago. It was the world’s brightest gamma ray source then and it is the brightest now. So what have you achieved other than demonstrating the arrogance of youth?

  14. #14 Sinisa Lazarek
    January 4, 2017

    @ camel

    giving a link to a paper behind a paywall… not cool. But did find this in intro:
    Presently, the HIγS facility produces the linearly polarized γ-rays with energies from 1.8 MeV to 58 MeV,

    58 MeV is a long way from claimed 938 MeV. So some explanation is required IMO.

  15. #15 Michael Kelsey
    SLAC National Accelerator Laboratory
    January 4, 2017

    @gallopingcamel #13: “Youth”? Sadly, I stopped being a “young physicist” about 15 years ago 🙁 I measured f(Ds) for my thesis, built (with a group) the BaBar drift chamber (DCH) as a post-doc, and was manager of the DCH group on BaBar for it’s entire run. After BaBar, I was the lead for the intranuclear cascade (Bertini-inspired) model in the Geant4 simulation toolkit. Now I’m working on the SuperCDMS dark matter search.

    When thermal protons and antiprotons interact, you’re *STILL* going to have quark-antiquark interactions and mostly pion produciton. You can’t get around the Feynman-x distributions in the two particles, which precludes getting a pair of perfect 938 MeV gammas.

    I’m in the middle of a fairly complex project today, but you can run the code yourself (http://geant4.cern.ch), just use a very low energy antiproton “beam” (something like 1 MeV or so) onto a hydrogen block. I’d recommend using either BIC or INCL++ as the cascade model in the physics list, as Bertini doesn’t handle anti-particle projectiles very well (it was originally written for pion-nucleus work, back in the 1960s, and it shows 🙁 ).

  16. #16 Michael Kelsey
    SLAC National Accelerator Laboratory
    January 4, 2017

    @Sinisa #14: The energies are entirely unrelated. Gallopingcamel was (quite rightly!) citing HIyS as bona fides for his/her having some expertise in the area we are discussing. The same way that I’ve cited my own work. Not a problem.

    The 1.5-58 MeV gammas from HIyS are the output of a free-electron laser (FEL). He/she is asserting that “938 MeV gammas” come from proton-antiproton annihilation. My point is that because protons are composite entities, the actual annihilation happens among the individual quarks, which themselves have a distribution of momenta within the proton. What you get out is a mix of pions, gammas from pi0 decays, and occasionally a kaon pair. You don’t get a single gamma with the proton mass.

  17. #17 Sinisa Lazarek
    January 4, 2017

    Thank you for clarifying. The way I read the post was that he said that it indeed can be done, and then gave a reference for HIyS as gamma ray producer. So I (mistakenly) thought that he was comparing the two gamma ray outputs.

  18. #18 gallopingcamel
    January 4, 2017

    That is a little better. I don’t have my apology but at least you realize that “lack of knowledge” is not the problem. The electrons we were tickling at Duke had 2,000 times their rest mass so we were able to produce the particle soup you mentioned simply by letting them hit something.

    The Duke linear accelerator was built out of redundant equipment from SLAC. Our electron storage ring was designed by Wiedemann:

    Wiededemann’s design was improved by Vladimir Litvinenko who was awarded the FEL prize in recognition of the HIGS (High Intensity Gamma Source):

    That is what got me thinking about interstellar travel. At Duke our electrons had a gamma of 2,000 so if you were traveling with that electron, time would slow down by a factor of 2,000. Of course your mass would increase by the same factor but in the electron’s time frame everything would appear to be normal.

    My PAR (Proton Annihilation Rocket) is absurd according to physics as we know it which is why I quoted Lord Kelvin. If anyone as smart as he was could be so wrong why would you believe that our understanding of physics will not advance immensely in another 100 years?

    Remember that the electron was discovered in 1899 by J.J.Thomson. There is a plaque on the wall of the old Cavendish laboratory celebrating that achievement. For four years I walked by that plaque on my way to lectures.

    Like you I can’t imagine any way to “Beam” gamma rays and anything else that is produced when a proton is annihilated. In 100 years it may be trivial enough to appear on boxes of cereal.

  19. #19 gallopingcamel
    January 4, 2017

    Sorry about that!

    Here are links to a couple of relevant papers that should be easily accessible:

    I am not trying to hide my identity. The “gallopingcamel” is Peter Morcombe. While I waste some time blogging on science and energy related issues, my intense passion is directed at improving K-12 education in the USA.

    I am not trying to hide my identity

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