Space travel at light speed is impossible. Space travel at near light speed is impossible with current engineering, but not theoretically impossible. We can accelerate particles to near light speed, but not something of everyday size.JerryF said:So I wonder, aside from the impractical side of intergalactic exploration (re: much time passing on Earth whilst the travellers are away), is it stated that travel at or near light speed is impossible for a spaceship?
This is an important observation and one of the basic ideas of modern physics. We spend a lot of time on here arguing with those who cannot accept this!JerryF said:To the moving object, in its own reference frame, why would anything change regardless of how close to c it moves?
Velocity zero means velocity zero. Which is different from light speed.JerryF said:Thanks anuttarasammyak! So I wonder, aside from the impractical side of intergalactic exploration (re: much time passing on Earth whilst the travellers are away), is it stated that travel at or near light speed is impossible for a spaceship? I keep coming back to the thought that it is possible but everything I read seems to repeat that it's not.
JerryF said:To the moving object, in its own reference frame, why would anything change regardless of how close to c it moves?
How near is near?fresh_42 said:Near light speed is impossible. Space isn't empty and the bow wave will make it a no go.
https://www.physicsforums.com/threads/today-i-learned.783257/page-140#post-6424558
How big is the ship?PeroK said:How near is near?
Rather than bump the other thread, would you or someone else explain why the CMB would work like a resistor and why added energy would be used for particle production?fresh_42 said:Near light speed is impossible. Space isn't empty and the bow wave will make it a no go.
https://www.physicsforums.com/threads/today-i-learned.783257/page-140#post-6424558
Small enough and streamlined enough to achieve the speed it wants to achieve!fresh_42 said:How big is the ship?
It was on a tv show, so no valid reference. At least it was an astronomer who said it. The photons of the CMB are everywhere, so there will be no way to escape them. They make space a fluid with viscosity. Thus depending on mass and surface area we will get a thermodynamic effect. I don't know at which temperature particle production begins, and whether it is pair production, or radioactivity due to collisions with the ship's material, or due to the existing matter in space.Grasshopper said:Rather than bump the other thread, would you or someone else explain why the CMB would work like a resistor and why added energy would be used for particle production?
This is obviously something almost no one ever talks about and I’m now very must interested in how it would work.
Any constraints and problems associated with the CMB are already well beyond the other engineering constraints we face. Currently our space probes leave the solar system at less than ##20 km/s##.fresh_42 said:It was on a tv show, so no valid reference. At least it was an astronomer who said it. The photons of the CMB are everywhere, so there will be no way to escape them. They make space a fluid with viscosity. Thus depending on mass and surface area we will get a thermodynamic effect. I don't know at which temperature particle production begins, and whether it is pair production, or radioactivity due to collisions with the ship's material, or due to the existing matter in space.
Grasshopper said:would you or someone else explain why the CMB would work like a resistor and why added energy would be used for particle production?
The point is, that long before you reach 0.999999991 c, the necessary energy to get to this level will be used for particle production, hence you cannot use it as kinetic energy anymore. There is no way to get arbitrary close to c.DrStupid said:The CMB would be blueshifted in front of the ship. In case of the 0.999999991 c, mentioned above for protons in an accelerator, it would be a factor of 15000. That means that the sky in front of the ship has a temperature of 40000 K, resulting in a heat influx of 150 GW/m² and a radiation pressure of 500 N/m². This shouldn't be an issue for somebody who is able to accelerate a spaceship to this speed. However, there is no upper limit and it is just one of the minor problems. The interstellar medium is much worse.
The CMB provides low energy photons which will be scattered at the electrons and protons of the spaceship. You will get a particle accelerator!Wikipedia said:Inverse Compton scattering is important in astrophysics. In X-ray astronomy, the accretion disk surrounding a black hole is presumed to produce a thermal spectrum. The lower energy photons produced from this spectrum are scattered to higher energies by relativistic electrons in the surrounding corona. This is surmised to cause the power law component in the X-ray spectra (0.2–10 keV) of accreting black holes.[clarification needed]
The effect is also observed when photons from the cosmic microwave background (CMB) move through the hot gas surrounding a galaxy cluster. The CMB photons are scattered to higher energies by the electrons in this gas, resulting in the Sunyaev–Zel'dovich effect. Observations of the Sunyaev–Zel'dovich effect provide a nearly redshift-independent means of detecting galaxy clusters.
Some synchrotron radiation facilities scatter laser light off the stored electron beam. This Compton backscattering produces high energy photons in the MeV to GeV range subsequently used for nuclear physics experiments.
fresh_42 said:Edit: He speaks English, so you can send an email and ask for details.
Edit: And this is still far more evidence than I have seen from anybody else in this thread!
I think this is wrong, as the energy for particle production is much higher than that speed will give you. See the above link on high-energy cosmic rays.fresh_42 said:The point is, that long before you reach 0.999999991 c, the necessary energy to get to this level will be used for particle production, hence you cannot use it as kinetic energy anymore.
fresh_42 said:The point is, that long before you reach 0.999999991 c, the necessary energy to get to this level will be used for particle production, hence you cannot use it as kinetic energy anymore. There is no way to get arbitrary close to c.
Since when? The speeds in our accelerators are far less than that - and we produce particles. How do you discuss the inverse Compton scattering away? We also have a positive area, and with increasing speed really many photons from CMB per square inch per time. Why shouldn't arise viscosity from that? The physics is still the same.PeroK said:I think this is wrong, as the energy for particle production is much higher than that speed will give you. See the above link on high-energy cosmic rays.
DrStupid said:Which energy are you talking about? The energy of the blueshifted CMB is definitely not sufficient for pair production. It is just in the range of UVC.
fresh_42 said:How do you discuss the inverse Compton scattering away?
fresh_42 said:How do you discuss the inverse Compton scattering away?
That's a different sort of collision: that's particle-particle collisions. Lesch's point is about particle-photon collisions.fresh_42 said:Since when? The speeds in our accelerators are far less than that - and we produce particles.
Here are some ideas (they might not work) to use the CMB to advantage.fresh_42 said:The physics is still the same.
This is wrong, because of relativity. The assumed speed of the spaceship makes the low energy photons hard (see the Wikipedia quote above). If we can even measure the bow wave of solar winds at the boundary of the heliosphere, then CMB definitely becomes an issue at near c speeds. But again: I provided the contact data of the person who made this claim. Send an email and ask the theoretical physicist who made this claim. If I had to choose whom I trust more ...DrStupid said:With the principle of relativity. Pair production requires hard gamma radiation. But in the rest frame of the ship there is just UVC radiation. If there is no pair production in the inertial rest frame of the ship than there is no pair production in any other inertial frame. That also excludes inverse Compton scattering from CBM to photons with sufficient energy.
That's a pop-science source and, critically, he doesn't give the energies (or spaceship speeds) he is talking about. I've estimated the speed in a previous post.fresh_42 said:This is wrong, because of relativity. The assumed speed of the spaceship makes the low energy photons hard (see the Wikipedia quote above). If we can even measure the bow wave of solar winds at the boundary of the heliosphere, then CMB definitely becomes an issue at near c speeds. But again: I provided the contact data of the person who made this claim. Send an email and ask the theoretical physicist who made this claim. If I had to choose whom I trust more ...
The medium is, the source is not. You didn't provide any source at all, and I provided the contact data as only possible reference (due to language issues), so that you can simply ask the author of the argument. And still, inverse Compton scattering is not pop science.PeroK said:That's a pop-science source and, critically, he doesn't give the energies (or spaceship speeds) he is talking about. I've estimated the speed in a previous post.
It's up to you to provide evidence of what energies we are talking about here.
This IS wrong. The CMB has a typical frequency of ##1.6 \times 10^{11} \ Hz##. Hard gamma rays have a frequency of about ##1.6 \times 10^{19} \ Hz##. That requires a gamma factor of about ##10^8## or a speed of ##(1 - \epsilon)c##, where ##\epsilon \approx 10^{-16}##.fresh_42 said:This is wrong, because of relativity. The assumed speed of the spaceship makes the low energy photons hard (see the Wikipedia quote above). If we can even measure the bow wave of solar winds at the boundary of the heliosphere, then CMB definitely becomes an issue at near c speeds. But again: I provided the contact data of the person who made this claim. Send an email and ask the theoretical physicist who made this claim. If I had to choose whom I trust more ...
Guess British (see quotation above) and German (see quotation above) universities are all wrong then and some guys on the internet are right. I have quoted two sources (one if we omit Wikipedia) that CMB is a source for inverse Compton scattering. Where is your proof it is not?PeroK said:This IS wrong. The CMB has a typical frequency of ##1.6 \times 10^{11} \ Hz##. Hard gamma rays have a frequency of about ##1.6 \times 10^{19} \ Hz##. That requires a gamma factor of about ##10^8## or a speed of ##(1 - \epsilon)c##, where ##\epsilon \approx 10^{-16}##.
The speed quoted by @DrStupid above, would boost the CMB to UV light - subject to checking that calculation.
Inverse Compton scattering is not particularly relevant for the limitations on spaceship travel. Your lack of knowledge and your pop-science source are leading you astray here.fresh_42 said:Guess British (see quotation above) and German (see quotation above) universities are all wrong then and some guys on the internet are right. I have quoted two sources (one if we omit Wikipedia) that CMB is a source for inverse Compton scattering. Where is your proof it is not?
PeroK said:Inverse Compton scattering is not particularly relevant for the limitations on spaceship travel. Your lack of knowledge and your pop-science source are leading you astray here.
Stay calm and learn some physics!
Do you really want to discuss on an ad hominem level? Well, I'm ready to do so. I think you should publish your knowledge then, if British and German professional astronomers teach it all wrong.PeroK said:Stay calm and learn some physics!