I Understanding Relativity: How Moving Objects Experience Near Light Speed Travel

[JerryF]

TL;DR Summary

The impossibility of mass travelling at c is given from an observers point of view, not the moving object's point of view

Light speed is impossible for anything with mass as more and more energy is required with increasing velocity. But this is only to an observer in a different reference frame. To the moving object, in its own reference frame, why would anything change regardless of how close to c it moves?

 

[anuttarasammyak]

Any object in its own frame has velocity zero. Nothing extraordinary takes place on him/her. Acceleration is another story.

 

[JerryF]

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.

 

[anuttarasammyak]

To launch a rocket and accelerate it to near light speed is much energy consuming task but possible in principle it is. For an example we succeed to get 0.999999991c speed proton in accelerator.　

 

[JerryF]

Thank you

 

[PeroK]

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?

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.

 

[PeroK]

To the moving object, in its own reference frame, why would anything change regardless of how close to c it moves?

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!

 

[jbriggs444]

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.

Velocity zero means velocity zero. Which is different from light speed.

 

[DrStupid]

To the moving object, in its own reference frame, why would anything change regardless of how close to c it moves?

It wouldn't change anything to the moving object, but to everything else. In that frame the rest of the universe is moving close to c and that makes a huge difference. Even the cosmic background radiation becomes a problem if you are fast enough.

 

[fresh_42]

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

 

[PeroK]

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 near is near?

 

[fresh_42]

How near is near?

How big is the ship?

 

[Grasshopper]

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

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.

 

[PeroK]

How big is the ship?

Small enough and streamlined enough to achieve the speed it wants to achieve!

 

[fresh_42]

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.

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.

My suspicion: inverse Compton effect.

 

[PeroK]

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.

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 $20 km/s$.

The theoretical limit for space travel is $< c$. I don't see how you can have another number like $0.97c$ as an absolute maximum. What's true is that any particular craft leaving Earth will have its own specific limit, depending on its engineering. That limit may be anything - and it's entirely possible the human race wil never do better than $0.1c$, or something like that.

 

[DrStupid]

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 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.

 

[fresh_42]

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 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.

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.

The CMB provides low energy photons which will be scattered at the electrons and protons of the spaceship. You will get a particle accelerator!

 

[fresh_42]

Wrong language, but here is the source and here are the contact data:
https://www.usm.uni-muenchen.de/people/lesch/lesch.html



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!

 

[PeroK]

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!

My German is no longer up to following everything he says, but he appears to be talking about the limit on high-energy cosmic ray protons, due to scattering the CMB:

https://en.wikipedia.org/wiki/Ultra-high-energy_cosmic_ray

The limit is $5 \times 10^{19}eV$, which implies a gamma factor of $5 \times 10^{10}$, which would give a speed of $(1 - \epsilon)c$, where $\epsilon = 2 \times 10^{-20}$.

That could be your new (realistic) cosmic speed limit. Although, that speed is already something that is so far beyond any engineering possibility as to be irrelevant. In particular

In your other post, to get to Andomeda in 28 years, requires a gamma factor of only $100,000$. The Cosmic ray limit above is, therefore, irrelevant to this and Andromeda in 28 spaceship years is still (theoretically) possible.

 

[PeroK]

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.

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]

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.

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]

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.

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.

 

[fresh_42]

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.

How do you discuss the inverse Compton scattering away?

 

[DrStupid]

How do you discuss the inverse Compton scattering away?

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.

 

[PeroK]

Since when? The speeds in our accelerators are far less than that - and we produce particles.

That's a different sort of collision: that's particle-particle collisions. Lesch's point is about particle-photon collisions.

The inverse Compton isn't relevant here. That's just photon scattering, not producing additional particles.

It's already clear that the CMB will melt any known material at speeds well below what we are talking about - this new constraint from Lesch applies at far higher energies.

Whether it's an engineering or a theoretical limit is a moot point.

 

[PeroK]

The physics is still the same.

Here are some ideas (they might not work) to use the CMB to advantage.

https://physics.stackexchange.com/q...ic-spaceship-cmb-radiation-and-thermodynamics

The point is that - by some unknown engineering ingenuity - you might get past Lesch's limit. It's not necessarily a limit the way $c$ is.

And, of course, NASA is seriously looking at Alcubierre ideas to circumvent that limitation.

 

[fresh_42]

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.

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 ...

 

[PeroK]

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 ...

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.

 

[fresh_42]

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.

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.
http://www-astro.physics.ox.ac.uk/~garret/teaching/lecture8-2014-2015.pdf (p.13f.)

 

[PeroK]

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 ...

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.

 

[fresh_42]

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.

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]

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?

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!

 

[fresh_42]

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!

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.

 

[fresh_42]

5 Joule are necessary to scatter CMB.
https://cds.cern.ch/record/384407/files/9904108.pdf (p.35)

 

[fresh_42]

Inverse Compton scattering is not particularly relevant for the limitations on spaceship travel.

Near speed of light? Where is your evidence for that claim? And some hand wavy numbers do not count. They are worth even less than pop science.

 

[DrStupid]

The assumed speed of the spaceship makes the low energy photons hard (see the Wikipedia quote above).

I read the quote and didn't find enything that supports your claim.

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.

That depends on the definition of "near c speeds". At least 0.999999991 c is nowhere near the limit you are talking about.

I provided the contact data of the person who made this claim.

I watched the video and didn't find enything that supports your claim.

 

[fresh_42]

I watched the video and didn't find enything that supports your claim.

Seen, but probably not understood.
0:10
3:50
5:00
5:30-6:30
etc.

 

[weirdoguy]

If I had to choose whom I trust more ...

The most important thing that PF tought me is that you can't trust pop-sci sources, no matter who is talking. Even Hawking wrote dobious stuff in his pop-sci books. I would've expected that Mentors know this...

 

[fresh_42]

The most important thing that PF tought me is that you can't trust pop-sci sources, no matter who is talking. Even Hawking wrote dobious stuff in his pop-sci books. I would've expected that Mentors know this...

I cited a total of 4 sources, 2 of them scientific sources, one an ordinary professor.

And I haven't seen a single reference that they are all wrong. Those who can read have a clear advantage. Sorry, but I really doubt that a spaceship can be accelerated close to c without providing more energy to its own electrons than 5 Joule. However, I would appreciate to see such a calculation. Maybe in the science jokes forum?

 

[PeroK]

Sorry, but I really doubt that a spaceship can be accelerated close to c without providing more energy to its own electrons than 5 Joule. However, I would appreciate to see such a calculation. Maybe in the science jokes forum?

$5J$ is a lot of energy for an electron.

 

[DrStupid]

Seen, but probably not understood.

You or me?

0:10 - SRT supports interstellar (or even intergalactic) space travel by time dilation and length contraction
3:50 - about CMB
5:00 - blueshift of CMB in front of the ship
5:30-6:30 - pair production if energy is high enough (example: ultra-high-energy cosmic rays)

I still do not see how that supports your claim.

 

[fresh_42]

$5J$ is a lot of energy for an electron.

An entire spaceship accelerated to say .8c, too. The spaceship itself becomes a cosmic ray, not the CMB. This blue shifts to X-ray frequencies. Both together are enough scattering energy for particle production.

 

[fresh_42]

You or me?

0:10 - SRT supports interstellar (or even intergalactic) space travel by time dilation and length contraction
3:50 - about CMB
5:00 - blueshift of CMB in front of the ship
5:30-6:30 - pair production if energy is high enough (example: ultra-high-energy cosmic rays)

I still do not see how that supports your claim.

I'm happy that CERN does. Still better than some guys on the internet. This is getting ridiculous. I want to see the proof of your statement: "CMB is irrelevant for a rocket accelerated to say 0.8c"

 

[vanhees71]

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 ...

Sure, if you travel with a speed $v=\beta c$ relative to the restframe of the CMBR the CMBR em. waves propagating directly against the direction of $v$ lead to the (maximal) blue shift of
$$\omega'=\sqrt{\frac{1+\beta}{1-\beta}} \omega.$$
In fact in this direction you observe a black-body spectrum with the correspondingly higher "effective temperature"
$$T_{\text{eff}}'=\sqrt{\frac{1+\beta}{1-\beta}} T,$$
where $T$ is the proper invariant (scalar) temperature measured by a thermometer in the CMBR rest frame. If $\beta \rightarrow 1$ you get arbitrary high temperatures leading to a blue shift of the CMBR to any hard-$\gamma$-ray range you like.

[EDIT: To clarify in view of #46] Of course the quantities with primes refer to what's observed in the space ship's restframe and the unprimed ones refer to the CMBR restframe.

 

[PeterDonis]

The spaceship itself becomes a cosmic ray, not the CMB. This blue shifts to X-ray frequencies. Both together are enough scattering energy for particle production.

You're double counting here. You can't blueshift the CMB and also treat the spaceship itself as a cosmic ray. If you are blueshifting the CMB, then you are working in the spaceship's rest frame, which means the spaceship's kinetic energy is zero. Or if you call the spaceship a cosmic ray, then you are working in the CMB rest frame, which means the CMB temperature is 2.7 K. You have to pick one; you can't take the higher energy value from both.

 

[PeroK]

An entire spaceship accelerated to say .8c, too. The spaceship itself becomes a cosmic ray, not the CMB. This blue shifts to X-ray frequencies. Both together are enough scattering energy for particle production.

$0.8c$ relative to the comoving frame is not, however, a barrier which a spacecraft could not possibly overcome.

 

[fresh_42]

You're double counting here. You can't blueshift the CMB and also treat the spaceship itself as a cosmic ray. If you are blueshifting the CMB, then you are working in the spaceship's rest frame, which means the spaceship's kinetic energy is zero. Or if you call the spaceship a cosmic ray, then you are working in the CMB rest frame, which means the CMB temperature is 2.7 K. You have to pick one; you can't take the higher energy value from both.

Yeah, that's true. Let's take the protons then. The question remains: How far can we accelerate a massive object before inverse Compton scattering from CMB becomes an issue? My claim is, long before .8c, others say it's irrelevant. CERN says 5 Joule are necessary to start the process. The cosmological background is that we observed hard cosmic rays scattering at CMB. Is there any reason a rocket would make an exception?

 

[fresh_42]

$0.8c$ relative to the CMB is not, however, a barrier which a spacecraft could not possibly overcome.

The hypothesis is, that energy invested in acceleration will be consumed by the production of pions at some stage, and thus not available anymore for further acceleration. The question can only be: where is that barrier? If Lesch is right and there are 400 photons CMB in every cubic centimeter of space, then I only claim that this is enough to become an issue long before c.

 

[DrStupid]

I'm happy that CERN does.

Provide a proper reference.

I want to see the proof of your statement: "CMB is irrelevant for a rocket accelerated to say 0.8c"

Just do the math. With

$\beta = 1 - \varepsilon$

blueshift is

$f' = f \cdot \sqrt {\frac{{1 + \beta }}{{1 - \beta }}} = f \cdot \sqrt {\frac{2}{\varepsilon } - 1} \approx f \cdot \sqrt {\frac{2}{\varepsilon }}$

The maximum of the CMB is at 282 GHz and 0.8 c corresponds to $\varepsilon = 0.2$. That results in 846 GHz for the maximum of the blueshifted CMB. That means we are talking about IR radiation corresponding to a temperature of 8.6 K. How is that relevant for a rocket?