6 ms·

I think that when you send out photons, there is energy in them. Like, energy you would have rather have converted into momentum. ...IANAP.

fixermark 12y ago
The momentum those photons are carrying implies a change in momentum to the emitter that is equal and opposite. Conservation of momentum applies even though the emitted substance is electromagnetic radiation, not matter particles. What's weird about this device is that the radiation chamber is completely closed---the microwaves emitted are then re-absorbed by the chamber, but the claim is that the net momentum of the chamber has changed... Somehow. If it is true, it'd be a violation of conservation of momentum (unless the microwaves are somehow passing momentum to something else, which is why people are talking about quantum mechanics possibly coming into play).
- daveloyall 12y ago
  We're well beyond my physics knowledge here. (See IANAP above!) So, I have questions. Is momentum the only type of energy a photon has? By allegory: Suppose a cannonball can be red hot, or cool. Separately, it can be flying or at rest. So, there are four cannonball types: hf, hr, cf, and cr. The hot, flying cannonball has the most energy and the cool, resting cannonball has the least. Yet, the momentum of both flying cannonballs is equal--temperature is orthogonal. Is all that right? If it is, then... I thought photons contain some energy that is similarly orthogonal to momentum. ...I guess I don't recall learning that, so maybe I just made it up. But, I have seen videos of laser pointers that can burn paper--they don't seem to have recoil. If the energy that is burns that paper is ... momentum, then I've clearly misunderstood the SCALE of mechanical energy vs. thermal energy. But if the thermal (or something?!) energy is ... separate, then... Well, then we're back to the original point: most of the energy you use to create a photon isn't translated into momentum, so light isn't the propellant you're looking for. NB I don't have a comment about the microwave chamber other than "NFW.".
  - MereInterest 12y ago
    IAAP, so let me take a stab at answering it. Each photon can be completely described by three values. Where is it? Which direction is it traveling? What is its energy? For photons, unlike the cannonball, there are no additional places to store energy. So, what does that mean for the laser example? Because you are absolutely right that lasers can ignite paper. Photons carry both energy and momentum, but the two cannot be separated. If the paper takes the energy, it must also take away the momentum. The key equation for light is "E = pc", energy equals momentum times the speed of light. The more the energy, the more the momentum. Reduce the energy (or transfer it into the paper Let's start playing with some numbers. I like drinking tea, and I don't like waiting very long for the water to boil. So, I take out my 1 megawatt laser, and bring a cup of water to boil in about 0.5 second. The lasers you have seen igniting paper are usually 50 milliwatt, so we are talking 20 million times stronger. If there is an effect, surely it would be visible here. So, that E=pc formula still works here. Because I am transferring energy to my cup of water, I must also be transferring momentum. The question is, how much momentum? To determine this, I rearrange the formula to p = E/c. Using the magic of google to handle the unit conversions, I find that I have about 0.001 pounds of force. The friction of the cup against the table is far, far more than enough to keep the cup from showing any reaction whatsoever. https://www.google.com/?gws_rd=ssl#q=1+megawatt+%2F+speed+of+light+in+pounds https://www.google.com/?gws_rd=ssl#q=1+megawatt+%2F+speed+of... You are asking absolutely great questions. It is fun to hear somebody not accepting things at face value, comparing to known experiences, and figuring out how to make them fit. Keep up the great work.
    - daveloyall 12y ago
      Thank you for this. Let's see, I think a million watts is a bit more than we need to 235 mL of water in 0.5 seconds. A calorie (not a Calorie) is the amount of energy it takes to raise 1 ml of water 1 deg C. Suppose 15C water (pipes run through the ground which is cool). https://www.google.com/search?q=(85+calories+*+235)+%2F+0.5+seconds https://www.google.com/search?q=(85+calories+*+235)+%2F+0.5+... Is that right? While a megawater later is overkill for your cup of tea, it is NOT overkill for ten cups of tea (in half a second)? For some reason, I always thought of a megawatt as BIGGER. Or, I guess I thought of boiling water as SMALLER. In any case, there's still something I don't get here. Obviously if you are trying to change the velocity of your spacecraft and all you have is a cup of water and a megawatt laser, you're not going to point the laser into space, you're going to point it at the cup. (Which happens to be roughly and conveniently conical.) Picture this: point your megawatt laser at your friend's spacecraft and her cup of water... She'll get delta-V when her water boils, but that's AFTER you already got a little (very little) recoil from firing the laser. I guess I don't understand the relationship between momentum and mass when mass is zero.

fixermark 12y ago

The momentum those photons are carrying implies a change in momentum to the emitter that is equal and opposite. Conservation of momentum applies even though the emitted substance is electromagnetic radiation, not matter particles. What's weird about this device is that the radiation chamber is completely closed---the microwaves emitted are then re-absorbed by the chamber, but the claim is that the net momentum of the chamber has changed... Somehow. If it is true, it'd be a violation of conservation of momentum (unless the microwaves are somehow passing momentum to something else, which is why people are talking about quantum mechanics possibly coming into play).

daveloyall 12y ago
We're well beyond my physics knowledge here. (See IANAP above!) So, I have questions. Is momentum the only type of energy a photon has? By allegory: Suppose a cannonball can be red hot, or cool. Separately, it can be flying or at rest. So, there are four cannonball types: hf, hr, cf, and cr. The hot, flying cannonball has the most energy and the cool, resting cannonball has the least. Yet, the momentum of both flying cannonballs is equal--temperature is orthogonal. Is all that right? If it is, then... I thought photons contain some energy that is similarly orthogonal to momentum. ...I guess I don't recall learning that, so maybe I just made it up. But, I have seen videos of laser pointers that can burn paper--they don't seem to have recoil. If the energy that is burns that paper is ... momentum, then I've clearly misunderstood the SCALE of mechanical energy vs. thermal energy. But if the thermal (or something?!) energy is ... separate, then... Well, then we're back to the original point: most of the energy you use to create a photon isn't translated into momentum, so light isn't the propellant you're looking for. NB I don't have a comment about the microwave chamber other than "NFW.".
- MereInterest 12y ago
  IAAP, so let me take a stab at answering it. Each photon can be completely described by three values. Where is it? Which direction is it traveling? What is its energy? For photons, unlike the cannonball, there are no additional places to store energy. So, what does that mean for the laser example? Because you are absolutely right that lasers can ignite paper. Photons carry both energy and momentum, but the two cannot be separated. If the paper takes the energy, it must also take away the momentum. The key equation for light is "E = pc", energy equals momentum times the speed of light. The more the energy, the more the momentum. Reduce the energy (or transfer it into the paper Let's start playing with some numbers. I like drinking tea, and I don't like waiting very long for the water to boil. So, I take out my 1 megawatt laser, and bring a cup of water to boil in about 0.5 second. The lasers you have seen igniting paper are usually 50 milliwatt, so we are talking 20 million times stronger. If there is an effect, surely it would be visible here. So, that E=pc formula still works here. Because I am transferring energy to my cup of water, I must also be transferring momentum. The question is, how much momentum? To determine this, I rearrange the formula to p = E/c. Using the magic of google to handle the unit conversions, I find that I have about 0.001 pounds of force. The friction of the cup against the table is far, far more than enough to keep the cup from showing any reaction whatsoever. https://www.google.com/?gws_rd=ssl#q=1+megawatt+%2F+speed+of+light+in+pounds https://www.google.com/?gws_rd=ssl#q=1+megawatt+%2F+speed+of... You are asking absolutely great questions. It is fun to hear somebody not accepting things at face value, comparing to known experiences, and figuring out how to make them fit. Keep up the great work.
  - daveloyall 12y ago
    Thank you for this. Let's see, I think a million watts is a bit more than we need to 235 mL of water in 0.5 seconds. A calorie (not a Calorie) is the amount of energy it takes to raise 1 ml of water 1 deg C. Suppose 15C water (pipes run through the ground which is cool). https://www.google.com/search?q=(85+calories+*+235)+%2F+0.5+seconds https://www.google.com/search?q=(85+calories+*+235)+%2F+0.5+... Is that right? While a megawater later is overkill for your cup of tea, it is NOT overkill for ten cups of tea (in half a second)? For some reason, I always thought of a megawatt as BIGGER. Or, I guess I thought of boiling water as SMALLER. In any case, there's still something I don't get here. Obviously if you are trying to change the velocity of your spacecraft and all you have is a cup of water and a megawatt laser, you're not going to point the laser into space, you're going to point it at the cup. (Which happens to be roughly and conveniently conical.) Picture this: point your megawatt laser at your friend's spacecraft and her cup of water... She'll get delta-V when her water boils, but that's AFTER you already got a little (very little) recoil from firing the laser. I guess I don't understand the relationship between momentum and mass when mass is zero.